Title: Revision of Enthalpy

Learning Objectives:By the end of the lesson I will be able……..•To understand and explain enthalpy•To describe the difference between endo and exothermic reactions•Be able to draw and understand Activation Energy and enthalpy profile diagrams•And be able to do enthalpy calculations

http://youtu.be/XgiCn1IpvzM

Endo Exo thermic youtube song

Endo exothermic Song

Pub quiz answers

1.An exothermic reaction is when a chemical reaction releases or gives out heat. TRUE

2.Breaking a bond needs energy. TRUE

3.In enthalpy calculations which Greek letter symbolises standard states and measurements under standard conditions.

Θ, Δ, or Ω

4.The standard conditions in a reaction are 100kPa (1 atm) pressure and a temp of 273K (0οC) False

5.Enthalpy change ΔH is measured in KJ mol-1 TRUE

6.All combustion reactions are endothermic. FALSE

7.Is this a balanced equation for combustion of methane?

CH4(g) + 2O2(g) - CO2(g) + 2H20(l) TRUE

8. If heat is lost by reacting chemicals is less than the amount and gained by the surroundings then ΔH is negative. TRUE

9. The quantity of energy needed to break a particular bond in a molecule is called bond enthalpy. TRUE

10. Write down an example of bond enthalpy using H 2

Answer H2(g) 2H (g)

11. If you need more energy to break the bonds than is released when bonds are formed then ΔH is negative ? FALSE

12. This is an enthalpy profile diagram for an endothermic reaction. FALSE

13. The activation energy is the largest amount of energy needed to break bonds and start a chemical reaction. FALSE it’s the smallest

14. Standard enthalpy change of formation ΔHf Θ is the enthalpy change when 1

mole of an element is formed from its compounds in their standard states under standard conditions. False

Demo• Endothermic and Exothermic• Endo: requires heat energy• Exo: releases heat energy

Activity 1: Ice melting: What is causing this ice cube to melt?Is this an endothermic or exothermic reaction?

• Is heat energy being absorbed from the surroundings (endo) causing the ice to melt?

Or• is heat energy given out from the ice cube

(exo) to the surroundings causing the ice to melt?

So to break a bond…….

Mini whiteboard activity

To break a bond………….

We need energy to break a bond!

So is this an endo or exothermic reaction?…………………………………………………………….

When a bond is made………….

Energy is released or given out!Use your data book to calculate the bond energies of the above; (H-H), (N-N), (N-H) So is this an endo or exothermic reaction?…………………………………………………………….

1 Nitrogen molecule

What is enthalpy (H)?

• Heat content stored in a chemical system

What do these symbols mean?

• ΔHѳ

• Δ means ……………– H means ……………– Ѳ means …………..

Enthalpy is measured in …………..

AFL: To break a bond………….

So is (∆H) POSITIVE OR NEGATIVE?…………………………………………………………….

Bond energiesBond energies=The energy needed to break a chemical bond

Different chemical bonds have different bond energies

Do activity 1

Activity 1• Work out the enthalpy change (∆H) for

this reaction2H2 + O2 = 2H2O• Hint: Enthalpy change of reaction = products - reactants

• Is it endothermic or exothermic reaction?• See if you can sketch an energy profile

diagram

Examples of Exothermic reactions

• Oxidation and neutralisation reactions are all exothermic reactions.

-they give out heat energy

so ∆H is positive or negative?

AFL: When a bond is formed in a reaction………….

Energy is released or given out! So is (∆H) POSITIVE OR NEGATIVE?………………………………….

activity 2

1 Nitrogen molecule

Activity 2 REACTANTS -> PRODUCTS HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)Energy of the PRODUCTS = +153 kJEnergy of the REACTANTS = + 500 kJ What is the enthalpy change ΔH of this reaction?Give the units…..Is this reaction endothermic or exothermic?Draw an energy profile diagramHint: Enthalpy change of reaction = products - reactants

Energy Profile DiagramLook at the figures on the Y-axis to explain why this isan endothermic reaction.Write down your explanation using words: bond breaking, bond forming, energy

Endo or Exo

Endo or Exo

activity 3

Activity 3: Fill in the following words on the diagram

reactants, products, activation energy arrow, ΔH arrow +ΔH/ – ΔH, correct units for enthalpy

Enthalpy

What’s the difference between these two diagrams?

Activity 4Sketch an enthalpy profile diagram for this reaction

CO (g) + NO2 -> CO2(g) + NO(g) ΔH = -226kJ mol-1 Ea = 134kJ mol-1

Show activation energy arrow and ΔH arrowState whether it’s an endo or exothermic reaction.

Activity 1 answers• Work out the enthalpy change (∆H) for

this reaction2H2 + O2 = 2H2O• 2X (436) + 498 2 X (464) ∆H = +442 kj mol -1

• Is it an endothermic reaction

Activity 2 answers

REACTANTS -> PRODUCTS HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)Energy of the PRODUCTS = +153 kJEnergy of the REACTANTS = + 500 kJ What is the enthalpy change ΔH of this reaction? -347 kJ mol -1

It is an exothermic reactionenergy profile diagram

Activity 3 answer: Fill in the following words on the diagram

reactants, products, activation energy arrow, ΔH arrow +ΔH/ – ΔH, correct units for enthalpy

Enthalpy kJ mol -1

– ΔHreactants

products

Activity 4 answersSketch an enthalpy profile diagram for this reaction

CO (g) + NO2 -> CO2(g) + NO(g)

ΔH = -226kJ mol-1 Ea = 134kJ mol-1

Show activation energy arrow and ΔH arrowState whether it’s an endo or exothermic reaction.

AFL: thumbs up/thumbs downHow confident do I feel about…………….I can explain enthalpy to the person beside meI understand the difference between endo and exothermic reactions I can draw Activation Energy arrows and ∆H arrows on energy profile diagrams

Enthalpy Change of Combustion ΔHcѳ

Fill in the blanks CH4(g) + …….. (g) -> …… (g) + …… (l)

Do you think combustion reactions are endothermic or exothermic? ……………

CH4(g) + 2O2 (g) -> …CO2… (g) + 2H2O (l)ΔHc

ѳ is exothermic -890kJ mol-1

Standard Enthalpy Change of Reaction ΔHѳr

Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s)

ΔHѳr is the enthalpy change when one mole

of solid zinc reacts with 1 mole of copper sulphate (of concentration 1 mol dm-3) at 298K and 100kPa (1atm)

• Is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.

Standard Enthalpy Change of Combustion ΔHѳ

c