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The rate of reaction refers to the speed that products take to form through the reactants when a chemical reaction occurs. Furthermore, it also provides essential insights into the timeframe needed to complete a reaction. For instance, the reaction rate of the combustion of cellulose in fire is much higher, and the reaction completes in under a second.
Definition of rate of reaction
As mentioned earlier, the rate of reaction defines the speed that reactants take to become products. However, when we consider a chemical reaction, their rate of reaction usually varies drastically. Although a few chemical reactions complete immediately, others might take a prolonged period to meet the condition of equilibrium.
According to the general rate of reaction definition, wood combustion maintains a more significant reaction rate as the process is quick. On the other hand, the process of rusting of iron maintains a low reaction rate as it is a slower process.
Factors affecting the rate of reaction
Many factors influence the chemical reaction rate. A few of them are mentioned below:
- Concentration of reactants
As the concentration of the reactants taking part in the chemical reaction increases, the number of collisions or activated collisions tends to increase.
This, in turn, enhances the rate of a chemical reaction. This concept is based on collision theory, in which the number of collisions increases due to the increase in concentration. Thus, we can say that the rate of reaction is directly proportional to the increase in concentration.
That is, rate of reaction ∝ collision frequency (Z)
Formula = rate (r) ∝ cn or r = kcn
Where r = rate of the chemical reaction
c = concentration of the reactants
n = order of reaction
k = specific rate
Pressure
When the pressure in a particular gaseous system is increased, it directly enhances the number of collisions occurring between the reacting substances. This, in turn, affects the rate of the reaction in a positive way. This means the rate of reaction in which the reacting substances are present in gaseous form also increases.
- Temperature
Temperature is the third factor affecting the rate of reactions. This is a little complicated to understand, as there are a few reactions in which the rate of reaction increases with the increase in temperature.
In such reactions, the substances reacting gain kinetic energy and become more activated. This, in turn, increases the collision of the reacting substances, increasing the rate of a chemical reaction.
However, this isn’t true in every case. The rate of biological reactions in which enzymes are used as a catalyst is known to decrease with the increase in temperature. The reason is that enzymes lose their functioning and activity due to an increase in temperature.
- Presence of catalyst
A catalyst is a substance that helps the reaction to enhance the rate of the reaction. One thing to remember is that the catalyst itself doesn’t take part in the reaction. This means it comes out the same form in which it was added.
A catalyst provides an alternative path for the reaction to take place. The new path has lower activation energy, meaning the reactants are easily shifted to products.
Catalysts play an important role in reversible reactions. They are known to lower the activation energy of both forward and backward reactions.
However, the catalysts are of two types: positive and negative. Positive catalysts are known to enhance the rate of reaction, whereas negative catalysts interfere with the reaction and decrease it.
- Nature of the reactants
The nature of reactants is another factor affecting the rate of reactions. The two types of bonding – covalent and ionic – are known to cause different rates of reaction.
Ionic compounds tend to react faster, enabling the rate of reaction to increase. Alternatively, covalent compounds are known to react slowly, enabling the rate of reaction to decrease. The reason is that in ionic compounds, there is only ion transfer taking place. But on the other side, covalent compounds have to undergo bond cleavage before they form products.
- Orientation of reactants
The rate of reaction depends on the orientation in which the reactants are reacting and forming products. If the reactants are present in the right orientation, it will lead to an increased rate of reaction.
On the other hand, if the orientation of the reactant makes it impossible to attack, it will lead to a decrease in the rate of reaction.
In other words, simple reactants have a higher rate of reaction as compared to complex ones because of the higher probability of proper orientation.
Surface area
The surface area of the reactant is directly related to the increase in the rate of reaction. To increase the rate of reaction, the reactants and catalysts are used in finely powdered form. This increases the surface area of the reactants, which, in turn, increases the rate of reaction.
For example, in zinc and hydrochloric reactions, the reaction speed increases when zinc is used in powder form as compared to when zinc is used in its zinc wire form.
- Intensity of light
The average rate of reaction is directly proportional to the intensity of light. When suitable light is used, the rate of photochemical reaction is increased.
With the increase in the intensity of light, the number of photons also increases, which again increases the rate of reaction.
Conclusion
So these are the factors affecting the rate of reactions. However, the nature of the reactants is one of the most important factors that affect the rate of reaction, and the order of reactions.
One of the most common types of reactions in which the rate of reaction depends on only one reactant is known as a first-order reaction. However, there are other order reactions: second-order reaction, third-order reaction, pseudo-first-order reaction, and so on.
