Chemical kinetics is like a speedometer of any reaction. It tells us at which speed the reaction takes place. Thermodynamics tells us the feasibility of reaction ie. whether it is spontaneous or non spontaneous. Equilibrium gives us an idea about the extent of a reaction, but none of them tells the speed or rate at which the reaction takes place. Kinetics helps you to determine the speed of a reaction. Let’s take an example:
A ⟶ B
Time = 0 a moles 0
Time = t (a-x) moles x moles
Here A gets converted to B. What is the rate at which this reaction takes place? At time zero ‘a’ moles of ‘A’ are present and zero moleof ‘B’. After some time ‘t’ x moles of ‘A’ get converted to ‘B’ so there will be (a-x) moles of ‘A’ left.
You can define the average rate either in terms of disappearance of A with time or appearance of B with time.
Rateaverage = - Δ [Reactant] / Δtime
Rateaverage = Δ [Product] / Δtime
Rate of reaction is a positive quantity, minus sign shows only decrease in concentration of reactant. Its unit is mol L-1 sec-1.
Let’s take an example:
Hg(l) + Cl2(g) ⟶ HgCl2(s)
You can define the rate in terms of reactants (Hg or Cl2) or in terms of product HgCl2.
Rate = - Δ [Hg] / Δtime = - Δ [Cl2] / Δtime = Δ [HgCl2] / Δtime
Let’s take another example:
2HI(g) ⟶ H2(g) + I2(g)
Rate = Δ [H2] / Δtime = Δ [Cl2] / Δtime ≠ Δ [HI] / Δtime
In this example, the rate of appearance of H2is equal to the rate of appearance of I2 but it cannot be equal to the disappearance of HI. Because when 2moles of HI are decomposed, 1 mole each of H2 and I2 is produced.
- ½ Δ [HI] / Δtime = Δ [H2] / Δtime = Δ [Cl2] / Δtime
To get the correct rate of reaction from balance equation, we divide the rate by stoichiometric coefficient. For example:
2N2O5(g) ⟶ 4NO2(g) + O2(g)
Rateaverage = ½ {-[Δ N2O5]/Δt} = ¼ {[Δ NO2]/Δt} = ½ {[ΔO2]/Δt}
Let's try to do a simple problem. If initially the concentration of N2O5 is 2.33mol L-1 and after 184 minuts it is reduced to 2.08 mol L-1. Calculate the rate of reaction.
Initial concentration [N2O5]i= 2.33mol L-1
Final concentration [N2O5]f= 2.08mol L-1
Δt = 184min is given. And,
Rate = ½ {-[Δ N2O5]/Δt}
= ½ {-[2.33mol L-1 - 2.08mol L-1] / 184min
= 6.7 ×10-4mol L-1 min-1
Or,
= ½ {-[2.33mol L-1 - 2.08mol L-1] / (184) 60 sec
=1.13×10-5 mol L-1 sec-1
Rate of reaction can only be determined experimentally, you cannot predict it by balance chemical equation. It is totally an experimental quantity. It may depend on the concentrations of reactants or products. When it is represented by reactant concentration, it becomes the rate law. Let’s try to understand it by taking an example:
aA + bB ⟶ cC + dD
a, b, c and d are stoichiometric coefficients of balance equation. Rate of the reaction depends on the concentrations of A and B.
Rate ∝ [A]x [B]y
Rate of reaction depends on the concentration of A to the power x and B to the power y, where x and y has to be found out experimentally. They may be equal to a and b by coincidence but have no relation with stoichiometric coefficients of balance equation.
Rate = k [A]x [B]y
-d[R]/ dt = k [A]x [B]y
‘k’ is the proportionality constant known as rate constant. It only depends on temperature of the reaction and it has specific value for a particular reaction.
By adding the x and y you will get the order of the reaction.
Order of the reaction = (x +y)
Let’s see what it means. If the rate law for a reaction is given as,
Rate law = k [A]1 [B]1
Then the order of the reaction will become (1+1) = 2, which means it is a 2nd order reaction.
Order of reaction is the extent of dependency of the rate of reaction on the reactant concentration. It is based on experimental data, so it can be fraction, integer or zero. For example:
CHCl3 + Cl2 ⟶ CCl4 + HCl
Rate law for this reaction is = k [CHCl3]1[Cl2]1/2
Hence the order of reaction = 1+ ½ = 3/2 order
2NH3 ⟶ N2+ H2
Rate = k [NH3]0
Order of reaction= 0 order
It is quite normal for us to assume that rate of reaction depends the reactant concentration. Order of reaction tells us how the rate is related to the reactant concentration. But what does it mean if its order is zero? It means that rate is independent of reactant concentration. Rate remains constant throughout the reaction. In the coming post we will discuss it in detail.
Both rate of reaction and order of reaction are experimental terms and we cannot predict them by balance chemical equation. So what information can we draw from balance chemical equation? Balance chemical equation gives us clues about how a particular reaction takes place. For example:
N2 + H2 ⟶ 2NH3
One molecule of N2 and one molecule of H2participate in the formation of NH3. When these two molecules collide with each other, the reaction takes place and NH3 is formed. In the language of chemistry you can say that the molecularity of this reaction is two. Let’s take another example:
2HI(g) ⟶ H2(g) + I2(g)
Here, two molecules of HI react with each other. So the molecularity of the reaction is 2.
Molecularity tells us how many molecules/ ions/ atoms are needed to collide at once for a reaction to happen. Since it is the number of molecules/ ions/ atoms, it can only be an integer. It can never be a fraction or zero. Molecularity can be 1, 2, 3, 4..... but how is it possible for 5 or 6 or more than 6 molecules to collide at the same time? It may happen but the probability of its happening is very rare. Then how do such reactions take place? These reactions are called as complex reactions. Actually, they don’t happen in a single step, they take place in a number of small reactions or elementary reactions. To calculate the Molecularity of such complex reactions, you have to sum up the Molecularies of all the elementary reactions. Molecularity of a complex reaction doesn’t make sense. Molecularity is applicable to elementary reactions only.
In the coming post we will discuss zero order and first order reactions in detail.
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