rate of reaction

7. How far? 7. How far? How fast?How fast?

7.1 Energy changes in 7.1 Energy changes in chemical reactionschemical reactions

Chemical reactions are capable of releasing vast amounts of energy.

Hydrocarbon molecules contain only the elements carbon and hydrogen.

Methane + oxygen → carbon dioxide + water

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

The reaction between methane and oxygen

During this reaction, as with all others, bond are first broken and then new bonds are made.

In methane molecules, carbon atoms are covalently bonded to hydrogen atoms. In oxygen gas, the atoms are held together in diatomic molecules.

During the reaction, all these bonds must be broken. Chemical bonds are forces of attraction between atoms or ions.

To break these bonds requires energy; Energy must be taken in to pull the atoms apart.

Breaking chemical bonds takes in energy from the surroundings. This is an endothermic process.

New bonds are then formed: between carbon and oxygen to make carbon dioxide, and between hydrogen and oxygen to form water.

Forming these bonds gives out energy.

Making chemical bonds gives out energy to the surroundings. This is an exothermic process.

When methane reacts with oxygen, the total energy given out is greater than the total energy taken in.

So, overall, this reaction gives out energy – it is an exothermic reaction. The energy is released as heat.

The overall change in energy for this exothermic reaction can be shown in an energy level diagram.

Exothermic ReactionExothermic ReactionReactants Products + Energy

Reactants

Products

-HEn

erg

y

Energy of reactants

Energy of products

Reaction Progress

Energy/ kJ CH4 (g) + 2O2

(g)

CO2 (g) + 2H2O (g)

Heat given out

Progress of reaction

In this reaction, energy is given out because the bonds in the products (CO2 and H2O) are stronger than those in the reactants (CH4 and O2)

This means that the products are more stable than the reactants.

Some bonds are stronger than others. They require more energy to break them, but they give out more energy when they are formed.

The combustion reactions of fossil fuels such as oil and gas are exothermic.

The major characteristics that make these fuels so useful are that: • they are easy to ignite and burn•They are capable of releasing large amounts of energy as heat.

Endothermic reactions are far less common than exothermic ones.

The reaction between nitrogen and oxygen

Here, energy is absorbed from the surroundings.

The reaction between nitrogen and oxygen is endothermic.

It is one of the reaction that take place when fuel is burnt in car engines.

Nitrogen + oxygen → nitrogen monoxide

N2 (g) + O2 (g) → NO (g)Here the bonding in the products is weaker than in the reactants. Overall, energy is taken in by the reaction.

The energy change in going from reactants to products in a chemical reaction is known as the heat of reaction.

Heat of reaction

It is given the symbol ∆ H ( the symbol ∆ means ‘change in”

The energy given out or taken in is measured in kilojoules (kJ);1 kJ = 1000 J.

Heat of reaction

It is usually calculated per mole of a specific reactant or product (kJ / mol)

The starting point for the calculation is the reacting mixture. If a reaction gives out heat to the surroundings, the mixture has lost energy.

Exothermic reaction

It is an exothermic reaction.

In EXothermic reaction, heat EXits the reaction mixture.An exothermic reaction has a negative value of ∆ H.

Exothermic reaction

Exothermic ReactionExothermic ReactionReactants Products + Energy

Reactants

Products

-HEn

erg

y

Energy of reactants

Energy of products

Reaction Progress

If a reaction takes in heat from the surroundings, the mixture has gained energy.

Endothermic reaction

It is an endothermic reaction.

In ENdothermic reaction, heat ENters the reaction mixture.An endothermic reaction has a positive value of ∆ H.

Exothermic reaction

Endothermic ReactionEndothermic ReactionEnergy + Reactants Products

+H

Reaction progress

En

erg

y

Reactants

ProductsActivation Energy

Heat of reaction: for exothermic reactions, heat

energy is given out (exits) and ∆H is negative

for endothermic reactions, heat energy is taken in (enters) and ∆H is positive

These ideas fit with the direction of the arrows shown in the energy diagrams.

Energy/ kJ CH4 (g) + 2O2

(g)

CO2 (g) + 2H2O (g)

Heat given out (-

728kJ/mol)

Progress of reaction

Exothermic reaction

Experiments have been carried out to find out how much energy is needed to break various covalent bonds in compounds.

Making and breaking bonds

The average value obtained for a particular bond is known as the bond energy.It is a measure of the strength of the bond.

Energy/ kJ

N2 (g) + O2 (g)

2NO2 (g)

Heat taken in

Progress of reaction

ENERGY LEVEL DIAGRAMSENERGY LEVEL DIAGRAMS• We can show the energy transfers in

reactions on an energy level diagram.

• These show us the energy stored in the reactants compared to the energy stored in the products.

Exothermic energy level diagramExothermic energy level diagram

Exothermic energy level diagramExothermic energy level diagram• ∆H (‘delta H’) is the

symbol for the ‘change in energy’.

• In an exothermic reaction the products have less energy than the reactants.

• ∆H is negative for an exothermic reaction.

Endothermic energy level diagramEndothermic energy level diagram

Endoothermic energy level Endoothermic energy level diagramdiagram

• In an endothermic reaction the products have more energy than the reactants.

• ∆H is positive for an endothermic reaction.

Making and breaking bondsMaking and breaking bonds• We have seen how bonds are formed

(both ionic and covalent)• Do you think energy is needed to

break bonds?

• Breaking bonds requires energy. It is endothermic.

• Breaking bonds requires energy. It is endothermic.

• Making new bonds gives out energy. It is exothermic.

Heat of combustion

The energy change of a reaction when a substance is burnt.

Heat of neutralisation

The energy change of a reaction when an acid react with an alkali to form water.

•the amount of energy that is required to start a chemical reaction.

•Once activation energy is reached the reaction continues until you run out of material to react.  

.

Activation energy

∆H

•More usually, energy is required to start the reaction.

•When fuels are burnt, for example, energy is needed to ignite them.

Activation energy

•This energy may come from a spark, a match or sunlight.

•It is called the activation energy (given the symbol EA)

•It is required because initially some bonds must be broken before any reaction can take place.

Activation energy

∆H

•Sufficient atoms or fragments of molecules must be freed for the new bonds to begin forming.

•Once started, the energy released as new bonds are formed causes the reaction to continue.

Activation energy

•All reactions require some activation energy.

•For the reaction of sodium or potassium with water the activation energy is low, and there is enough energy available from the surroundings at room temperature for the reaction to begin spontaneously.

Activation energy

•Reactions can be thought of as the result of collisions between atoms, molecules or ions.

•In many of these collisions, the colliding particles do not have enough energy to react, and just bounce apart, rather like ‘dodgem cars’.

Activation energy

•A chemical reaction will only happen of the total energy of the colliding particles is greater than the required activation energy of the reaction.

Activation energy

•A chemical reaction will only happen of the total energy of the colliding particles is greater than the required activation energy of the reaction.

7.2 Rate of reaction

What does rate of reaction mean?What does rate of reaction mean?The speed of different chemical reactions varies hugely. Some reactions are very fast and others are very slow.

What is the rate of these reactions?

The speed of a reaction is called the rate of the reaction.

rusting baking explosion

slow fast very fast

Rates of reactionRates of reactionWhy are some reactions faster than others?

Reactions, particles and collisionsReactions, particles and collisions

Reactions take place when particles collide with a certain amount of energy.

The minimum amount of energy needed for the particles to react is called the activation energy, and is different for each reaction.

The rate of a reaction depends on two things:

the frequency of collisions between particles

the energy with which particles collide.

If particles collide with less energy than the activation energy, they will not react. The particles will just bounce off each other.

Changing the rate of reactionsChanging the rate of reactions

increased temperature

increased concentration of dissolved reactants, and increased pressure of gaseous reactants

increased surface area of solid reactants

use of a catalyst.

Anything that increases the number of successful collisions between reactant particles will speed up a reaction.

What factors affect the rate of reactions?

Slower and slower!Slower and slower!Reactions do not proceed at a steady rate. They start off at a certain speed, then get slower and slower until they stop.

As the reaction progresses, the concentration of reactants decreases.

This reduces the frequency of collisions between particles and so the reaction slows down.

percentage completion of reaction

100%0% 25% 50% 75%

reactants

product

Graphing rates of reactionGraphing rates of reaction

ReactantReactant––product mixproduct mix

How can rate of reaction be How can rate of reaction be measured?measured?

Measuring the rate of a reaction means measuring the change in the amount of a reactant or the amount of a product.

What can be measured to calculate the rate of reaction between magnesium and hydrochloric acid?

The amount of hydrochloric acid used up (cm3/min). The amount of magnesium chloride produced

(g/min). The amount of hydrogen product (cm3/min).

+magnesiumhydrochloric

acid+

magnesiumchloride hydrogen

Setting up rate experimentsSetting up rate experimentsWhat equipment is needed to investigate the rate of hydrogen production?

gas syringe

rubber bung

rubber connecterglass tube

conicalflask

magnesium

hydrochloricacid

hydro

gen p

roduce

d (

cm3)

time (seconds)10 20 30 40 50

10

20

30

40

50

60

70

00

x

y

Calculating rate of reaction from graphsCalculating rate of reaction from graphs

rate of reaction =

x

y

rate of reaction =

20 s

45 cm3 rate of reaction = 2.25 cm3/s

The gradient of the graph is equal to the initial rate of reaction at that time

How can the rate of reaction be calculated from a graph?

The reactant/product mixThe reactant/product mix

Collisions and reactions: Collisions and reactions: summarysummary

Temperature and collisionsTemperature and collisionsHow does temperature affect the rate of particle collision?

Effect of temperature on rateEffect of temperature on rateThe higher the temperature, the faster the rate of a reaction. In many reactions, a rise in temperature of 10 °C causes the rate of reaction to approximately double.

Why does increased temperature increase the rate of reaction?

At a higher temperature, particles have more energy. This means they move faster and are more likely to collide with other particles.

When the particles collide, they do so with more energy, and so the number of successful collisions increases.

Temperature and particle Temperature and particle collisionscollisions

Temperature and batteriesTemperature and batteriesWhy are batteries more likely to rundown more quickly in cold weather?

At low temperatures the reaction that generates the electric current proceeds more slowly than at higher temperatures.

This means batteries are less likely to deliver enough current to meet demand.

How does temperature affect rate?How does temperature affect rate?

The reaction between sodium thiosulfate and hydrochloric acid produces sulfur.

Sulfur is solid and so it turns the solution cloudy.

How can this fact be used to measure the effect of temperature on rate of reaction?

hydrochloric

acid

sodium

chloride

sulfur

sodiumthiosulfa

te

+ + water

sulfurdioxid

e

++

Na2S2O

3

(aq)

2HCl

(aq)

2NaCl

(aq)

S(s)

++ SO2

(g)H2O(l)

+ +

The effect of temperature on rateThe effect of temperature on rate

Effect of concentration on rate of reactionEffect of concentration on rate of reactionThe higher the concentration of a dissolved reactant, the faster the rate of a reaction.

Why does increased concentration increase the rate of reaction?

At a higher concentration, there are more particles in the same amount of space. This means that the particles are more likely to collide and therefore more likely to react.

higher concentrationlower concentration

Concentration and particle Concentration and particle collisionscollisions

The effect of concentration on rateThe effect of concentration on rate

Effect of pressure on rate of Effect of pressure on rate of reactionreaction

The gas particles become closer together, increasing the frequency of collisions. This means that the particles are more likely to react.

Why does increasing the pressure of gaseous reactants increase the rate of reaction?

As the pressure increases, the space in which the gas particles are moving becomes smaller.

lower pressure higher pressure

Effect of surface area on Effect of surface area on rate of reactionrate of reactionAny reaction involving a solid can only take place at the surface

of the solid.

If the solid is split into several pieces, the surface area increases. What effect will this have on rate of reaction?

The smaller the pieces, the larger the surface area. This means more collisions and a greater chance of reaction.

This means that there is an increased area for the reactant particles to collide with.

low surface area high surface area

Surface area and particle Surface area and particle collisionscollisions

Reaction between a Reaction between a carbonate and acidcarbonate and acidMarble chips are made of calcium carbonate. They react with

hydrochloric acid to produce carbon dioxide.

The effect of increasing surface area on the rate of reaction can be measured by comparing how quickly the mass of the reactants decreases using marble chips of different sizes.

hydrochloric

acid

calcium

chloride

calciumcarbonat

e

+ + water

+ carbon

dioxideCaCO3

(aq)2HCl(aq)

CaCl2

(aq) ++ H2O(aq) +

CO2

(g)

The effect of surface area on rateThe effect of surface area on rate

reaction (time)

en

erg

y (

kJ)

What are catalysts?What are catalysts?Catalysts are substances that change the rate of a reaction without being used up in the reaction.

Catalysts never produce more product – they just produce the same amount more quickly.

Different catalysts work in different ways, but most lower the reaction’s activation energy (Ea).

Ea withcatalyst

Ea withoutcatalyst

Everyday catalystsEveryday catalysts Nickel is a catalyst in the production of margarine

(hydrogenation of vegetable oils).

Many catalysts are transition metals or their compounds. For example:

Platinum is a catalyst in the catalytic converters of car exhausts. It catalyzes the conversion of carbon monoxide and nitrogen oxide into the less polluting carbon dioxide and nitrogen.

Iron is a catalyst in the production of ammonia from nitrogen and hydrogen (the Haber process).

Catalysts in industryCatalysts in industry

Catalysts are also essential for living cells. Biological catalysts are special types of protein called enzymes.

Why are catalysts so important for industry?

Products can be made more quickly, saving time and money.

Catalysts reduce the need for high temperatures, saving fuel and reducing pollution.

GlossaryGlossaryactivation energy – The amount of energy needed to start a reaction.

catalyst – A substance that increases the rate of a chemical reaction without being used up.

concentration – The number of molecules of a substance in a given volume.

enzyme – A biological catalyst.

rate of reaction – The change in the concentration over a certain period of time.

AnagramsAnagrams

Rates of reaction: summaryRates of reaction: summary

Multiple-choice quizMultiple-choice quiz

Surface areaSurface area• If we make the

pieces of the reactants smaller we increase the number of particles on the surface which can react.

• This makes the reaction faster.

The particles on the surface can react

When cut into smaller pieces the particles on the inside can react

How do we make the How do we make the reaction go faster?reaction go faster?

• There are four things that we can change to make the reaction go faster.

• They are • Temperature• Surface area• Concentration• Using a catalyst

TemperatureTemperature

•When we increase the temperature we give the particles energy

•This makes them move faster •This means they collide with

other particles more often•So the reaction goes faster.

ConcentrationConcentration• If we make one reactant

more concentrated (like making a drink of orange squash more concentrated)

• There are more particles in the same volume to react

• So the reaction goes faster.

• Click here to complete exercise

There are less red particles in the same volume so there is less chance of a collision

There are more red particles in the same volume so there is more chance of a collision so the reaction goes faster

Using a catalystUsing a catalyst

• A catalyst is a chemical which is added to a reaction.

• It makes the reaction go faster.• The catalyst does not get used

up in the reaction.• It gives the reaction the energy

to get started

Endothermic Reaction:

- system absorbs E

- more energy needed to break bonds than released by creating bonds

- change in enthalpy is positive

Exothermic Reaction:

- system releases E

- more energy released by creating bonds than needed to break bonds

- change in enthalpy is negative

100

Cold and hot packsCold and hot packs

• How do instant hot and cold packs work?

101

Hot packHot pack

• Pressing the bottom , the diaphragm breaks.• Calcium chloride dissolves in water and warms

it.• The beverage gets warm.

102

EnergySurroundings

Exothermic processExothermic process• Heat flows into the surroundings

from the system in an exothermic process.

Temperature rises

Hot pack

103

Hot packHot pack

• We will repeat the process in a beaker with calcium chloride ( 25 g CaCl2) + water (25 ml) and a thermometer.

• We will record the initial temperature of the water and the temperature after the dissolution of the salt.

• Observation:• a temperature rise of…….

104

Cold packCold pack

• Water and ammonium nitrate are kept in separate compartments.

• Pressing the wrapper, the ammonium nitrate dissolves in water and absorbs heat.

• The pack becomes cold.

It is used to treat sports injuries.

105

Cold pack

Energy

Surroundings

Endothermic processEndothermic process

• Heat flows into the system from the surroundings in an endothermic process.

Temperature falls

106

Cold packCold pack

• We will repeat the process in a beaker with ammonium nitrate ( 25 g NH4NO3) + water (25 ml) and a thermometer.

• We will record the initial temperature of the water and the temperature after the dissolution of the salt.

• Observation:• a temperature drop of…….

Exothermic ReactionExothermic ReactionReactants Products + Energy 10 energy = 8 energy + 2 energy

Reactants

Products

-HEn

erg

y

Energy of reactants

Energy of products

Reaction Progress

Endothermic ReactionEndothermic ReactionEnergy + Reactants Products

+H Endothermic

Reaction progress

En

erg

y

Reactants

ProductsActivation Energy

Effect of Catalyst on Reaction Effect of Catalyst on Reaction RateRate

reactants

products

En

erg

y

activation energy for catalyzed

reaction

Reaction Progress

No catalyst

Catalyst lowers the activation energy for the reaction.What is a catalyst? What does it do during a chemical reaction?

Basic RevisionBasic Revision• For a chemical reaction to occur,

bonds must be both formed and broken

• Fe + CuSO4 Cu + FeSO4

Basic RevisionBasic Revision• Chemical reactions always involve

energy changes.

• Making and breaking bonds involves energy changes

ENERGY AND CHEMICAL ENERGY AND CHEMICAL REACTIONSREACTIONS

Exothermic reactions• These reactions give out heat

energy.• Combustion is an exothermic

reaction.

ENERGY AND CHEMICAL ENERGY AND CHEMICAL REACTIONSREACTIONS

Endothermic reactions• These reactions take in heat energy

from their surroundings.• These cause temperatures to fall.

ENERGY AND CHEMICAL ENERGY AND CHEMICAL REACTIONSREACTIONS

• Use the apparatus as shown.

• Work with 20ml of each substance in turn.

• Copy the results table before you start

ENERGY AND CHEMICAL ENERGY AND CHEMICAL REACTIONSREACTIONS

Reaction temp. before mixing (0C)

temp. after mixing (0C)

Exothermic or Endothermic?

sodium hydroxide sol’n + hydrochloric acid

sodium hydrogen carbonate sol’n + citric acid

copper sulphate sol’n + magnesium powder

sulphuric acid + magnesium ribbon

barium chloride + sodium sulphate

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The Energy of Physical, Chemical and Nuclear The Energy of Physical, Chemical and Nuclear Reactions Reactions

• Thermodynamics - the study of energy and energy transfer

• Thermochemistry - the study of energy involved in chemical reactions

Energy (symbol: E – unit: J joules) Law of Conservation of Energy

- total energy of the universe is constant

- energy can neither be created nor destroyed Δ universe=0 𝐸- energy can be transferred from one substance to another

- energy can be converted into various forms

System - part of the universe that is being studied and

observed (reactants & products)

Surroundings - everything else in the universe

- part of the universe that is likely to be affected by

energy changes in the system

Universe = System + Surroundings Δ𝐸univeres= Δ𝐸system+Δ𝐸surrounding = 0

Any change in the system is accompanied by an equal

and opposite change in the surroundings. Δ𝐸system =−Δ𝐸surrounding

Heat (symbol: Q – unit: J joules) - transfer of kinetic

energy (Ek) spontaneously from warmer to cooler

objects

Temperature (symbol: T – unit: K Kelvin, °C

Celsius degrees) TK = T°C + 273.15

- measure of the average kinetic energy of the

particles that make up a substance or system

Celsius scale – relative to water (0°C water melting

point, 100°C water boiling point)

Kelvin scale – absolute scale (O K is temp. when a

substance has no kinetic energy)

Enthalpy (symbol: H – unit: J joules)

- total internal energy of a substance at a constant pressure

- hard to measure; includes (1) energy of physical state, (2)

energy in bonds, (3) energy in nucleus; includes:

- moving electrons within atoms

- vibration of atoms connected by chemical bonds

- rotation and translation of molecules

- nuclear potential energy of protons and neutrons in atomic

nuclei

- electronic potential energy of atoms connected by chemical

bonds

- easily measure and study change in enthalpy (ΔH) in a

reaction

- enthalpy change of a process is equivalent to its heat change

at constant pressure

- chemical bonds are sources of stored energy

- breaking a bond is a process that requires energy

- creating a bond is a process that releases energy

Representing Enthalpy Changes Representing Enthalpy Changes

• ΔHrxn enthalpy of reaction (depends on

temperature and pressure)

• ΔH°rxn standard enthalpy of reaction at

SATP (25°C, 100 kPa)

(1) Thermochemical Equation (1) Thermochemical Equation – balanced – balanced equation that indicates enthalpy change equation that indicates enthalpy change

(2) Separate Expression (2) Separate Expression

(3) Enthalpy Diagram (3) Enthalpy Diagram

1. Physical changes - Small change in enthalpy (tens of kJ/mol) - Change in intermolecular bonds between molecules as they change phase o ΔHvap enthalpy of vaporization (liquid gas) o ΔHcond enthalpy of condensation (gas liquid) o ΔHmelt enthalpy of melting (solid liquid) o ΔHfre enthalpy of freezing (liquid solid)

o ΔHsoln enthalpy of solution (solid aqueous – dissolving)

2. Chemical changes

- Moderate change in enthalpy (hundreds of kJ/mol)

- Change in intramolecular bonds within molecules as reactants break apart to form products o ΔHcomb enthalpy of combustion

o ΔHneut enthalpy of neutralization

o ΔHf enthalpy of formation

3. Nuclear changes

- Enormous change in enthalpy (billions of kJ/mol)

- Change in nuclear binding energy holding together the nucleus of an atom o A significant amount of mass of the reactants is actually converted to energy

o Einstein: E=mc2, where m=mass and c=speed of light (3.0x108m/s) so a tiny mass is equivalent to a significant amount of energy

Mass Defect o difference in mass between a nucleus and its nucleons (particles found in the nucleus)

o caused by the energy associated with the strong force that holds a nucleus together

o the higher the nuclear binding energy, the more stable the nucleus

Nucleus + Nuclear binding energy Nucleons

References• McGraw-Hill Ryerson Chemistry 12• Nelson Chemistry 12

Rates of Rates of Reactions and Reactions and

EnzymesEnzymes

Rates of ReactionRates of ReactionChemical reactions occur when different atoms or molecules collide:

For the reaction to happen the particles must have a certain amount of energy – this is called the ACTIVATION ENERGY.

The rate at which the reaction happens depends on four things:

1) The temperature of the reactants,

2) Their concentration

3) Their surface area

4) Whether or not a catalyst is used

Measuring rate of reactionMeasuring rate of reactionTwo common ways:

1) Measure how fast the products are formed

2) Measure how fast the reactants are used up

Rate of reaction graphRate of reaction graph

Amount of product formed

Time

Slower reactionFast rate

of reaction here

Slower rate of reaction here due to reactants being used

up

Enzymes are biological catalysts. They help the reactions that occur in our bodies by controlling the rate

of reaction.

Enzymes are denatured

beyond 40OC

EnzymesEnzymes

Yeast is an example of an enzyme. It is used to help a process called fermentation:

Sugar Alcohol + carbon dioxide

The alcohol from this process is used in making drinks and the carbon dioxide can be used to make bread rise.

Enzymes work best in certain conditions:

Enzyme activity

Temp pH pH400C

Could be protease (found in the stomach)

Could be amylase (found in the intestine)

Uses of enzymesUses of enzymes1) Enzymes are used in washing powders to help digest food stains. Biological washing powders will only work on 400C or lower.

2) Enzymes are used in baby foods to “pre-digest” the proteins.

3) Enzymes are used to convert starch into sugar which can then be used in food.

4) Conversion of glucose into fructose – glucose and fructose are “isomers” (they have the same chemical formula), but fructose is sweeter.