chapter 1 rate of reactions

CHAPTER 1 RATE OF REACTION

Contents

1.1 Understanding of Rate of Reaction1.2 Factors Affecting Rate of Reaction1.3 Collision Theory 1.4 Scientific Knowledge to Enhance Quality

of Life

1.1 Understanding of Rate of Reaction Rate of reaction is defined as the change in the amount

of reactants or products per unit time. We usually use water displacement method to collect

gas

Method of Gas Collection Using a Gas Syringe System

• The reaction is fastest at the start when the reactants are at a maximum (steepest gradient)

• The gradient becomes progressively less as reactants are used up and the reaction slows down.

• Finally the graph levels out when one of the reactants is used up and the reaction stops.

• The amount of product depends on the amount of reactants used.

• The initial rate of reaction is obtained by measuring the gradient at the start of the reaction. A tangent line is drawn to measure rate of reaction at instantaneous time.

Different chemical reactions occur at different rates. • Fast Reaction• Slow Reaction

Fast Reaction Slow Reaction

The time taken for a fast reaction is short.

The time taken for a slow reaction is long.

The rate of reaction is said to be high.

The rate of reaction for a slow reaction is low.

Example: •Fading of dyes on a shirt under hot sun •Burning of petrol in a car engine •Striking a match •Ripening of tomatoes

Example:•A piece of newspaper turning yellow •The weathering of limestone by acid rain •Rusting of a water pipe

Fast Reaction

Slow Reaction

Observable changes for measuring rate of reaction

Finding Average Rate Of Reaction

Example: In a chemical reaction, 2.5g of calcium carbonate react completely with excess hydrochloric acid to produce 600cm³ of carbon dioxide gas in 1.5 minutes. Find the rate of reaction in term of

a. decreasing mass of calcium carbonateb. increasing volume of carbon dioxide gas produced

Finding Average Rate Of Reaction From Measurable Quantities

During a chemical reaction, two things happen1. The quantities of reactants reduce.2. The quantities of products increase.

Therefore, the rate of the reaction can be determined by• measuring the decrease of the amount of the reactants over time.• measuring the increase of the amount of the products over time.

However, the quantity (or change) of the reactants/product may be measurable or immeasurable.

The easily measured quantity changes include• Mass• Concentration (Conductivity)• Volume of gas

Example:In a reaction, 5 g of calcium carbonate takes 250 seconds to completely react with solution of hydrochloric acid. Calculate the average rate for this reaction in unitsa)g s-1 andb)mol s-1

[ Relative atomic mass: C 12; O, 16; Ca, 40]

Finding Average Rate Of Reaction From Immeasurable Quantities

Example:When the aqueous of ethanadioic is mixed with acided potassium manganate(VII) , the reaction happen slowly at room temperature. The purple colour of the solution is bleached after 20 seconds. Calculate the average rate of reaction.

Analysing Rate of Reaction from Graph

Graph Of Product/Reactant Change Against TimeIn a chemical reaction, 

• the reactants will decrease over time • the product will increase over time

the rate of reaction will decrease over time because of the decrease in concentration and total surface area of reactants.In a graph of quantity of product/reactant over time, the rate of reaction is equal to the gradient of the graph.

Example:

The reaction between dilute hydrochloric acid and excess marble will produce calcium chloride and gas of carbon dioxide. Sketch the graph of

1. the mass of the marble against time.2. the volume of carbon dioxide against time.3. the concentration of hydrochloric acid against time.4. the concentration of calcium chloride against time.

Finding The Average Rate Of Reaction From A GraphExample:

In a reaction between calcium carbonate and liquid hydrochloric acid, carbon dioxide gas that is released is collected in a burette. The graph shows the volume of carbon dioxide collected over time. Find the average rate of reaction in the first 60s.

Finding Instantaneous Rate Of Reaction From A Graph

The rate of reaction changes from time to time as the reaction happens.

The rate of reaction at a particular time is called the instantaneous rate.

The instantaneous rate of a reaction is equal to the gradient of tangent at a particular time.

Example:

The graph shows the volume of carbon dioxide gas released over time in a chemical reaction. Find the rate of reaction at t = 40s

1.2 Factors Affecting Rate of Reaction

1. Size of Solid Reactants/Total Surface Area

The smaller the size of the particle, the bigger the total surface area. 

The bigger the total surface area, the higher the rate of reaction.

Experiment 125 cm3 of 0.5 mol dm–3 hydrochloric acid + calcium carbonate chips. The carbon dioxide gas released is collected in a burrete. The volume of the gas released is recorded in every 30s. The result is plotted in a graph.

Experiment 225 cm3 of 0.5 mol dm–3 hydrochloric acid + calcium carbonate powder. As in experiment 1, the carbon dioxide gas released is collected in a burrete and the volume of the gas released is recorded in every 30s. The result is plotted in the same graph in experiment 1.

Conclusion

• The gradient of the curve for experiment 2 is greater than the curve for experiment 1. This indicate that the rate of reaction in experiment 2 is higher than experiment 1.

• Conclusion: the smaller the particle size of the reactant, the bigger the total surface area, and the bigger the total surface area, the higher the rate of the reaction will be.

2. Concentration Of Reactants

ExperimentBy measuring the time taken for the formation of sulphur precipitate (yellow solid) when sulphuric acid, H2SO4 reacts with sodium thiosulphate(VI), Na2S2O3 of different concentration , we can investigate the effect of concentration of the reactant on the rate of reaction.

The higher the concentration of the solution, the higher the rate of reaction.

Procedure:•50 cm3 of 0.2 mol dm-3 sodium thiosulphate solution + 10 cm3 of 0.5 mol dm-3 sulphuric acid.•Time taken for the ̔X’ sign placed under the conical flask to disappear from view is recorded.•The experiment is repeated by using sodium thiosulphate solution with concentration 0.4 mol dm-3, 0.6 mol dm-3, 0.8 mol dm-3 and 1.0 mol dm-3.

Conclusion :•The graph for concentration of sodium thiosulphate (VI), Na2S2O3 against time taken for the sulphur precipitate to formed is plotted.•As the concentration of sodium thiosulphate solution decreases, the longer the time is needed for the marked ̔X’ to disappear.

3. Temperature Of The Reactant

ExperimentBy measuring the time taken for the formation of sulphur precipitate (yellow solid) when sulphuric acid, H2SO4 reacts with sodium thiosulphate(VI), Na2S2O3 of different temperature, we can investigate the effect of temperature of the reactant on the rate of reaction.

The higher the temperature of the solution, the higher the rate of reaction.

Procedure:•50 cm3 of 0.2 mol dm-3 sodium thiosulphate solution at 30ºC + 10 cm3 of 0.5 mol dm-3sulphuric acid.•Time taken for the ̔X’ sign placed under the conical flask to disappear from view is recorded.•The experiment is repeated by using sodium thiosulphate solution with temperature 35ºC, 40ºC, 45ºC and 50ºC.

Conclusion :

• The graph for temperature of sodium thiosulphate (VI), Na2S2O3 against time taken for the sulphur precipitate to formed is plotted.

• As the temperature of sodium thiosulphate solution decreases, the longer the time is needed for the marked ̔X’ to disappear.

Pressure Of Gas

For reactions involve gas, the rate of reaction is affected by the pressure of the gas.

Pressure DOES NOT affect the rate of reaction where the reactants are in the form of solids or liquids.

The higher the pressure of the gas, the higher the rate of reaction

The higher the pressure of the gas, the higher the rate of reaction

4. Catalyst Catalyst is a chemical substance that change the rate of

chemical reaction Characteristics of catalyst:

Catalyst

Only a small amount of catalyst is needed to increases the rate of reaction. An increase in the quantity of catalyst will increase the rate of reaction slightly

During a reaction, catalyst remains chemically unchanged but may undergo physical changes. For example, catalyst may turn into powder during the reaction

Change the rate of reaction

Does not change thequantity of products formed

It is specific in its action. It can only catalyse a particular reaction

Catalysts provide a different reaction path with a low activation energy

List of Reactions and the Catalyst

Chemical Reaction CatalystDecomposition of hydrogen peroxide:

2H2O2 → 2H2O + O2

Manganese(IV) oxide, MnO2

Haber Process

N2 + 3H2 → 2NH3Iron, Fe

Contact Process

2SO2 + O2 → 2SO3Vanadium(V) oxide, V2O5

Ostwald Process

4NH3 + 5O2 → 4NO + 6H2O

Platinum, Pt

Factors Affecting Catalyst

A catalyst is a substance which can change the rate of reaction. 

There are 2 types of catalyst: • Positive catalyst – Increase the rate of

reaction. • Negative catalyst – Reduce the rate of

reaction.

Copper(II) sulphate acts as a catalyst to increase the rate of reaction between zinc and hydrochloric acid

ConclusionThe presence of catalyst increases the rate of reaction.

Manganese(IV) oxide acts as a catalyst to increase the rate of reaction between zinc and hydrochloric acid

Conclusion:The presence of catalyst increases the rate of reaction

Application of Catalysts in Industry a) Haber Process (Produces Ammonia)

• In the Haber process, a mixture of nitrogen and hydrogen in the ratio 1:3 is conducted through the powdered iron as catalyst at a temperature of 450°- 550°C and a pressure of 200 -300 atmospheres.

• Powdered iron is used as the catalyst to raise the rate of reaction.

• Also, the reaction is conducted at high temperature to increase the rate of reaction.

1.3 Collision Theory The collision theory states

that:• The particles of the reacting

need to touch to enable bond formation or breaking to happen.

• Collisions of particles of a reacting substance need to achieve a certain minimum energy (Activation Energy) in order to produce a reaction.

• Particles that collide also need to have the correct orientation of collision.

According to collision theory, atoms, ions, and molecules can react to form products when they collide with one another, provided that the colliding particles have enough kinetic energy.

Effective Collision

Ineffective Collision

Activation Energy• The minimum energy that the reactants particles must achieve at the

time of collision in order for a chemical reaction to take place.• The value of the activation energy is different for different reactions.• A reaction with high activation energy occurs slowly whereas a

reaction with a low activation energy occurs fast.

Energy Profile DiagramIn this diagram, the activation energy is shown by the difference in energy between the peak of the graph and the level of the energy of the reacting substance.

Exothermic Reaction Endothermic Reaction

Factors Affecting Rate Of Reaction - Explanation By Collision Theory

Total Surface Area of Reactants• When the size of the solid substance that reacts is smaller, its total

surface area exposed becomes larger.• This causes the collisions frequency between the reactants increases.• As a result, the frequency of effective collisions also increases and

hence increases the rate of reaction.

Presence of Catalyst• When a positive catalyst is used in a reaction, the catalyst

prepares an alternative path with lower activation energy for the reaction.

• As a result, the frequency of effective collisions increases and hence increases the rate of reaction.

Concentration• Solution with higher concentration has more particles per unit

volume in the solution.• As a result, the collisions frequency between the reactants

increases.• Consequently, the frequency of effective collisions also

increases and hence the rate of reaction increases.

Temperature• When the temperature of a reaction increases, the particles of

the reacting substances move faster.• This causes the collisions frequency between the reactants

increases.• As a result, the frequency of effective collisions also increases

and hence increases the rate of reaction.

Pressure of Gas• For a reaction that involves a gas, when pressure increases,

the particles of gas are compressed to fill the spaces which are small. This makes the number of particles of gas per unit of volume to increase.

• This causes the collisions frequency between the reactants increases.

• As a result, the frequency of effective collisions also increases and hence increases the rate of reaction

1.4 Scientific Knowledge to Enhance Quality of Life

1. Keeping food in a refrigerator• If food is kept in the fridge, the food will keep longer

because the low temperature will slow down the rate of the chemical reaction which destroys food.

2. Cooking food in a pressure cooker• In a pressure cooker, the high pressure causes the

water in the cooker to boil at a temperature of more than 100°C.

• At a higher temperature, the time for the food to get cooked is decreased.

3. Cooking Food in Small Pieces• Food in the shape of big pieces has a surface area

per volume which is small, so the heat takes a longer time to reach the inside of the food. So, to cook faster, the food needs to be cut into smaller pieces.

4. Making Margarine• Vegetable oil is an organic compound that is

not saturated and exists in liquid state at room temperature.

• Vegetable oil can be changed to margarine through the process of hydrogenation using nickel as catalyst at a temperature of 180°C

5. Burning of Coal• Coal contains the element carbon. • A big piece of coal takes a long time to burn because the total

surface area that is touched by the fire is small.• The rate of burning pieces of coal which are small is higher

because the total surface area is bigger. With this, it provides a lot of heat energy in a short period of time.

The End