Reaction Kinetics

Define rate of reaction

Rate of reaction is defined as the change in concentration of reactant or product per unit time.

Define instantaneous rate of reaction

Instantaneous rate of reaction is defined as the rate of reaction at a specified time. It can be determined from the gradient of the tangent of a concentration-time graph at the specified time.

Define initial rate of reaction

Initial rate of reaction is defined as the rate at the start of the reaction where an infinitely small amount of reactant has been used up. It can be determined from the gradient of the tangent of a concentration-time graph at t=0.

Define rate equation

Rate equation is defined as an expression that shows the exact dependence of reaction rate on the concentration of all reactants. It takes the form k=[A]^m[B]^n, where k is the rate constant.

Define rate constant

Rate constant, k is defined as the proportionality constant in the rate equation which relates reaction rate to the concentration of reactants.

State Arrehenius Equation

k= Ae^ (-Ea/RT).

lnk= lnA - Ea/RT.

lnk= (-Ea/R)(1/T) + lnA.

Graident= -Ea/R and y-intercept= lnA.

Define order of reaction

Order of reaction with respect to a reactant is defined as the power to which the concentration of reactant is raised in the rate equation.

Describe zero order reactions

1. Rate equation: rate= k[A]^0, where k=moldm^-3s^-1.

2. Rate of reaction is independent of the concentration of A.

3. Graph of rate against [A]:

4. Graph of [A] against time:

Describe first order reactions

1. Rate equation: rate=k[A]^1, where k=s^-1.

2. Rate of reaction is directly proportional to the concentration of reactant A.

3. Graph of rate against [A]:

4. Graph of [A] against time:

Define half-life

Half-life, t1/2 is defined as the time required for the concentration of reactant to decrease to half of its initial value. It can be determined graphically from a concentration-time graph or from the equation t1/2= ln2/k.

State how number of half-life can be determined

Amount of sample after t minutes/ Initial amount of sample= (1/2)^ Number of half-life elapsed.

Describe second order reactions

1. Rate equation: rate=k[A]², where k= mol^-1dm³s^-1.

2. Rate of reaction is directly proportional to the square of the concentration of reactant A.

3. Graph of rate against [A]:

4. Graph of [A] against time:

Describe pseudo order reactions

If a particular reactant is present in large excess with respect to other reactants, its concentration will not change significantly and can be regarded as constant. The order of reaction when determined under such a condition is referred to as pseudo order.

State how order of reaction can be determined

1. Initial rates method.

2. Concentration-time graph method.

Define reaction mechanism

Reaction mechanism is defined as the sequence of single reaction steps that occur during the conversion of reactants to products. These steps sum to the overall reaction.

Define molecularity

Molecularity is defined as the number of elementary particles taking part in the elementary step.

Define rate-determining step

Rate-determining step is defined as the rate of the slowest step in the reaction mechanism. The rate of the overall reaction is controlled by the rate of this rate-determining step.

Describe how concentration of reactants affect rate of reaction

When concentration of reactants increases, number of reactant particles per unit volume increases. Reactant particles are closer together and collide more frequently. Therefore, frequency of effective collisions increases and rate of reaction increases.

Describe how state of reactants affect rate of reaction

When reactant particle size decreases, total exposed surface area of contact increases. Reactant particles collide more frequently. Therefore, frequency of effective collisions increases and rate of reaction increases.

Describe how temperature affects rate of reaction

When temperature increases, average kinetic energy of reactant particles increases. Reactant particles collide more frequently. Number of reactant particles with energy>= Ea significantly increases. Therefore, frequency of effective collisions increases and rate constant, k increases and rate of reaction increases.

Define catalyst

Catalyst is defined as a substance that increases the rate of reaction by providing an alternate pathway with lower activation energy and without itself being chemically changed at the end of the reaction.