●
● Concentration of the reactants and gas pressure. ●
● Temperature at which the reaction is carried out. ●
● Light. ●
● Use of a catalyst, including enzymes.
Collision theory
For a chemical reaction to occur, reactant particles need to collide with one another. Not every collision results in the formation of products. For products to be formed, the collision has to have a certain minimum amount of energy associated with it. This minimum amount of energy is known as the activation energy, Ea (Figure 7.2). Collisions which result in the formation of products are known as successful collisions. progress of reaction 0 products energy activation energy
reactants overallenergy
change
Figure 7.2 Energy level diagram showing activation energy.
Surface area
In Chapter 13, we shall see that limestone (calcium carbonate) is a substance which can be used to neutralise soil acidity. Powdered limestone is used as it neutralises the acidity faster than if lumps of limestone are used. Why do you think this is the case?
In the laboratory, the reaction between acid and limestone in the form of lumps or powder can be observed in a simple test-tube experiment. Figure 7.3 shows the reaction between dilute hydrochloric acid and limestone in lump and powdered form. hydrochloric acid 2HCl(aq) + + + + + + calcium carbonate CaCO3(s) →● ● → calcium chloride CaCl2(aq) carbon dioxide CO2(g) water H2O(l)
Figure 7.3 The powdered limestone (left) reacts faster with the acid than
the limestone in the form of lumps.
The rates at which the two reactions occur can be found by measuring either:
●
● the volume of the carbon dioxide gas which is
produced, or
●
● the loss in mass of the reaction mixture with time.
These two methods are generally used for measuring the rate of reaction for processes involving the formation of a gas as one of the products.
The apparatus shown in Figure 7.4 (p. 106) is used to measure the loss in mass of the reaction mixture. The mass of the conical flask plus the reaction mixture is measured at regular intervals. The total loss in mass is calculated for each reading of the balance, and this is plotted against time. Some sample results from experiments of this kind have been plotted in Figure 7.5.
The reaction is generally at its fastest in the first minute. This is indicated by the slopes of the curves during this time. The steeper the slope, the faster the rate of reaction. You can see from the two traces in Figure 7.5 that the rate of reaction is greater with the powdered limestone than the lump form.
Figure 7.4 After 60 seconds the mass has fallen by 1.24 g. 0 1 2 1 2 3 4 5 6 7 1 powdered limestone 2 lumps of limestone time/min loss in mass/g
Figure 7.5 Sample results for the limestone/acid experiment.
An increase in the surface area of a solid reactant results in an increase in the number of collisions, and this results in an increase in the number of successful collisions. Therefore, the increase in surface area of the limestone increases the rate of reaction.
cut up cut up powder
etc.
Figure 7.6 A powder has a larger surface area.
In certain industries the large surface area of fine powders and dusts can be a problem. For example, there is a risk of explosion in flourmills and mines, where the large surface area of the flour or coal dust can – and has – resulted in explosions through a reaction with oxygen gas in the air when a spark has been created by machinery or the workforce (Figure 7.7). On 26 September 1988, two silos containing wheat exploded at the Jamaica Flour Mills Plant in Kingston, Jamaica, killing three workers, as a result of fine dust exploding.
Figure 7.7 The dust created by this cement plant in India is a potential
hazard.
The surface area has been increased by powdering the limestone (Figure 7.6). The acid particles now have an increased amount of surface of limestone with which to collide. The products of a reaction are formed when collisions occur between reactant particles.
107
Questions
1 What apparatus would you use to measure the rate of
reaction of limestone with dilute hydrochloric acid by measuring the volume of carbon dioxide produced?
2 The following results were obtained from an experiment of
the type you were asked to design in question 1.
Time/min 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Total volume of CO2 gas/cm3 0 15 24 28 31 33 35 35 35 35 35
a Plot a graph of the total volume of CO2 against time.
b At which point is the rate of reaction fastest?
c What volume of CO2 was produced after 1 minute
15 seconds?
d How long did it take to produce 30 cm3 of CO 2?
Concentration
A yellow precipitate is produced in the reaction between sodium thiosulfate and hydrochloric acid.
sodium thiosulfate Na2S2O3(aq) + + + + + + + + hydrochloric acid 2HCl(aq) → → sodium chloride 2NaCl(aq) sulfur dioxide SO2(g) water H2O(l) sulfur S(s)
The rate of this reaction can be followed by recording the time taken for a given amount of sulfur to be precipitated. This can be done by placing a conical fl ask containing the reaction mixture on to a cross on a piece of paper (Figure 7.8). As the precipitate of sulfur forms, the cross is obscured and fi nally disappears from view. The time taken for this to occur is a measure of the rate of this reaction. To obtain suffi cient information about the effect of changing the concentration of the reactants, several experiments of this type must be carried out, using different concentrations of sodium thiosulfate or hydrochloric acid.
Figure 7.8 The precipitate of sulfur obscures the cross.
Some sample results of experiments of this kind have been plotted in Figure 7.9. You will note from the graph that when the most concentrated sodium thiosulfate solution was used, the reaction was at its fastest. This is shown by the shortest time taken for the cross to be obscured.
120 100 80 60 40 20 0 0.01 0.015
concentration of sodium thiosulfate/mol dm–3
time for cross to disappear/s
0.02 0.025 0.03
Figure 7.9 Sample data for the sodium thiosulfate/acid experiment at
different concentrations of sodium thiosulfate.
From the data shown in Figure 7.9 it is possible to produce a different graph which directly shows the rate of the reaction against concentration rather than the time taken for the reaction to occur against concentration. To do this, the times can be converted to a rate using:
rate = 1
reaction time (s)
This would give the graph shown in Figure 7.10 (p. 108). As discussed earlier, the products of the reaction are formed as a result of the collisions between reactant particles. There are more particles in a more concentrated solution and the collision rate between reactant particles is higher. The more often the particles collide, the greater the chance they have of having suffi cient energy to overcome the activation energy of the reaction, and of a successful collision occurring. This means that the rate of a chemical reaction will increase if the concentration of reactants is increased, because there are more particles per unit volume.
In reactions involving only gases, for example the Haber process (Chapter 11, p. 177), an increase in the overall pressure at which the reaction is carried out increases the rate of the reaction. The increase in pressure results in the gas particles being pushed closer together. This means that they collide more often and so react faster.
rate of reaction/s –1 0.01 0.005 0 0.01 0.015 0.02 0.025 0.03 0.015
concentration of sodium thiosulfate/mol dm–3
0.02 0.025 0.03
Figure 7.10 Graph to show the rate of reaction against concentration.
Question
1 Devise an experiment to show the effect of changing
the concentration of dilute acid on the rate of reaction between magnesium and hydrochloric acid.
Temperature
Why do you think food is stored in a refrigerator? The reason is that the rate of decay is slower at lower temperatures. This is a general feature of the majority of chemical processes.
The reaction between sodium thiosulfate and hydrochloric acid can also be used to study the effect of temperature on the rate of a reaction. Figure 7.11 shows some sample results of experiments with sodium thiosulfate and hydrochloric acid (at fi xed concentrations) carried out at different temperatures. You can see from the graph that the rate of the reaction is fastest at high temperatures.
140 120 100 80 60 40 20 0 10 60 temperature/ C
time for cross to disappear/s
20 30 40 50
Figure 7.11 Sample data for the sodium thiosulfate/acid experiment at
different temperatures.
As the temperature increases, the reactant particles increase their kinetic energy and they move faster. The faster movement results in more collisions between the particles. Some of these extra collisions, which result from the temperature increase, will be successful collisions. This causes the reaction rate to increase.
Questions
1 Explain why potatoes cooked in oil cook faster than those
cooked in water.
2 Devise an experiment to study the effect of temperature on
the reaction between magnesium and hydrochloric acid.
3 Explain why food cooks faster in a pressure cooker.
Light
Some chemical reactions are affected by light. When particles absorb light energy, the energy can be used to break bonds, overcoming the activation energy of the reactions and causing a chemical reaction to occur faster (see p. 105). Reactions which occur as a result of the absorption of light are known as photochemical reactions. The absorption of light in these reactions causes bonds to break, producing reactive particles known as radicals. These radicals are responsible for many of the chemical reactions which happen in the stratosphere (see p. 221).