13. Speed of Reaction
Speed of Reaction
Speed of Reaction is a measure how fast a reaction takes place. It is also referred to as the speed with which a reactant is used up or the speed at which a product is formed.
For a reaction to occur,
Any factor that increases the frequency of effective collision will increase the speed of reaction.
Inferring Speed of Reaction from Graphs
Speed of Reaction is a measure how fast a reaction takes place. It is also referred to as the speed with which a reactant is used up or the speed at which a product is formed.
For a reaction to occur,
- the reacting particles must collide with one another
- the particles must possess sufficient activation energy
Any factor that increases the frequency of effective collision will increase the speed of reaction.
Inferring Speed of Reaction from Graphs
- The rate of the reaction at any particular time may be estimated by the gradient of the curve at that time.
- Gradient is largest (steepest) at the start of the experiment. Hence, the rate of reaction is fastest at the beginning because the amount of reactants is the highest.
- Gradient decreases with time. Hence, the rate of reaction decreases with time. This is because as the reactants are used up in the reaction.
- Gradient becomes zero. Hence, the reaction has stopped as all the limiting reactant is used up.
- Temperature of the reactants: An increase in temperature of the reactants increases the speed of reaction. At a higher temperature, reacting particles have more energy and move faster. The frequency of collisions between the reacting particles thus increases. This leads to an increase in the frequency of effective collisions. The speed of reaction thus increases.
- Concentration of the reactants: An increase in concentration of the reactants increases the speed of reaction. When the concentration of a reactant is increased, there are more reacting particles per unit volume. The frequency of collisions between the reacting particles thus increases. This leads to an increase in the frequency of effective collisions. The speed of reaction thus increases. If the number of moles of reactants increases, the number of moles of the products will also increase.
- Pressure for gases: An increase in pressure of the gas increases the rate of reaction. When the pressure of a gas is increased, there are more gas particles per unit volume. The frequency of collisions between the gas particles thus increases. This leads to an increase in the frequency of effective collisions. The speed of reaction thus increases.
- Particle size ( surface area ) of the reactants: A decrease in the size of the particles of the reactants (larger surface area) increases the rate of reaction. Smaller pieces of solids have a larger total surface area than bigger pieces of solids of the same mass. As the area of contact between the reacting particles increases, the frequency of effective collisions also increases. The speed of reaction thus increases.
- Presence of a catalyst: Only a small amount of catalyst is required to increases the rate of reaction. It remains chemically unchanged at the end of the reaction. Yield of the chemical reaction remains the same. A catalyst provides an alternative pathway of lower activation energy. This means that the minimum energy required for the reaction is lowered and there would be more particles possessing this minimum energy required for the reaction to occur. This leads to an increase in the frequency of effective collisions. The speed of reaction thus increases.