Question

A. Define reaction rate with example.

B. What is a rate equation? Provide an example with a balanced chemical equation.

C. What is half-life of a reaction? Write the half-life expression for first order and second order reaction.

D. Define the order of a reaction. Why order of a reaction does not depend on the stoichiometric value of a reactant?

E. What is a catalyst? What specific function a catalyst play in a reaction?

Answer 1

A) The reaction rate is defined as the rate of change of the concentration of a reactant or product divided by its
stochiometric coefficient.The stoichiometry refers to the number of moles of each reactant and product appearing in the reaction equation. The rate therefore has units of concentration per unit time, mol dm-3 s-1.

For example, the reaction equation for Haber process, used industrially to produce ammonia, is:

N₂ + 3H₂ 2NH₃
N₂ has a stochiometric coefficient of 1, H₂ has a coefficient of 3, and NH₃ has a coefficient of 2.

For the above reaction, the rate is :
rate = -d[N₂] = -1 d[H₂]    =1 d[NH₃] dt 3 dt 2 dt
negative sign indicates that amount of reactant is decreasing with time.

B) The rate equation is an expression relating the rate of a reaction to the concentrations of the reactants.

For a general reaction: A + B -> P

rate = k [A]^a[B]^b

i.e. the rate is proportional to the concentrations of the reactants each raised to some power.The constant of proportionality, k, is called the rate constant. The 'power' a particular concentration is raised to, is the order of the reaction with respect to that reactant.The sum of the powers is called the overall order.

overall order of reaction = a+b

For example,
H₂+ I₂ → 2HI rate = k [H₂][I₂]
3ClO⁻ → ClO3⁻ + 2Cl⁻ rate = k [ClO⁻]²

2H_2(g) + 2NO_(g) 2H₂O_(g) + N_2(g)   rate = k[NO_(g)]² [H_2(g)]

C) The half life, t_1/2, of a reaction is defined as the time it takes for the concentration of the reactant to decrease to half of its initial value.We can obtain equations for the half lives for reactions of various orders by substituting the values t = t_1/2 and [A] = ½ [A]₀ into the integrated rate laws.

First order reaction: t_1/2 = ln2    = 0.693/k where k = rate constant for 1st order reaction
k Second order reaction : t_1/2 = 1    where k = rate constant for 2nd order reaction
k[A]₀

D) The sum of the powers or exponents to which the concentration terms are raised in the rate equation is called the order of that chemical reaction.

2H_2(g) + 2NO_(g) 2H₂O_(g) + N_2(g)   rate = k[NO_(g)]² [H_2(g)]

Order of this reaction = 2+1 = 3 , it is 3rd order reaction

Order of a reaction can be 0,1,2,3 and even a fraction.Order of any reaction is experimentally determined.

the order of the reaction does not depent on the stoichiometric value of a reactant because the stoichiometric coefficient relates the number of reactant and product molecules involved in the reaction. the order of the reaction depends on the mechanism of the reaction and not on the number of molecules involved in overall reaction.

E) A catalyst is a substance which alters the rate of a reaction without itself undergoing any permanent chemical change.catalysts are reaction specific.for example, MnO2 catalyses the following reaction so as to increase its rate considerably.

2KClO₃ ----> 2KCl + 3O₂ catalyzed by MnO₂

k_cat > k_uncat

Chemical reactions need a certain amount of energy in order to start a reaction.This is called the "activation energy" of the reaction and is different for different reactions.Catalyst provides an alternate reaction pathway with a lower activation energy.