In the previous section, we completed a discussion on Molarity. In this chapter we will see Rate of chemical reactions and Chemical equilibrium.
We see different chemical reactions in our daily life. Some examples are:
•Burning of firewood •Rusting of iron •Photosynthesis •Reaction between acid and alkali •Reaction between acid and metals •Reaction between cement and water and the setting of cement
■ Burning of firewood and rusting of iron are two different chemical reactions. Also, they occur at different speeds.
• A small piece of iron reacts with the oxygen in the atmosphere for a long time to produce rust.
• But a small piece of firewood will burn out completely in a short time.
■ There are situations where we will want to increase or decrease the speed of chemical reactions.
• For example, if we can reduce the speed at which rusting of a piece of iron takes place, we can use that iron piece for a longer time.
• Similarly, if we can increase the speed at which the firewood burns, we will be able to cook food faster.
■ Following are some methods which are commonly used to make the firewood burn faster:
• Provide more air
• Make the firewood into smaller pieces
• Remove moisture from the firewood by drying in sunlight before burning
So factors like: provision of more air, making of smaller pieces, removal of moisture etc., helps to increase the rate (speed) of burning of firewood. In this way, there are many factors that influence a chemical reaction. Let us now learn about such factors:
Let us do a simple experiment:
1. Take magnesium ribbons of equal mass in two test tubes.
2. Add concentrated HCl in one test tube (fig 11.1a) and dilute HCl to the other (fig 11.1b).
The volumes of HCl must be equal.
3. Let us note down the observations:
• Test tube 1: Many gas bubbles are formed quickly. The magnesium is consumed within a very short time.
• Test tube 2: Gas bubbles are formed slowly. It takes longer time for the magnesium to get consumed.
■ So we find that reaction is faster in one test tube and slower in the other.
• We want to know the reason. For that, first we want to learn about effective collisions.
Collision Theory
• According to this theory, reactant molecules have to collide against each other for chemical reaction to occur.
• But all collisions between between reactant molecules need not result in chemical reaction.
• If a collision results in a reaction, it is called an effective collision.
■ But if a collision is to be effective, the colliding molecules should have an energy level above a specified level. It can be explained as follows:
• Consider two molecules. Let each of them possess an energy E.
• Let Es be the specified energy level.
• Let the two molecules collide. If E is less than Es, no reaction will take place between them.
• If E is greater than Es, a reaction will take place.
• So it is clear that, we must have a large number of effective collisions within a short span of time. Then only the reaction will take place at a greater speed.
■ How can we increase the number of effective collisions?
If there are a large number of molecules, each of them with energy greater than Es, there will be an increased number of effective collisions.
Now let us check our present experiment. The balanced chemical equation for the reaction is:
Mg + 2HCl ⟶ MgCl2 + H2
• One molecule of magnesium reacts with two molecules of HCl to produce one molecule of magnesium chloride and one molecule of hydrogen.
■ The amount of a substance contained in unit volume is called concentration of that substance.
• In the first test tube, the concentration of HCl was greater. That means, the number of HCl molecules was greater.
• This resulted in the increase in number of effective collisions. So the speed of the chemical reaction increased.
1. Equal masses of magnesium was taken in the test tubes.
2. During the reaction, using a stop watch, find the time required for the complete consumption of magnesium in test tube 1. Then we will be able to write the following ratio:
Amount of magnesium consumed⁄Time required for the consumption
• Note that 'time' is in the denominator. So this ratio gives the amount of magnesium consumed in unit time. So this ratio is the 'speed (rate) of the reaction'.
■ In general, we can write:
Speed of reaction = Amount of magnesium consumed⁄Time required for the consumption
• In test tube 1, the magnesium was consumed quickly. So the 'time' which comes in the denominator is less. Thus the ratio gives a large value. That means the speed is high.
Another method:
1. In the above experiment, hydrogen is one of the products. using special apparatus, we can collect all the hydrogen that is formed.
2. We can also determine the actual quantity of hydrogen thus collected. Also, using a stop watch, we can determine the time required to collect that much hydrogen.
3. So we can take the ratio:
Amount of hydrogen formed⁄Time required for the formation
• Note that 'time' is in the denominator. So this ratio gives the amount of hydrogen formed in unit time. So this ratio is the speed of the reaction.
■ In general, we can write:
Speed of reaction = Amount of hydrogen formed⁄Time required for the formation
• In test tube 1, the hydrogen was formed quickly. So the time which comes in the denominator is less. Thus the ratio gives a large value. That means the speed is high.
So the reaction between A and C will be faster than the reaction between B and C. This is because of the 'difference in nature' between A and B. Let us do an experiment:
1. Take equal volumes of dilute HCl in two test tubes.
2. Take magnesium metal and zinc metal of almost equal shape and mass. Put the magnesium and zinc into the two separate test tubes at the same time.
3. Let us note down the observations:
• The reaction between magnesium and HCl is faster.
We want to know the reason. Let us analyse:
• The balanced chemical equations are:
Test tube 1: Zn + 2HCl ⟶ ZnCl2 + H2
Test tube 2: Mg + 2HCl ⟶ MgCl2 + H2
• Dilute HCl was taken in both the test tubes. That means, the concentration of HCl was same in both cases.
• So it is clear that, the nature of metals influenced the rate of the chemical reactions. Zinc and magnesium has different natures. Magnesium reacts readily with HCl than zinc.
1. When pressure is increased, the gas molecules come closer to each other. So the possibility of effective collision increases. Thus the speed of the reaction increases. Let us see an example:
2. Fig.11.3(a) shows nitrogen and hydrogen taken in a cylinder. The pressure can be varied using the piston.
3. In fig.11.3(b), the pressure is increased to 2 atm. Because of that, the volume is reduced.
4. Now consider figs (c) and (d). In fig (c), a blue square is shown. It represents unit volume. It may be 1 mm3 or 1 cm3 or any other convenient unit volume.
• The number of molecules within the unit volume is low in fig (c).
• But in fig (d), it is high.
5. That means, when the pressure is increased, volume becomes less, and the number of molecules within unit volume is increased.
• When the number of molecules within unit volume increases, more effective collisions take place within a specified time. Thus the speed of reaction increases.
In the next section, we will see a few more important factors.
We see different chemical reactions in our daily life. Some examples are:
•Burning of firewood •Rusting of iron •Photosynthesis •Reaction between acid and alkali •Reaction between acid and metals •Reaction between cement and water and the setting of cement
■ Burning of firewood and rusting of iron are two different chemical reactions. Also, they occur at different speeds.
• A small piece of iron reacts with the oxygen in the atmosphere for a long time to produce rust.
• But a small piece of firewood will burn out completely in a short time.
■ There are situations where we will want to increase or decrease the speed of chemical reactions.
• For example, if we can reduce the speed at which rusting of a piece of iron takes place, we can use that iron piece for a longer time.
• Similarly, if we can increase the speed at which the firewood burns, we will be able to cook food faster.
■ Following are some methods which are commonly used to make the firewood burn faster:
• Provide more air
• Make the firewood into smaller pieces
• Remove moisture from the firewood by drying in sunlight before burning
Influence of Concentration on the rate of chemical reactions
1. Take magnesium ribbons of equal mass in two test tubes.
2. Add concentrated HCl in one test tube (fig 11.1a) and dilute HCl to the other (fig 11.1b).
3. Let us note down the observations:
• Test tube 1: Many gas bubbles are formed quickly. The magnesium is consumed within a very short time.
• Test tube 2: Gas bubbles are formed slowly. It takes longer time for the magnesium to get consumed.
■ So we find that reaction is faster in one test tube and slower in the other.
• We want to know the reason. For that, first we want to learn about effective collisions.
• According to this theory, reactant molecules have to collide against each other for chemical reaction to occur.
• But all collisions between between reactant molecules need not result in chemical reaction.
• If a collision results in a reaction, it is called an effective collision.
■ But if a collision is to be effective, the colliding molecules should have an energy level above a specified level. It can be explained as follows:
• Consider two molecules. Let each of them possess an energy E.
• Let Es be the specified energy level.
• Let the two molecules collide. If E is less than Es, no reaction will take place between them.
• If E is greater than Es, a reaction will take place.
• So it is clear that, we must have a large number of effective collisions within a short span of time. Then only the reaction will take place at a greater speed.
■ How can we increase the number of effective collisions?
If there are a large number of molecules, each of them with energy greater than Es, there will be an increased number of effective collisions.
Mg + 2HCl ⟶ MgCl2 + H2
• One molecule of magnesium reacts with two molecules of HCl to produce one molecule of magnesium chloride and one molecule of hydrogen.
■ The amount of a substance contained in unit volume is called concentration of that substance.
• In the first test tube, the concentration of HCl was greater. That means, the number of HCl molecules was greater.
• This resulted in the increase in number of effective collisions. So the speed of the chemical reaction increased.
Next we want a method to write the actual speed of a chemical reaction. We know that, the speed of a vehicle is written by considering the distance travelled in unit time. For example, if the vehicle travelled 1500 m in 20 seconds, it's speed is 1500⁄20 = 75 m/s
We follow a similar method for the 'speed of reactions' also. Consider the experiment that we did above. 1. Equal masses of magnesium was taken in the test tubes.
2. During the reaction, using a stop watch, find the time required for the complete consumption of magnesium in test tube 1. Then we will be able to write the following ratio:
Amount of magnesium consumed⁄Time required for the consumption
• Note that 'time' is in the denominator. So this ratio gives the amount of magnesium consumed in unit time. So this ratio is the 'speed (rate) of the reaction'.
■ In general, we can write:
Speed of reaction = Amount of magnesium consumed⁄Time required for the consumption
• In test tube 1, the magnesium was consumed quickly. So the 'time' which comes in the denominator is less. Thus the ratio gives a large value. That means the speed is high.
Another method:
1. In the above experiment, hydrogen is one of the products. using special apparatus, we can collect all the hydrogen that is formed.
2. We can also determine the actual quantity of hydrogen thus collected. Also, using a stop watch, we can determine the time required to collect that much hydrogen.
3. So we can take the ratio:
Amount of hydrogen formed⁄Time required for the formation
• Note that 'time' is in the denominator. So this ratio gives the amount of hydrogen formed in unit time. So this ratio is the speed of the reaction.
■ In general, we can write:
Speed of reaction = Amount of hydrogen formed⁄Time required for the formation
• In test tube 1, the hydrogen was formed quickly. So the time which comes in the denominator is less. Thus the ratio gives a large value. That means the speed is high.
Nature of reactants and the rate of chemical reaction
The ability of each substance to take part in chemical reaction is different. A substance A may readily react with another substance B. But a substance C may not show much readiness to react with substance C. This is shown in fig.11.2 below:
1. Take equal volumes of dilute HCl in two test tubes.
2. Take magnesium metal and zinc metal of almost equal shape and mass. Put the magnesium and zinc into the two separate test tubes at the same time.
3. Let us note down the observations:
• The reaction between magnesium and HCl is faster.
We want to know the reason. Let us analyse:
• The balanced chemical equations are:
Test tube 1: Zn + 2HCl ⟶ ZnCl2 + H2
Test tube 2: Mg + 2HCl ⟶ MgCl2 + H2
• Dilute HCl was taken in both the test tubes. That means, the concentration of HCl was same in both cases.
• So it is clear that, the nature of metals influenced the rate of the chemical reactions. Zinc and magnesium has different natures. Magnesium reacts readily with HCl than zinc.
Influence of pressure on the speed of reactions
In those reactions in which gases are involved, pressure is an important factor which influences the speed. Let us see the reason:1. When pressure is increased, the gas molecules come closer to each other. So the possibility of effective collision increases. Thus the speed of the reaction increases. Let us see an example:
2. Fig.11.3(a) shows nitrogen and hydrogen taken in a cylinder. The pressure can be varied using the piston.
4. Now consider figs (c) and (d). In fig (c), a blue square is shown. It represents unit volume. It may be 1 mm3 or 1 cm3 or any other convenient unit volume.
• The number of molecules within the unit volume is low in fig (c).
• But in fig (d), it is high.
5. That means, when the pressure is increased, volume becomes less, and the number of molecules within unit volume is increased.
• When the number of molecules within unit volume increases, more effective collisions take place within a specified time. Thus the speed of reaction increases.
In the next section, we will see a few more important factors.
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