Reaction Rate Reaction Rate

• The rate of reaction is how fast the The rate of reaction is how fast the reaction is.reaction is.

• To measure rate we must take into To measure rate we must take into consideration 2 measurable quantities :consideration 2 measurable quantities :

• one of these must be time one of these must be time • the other could be the decrease in mass the other could be the decrease in mass

of the products of the products oror the increase in mass the increase in mass of the reactants.of the reactants.

Calculating RateCalculating Rate

Average rate = Change in Average rate = Change in reactant or reactant or product /product /

time taken time taken for for change.change.

ExampleExample

Example – Example – calculate calculate the rate of the rate of reaction in reaction in the first 20 the first 20 secsec

Average Average rate =rate =

Change in Change in mass /time mass /time takentaken

= 50 – 20/= 50 – 20/

20 – 020 – 0

= 30/20= 30/20

==1 .5g s 1 .5g s -1-1

Time Time SecSec

Mass of Mass of reactantreactant

00 50g50g

1010 30g30g

2020 20g20g

3030 10g10g

4040 10g10g

Collision Theory!Collision Theory!

• For a chemical reaction to take place the For a chemical reaction to take place the reactant particles must collide with each reactant particles must collide with each other.other.

• The more frequently they collide – the The more frequently they collide – the greater chance they will be in the greater chance they will be in the correct “ geometry” and so the collision correct “ geometry” and so the collision will be successful and a reaction can will be successful and a reaction can take place!take place!

Factors that affect rateFactors that affect rate

• TemperatureTemperature

• ConcentrationConcentration

• Particle sizeParticle size

• CatalystCatalyst

ConcentrationConcentration

• When we increase the concentration of When we increase the concentration of reactants the rate increases.reactants the rate increases.

This is because the more particlesThis is because the more particles- The greater chance they will collide - The greater chance they will collide - The greater chance the collision will be - The greater chance the collision will be

successful i.e. correct geometrysuccessful i.e. correct geometry- The greater chance of a reaction- The greater chance of a reaction!! ( PPA)( PPA)

Particle SizeParticle Size

• In order for particles to collide their In order for particles to collide their surfaces must come into contact.surfaces must come into contact.

• The bigger the surface – the bigger the The bigger the surface – the bigger the chance of collision.chance of collision.

• The greater chance of successful The greater chance of successful collisionscollisions

• The greater chance of a reaction.The greater chance of a reaction.• The smaller the particle – the bigger the The smaller the particle – the bigger the

surface area.surface area.

Activation EnergyActivation Energy

• This is the minimum amount of This is the minimum amount of energy particles must have in order energy particles must have in order to react with each other.to react with each other.

• The lower the activation energy,The lower the activation energy,

of a reaction, the greater chance of a reaction, the greater chance the reaction will happen the reaction will happen successfully. successfully.

TemperatureTemperature

• When we heat up particles – they move! When we heat up particles – they move! (kinetic energy)(kinetic energy)

• Temperature is a measure of the Temperature is a measure of the average kinetic energy.average kinetic energy.

• If we increase the temperature more If we increase the temperature more particles will have equal to or greater particles will have equal to or greater than Activation Energy.than Activation Energy.

• More collisions – more successful More collisions – more successful collisions – increase in reaction rate!collisions – increase in reaction rate!

( PPA)( PPA)

CatalystsCatalysts

• Catalyst lower the activation energy of a Catalyst lower the activation energy of a reaction.reaction.

• Therefore there is more chance of Therefore there is more chance of collisions – more successful collisions – collisions – more successful collisions – increase in rate!increase in rate!

• Catalysts are not used in the reaction – Catalysts are not used in the reaction – they can be used again!they can be used again!

• Catalyst inhibitors will slow down rate.Catalyst inhibitors will slow down rate.

Types of CatalystsTypes of Catalysts

• Homogeneous – Catalyst and Homogeneous – Catalyst and reactants are in same state.reactants are in same state.

• Heterogeneous – Catalyst and Heterogeneous – Catalyst and substrate are in a different state.substrate are in a different state.

• Biological - EnzymesBiological - Enzymes

Homogeneous CatalystsHomogeneous Catalysts

• A homogenous catalyst takes part in the A homogenous catalyst takes part in the reaction and is then reformed at the reaction and is then reformed at the end.end.

• The reactants and catalyst are in the The reactants and catalyst are in the same physical state!same physical state!

• Example –Example – Cobalt ions are the catalyst in Cobalt ions are the catalyst in the reaction between Hydrogen peroxide the reaction between Hydrogen peroxide and Potassium sodium tartrate.and Potassium sodium tartrate.

Heterogeneous catalystsHeterogeneous catalysts

• The catalyst provides a surface for the The catalyst provides a surface for the reaction.reaction.

• The reactant particles are absorbed onto The reactant particles are absorbed onto “active sites” on the catalyst surface.“active sites” on the catalyst surface.

• The reactant bonds weaken.The reactant bonds weaken.• Particles are in the correct position for “ Particles are in the correct position for “

collisions”collisions”• Collision occur – new product bonds form.Collision occur – new product bonds form.• The new product particles leave catalyst The new product particles leave catalyst

surface.surface.

Biological catalystsBiological catalysts

• Enzymes are catalysts inside living Enzymes are catalysts inside living tissue.tissue.

• They are very specific – usually only They are very specific – usually only catalyse one reaction.catalyse one reaction.

• They work at “ optimum conditions” – They work at “ optimum conditions” – i.e. pH, temperature.i.e. pH, temperature.

• E.g – amylase helps the break down of E.g – amylase helps the break down of starch to sugar.starch to sugar.

Denaturing CatalystsDenaturing Catalysts

• This is when a catalyst is destroyed.This is when a catalyst is destroyed.• It can be caused by the “ active site” It can be caused by the “ active site”

being blocked.being blocked.• E.g. lead petrol denatures the active site E.g. lead petrol denatures the active site

in a catalytic converter.in a catalytic converter.

OrOr• Too high a temperature or the wrong pH Too high a temperature or the wrong pH

would denature an enzyme.would denature an enzyme.

Exothermic ReactionsExothermic Reactions

• An exothermic reaction is one An exothermic reaction is one where heat is released. This means where heat is released. This means that the energy released when the that the energy released when the product bonds were made is product bonds were made is greater then the energy put in to greater then the energy put in to break the reactant bonds.break the reactant bonds.

• E.g. combustion, neutralisation.E.g. combustion, neutralisation.

Endothermic reactionEndothermic reaction

• This is when overall energy is taken This is when overall energy is taken in.in.

• The energy released when the The energy released when the product bonds were made is less product bonds were made is less than the energy put in to break the than the energy put in to break the reactant bonds.reactant bonds.

• E.g. Dissolving some salts – E.g. Dissolving some salts – ammonium carbonate.ammonium carbonate.

Enthalpy ChangeEnthalpy Change

• The difference between potential The difference between potential energy of reactants and products is energy of reactants and products is called – ENTHALPY CHANGE.called – ENTHALPY CHANGE.

• It is denoted by the symbol ΔHIt is denoted by the symbol ΔH• ΔH = Hp – HrΔH = Hp – Hr• Hp = enthalpy of productsHp = enthalpy of products• Hr = enthalpy of reactants.Hr = enthalpy of reactants.

ΔHΔH

• The ΔH of exothermic reaction is The ΔH of exothermic reaction is always a negative value.always a negative value.

• The ΔH of endothermic reactions is The ΔH of endothermic reactions is always a positive value.always a positive value.

( graphs)( graphs)

Calculations using Calculations using equations!equations!

• ExampleExample• What mass of water is produced What mass of water is produced

when 1g of Methane is burned.when 1g of Methane is burned.• Step 1Step 1 – Balanced equation – Balanced equation• CHCH4 4 + 2O+ 2O22 —> CO —> CO22 + 2H + 2H22OO• Step 2Step 2 – Mole ratio – Mole ratio • 1 mole of CH1 mole of CH44 produces 2 moles of produces 2 moles of

HH22O.O.

• Step 3Step 3 – Mass – Mass

• 1 mole = 16g of CH1 mole = 16g of CH44 —>2moles —>2moles of H2O of H2O = 2 x 18= 2 x 18

36g36g

1g of CH1g of CH44 —> 1 / 16 x 36 —> 1 / 16 x 36

==2.25g2.25g

Excess!Excess!

• ExampleExample• 8g of Methane reacts with 16g of 8g of Methane reacts with 16g of

Oxygen. Which reactant is in excess?Oxygen. Which reactant is in excess?• Step 1Step 1 – Balanced Equation. – Balanced Equation.

• CHCH44 + 2O + 2O2 2 —>2H—>2H22O + COO + CO22

• Step 2Step 2 – Mole ratio. – Mole ratio.

• 1 mole of CH1 mole of CH44 reacts with 2 moles of O reacts with 2 moles of O22

• Step 3Step 3 – Put in Mass – Put in Mass• CHCH44:O:O22

• 1 : 21 : 2• 16g —>2 x 16g = 32g16g —>2 x 16g = 32g• Step 4Step 4 – Put in actual moles – Put in actual moles• 8g of CH4 = 0.5 moles8g of CH4 = 0.5 moles• 16g of O2 = 0.5 moles16g of O2 = 0.5 moles• Since we only need 0.25 moles of CHSince we only need 0.25 moles of CH44 to to

react with 16g of Oreact with 16g of O22 – CH – CH44 is in excess. is in excess.