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Chapter 1: Rate of Reaction
1.1 Rate of Reaction
Which reaction is faster?
The meaning of rate of reaction
1 A 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. 3.
The meaning of rate of reaction
The rate of reaction is inversely proportional to the time taken for the reaction to be completed. Rate of reaction
The reaction is if it takes a to complete. Conversely, the reaction is slow if it takes a for the reaction to complete.
fast
1 Time taken
short time
long time
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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.
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 = Amount of product obtained
Time Taken
Rate of reaction =
Amount of reac tan t used up Time Taken
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 =
Volume of produced Time Taken
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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) cm3 per unit time (second or minute) for a gas evolved
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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 solution
dm-3 per unit time for a reactant in aqueous
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 Fast
Combustion Precipitation of silver chloride Reaction between reactive metal and water
Slow
Rusting Photosynthesis Fermentation
Measuring reaction rates
1. The rate of reaction can be express in two ways: (a) the or
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
average rate of reaction over a period of time, given time.
(b) the rate of reaction at any
period of time.
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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?
0.1g
Measuring reaction rates
Example 2
Calcium carbonate reacts with dilute hydrochloric acid according to the equation: CaCO3(s) + 2HCl(aq) CaCl2(aq) + H2O(l) + CO2(g) After 1.2 minutes, the volume of gas produced is 100 cm3. Calculate the average rate of reaction in the units of (a) cm3 min-1, (b) cm3 s-1.
1 average rate of reaction= 5s 0.02 gs
Measuring reaction rates
Example 2 Solution:
average reaction rate volume of CO 2 produced time taken 100(cm 3 )
Measuring reaction rates
Example 2 Solution:
Average rate volume of CO2 produced time taken
100cm 3 1.39cm 3 s 1 72s
1.2(min) 83.3cm 3 min 1
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. 4.
Measuring reaction rates
The rate of reaction at a given time can determined by
measuring the against time (Figure1.2)
gradient of the graph mass of reactant
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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 2O2 (aq) 2H2O(l) + O2(g) The results of an experiment on the decomposition of hydrogen peroxide are given below.
Time(s) Volume of O2(cm3) 0 0 15 16 30 30 45 40 60 48 90 56
a b
Calculate the rate of reaction at 40 seconds in the units of (a) cm3 s-1, (b) cm3 min -1.
Solving numerical problems involving rate of reaction
Example 3: Solution
(a) The rate of reaction at 40 s = Gradient at 40 s 49
Solving numerical problems involving rate of reaction
Example 4: Solution (b) time =
(58 18) 40 2 min min min 60 60 3
a 49 21 0.70cm 3 s 1 b 58 18
21
Rate in cm3 min-1 Rate of reaction in cm3 min-1
3 = 28cm
18
2 min 3
= 42cm3 min-1
58
Measuring reaction rates
5 Analysing a reaction rate curve:
Measuring reaction rates
5 Analysing a reaction rate curve: (b) Figure 1.5 shows that the rate of reaction at t2, is
(a) The steeper the gradient, the faster the rate of reaction.
slower than the rate of
reaction at t1. The gradient at t2 is less steep than the gradient at t1. As the reaction proceeds, the curve becomes less steep because the rate of reaction decreases.
Steeper
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Measuring reaction rates
6 Comparing reaction rates at a give time
Measuring reaction rates
Activity: To find the reaction rates at (a) 90 s, (b)180 s a (c) the average rate of the reaction between zinc and dilute sulphuric acid
Measuring reaction rates
Apparatus: Conical flask, measuring cylinder, delivery tube, burette, basin, retort stand, retort clamp and stopwatch.
Measuring reaction rates
Materials:
Granulated zinc and 0.3 mol dm-3 sulphuric acid.
Measuring reaction rates
Procedure: 1 The burette is filled with water and inverted over a basin of water.
Measuring reaction rates
Procedure: 2 Using a measuring cylinder, 20.0 cm3 of 0.3 mol dm-3 sulphuric acid is measured out and poured into a conical flask.
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Measuring reaction rates
Procedure: 3 5.0 g of granulated zinc are then added to the sulphuric acid in the conical flask.
Measuring reaction rates
Procedure: 4 The conical flask is then closed and the hydrogen gas produced is collected in the burette by the displacement of water as shown in Figure 1.5.
Measuring reaction rates
Procedure: 5 The stopwatch is started immediately. 6 The volume of hydrogen gas collected in the burette is recorded at 30second intervals.
Measuring reaction rates
Results:
Measuring reaction rates
Calculation: (a) The rate of reaction at 90 s = slope at 90 s
Measuring reaction rates
Calculation: (b) The rate of reaction at 180 s = slope at 180 s
YZ XY (52 20)cm 3 (180 30) s
YZ XY (48 30)cm 3 (240 18) s
32cm 3 0.213cm3 s 1 150s
18cm 3 0.081cm3 s 1 222s
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Measuring reaction rates
Calculation: (c) The average rate of reaction
Measuring reaction rates
Conclusion: The rate of reaction decreases as the reaction proceeds.
total volume of H 2 produced total time taken 47 0.157cm 3 s 1 300
Measuring reaction rates
Activity: To measure the rate of reaction between calcium carbonate (CaCO3) and excess hydrochloric acid
Measuring reaction rates
Apparatus: Conical flask, electronic balance, measuring cylinder and stopwatch.
Measuring reaction rates
Material: Calcium carbonate (CaCO3) pieces, 2.0 mol dm-3 hydrochloric acid and cotton wool.
Measuring reaction rates
Procedure 1 Using a measuring cylinder, 50 cm3 of 2 mol dm-3 hydrochloric acid are measured out and poured in a dry conical flask. The mouth of the conical flask covered with some of cotton wool. The cotton wool is inserted into the mouth of the conical flak is prevent liquid from splashing out during the reaction.
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Measuring reaction rates
Procedure 2 The conical flask is placed on the electronic balance as shown in Figure 1.6.
Measuring reaction rates
Procedure 3 The mass of conical flask and its contents (calcium carbonate, hydrochloric acid and cotton wool) is recorded.
Measuring reaction rates
Procedure 4 The calcium carbonate is then transferred to the hydrochloric acid in the conical flask and the stopwatch is started immediately.
Measuring reaction rates
Procedure 5 The mass of the conical flask (and its contents) is recorded at oneminute intervals.
Measuring reaction rates
Results: Time(s) Mass of conical flask + content(g) 0 60.0 1 59.1 2 58.3 3 57.9 4 57.4 5 57.0 6 56.8 7 56.5 8 56.3
Measuring reaction rates
Based on the experimental results, a graph of the mass of conical flask and its contents against time is plotted (Figure 1.7)
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Measuring reaction rates
Calculation The average rate of reaction for the first minute. Decrease in mass = mass of carbon dioxide produced = (60.0-59.1) g = 0.9g Average rate of reaction for the first minute
Measuring reaction rates
Calculation the average rate of reaction between 1.4 minutes and 2.2 minutes Rate of decrease in mass
mass of CO 2 produced 0.9 g 0.9 g min 1 time taken 1.0 min
Time(s) Mass of conical flask + content(g) 0 60.0 1 59.1 2 58.3 3 57.9 4 57.4 5 57.0 6 56.8 7 56.5 8 56.3
58.8 58.3 0.625 g min 1 2.2 1.4
Measuring reaction rates
Calculation
(c) The reaction rate at the 5th minute =Gradient of the graph at 5.0 minutes =
Measuring reaction rates
Conclusion: The rate of reaction decreases as the reaction proceeds. Finally, the reaction will stop when all the calcium carbonate added have reacted.
a 57.5 56.4 1.1g b 7.0 3.4 3.6minutes 1.1 Gradient 0.306 g min 1 3.6
a b
Solving numerical problems involving rate of reaction
Example 4: 3.0 g of excess marble (CaCO3) are added to 100 cm3 of dilute hydrochloric acid. Figure 1.8 shows the graph of volume of carbon dioxide produced against time.
Solving numerical problems involving rate of reaction
Example 4:
Calculate (a) the average rate of reaction (b) the concentration of hydrochloric acid in mol dm-3. (1 mole of any gas occupies 24 dm3 at room conditions).
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Solving numerical problems involving rate of reaction
Example 4: Solution (a) Total volume of carbon dioxide evolved = 360 cm 3 Time taken = 8.0 minutes Average rate of reaction
Solving numerical problems involving rate of reaction
Example 4:
Solution (b) Number of moles of CO2 evolved
360 45cm 3 min 1 8
360 0.015mol 24 1000
CaCl2 + H2O + CO2 =2: 1
CaCO3 + 2HCl
Mole ratio of HCl : CO2
Solving numerical problems involving rate of reaction
Example 4:
Solving numerical problems involving rate of reaction
Example 4:
Solution (b) According to the equation, number of moles of hydrochloric acid used
= 2 x 0.015 = 0.03 mole
CaCO3 + 2HCl CaCl2 + H2O + CO2 2 moles HCl produces 1 mol CO2 ? moles HCl produces 0.015 mol CO2
Solution (b) According to the equation, number of moles of hydrochloric acid used = 2 x 0.015 N = 0.03 mole M
N MV
Concentration of hydrochloric acid, M
number of moles volume( in dm 3 ) 0.03 0.3moldm 3 0.1
V = 100 cm3 =0.1 dm3
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