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Rate of Reaction is the rate at which a chemical reaction occurs. It is proportional to the rate of increase in the product concentration per unit of time and the decrease in the reactant concentration per unit of time. This article deals with the rate of reaction, its formula, factors affecting the rate of reaction and Important questions.
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Key Terms: Rate of reaction, chemical reactions, presence of a catalyst, reagents, order of reaction, equilibrium
Rate of Reaction
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Reaction rate or rate of reaction is the rate at which reactants are converted into products. For chemical reactions, the speed at which the reaction occurs varies widely. Some chemical reactions are almost instantaneous, while others take some time to reach final equilibrium.
According to a general definition, the speed at which a reaction takes place is called the reaction rate. For example, burning wood has a high reaction rate because the process is fast whereas the oxidation of iron has a low reaction rate because the process is slow.
Also Read:
| Related Articles | ||
|---|---|---|
| Rate of a Chemical Reaction | Collision Theory | Half Life |
| Surface Chemistry | Anode and Cathode | Integrated Rate Equatations |
Factors Affecting the Rate of Reaction
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There are various factors that affect the rate of reaction. Some of them are given below:
Type of Reaction
- The rate of reaction depends greatly on the type and nature of the reaction. As noted above, few reactions are inherently faster than others, while some reactions are very slow.
- The physical state of the reagents, the number of reagents, the complexity of the reaction, and other factors also affect the rate of reaction.
- The reaction rate is generally slower in liquids than in gas and slower in solids than in liquids.
- The size of the reagent also plays a major role. The smaller the reagent size, the faster the reaction.
Effect of concentration on the reaction rate
- According to the collision theory, the rate of the reaction increases with the concentration of the reactants.
- According to the law of mass action, the concentration of the reactants is directly proportional to the rate of the chemical reaction.
- This implies that the speed of the chemical reaction increases with increasing concentration and decreases with decreasing concentration of the reagents.
- Time plays an important role in the change of the concentration of reactants and products. Therefore, consistent time is a crucial factor that affects the reaction speed.
Pressure
Pressure increases the concentration of a gas which results in the increase of the rate of reaction. The reaction rate increases in the direction of fewer gas molecules and decreases in the opposite direction.
Temperature
According to collision theory, chemical reactions occurring at higher temperatures produce more energy than reactions at lower temperatures. This is because colliding particles will have the required activation energy at high temperatures and more successful collisions will occur.
Solvent
Rate of reaction depends on the type of solvent. Solvent properties and ionic strength strongly influence the rate of reaction.
Order
The order of the reaction controls how the pressure or concentration of the reactant affects the rate of the reaction.
Electromagnetic radiation
Electromagnetic radiation is a form of energy. The presence of electromagnetic radiation in a chemical reaction can speed up the rate of reaction because it gives more energy to the reactants.
Presence of a catalyst
A catalyst can be defined as a substance that increases the rate of reaction without actually participating in the reaction. The definition itself describes its effect on chemical reactions.
The presence of a catalyst increases the rate of reaction in forward and reverse reactions by providing an alternate pathway with lower activation energy.
Surface of Reactant
The surface of the reactant affects the rate of reaction. If the size of a particle is small, the surface area will be larger and this will increase the speed of heterogeneous chemical reactions.
Formula of Rate of Reaction
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Consider a chemical reaction.
aA + bB → cC + dD
Here, A and B denote reactants and C and D denote products
a, b, d, d denotes stoichiometric coefficients.
According to the IUPAC, the reaction rate r, which takes place in a closed system without the formation of reaction intermediates under isochoric conditions, is defined as:
The negative sign given in the formula is indicating the decreasing concentration of the reactants.
Average Rate of Reaction
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Let us now consider the following reaction to better understand it.
A → B
In this reaction, reactant A undergoes a chemical reaction to give a product B. It is a general conversion to represent the concentration of a reactant as [reactant] or the concentration of product as [product]. So the product of the concentration of A can be represented as [A] and that of B as [B]. The moment at which the reaction begins is the starting time,
i.e. t = 0.
Let’s consider a situation:
At t = t1,
The concentration of A = [A]1
The concentration of B = [B]1
At t = t2
The concentration of A = [A]2
The concentration of B = [B]2
Now, to know the rate at which A (reactant) disappears and the rate at which that Product B appears in the time interval between t1 and t2. Therefore,
The rate of disappearance of A = [A]2– [A]1/( t2 - t1) = –Δ [A] / Δt
The negative sign given in the formula shows that the concentration of A is decreasing.
Similarly,
the rate of disappearance of B = [B]2 - [B]1/ (t2 - t1) = Δ [B] / Δt
Since A is the only reactant involved in the reaction and B is the only product already formed as the mass is preserved. The amount of A that has disappeared in the time interval Δt is the same as the amount of B that is formed in the same time interval.
We can therefore say,
Rate of Reaction = - disappearance rate of A = appearance rate of B,
i.e. Rate of Reaction = −Δ [A] / Δt = Δ [B] / Δt
The above terms for the disappearance rate of A and the appearance rate of B are average reaction rates. These speeds indicate the rate of reaction for the entire time interval Δt and are therefore referred to as the average rate of reaction.
Instantaneous Rate of Reaction
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The instantaneous rate of reaction is the speed at which the reaction takes place at a given time.
Assume that the value of the term Δt is very small and approaches zero. Now, we have an infinitesimally small Δt, which is a very small period of time and can be viewed as a specific point of time. The average reaction speed is the instantaneous reaction speed.
Average rate of Reaction = −Δ[A] / Δt = Δ[B] / Δt
When Δt →0
Instantaneous Rate of Reaction = −Δ[A] / dt = Δ[B] / dt
Instantaneous Rate of Reaction = −d[A] / dt = d[B] / dt
The unit of rate of reaction is given by concentration or time that is (molL-1)sec-1.
Rate Constant
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The constant of proportionality between the reactants concentration and the rate of a chemical reaction is known as rate constant.
It is denoted by the symbol ‘k’. It is also known as the reaction rate constant or reaction rate coefficient.
In other words, the rate constant is the ratio of the rate of reaction and the molar concentration of the reactants.
The units of rate constant depend on the type of order of reaction, that is, zero order, first order, second order, third order reaction.
The following table gives the units of rate constant and the type of order of the reaction:
| Order of the Reaction | Unit of the Rate Constant |
|---|---|
| Zero-order reaction | Mol L-1 s-1 |
| First-order reaction | s-1 |
| Second-order reaction | mol-1s-1 |
| Third-order reaction | mol-2 s-1 |
Where,
A is the frequency of particle collision
Ea is the activation energy
R is the universal gas constant
T is the absolute temperature
Things to Remember
- Rate of reaction is the rate at which reactants are converted into products.
- Some chemical reactions attain equilibrium instantaneously, while others take some time to reach final equilibrium.
- The factors that affect the rate of reaction are- Pressure, type of reaction, temperature, concentration, the surface of reactant, catalyst, electromagnetic radiation, etc.
- The instantaneous rate of reaction is the speed at which the reaction takes place at a given time.
- Rate constant is the constant of proportionality between the reactants concentration and the rate of a chemical reaction.
Also Read:
Sample Questions
Ques.1: Define ‘rate of a reaction’. (Delhi 2010, 1 Mark)
Ans: The change in the concentration of either reactants or products per unit time is called rate of reaction.
In other words, the rate of reaction can be defined as the change in the molar concentration of the species taking part in a reaction per unit time.
Ques.2: Express the rate of the following reaction in terms of the formation of ammonia :
N2(g) + 3H2(g) → 2NH3(g) (Comptt. All India 2013, 1 Mark)
Ans: The rate of reaction, N2(g) + 3H2(g) → 2NH3(g) can be given as 
Ques.3: A reaction is of second order with respect to a reactant. How will the rate of reaction be affected if the concentration of this reactant is
(i) doubled, (ii) reduced to half? (Delhi 2009, 2 Marks)
Ans: Since Rate = K[A]2
For a second-order reaction,
Let [A] = a then Rate = Ka2
(i) If [A] = 2a then Rate = K (2a)2 = 4 Ka2
∴Rate of reaction becomes 4 times
(ii) If [A] = a/2, then Rate = K (a/2)2 = Ka2/4
∴ Rate of reaction will be ¼ th.
Ques.4: A reaction is of first-order in reactant A and of second-order in reactant B. How is the rate of this reaction affected when (i) the concentration of B alone is increased to three times (ii) the concentrations of A as well as B are doubled? (Delhi 2010, 2 Marks)
Ans: r = K[A]1 [B]2
(i) When the concentration of B increases to 3 times,
r = KA(3B)2
∴ r = 9KAB2 = 9 times
When the concentration of B increases to 3 times, the rate of reaction becomes 9 times
(ii) the concentrations of A as well as B are doubled
r = K(2A) (2B)2
∴ r = 8KAB2 = 8 times
When the concentrations of A as well as B are doubled, the rate of reaction becomes 9 times.
Ques.5: Define the following :
(i) Elementary step in a reaction
(ii) Rate of a reaction (All India 2009, 2 Marks)
For example- Reaction between H2, and I2 to form 2HI
H2 + I2 → 2HI
(ii) Rate of a reaction: The rate of change in the concentration of either the reactants or the products per unit time is called rate of reaction.
Ques.6: Distinguish between ‘rate expression’ and ‘rate constant’ of a reaction. (Delhi 2011, 2 Marks)
Ans: Rate expression: The expression at which the rate of reaction is expressed in terms of molar concentrations of the reactants having each term raised to their power, which may or may not be same as the stoichiometric coefficient of that reactant in the balanced chemical equation.
Rate constant: The rate of change in the concentration of either the reactants or the products per unit time is called rate of reaction.
Ques.7: What is meant by rate of a reaction? Differentiate between the average rate and instantaneous rate of a reaction. (Comptt. All India 2012, 2 Marks)
Ans: Rate of reaction: The rate of change in the concentration of either the reactants or the products per unit time is called rate of reaction.
Average rate: Average rate is defined as the rate that depends upon the change in concentration of reactants or products and the time taken for the change to occur.
R → P
Instantaneous rate: It is defined as the change in the rate of concentration of either reactant or product at a particular moment of time.
Ques.8: A first order gas phase reaction : A2B2(g) → 2A(g) + 2B(g) at the temperature 400°C has the rate constant k = 2.0 × 10-4 sec-1. What percentage of A2B2 is decomposed on heating for 900 seconds? (Antilog 0.0781 = 1.197) (Comptt. All India 2013, 3 Marks)
Ans: Since the reaction is of the first order
Ques.9: (a) Explain the following terms :
(i) Rate of a reaction
(ii) Activation energy of a reaction (b) The decomposition of phosphine, PH3, proceeds according to the following equation:
4 PH3 (g) → P4 (g) + 6 H2 (g)
It is found that the reaction follows the following rate equation :
Rate = K [PH3].
The half-life of PH3 is 37.9 s at 120° C.
(i) How much time is required for 3/4th of PH3 to decompose?
(ii) What fraction of the original sample of PH3 remains behind after 1 minute? (All India 2010, 5 Marks)
Ans: (a) (i) Rate of a reaction: The rate of change in the concentration of either the reactants or the products per unit time is called rate of reaction.
(ii) The minimum extra amount of energy absorbed by the reactant molecules to form the activated complex is called activation energy. The activation energy of the reaction decreases by the use of a catalyst.
(b) (i) According to the formula :
Ques.10: (a) For a reaction A + B → P, the rate is given by Rate = k[A] [B]2
(i) How is the rate of reaction affected if the concentration of B is doubled?
(ii) What is the overall order of reaction if A is present in large excess?
(b) A first-order reaction takes 30 minutes for 50% completion. Calculate the time required for 90% completion of this reaction. (log 2 = 0.3010) (Delhi 2015, 5 Marks)
Ans: (a) For the reaction A + B → P rate is given by
Rate = k[A]1[B]2
(i) r1 = k[A]1 [B]2
r2 = k[A]1[2B]2
r2 = k[A]1[2B]2 = 4k[A]1[B]2
r1 = 4r2, rate will increase four times of actual rate.
(ii) When A is present in a large amount, the order with respect to A is zero.
Hence overall order = 0 + 2 = 2, second order reaction.
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