Chapter 1: Rate of Reaction Rate of Reaction

Which reaction is faster?

The meaning of rate of reaction 1A chemical reaction takes place when reactants are mixed together under suitable conditions.

The meaning of rate of reaction 2 The speed of a chemical reaction is called the rate of reaction.

The meaning of rate of reaction 3.The rate of reaction is inversely proportional to the time taken for the reaction to be completed.

The meaning of rate of reaction 3.The rate of reaction is inversely proportional to the time taken for the reaction to be completed. Rate of reaction  The reaction is fast if it takes a short time to complete. Conversely, the reaction is slow if it takes a long time for the reaction to complete.

The meaning of rate of reaction 4 During a chemical reaction, the reactants are used up as the products are formed. Thus, the amounts of reactants decrease (Figure 1.1(a)) while the amounts of products increase as the reaction proceeds (Figure 1.1(b)).

The meaning of rate of reaction 5 Therefore, the rate of reaction can be determined in one of the following ways: (a) the rate of disappearance of a reactant, or (b) the rate of formation of a product

The meaning of rate of reaction 6 (a) The rate of reaction can be defined as the amount of a reactant used up per unit time. Rate of reaction =

The meaning of rate of reaction 6 (b) The rate of reaction can also be defined as the amount of a product obtained per unit time. Rate of reaction =

The meaning of rate of reaction 7 Methods of measuring reaction rates (a) The amount of a reactant used up or a product obtained can be measured in terms of the mass of the substance or the concentration of the substance.

The meaning of rate of reaction 7 Methods of measuring reaction rates (b) For chemical reactions that produce gases, the rate of reactions can be measured as the volume of gas produced per unit time. Rate of reaction =

The meaning of rate of reaction 8 Reaction between calcium carbonate and dilute hydrochloric acid. (a) The reaction between calcium carbonate (marble chips) and dilute hydrochloric acid can be represented by the equation:

The meaning of rate of reaction (b) During the reaction, the following observable changes take place. (i) The mass of calcium carbonate (the reactant) decreases. (ii) The concentration of hydrochloric acid (the reactant) decreases. (iii) The volume of carbon dioxide (the product) produced increases.

The meaning of rate of reaction (c) Thus, the rate of reaction between calcium carbonate and hydrochloric acid can be determined by measuring (i) the decrease in mass of calcium carbonate per unit time, or (ii) the increase in volume of carbon dioxide per unit time.

The meaning of rate of reaction 9 (a) Besides changes in the mass of the reactants or the changes in the volume of the gaseous products, other changes that can be observed are as follows: (i) changes in colour (ii) formation of precipitates

The meaning of rate of reaction (b) Therefore, the time taken for the colour of a reactant to change or a certain amount of precipitate to form can be used to measure the rate of reaction.

The meaning of rate of reaction 10 The units used for the rate of reaction will depend on the changes measured. For example (a) cm 3 per unit time (second or minute) for a gas evolved

The meaning of rate of reaction 10 The units used for the rate of reaction will depend on the changes measured. For example (b) g per unit time or mol per unit time for a solid reactant

The meaning of rate of reaction 10 The units used for the rate of reaction will depend on the changes measured. For example (c) mol dm -3 per unit time for a reactant in aqueous solution

The meaning of rate of reaction 11 Different chemical reactions take place at different rates. Some reactions occur very rapidly and some very slowly. Table 1.1 shows some examples of very fast reactions and very slow reactions.

The meaning of rate of reaction FastSlow CombustionRusting Precipitation of silver chloridePhotosynthesis Reaction between reactive metal and water Fermentation

Measuring reaction rates 1. The rate of reaction can be express in two ways: (a) the average rate of reaction over a period of time, or (b) the rate of reaction at any given time.

Measuring reaction rates 2 The average rate of reaction is the average of the reaction rates over a given period of time. We can measure the average rate of reaction by measuring the change in amount (or concentration) of a reactant or a product over a period of time.

Measuring reaction rates Example 1 A piece of magnesium ribbon weighing 0.1 g is added to dilute hydrochloric acid. After 5 seconds, all the magnesium had dissolved. What is the average rate of reaction? average rate of reaction=

Measuring reaction rates Example 2 Calcium carbonate reacts with dilute hydrochloric acid according to the equation: CaCO 3 (s) + 2HCl(aq)  CaCl 2 (aq) + H 2 O(l) + CO 2 (g) After 1.2 minutes, the volume of gas produced is 100 cm 3. Calculate the average rate of reaction in the units of (a) cm 3 min -1, (b) cm 3 s -1.

Measuring reaction rates Example 2 Solution:

Measuring reaction rates Example 2 Solution:

Measuring reaction rates 3 The rate of reaction at any given time is the actual rate of reaction at a given time. The reaction rate at any given time is also known as the instantaneous rate of reaction.

Measuring reaction rates 4. The rate of reaction at a given time can determined by measuring the gradient of the graph mass of reactant against time (Figure1.2)

Measuring reaction rates Determining the gradient of the tangent time t: Rate of reaction at time t (gs -1 ) = Gradient of the graph =

Solving numerical problems involving rate of reaction Example 3: Hydrogen peroxide decomposes according to the equation: 2H 2 O 2 (aq)  2H 2 O(l) + O 2 (g) The results of an experiment on the decomposition of hydrogen peroxide are given below. Calculate the rate of reaction at 40 seconds in the units of (a) cm 3 s -1, (b) cm 3 min -1. Time(s) Volume of O 2 (cm 3 )

Solving numerical problems involving rate of reaction Example 3: Solution (a) The rate of reaction at 40 s = Gradient at 40 s

Solving numerical problems involving rate of reaction Example 4: Solution (b) time = Rate in cm 3 min -1 Rate of reaction in cm 3 min -1 = = 42cm 3 min -1

Measuring reaction rates 5 Analysing a reaction rate curve: (a) The steeper the gradient, the faster the rate of reaction. Steeper