14. Rates of Reaction

Learning Outcomes

Learning Outcomes

Experiments! To monitor the rate of production of oxygen from hydrogen peroxide using manganese dioxide as a catalyst To study the effect of concentration on the rate of reaction using sodium thiosulphate solution and hydrochloric acid To study the effect of temperature on rate of reaction using sodium thiosulphate and hydrochloric acid

Rate of Reaction The change in concentration per unit time of any one reactant or product. Full marks given for: Rate of reaction = change in concentration change in time

Total time for the reaction to go to completion Average Rate Total volume produced Total time for the reaction to go to completion Unit = cm3/min

The rate of reaction at any one particular time during the reaction. Instantaneous Rate The rate of reaction at any one particular time during the reaction. Steps: Draw graph (volume v time) Find tangent to curve at a particular time Find slope of tangent (rise/ run or y2-y1/ x2-x1) Slope = instantaneous rate

Monitoring the rate of production of oxygen

Graph Volume Oxygen v Time 160 V O L U M E G A S cm3 140 120 100 80 60 40 20 10 20 30 40 50 60 70 80 90 TIME sec

160 V O L U M E G A S cm3 140 120 100 80 60 40 20 10 20 30 40 50 60 70 80 90 TIME sec

What volume of gas was collected after 30 s ? 120cm3 What volume of gas was collected after 30 s ? 160 V O L U M E G A S cm3 140 120 100 80 60 40 20 10 20 30 40 50 60 70 80 90 TIME sec

When maximum volume of oxygen obtained 160 V O L U M E G A S cm3 140 120 After what time was the reaction finished ? 100 80 60 40 20 10 20 30 40 50 60 70 80 90 TIME sec

When half maximum volume of oxygen obtained 160 V O L U M E G A S cm3 140 120 When was the reaction half finished ? 100 80 60 40 20 10 20 30 40 50 60 70 80 90 TIME sec

What was the instanteous rate at 30 s ? 160 V O L U M E G A S cm3 Y2=150 140 120 Rate= Y2-Y1 X2-X1 What was the instanteous rate at 30 s ? 100 Y1=95 80 Rate= 55 40 Rate= 150-95 50-10 Rate= 1.375 cm3/sec 60 40 20 10 20 30 40 50 60 70 80 90 X1=10 X2=50 TIME

How would the graph change if the solution as more concentrated ? 160 V O L U M E G A S cm3 140 120 How would the graph change if the solution as more concentrated ? 100 80 60 40 20 10 20 30 40 50 60 70 80 90 TIME sec

How would the graph change if the temperature was higher ? 160 V O L U M E G A S cm3 140 120 How would the graph change if the temperature was higher ? 100 80 60 40 20 TIME

Rate of a Reaction The change in concentration per unit time of any one reactant or product Average Rate Total volume of gas produced (cm3) Total time to go to completion Instantaneous Rate The rate at a particular point in time during the reaction (slope of tangent to curve)

For a reaction to take place: The reacting particles must collide with each other A collision only results in the formation of products if a certain minimum energy is exceeded

Factors Affecting Rate of Reaction

Factors Affecting Rates of Reactions By experiment it is found that the rate of a reaction depends on five factors: Nature of reactants Particle size Concentration Temperature Catalysts

1. Nature of Reactants Covalent Compounds Ionic Compounds - Slow reactions - Fast reactions - Requires energy to break covalent bonds - Does not require energy to break bonds

2. Particle Size - Finer particles = greater the surface area The greater the surface area, the quicker the reaction E.g: Reaction of marble (calcium carbonate) with dilute HCl The smaller the particle size the larger the surface area. If the surface area of the substance is larger, more of it is available to react This increases the rate of reaction *As CO2 is realeased, the mass of the flask drops https://www.youtube.com/watch?v=yRw4ZRqmxOc&authuser=0 https://www.youtube.com/watch?v=IvPL7KC1DEA&authuser=0

2. Particle Size

2. Particle Size 16 places of attack- increased surface ares

Coal dust explosion Pittsburgh 2. Particle Size Dust EXPLOSIONS If very fine dust in the air builds up, an explosive reaction can occur Coal dust explosion Pittsburgh Grain silo explosion

2. Particle Size Conditions for dust explosion: The explosion must take place in an enclosed space The particles must be combustible There must be a source of ignition (spark, flame, etc) The particles must be dry There must be oxygen present

3. Concentration The higher the concentration, the faster the rate of reaction

To investigate the effect of concentration on reaction rate Add the HCl to the Na2S2O3 in the flask Record the time taken for the X to disappear Repeat with different concentrations of Na2S2O3 Inverse the time to get the rate

Effect of concentration on rate Conclusion ? R A T E Rate is directly proportional to concentration 1 Time CONCENTRATION

4. Temperature Hotter temperature = faster rate Cooler temperature = slower rate

To investigate the effect of temperature on reaction rate Add the HCl to the Na2S2O3 in the flask Record the temperature after mixing Record the time taken for the X to disappear Inverse the time to get the rate

Repeat at different temperatures

Effect of temperature on rate Conclusion ? R A T E Rate increases with increasing temperature 1 Time TEMPERATURE

What is the main factor increasing rate between A and B Effect of temperature on rate What is the main factor increasing rate between A and B R A T E 1 Time Rate increases due to increased collisions /sec B A TEMPERATURE

What other factor caused the large rate increase between B and C Effect of temperature on rate What other factor caused the large rate increase between B and C C R A T E More colliding particles have E Act 1 Time B TEMPERATURE

5. Catalysts Catalyst A catalyst is something that alters the rate of a chemical reaction but is not consumed in the reaction Negative catalyst: (inhibitor) can slow down the rate of a reaction You need to know: The general properties of catalysts Types of catalysis Mechanisms of catalysts Catalytic converter

a) Properties of Catalysts Remain chemically unchanged Only needed in small amounts In equilibrium reaction, a catalyst helps equilibrium to be achieved quicker

a) Properties of Catalysts 4. Specific – work for one reaction but not the other E.g: Enzymes Catalase – made in the liver, decomposes hydrogen peroxide Enzyme A substance that is produced by a living cell and acts as a biological catalyst

a) Properties of Catalysts 5. The action of a catalyst can be destroyed by catalyst poisons E.g: Lead: when lead was added to petrol, it was found that the lead destroyed the catalysts in the catalytic converter Arsenic: Inhibits the actions of certain enzymes in the body Catalyst poison A substance that makes a catalyst inactive

Learning Check

b) Types of Catalysis Types of Catalysis Homogeneous Catalysis Autocatalysis Heterogeneous Catalysis

Homogeneous Catalysis b) Types of Catalysis Homogeneous Catalysis Catalysis in which both the reactants and the catalyst are in the same phase, i.e: there is no boundary between the reactants and the catalyst ***You must say phase*** (-3 marks for saying state) Phase= a specific state of matter that has uniform chemical and physical properties Example: The iodine snake (aqueous potassium iodide and hydrogen peroxide)

Heterogeneous Catalysis b) Types of Catalysis Heterogeneous Catalysis Catalysis in which the reactants and the catalyst are in different phases i.e: there is a boundary between the reactants and the catalyst Example: oxidation of methanol by platinum catalyst Pt 2CH3OH + O2  2HCHO + 2H2O

https://www.youtube.com/watch?v=8lkq4vRkQ2Y https://www.youtube.com/watch?v=FSdBB1vBDKY https://www.youtube.com/watch?v=rsIhjinuBYA

Starts slow but speeds up as the product (catalyst) starts to form b) Types of Catalysis Autocatalysis Catalysis in which one of the products acts as a catalyst for the reaction Starts slow but speeds up as the product (catalyst) starts to form

c) Mechanisms of Catalysis The Intermediate Formation Theory The Surface Adsorption Theory

The Intermediate Formation Theory c) Mechanisms of Catalysis The Intermediate Formation Theory One or more of the reactants combine with the catalyst to form an intermediate compound

The Intermediate Formation Theory c) Mechanisms of Catalysis The Intermediate Formation Theory C= Catalyst Consider: W + X → Y + Z One of the reactants combines with the catalyst to form an intermediate compound- this does not last very long: W + C → [WC] Intermediate compound then reacts with the other reactant (X) to give the final product: [WC] + X → Y + Z + C Overall reaction: W + X → Y + Z C

Example of the Intermediate Formation Theory c) Mechanisms of Catalysis Example of the Intermediate Formation Theory Iodine Snake The decomposition of hydrogen peroxide (H2O2) catalysed by the presence of I- ions. 2H2O2 → 2H2O + O2 1. One of the hydrogen peroxide molecules reacts with an I- ion to form the IO- intermediate H2O2 + I- → H2O + IO- I-

Example of the Intermediate Formation Theory c) Mechanisms of Catalysis Example of the Intermediate Formation Theory 2. The intermediate then reacts with another hydrogen peroxide molecule to form the products and regenerate the catalyst H2O2 + IO- → H2O + O2 + I- Adding the equations in steps 1 & 2 gives us the overall equation: I- 2H2O2 → 2H2O + O2

The Intermediate Formation Theory c) Mechanisms of Catalysis The Intermediate Formation Theory One or more of the reactants combine with the catalyst to form an intermediate compound https://www.youtube.com/watch?v=kt26HLzV05E https://www.youtube.com/watch?v=bQhN0HtDrM8

c) Mechanisms of Catalysis Exam Q!! The oxidation of potassium sodium tartrate by hydrogen peroxide catalysed by cobalt (II) ions provides evidence for the intermediate formation theory of catalysis. State the observation you would make when carrying out the experiment. Explain how these observation provide evidence for the intermediate formation theory

c) Mechanisms of Catalysis Exam Q!! Observations: The solution is pink at the start of the experiment After adding 100-volume H2O2, fizzing is observed The solution changes to a green colour After a short while, the fizzing stops and the green colour changes back to pink Explanations: Pink → green indicates the formation of a new substance Green pink indicates the substance was an intermediate Fizzing when green indicates the intermediate is reacting Pink colour at the end indicates the cobalt (II) ions were not used up and have been regenerated

c) Mechanisms of Catalysis Exam Q!! Observations: The solution is pink at the start of the experiment After adding 100-volume H2O2, fizzing is observed The solution changes to a green colour After a short while, the fizzing stops and the green colour changes back to pink Explanations: Pink → green indicates the formation of a new substance Green pink indicates the substance was an intermediate Fizzing when green indicates the intermediate is reacting Pink colour at the end indicates the cobalt (II) ions were not used up and have been regenerated

The Surface Adsorption Theory- c) Mechanisms of Catalysis The Surface Adsorption Theory- Heterogeneous Catalysis Mechanism 3 steps: Adsorption: Molecules settle on the surface of the catalyst and are held there by temporary bonds Reaction on the surface: Chemical reaction occurs. High concentration of molecules on the surface makes it more likely that they will collide with each other (react) Desorption: Products leave the surface of the catalyst Absorption= when a substance moves in to another Adsorption= accumulation of substances only at the surface of another substance

The Surface Adsorption Theory- c) Mechanisms of Catalysis The Surface Adsorption Theory- Heterogeneous Catalysis Mechanism More finely divided the catalyst = the better its performance. Greater surface area on which adsorption can take place. When a catalyst is ‘poisoned’ a lot of the surface of the catalyst is blocked off due to permanent bond formation between the metal and the ‘poison’.

d) Catalytic Converters A device in the exhaust of a motor vehicle which contains catalysts to convert pollutants in the exhaust gases to less harmful substances ENGINE GASES NO2 CO CXHY (HYDROCARBONS) EXHAUST GASES N2 CO2 H2O

d) Catalytic Converters A catalytic converter consists of a thin coating of the metals platinum, palladium and rhodium (the catalysts) on a ceramic or metal ‘honeycomb’ inside a stainless steel base *Common exam q!* Name the catalysts used in a catalytic converter

d) Catalytic Converters The main reaction that occurs: Pt/Pd/Rh 2CO + 2NO → 2CO2 + N2 *This is heterogeneous catalysis *Common exam q!* Name the type of catalysis that occurs in catalytic converters

ENVIRONMENTAL BENEFIT OF REMOVAL d) Catalytic Converters REACTANT PRODUCT ENVIRONMENTAL BENEFIT OF REMOVAL Carbon monoxide Carbon dioxide Carbon monoxide is a poisonous gas. Nitrogen monoxide Nitrogen Nitrogen monoxide is poisonous and contributes to acid rain. Hydrocarbons Carbon dioxide and water Hydrocarbons cause smog and contribute to the greenhouse effect.

Collision Theory and Activation Energy

Collision Theory Attempts to explain why these five factors influence the rate of reaction For a reaction to occur, the reacting particles must collide with each other. A collision only results in the formation of products if a certain minimum energy is exceeded in the collision. Such a collision is called an effective collision An effective collision is a collision which results in the formation of products

Collision Theory Attempts to explain why these five factors influence the rate of reaction When an effective collision occurs between colliding particles, bonds are broken and new bonds are formed, i.e: the effective collision brings about a chemical reaction. This results in the formation of products Rate of reaction: depends on not only the number of collision, but how many of these are effective

Collision Theory Attempts to explain why these five factors influence the rate of reaction 3. For a collision to be effective, it must have a certain amount of energy – Activation energy Activation Energy The minimum energy that colliding particles must have for a reaction to occur, i.e: the minimum energy required for effective collisions between particles to occur

Reaction Profile Diagram A graph which shows the change in energy of a chemical reaction with time as the reaction progresses

Activation Energy and Reaction Profile Diagrams Large Activation Energy Only a small number of molecules have enough energy to change into products. Therefore the rate is slow

Activation Energy and Reaction Profile Diagrams Small Activation Energy Greater number of molecules have enough energy to change into products. Therefore the rate is fast.

Exothermic Reactions Exothermic: A chemical reaction that gives out heat. Products have less energy than the reactants

Endothermic Reactions A chemical reaction that takes in heat. Products have more energy than the reactants

Using the Collision Theory to Explain the 5 Factors: *NB for LC qs 1. Nature of Reactants: Reactions involving covalent bonds involve breaking bonds much higher activation energy slower rate of reaction Reactions involving ionic bonds lower activation energy faster rate of reaction. 2. Particle Size: Smaller the particle size: greater the surface area number of collisions increase number of effective collisions also increase increased rate of reaction.

Using the Collision Theory to Explain the 5 Factors: *NB for LC qs 3. Concentration Concentration of the reactants is increased number of collisions increased (particles closer together) number of effective collisions also increased increased rate of reaction.

Using the Collision Theory to Explain the 5 Factors: *NB for LC qs 4. Temperature At higher temperatures Particles have more energy A greater number of collisions reach the activation energy Increased rate of reaction.

Using the Collision Theory to Explain the 5 Factors: *NB for LC qs 5. Catalysts A catalyst works by providing an alternative reaction pathway with a lower activation energy. more reactant molecules now possess the energy required for effective collisions to occur increased rate of reaction.