ocr as level chemistry a delivery guide - theme: enthalpy...

CHEMISTRY A

AS LEVELDelivery Guide

H032

Theme: Enthalpy ChangesApril 2015

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CONTENTS

Introduction Page 4

Curriculum Content Page 5

Thinking Conceptually Page 7

Thinking Contextually Page 14

Learner Resources Page 16

AS LEVELCHEMISTRY A

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Delivery guides are designed to represent a body of knowledge about teaching a particular topic and contain:

• Content: A clear outline of the content covered by the delivery guide;

• Thinking Conceptually: Expert guidance on the key concepts involved, common difficulties students may have, approaches to teaching that can help students understand these concepts and how this topic links conceptually to other areas of the subject;

• Thinking Contextually: A range of suggested teaching activities using a variety of themes so that different activities can be selected which best suit particular classes, learning styles or teaching approaches.

If you have any feedback on this Delivery Guide or suggestions for other resources you would like OCR to develop, please email [email protected].

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Introduction

mailto:resourcesfeedback%40ocr.org.uk?subject=

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The specification states the following learning outcomes.

3.2.1 Enthalpy changes (a) explanation that some chemical reactions are accompanied by enthalpy changes that are exothermic (ΔH, negative)

or endothermic (ΔH, positive)

(b) construction of enthalpy profile diagrams to show the difference in the enthalpy of reactants compared with products

(c) qualitative explanation of the term activation energy, including use of enthalpy profile diagrams

(d) explanation and use of the terms:

(i) standard conditions and standard states (physical states under standard conditions)

(ii) enthalpy change of reaction (enthalpy change associated with a stated equation)

(iii) enthalpy change of formation (formation of 1 mol of a compound from its elements)

(iv) enthalpy change of combustion (complete combustion of 1 mol of a substance)

(v) enthalpy change of neutralisation (formation of 1 mol of water from neutralisation)

(e) determination of enthalpy changes directly from appropriate experimental results, including use of the relationship: q = mcΔT

(f ) (i) explanation of the term average bond enthalpy (breaking of 1 mol of bonds in gaseous molecules)

(ii) explanation of exothermic and endothermic reactions in terms of enthalpy changes associated with the breaking and making of chemical bonds

(iii) use of average bond enthalpies to calculate enthalpy changes and related quantities (see also 2.2.2 f )

(g) Hess’ law for construction of enthalpy cycles and calculations to determine indirectly:

(i) an enthalpy change of reaction from enthalpy changes of combustion

(ii) an enthalpy change of reaction from enthalpy changes of formation

(iii) enthalpy changes from unfamiliar enthalpy cycles

(h) the techniques and procedures used to determine enthalpy changes directly and indirectly.

Curriculum Content

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Students will need to be able to

Construct enthalpy profile diagrams to show the difference in the enthalpy of reactants compared with products

Give a qualitative explanation of the term activation energy, including use of enthalpy profile diagrams

Explain and use the terms exothermic and endothermic reactions in terms of enthalpy

Understand changes associated with the breaking and making of chemical bonds

Give an explanation of the term average bond enthalpy (breaking of 1 mol of bonds in gaseous molecules)

Use average bond enthalpies to calculate enthalpy changes and related quantities

Recognise and recall the techniques and procedures used to determine enthalpy changes directly and indirectly.

Use Hess’ law for construction of enthalpy cycles and calculations to determine indirectly:

• An enthalpy change of reaction from enthalpy changes of combustion

• An enthalpy change of reaction from enthalpy changes of formation

Curriculum Content

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In the following sections it is very important to remember that students still have issues differentiating between temperature and heat, i.e. one being the measurement (measured in oC or K depending upon the context) and the other being the energy term (measured in Joules (J). Some time should be spent beforehand probing students’ ideas before undertaking this section of work.

Experiments for this can be found:

http://www.nuffieldfoundation.org/practical-physics/mixing-hot-and-cold-water

http://www.nuffieldfoundation.org/practical-physics/heat-and-temperature

Approaches to teaching the content

Students need to be aware that when something feels cold that this a transfer of heat energy away from your hand and when something feels warm then this is a transfer of heat energy to the hand. With this in mind the following simple experiments can be carried out to allow the students to experience this.

The initial idea of the experiment is to determine if the temperature of the reaction increases or decreases. In other words if the reaction temperature increases then there is heat energy given out from the reaction so the reaction is EXOTHERMIC. Also, if the temperature of the reaction decreases then heat energy is being taken in from the surroundings so this reaction is ENDOTHERMIC.

Thinking Conceptually

Exothermic and Endothermic Reactions





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Reaction profile diagrams are a way of introducing students to show how the reaction has proceeded in a graphical/diagrammatical way. It is important for them to fully understand what is actually going on at the bond breaking level in order to fully appreciate the concept. In this concept it is very important that the student understands that the energy given out is what is left over after the new bonds have been formed in Exothermic reactions for example and vice versa.

Approaches to teaching the content

This section requires the uses of molecular models such as Molymods to show the concept.

(In this section it is assumed that the students can already construct and balance equations as this relies on the understanding that mass is always conserved and so this would follow that energy must also be conserved is a very important concept for the teaching of the whole of this module)

A simple model of a reaction should now be considered. The reaction between methane gas and oxygen - an everyday reaction. Students will be aware that the burning of methane in oxygen as in a Bunsen burner is an exothermic reaction.

A model of a methane molecule with 2 oxygen molecules should be constructed and then the products should then be constructed to show how to make carbon dioxide and 2 waters bonds of the methane and the oxygen must be broken and then reconstructed. Thus the excess energy given out is then as a result of the excess energy left over being given out.

CH4 + O2 ------------ CO2 + H20

This should then overcome the misconception.

Further information can be found at: www.rsc.org : Vanessa Kind – Students misconceptions about basic chemical ideas.

It should now be pointed out to the students that the energy given out in Exothermic reactions is given as a negative value and a positive value in Endothermic reactions

The different types of reactions, both Endothermic and Endothermic, can be shown diagrammatically by using reaction profile diagrams.

The following Youtube video will help to describe this:

https://www.youtube.com/watch?v=3GQ1ZLZB2i8

The students should then be encouraged to draw reaction profile diagrams for the reactions encountered in the reactions they carried out in the previous task.

Points to note as far as examination techniques are concerned:

• Both axes need to be labelled with time on the X-axis and energy on the Y-axis.

• If the reaction is exothermic then the final energy of the products will be less than the starting energy and vice versa.

• The Activation energy defined: the minimum quantity of energy which the reacting species must possess in order to undergo a specified reaction. This should be labelled with a double ended arrow with Ea

• The reactants and products should be labelled.


Reaction Profile Diagrams

www.rsc.org

https://www.youtube.com/watch?v=3GQ1ZLZB2i8

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Students should now have an idea about how reactions can be described as in terms of energy gained or energy lost depending upon how much energy is either lost or gained after the process of bonds being broken and formed has been carried out. They should now be able to describe this using the correct term of Enthalpy Change which is given the symbol ΔH (Δ meaning change in and H meaning enthalpy or energy). Students should be made aware at this point that this is measured in J mol-1.

In this unit there are four definitions that the students should be able to define in order to attain the marks on a paper. The usefulness and uses of enthalpy changes are explored further in the full A Level when the topic of thermodynamics is studied in further detail.

These definitions are often examined and as such the standard conditions and states are often required. Students need to know what they are and how they are defined. Students often miss these out when defining such quantities and lose marks.

Reference: Examiners reports found online

http://www.ocr.org.uk/i-want-to/download-past-papers/

Definitions

Standard Enthalpy Change of Reaction (ΔrH) - The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.

Enthalpy change of formation (ΔfH) - The enthalpy change that takes place when one mole of a compound in its standard state is formed from its constituent elements in their standard states under standard conditions.

Enthalpy change of combustion (ΔcH) - The enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions.

Enthalpy change of neutralisation (ΔnH) - The change in enthalpy that occurs when one equivalent of an acid and one equivalent of a base undergo a neutralization reaction to form water and a salt. It is a special case of the enthalpy of reaction. It is defined as the energy released with the formation of 1 mole of water.

Standard conditions for these are always:

• A pressure of 100kilopascals (102kPa)

• A temperature of 298K (25oC)

• Reactants and products in physical states, normal for the above conditions.

• A concentration of 1.0mol dm-3 for solutions.See attached card sort as an activity (Teacher Resource 1 and Learner Resource 1)

The activity will test the student’s ability to spot the correct definition, standard condition and nomenclature needed. There are some incorrect answers; these can be removed if the groups are struggling with the process. The incorrect answers could then be put in at a later date to check understanding as a starter for the next lesson for example.


Enthalpy Changes

http://www.ocr.org.uk/i-want-to/download-past-papers/

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Measurement of enthalpy changes directly by experimentationResearch has shown that students have a deep rooted issue with energy being used up rather than being conserved. This is gained from their everyday experiences such as petrol being used up in car etc. This misconception needs to be addressed before the next topic is considered. The teacher could revisit this by looking at the ways in which heat is absorbed by the hand from different sources including the differences between black and white surfaces.

http://www.nuffieldfoundation.org/practical-physics/absorbing-radiant-energy-different-surfaces

Alternatively having one student drink a hot drink and one drink a cold drink and use an infra red thermometer to show the differences in temperature of the outside of the throats of students before and after drinking the material. A Health and safety warning should be made here with regards to drinking hot liquids.



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Activities Resources

Teaching sequence

How does the manufacturer of a food stuff determine how much energy is contained within a bag of crisps for example?

A few bags of crisps could be examined here to find out which has the most energy within them, there could be an opportunity for students to research which bags contain the least amount of energy. Which would they recommend to be the ones with the least energy?

The Bomb Calorimeter

The following video explains how a Bomb Calorimeter works:

https://www.youtube.com/watch?v=ohyA9amFfsc

Rise in temperature of the water is recorded and the total energy is then calculated.

This can be replicated in the lab by carrying out a simple experiment and using the equation in Learner Resource 3 to calculate the energy given out per mole of chemical.

See Teacher Resource 2 and Learner Resource 2

Further examples:




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(b) Solid ammonium thiocyanate, NH4SCN, reacts with solid barium hydroxide, Ba(OH)2, as shown in the equation below.

2NH4SCN(s) + Ba(OH)2(s) Ba(SCN)2(s) + 2H2O(l) + 2NH3(g)

A research chemist carries out an experiment to determine the enthalpy change of this reaction.

In the experiment, 15.22g of NH4SCN is reacted with the slight excess of Ba(OH)2. The reaction absorbs energy, cooling the 50.0g of water from 21.9oC to 10.9oC.

(i) Calculate the energy absorbed, in KJ, during this reaction.

The specific heat capacity of the water = 4.2Jg-1K-1.

energy = ............................................kJ (2)

Question taken from OCR Chemistry A F322 Jan 11 question paper - question 3b


thermometer

50g of water

boiling tube

mixture of ammonium thiocynate and barium hydroxide

insulated beaker

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Question taken from OCR Chemistry A F322 Jan 12 question paper - question 3a


(a) A student investigates the reaction between magnesium and dilute hydrochloric acid.

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

The student determines the enthalpy change for this reaction.

In her experiment, she reacts 0.486g of magnesium with 50.0cm3 of 2.00mol dm-3 HCl(aq). The HCl(aq) is in the excess.

(i) Calculate the energy released, in KJ, during this reaction.

The specific heat capacity of the water = 4.18Jg-1K-1.

The density of the solution in 1.00g cm-3.

energy = ............................................kJ (2)

(ii) Calculate the amount, in moles of magnesium used by the student.

amount = ............................................mol (1)

(iii) Calculate the enthalpy change of the reaction.

Give your answer to three significant figures.

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ACTIVITIES

Thinking Contextually


Exothermic and Endothermic ReactionsThere are many examples in everyday life where both endothermic and exothermic reactions happen. For example, self-heating cans for expedition food as used by outdoors enthusiasts provide an example of exothermic reactions. The cooling packs used in sports injuries are examples of endothermic reactions where two chemicals mix to produce an endothermic reaction to reduce the swelling.

Reference: Journal of chemical education 1994 71 p1056 – Probing student’s misconceptions in student’s writing

Reaction Profile DiagramsFollowing the activity in the conceptual chapter on reaction profile diagrams, students should be made aware at this point that all reactions begin as endothermic processes as the diagrams show as energy is required to push the reaction over the activation energy boundary. This could be related to the initial idea that methane gas will burn in oxygen. However, this reaction will not proceed unless either a flame or a spark is present. This could be related to the misconception that mobile phones should not be used when filling up your car with petrol or diesel. However, you should not smoke whilst filling up your car. Should the clothing you wear also be taken into account?

https://www.youtube.com/watch?v=QkJdaU92Ln8



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Indirect measurement of enthalpyStudents have already been introduced to the concept of enthalpy changes being described as the energy left over when all of the bonds have been broken and reformed. It is important as the research shows that these topics are not taught as individual concepts as deep learning does not take place.

Ref : Chemical educational research and practice 2004, 5,301-305

So in this section the indirect measurement of enthalpy changes will be considered. This is done by considering both the use of average bond enthalpy calculations and Hess’s Law to calculate enthalpies of formations and combustion.

In the teaching of using Bond Enthalpies it is very important that the students know and keep referring back to the definition of a Bond Enthalpy.

The definition: the average enthalpy change that takes place when breaking 1 mole of a given bond in the molecule of a gaseous species under standard conditions.

The important part that is often left out of definitions given at A Level is the term gaseous species, this should be referred to regularly.

Students should also be made aware that they will not need to remember the actual values and that these will always be given.

See Learner Resources 3, 4 and 5.

Thinking Contextually

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Teacher Resource 1 – Enthalpy definitions Card Sort

Standard Enthalpy of a

reaction

The enthalpy change that takes place

when one mole of a compound is formed in its standard state from its constituent

elements in their standard states under standard conditions.

Enthalpy change of

neutralisation

Enthalpy change of combustion

The enthalpy change when

one mole of an element or a

compound reacts completely

with oxygen under standard

conditions.

The enthalpy change that occurs when

one mole of an acid and one mole of base undergo a

neutralisation reaction to form water and a

salt. It is defined as the energy released with the formation of one

mole of water.

The enthalpy change that

accompanies a reaction in the reaction

under standard conditions.

Group 1:

Group 3:

Group 2:

Group 4:

ΔrH

ΔrH

ΔcH

Enthalpy change of formation

ΔnH

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Teacher Resource 1 – Enthalpy definitions Card Sort


accompanies a reaction in the

molar quantities expressed

in a reaction under standard

conditions.





mole of salt.

Standard conditions

25OC 1 Atmosphere of pressure

100 kPa 1 mol dm-3

ΔformH


one mole of an acid and one mole of base

undergo a titration to form water and a


mole of salt.

Reactants and products in the

state they are in at room temperature

and pressure.

ΔoxH

Group 5:

298K

Incorrect cards:

1 mol dm-3

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Learner Resource 1 – Enthalpy definitions Card Sort

Standard Enthalpy of a

reaction

Enthalpy change of formation


accompanies a reaction in the

molar quantities expressed

in a reaction under standard

conditions.

The enthalpy change that takes place

when one mole of a compound is formed in its standard state from its constituent

elements in their standard states under standard conditions.

Enthalpy change of

neutralisation

Enthalpy change of combustion

The enthalpy change when

one mole of an element or a

compound reacts completely

with oxygen under standard

conditions.

The enthalpy change that occurs when one

mole of an acid and one mole of base undergo a neutralisation reaction

to form water and a salt. It is defined as the

energy released with the formation of one

mole of salt.

ΔrH

ΔfH

Standard conditions

25OC

ΔcH

298K

1 Atmosphere of pressure

100 kPa 1 mol dm-3

ΔformH

1 mol m-3





mole of water.


accompanies a reaction in the reaction

under standard conditions.


one mole of an acid and one mole

of base undergo a titration to form water and a salt. It

is defined as the energy released with the formation of one

mole of salt.

Reactants and products in the

state they are in at room temperature

and pressure.

ΔoxH

ΔnH

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Teacher Resource 2 – Measurement of enthalpy changes directly by experimentation (to be used with Learner Resource 2)

The following experiment can be carried out to find the temperature rise or drop of each reaction:

http://www.nuffieldfoundation.org/practical-chemistry/exothermic-metal-displacement-reactions

The equation used is:

q = mcΔT

m is the mass of the water used remember water has a density of 1 g cm-3

c is the specific heat capacity of water (4.2 kJ mol-1 K-1)

Whilst carrying out the experiment, the temperature of the copper sulphate should be measured every 30 seconds for 4 minutes. The metal should then be added and the temperature rise should be recorded every 30 seconds and noted. This is recorded until the temperature rises stops for at least three readings. The recordings should then be plotted on a graph of temperature rise on the y-axis vs time on the x-axis.

The following example can be used as a guide to calculate the enthalpy change per mole.

In the following reaction:

250cm3 of water was heated by a pentan-1-ol burner for a period of 10 minutes. The pentan-1-ol burner had a starting mass of 1.76g and after 10 minutes had a mass of 0.0g. The following results were obtained.

Time / seconds Temperature / oC

0 24

30 24

60 24

90 24

120 24

150 24

180 24

210 25

240 35

270 40

300 45

330 50

360 55

390 61

420 68

450 74

480 78

510 78

540 78

570 78

600 78



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Teacher Resource 2 – Measurement of enthalpy changes directly by experimentation

The calculation is as follows:

q = mcΔT

m -mass of water = 250g

c – specific heat capacity of water = 4.2

ΔT – change in temperature = 54oC

q = 250 x 4.2 x 54

q =56700 Joules for 1.76g of pentan-1-ol

To calculate into J mol-1

1.76 / 88 = 0.02 moles

56700 / 0.02 = 2835000 Jmol-1

Or 2835 kJ mol-1

This is then converted to -2835 kJ mol-1 as the reaction is exothermic.

This should be emphasised to the students as they will lose marks if the correct sign is not assigned.

This figure also differs from the actual value as heat is lost to the surroundings unlike the bomb calorimeter which is a closed system. This is an important stipulations as research shows students struggle with the difference of heat loss in systems and surroundings.

Ref: Physics education 1977 12 p248

Calculation taken from F322 Chem A paper OCR wed 3 June 2009

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Teacher Resource 2 – Measurement of enthalpy changes directly by experimentation

An example of a bomb calorimeter:

By courtesy of Encyclopaedia Britannica, Inc., copyright 1997; used with permission.

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Learner Resource 2 – Measurement of enthalpy changes directly by experimentation

The following experiment can be carried out to find the temperature rise or drop of each reaction:


The equation used is:

q = mcΔT

m is the mass of the water used remember water has a density of 1 g cm-3

c is the specific heat capacity of water (4.2 kJ mol-1 K-1)

Whilst carrying out the experiment, the temperature of the copper sulphate should be measured every 30 seconds for 4 minutes. The metal should then be added and the temperature rise should be recorded every 30 seconds and noted. This is recorded until the temperature rises stops for at least three readings. The recordings should then be plotted on a graph of temperature rise on the y-axis vs time on the x-axis.

The following example can be used as a guide to calculate the enthalpy change per mole.

In the following reaction:

250cm3 of water was heated by a pentan-1-ol burner for a period of 10 minutes. The pentan-1-ol burner had a starting mass of 1.76g and after 10 minutes had a mass of 0.0g. The following results were obtained.

Time / seconds Temperature / oC

0 24

30 24

60 24

90 24

120 24

150 24

180 24

210 25

240 35

270 40

300 45

330 50

360 55

390 61

420 68

450 74

480 78

510 78

540 78

570 78

600 78

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The calculation is as follows:

q = mcΔT

m -mass of water = 250g

c – specific heat capacity of water = 4.2

Using the graph work out ΔT and then calculate the heat energy q.

Tem

pera

ture

/0C

Time /s

Plot the data on the graph below


Tem

pera

ture

/0C

Time /s

Plot the data on the graph below

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Show all your working to give the value in kJ mol—1


25


An example of a bomb calorimeter:

By courtesy of Encyclopaedia Britannica, Inc., copyright 1997; used with permission.

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Teacher Resource 3 – Bond Enthalpies (to be used with Learner Resource 3)

Example calculation:

ΔH = the sum of the bonds broken – the sum of the bonds formed

Or

ΔH = Σ bonds broken – Σ bonds formed


This should be done in conjunction with molecular models to reinforce the idea that the Enthalpy change is the energy left over after the energy from the bonds being broken and reformed is calculated.

The table below shows the values of some average bond enthalpies.

Average bond enthalpy /kJ mol–1

C–H +410

O–H +465

O=O +500

C=O +805

C–O +336

The equation below shows the combustion of methanol, CH3OH, in the gaseous state.

CH3OH(g) + 1½O2(g)–CO2(g) + 2H2O(g)

Using the average bond enthalpies in the table above, calculate the enthalpy change of combustion, ΔcH, of gaseous methanol.

ΔcH = ............................................. kJ mol–1

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Teacher Resource 3 – Bond Enthalpies (to be used with Learner Resource 3)

Answer:

Bonds broken: 3(C–H) + (C–O) + (O–H) + 1.5 (O=O) = 2781 kJ

Bonds made: 2(C=O) + 4(O–H) = 3470 kJ

ΔcH =−689 (kJ mol–1)

This question was taken from the sample material Chemistry A F322 http://www.ocr.org.uk/Images/75089-unit-f322-chains-energy-and-resources-specimen.pdf

Example Calculations for bond enthalpies

Bond H-H C-C C=C N-H O-H O-O O=O C-H N=N C=O

MBE / kJ mol-1 436 348 612 388 463 146 496 412 944 743

Calculate the enthalpy changes in the following:

1. Ethene + hydrogen gas Ethane Answer = -136kJmol-1

2. Hydrogen Peroxide Hydrogen peroxide +1/2 Oxygen Answer = -102kJmol-1

3. The formation of ammonia from its constituent elements. Answer = -38kJmol-1

4. Propene + oxygen Carbon dioxide and water Answer = -1572kJmol-1

http://www.ocr.org.uk/Images/75089-unit-f322-chains-energy-and-resources-specimen.pdf

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Learner Resource 3 – Bond Enthalpies


ΔH = the sum of the bonds broken – the sum of the bonds formed

Or

ΔH = Σ bonds broken – Σ bonds formed


This should be done in conjunction with molecular models to reinforce the idea that the Enthalpy change is the energy left over after the energy from the bonds being broken and reformed is calculated.

The table below shows the values of some average bond enthalpies.

Average bond enthalpy /kJ mol–1

C–H +410

O–H +465

O=O +500

C=O +805

C–O +336

The equation below shows the combustion of methanol, CH3OH, in the gaseous state.

CH3OH(g) + 1½O2(g)–CO2(g) + 2H2O(g)

Using the average bond enthalpies in the table above, calculate the enthalpy change of combustion, ΔcH, of gaseous methanol.

ΔcH = ............................................. kJ mol–1

29

Learner Resource 3 – Bond Enthalpies

Example Calculations for bond enthalpies

Bond H-H C-C C=C N-H O-H O-O O=O C-H N=N C=O

MBE / kJ mol-1 436 348 612 388 463 146 496 412 944 743

Calculate the enthalpy changes in the following:

1. Ethene + hydrogen gas Ethane

2. Hydrogen Peroxide Hydrogen peroxide +1/2 Oxygen

3. The formation of ammonia from its constituent elements.

4. Propene + oxygen Carbon dioxide and water

30

Teacher Resource 4 – Using Hess’s law to calculate enthalpies of formation and enthalpies of combustion (to be used with Learner Resource 4)

Remember that energy cannot be destroyed but is merely changed from one form into another. Hess’s Law states the enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs.

ΔfH – Enthalpy of formation calculations

In order to carry out these calculations we have to draw a thermochemical cycle. See the example below:

According to Hess’s Law the enthalpy change will be the same for route 1 as it will be by adding the enthalpy changes for route 2 and route 3 together.

However because Route 2 is not a formation reaction we change the sign on the value and add them together.

A worked example.

Route 1

2C + 3H2

Ethane + Hydrogen

Route 2 Route 3

Ethane

Pb + C +O2

PbO(s) + CO(g)

ΔHf PbO = -219

ΔHf CO = -111

ΔHf Pb = 0

ΔHf CO2 = -394

Pb(s) + CO2 (g)

31

Teacher Resource 4 – Using Hess’s law to calculate enthalpies of formation and enthalpies of combustion (to be used with Learner Resource 4)

All values are in kJ mol-1. The value of solid Pb is 0 as it is in its standard form.

So because PbO and CO are not being formed their values have their sign changed

Therefore 219 +111 + -394 = -64kJ mol-1

It is important to remember that if the material is not being formed and is being broken down then the sign will change.

NB Easy way to remember the calculation in ΔfH always change the sign on the LHS of the thermochemical cycle.

Example calculations – Enthalpy of formulation calculations adapted from Calculations for A-Level Chemistry by E Ramsden (Nelson Thornes, 2001), reprinted by permission of the publishers, Oxford University Press.

The following are all standard enthalpies of formation, Δf H, in kJ mol-1

CH4(g) -76; CO2(g) -394; H20(l) -286; H20(g) -242; NH3(g), -46.2; HNO3(l) -176

Calculate the standard enthalpy change at 298K for the reaction

CH4(g) + 2O2(g) CO2(g) + H2O(l) Answer = -890 kJmol-1

Calculate the standard enthalpy change for the reaction

1/2N2(g) + 3/2H2O(g) NH3(g) + 3/4O2(g) Answer = +317 kJmol-1


1/2N2(g) + 1/2H2O(g) + 5/4O2(g) HNO3(l) Answer =-55 kJmol-1

Ф

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Learner Resource 4 – Using Hess’s law to calculate enthalpies of formation and enthalpies of combustion

Remember that energy cannot be destroyed but is merely changed from one form into another. Hess’s Law states the enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs.

ΔfH – Enthalpy of formation calculations

In order to carry out these calculations we have to draw a thermochemical cycle. See the example below:

According to Hess’s Law the enthalpy change will be the same for route 1 as it will be by adding the enthalpy changes for route 2 and route 3 together.

However because Route 2 is not a formation reaction we change the sign on the value and add them together.

A worked example.

Route 1

2C + 3H2

Ethane + Hydrogen

Route 2 Route 3

Ethane

Pb + C +O2

PbO(s) + CO(g)

ΔHf PbO = -219

ΔHf CO = -111

ΔHf Pb = 0

ΔHf CO2 = -394

Pb(s) + CO2 (g)

33

Learner Resource 4 – Using Hess’s law to calculate enthalpies of formation and enthalpies of combustion

All values are in kJ mol-1. The value of solid Pb is 0 as it is in its standard form.

So because PbO and CO are not being formed their values have their sign changed

Therefore 219+111 + -394 =

It is important to remember that if the material is not being formed and is being broken down then the sign will change.

NB Easy way to remember the calculation in ΔfH always change the sign on the LHS of the thermochemical cycle.

Example calculations – Enthalpy of formulation calculations adapted from Calculations for A Level Chemistry by E Ramsden (Nelson Thornes, 2001), reprinted by permission of the publishers, Oxford University Press.

The following are all standard enthalpies of formation, Δf H, in kJ mol-1

CH4(g) -76; CO2(g) -394; H20(l) -286; H20(g) -242; NH3(g), -46.2; HNO3(l) -176

Calculate the standard enthalpy change at 298K for the reaction

CH4(g) + 2O2(g) CO2(g) + H2O(l)


1/2N2(g) + 3/2H2O(g) NH3(g) + 3/4O2(g)


1/2N2(g) + 1/2H2O(g) + 5/4O2(g) HNO3(l)

Ф

34

Teacher Resource 5 – ΔcH Enthalpy of combustion calculations

As with the previous Enthalpy calculation it is helpful to draw a thermochemical cycle to help visualise what is going on:

With this calculation we are calculating the enthalpy change when one mole of substance is burned completely in oxygen under standard conditions.

As before, we can calculate the overall enthalpy change by finding the enthalpy of combustion for 2 and 3 and adding them to 4. This will give us the same answer as route 1 according to Hess’s Law. However, this time we are using combustion data as 2 and 3 are being combusted there is no need to change the sign on the RHS. The sign does need to be changed for 4 as this is not being combusted.

So the data for the combustion of ethyne is -1301 kJmol-1 and hydrogen is -286 kJmol-1 this needs to be multiplied by 2. And the enthalpy of combustion for ethane is -1560 kJmol-1.

Route 1

2CO2(g) + 3H2O(l)

C2H2(g) + 3.5O2(g) + 2H2

3 4

C2H6(g) + 3.5O2(g)

2

35

Teacher Resource 5 – ΔcH Enthalpy of combustion calculations

Hence the calculation is as follows:

(-1301 + -572) + 1560 (the sign has been changed) = ΔcH -313 kJ mol-1

So again as a rule of thumb in Enthalpy of combustion calculations are concerned you add all of the enthalpy values together making sure you take into account the stoichiometry but remembering to REVERSE the sign on the RHS of the equation.

Example calculations: Adapted from Mathematics for A Level Chemistry: A Course Companion by S Doyle, reprinted by permission of the publishers, Illuminate Publishing.

The enthalpy of formation cannot be directly measured for Glucose C6H12O6 so we measure it from the enthalpy of combustion data.

Substance ΔHc kJ mol-1

C(s) -394

H2(g) -286

C6H12O6(s) -2801

Write out a suitable equation and calculate the answer based upon the combustion cycle:

Answer= -1279 kJmol-1

36

Learner Resource 5 – ΔcH Enthalpy of combustion calculations

As with the previous Enthalpy calculation it is helpful to draw a thermochemical cycle to help visualise what is going on:

With this calculation we are calculating the enthalpy change when one mole of substance is burned completely in oxygen under standard conditions.

As before, we can calculate the overall enthalpy change by finding the enthalpy of combustion for 2 and 3 and adding them to 4. This will give us the same answer as route 1 according to Hess’s Law. However, this time we are using combustion data as 2 and 3 are being combusted there is no need to change the sign on the RHS. The sign does need to be changed for 4 as this is not being combusted.

So the data for the combustion of ethyne is -1301 kJmol-1 and hydrogen is -286 kJmol-1 this needs to be multiplied by 2. And the enthalpy of combustion for ethane is -1560 kJmol-1.

Route 1

2CO2(g) + 3H2O(l)

C2H2(g) + 3.5O2(g) +

3 4

C2H6(g) + 3.5O2(g)

2

37

Learner Resource 5 – ΔcH Enthalpy of combustion calculations

Hence the calculation is as follows:

(-1301 + -572) + 1560 (the sign has been changed) = ΔcH -313 kJ mol-1

So again as a rule of thumb in Enthalpy of combustion calculations are concerned you add all of the enthalpy values together making sure you take into account the stoichiometry but remembering to REVERSE the sign on the RHS of the equation.

Example calculations: Adapted from Mathematics for A Level Chemistry: A Course Companion by S Doyle, reprinted by permission of the publishers, Illuminate Publishing.

The enthalpy of formation cannot be directly measured for Glucose C6H12O6 so we measure it from the enthalpy of combustion data.

Substance ΔHc kJ mol-1

C(s) -394

H2(g) -286

C6H12O6(s) -2801

Write out a suitable equation and calculate the answer based upon the combustion cycle:

38

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