Reaction Kinetics ⏳️

Define rate of reaction

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

Define instantaneous rate of reaction

1. Instantaneous rate of reaction is the rate of reaction at a particular 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

1. Initial rate of reaction is the rate at the start of the reaction when 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

1. Rate equation is an expression that shows the exact dependence of reaction rate on the concentration of all reactants.

Define rate constant

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

State the Arrhenius equation

1. k= Ae^(-Ea/RT).

Define order of reaction

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

Define half life

1. Half life t1/2, of a reaction is 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 the equation for determination of number of half life

1. Amount of sample after time t/ initial amount of sample= (1/2)^n.

Define rate determining step

1. Rate determining step is 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/ pressure of reactants affects the rate of reaction

1. When concentration/ pressure of reactants increases, number of reactant particles per unit volume increases, hence reactant particles are closer together and collision frequency increases. The frequency of effective collisions increases, hence rate of reaction increases.

Describe how state of reactants affects the rate of reaction

1. When particle size increases, total surface area exposed increases, hence collision frequency increases. The frequency of effective collisions increases, hence rate of reaction increases.

Describe how temperature affects the rate of reaction

1. When temperature increases, kinetic energy of reactant particles increases, hence collision frequency increases. The number of reactant particles with energy>= activation energy increases significantly. The frequency of effective collisions increases, hence rate constant k increases and rate of reaction increases.

Describe how catalysts affect the rate of reaction

1. A catalyst is 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.

2. In the presence of a catalyst, number of reactant particles with energy>= activation energy increases significantly.

3. The frequency of effective collisions increases, hence rate constant k increases and rate of reaction increases.

State the types of catalyst

1. Homogenous catalyst.

2. Heterogenous catalyst.

3. Biological catalyst: Enzymes.

4. Autocatalyst.

Describe the structure and function of a homogenous catalyst

1. A homogenous catalyst is one which is in the same phase as the reactants.

2. A homogenous catalyst takes part in the reaction by being converted to an intermediate ion or compound which is subsequently consumed to form products. At the end of the reaction, the catalyst is regenerated.

3. The catalyst is involved in the rate determining step and the rate equation contains the catalyst.

4. The catalyst reaction involves two or more steps, each having a lower activation energy than the uncatalysed reaction.

Describe the catalytic role of Fe2+ in the I-/ S2O82- reaction

1. Step 1: 2Fe2+(aq) + S2O82-(aq)—> 2SO42-(aq) + 2Fe3+(aq).

2. Step 2: 2Fe3+(aq) + 2I-(aq)—> 2Fe2+(aq) + I2(aq).

3. Overall: S2O82-(aq) + 2I-(aq)—> 2SO42-(aq) + I2(aq).

Describe the catalytic role of Fe3+ in the I-/ S2O82- reaction

1. Step 1: 2Fe3+(aq) + 2I-(aq)—> 2Fe2+(aq) + I2(aq).

2. Step 2: 2Fe2+(aq) + S2O82-(aq)—> 2SO42-(aq) + 2Fe3+(aq).

3. Overall: S2O82-(aq) + 2I-(aq)—> 2SO42-(aq) + I2(aq).

Describe the catalytic role of atmospheric oxides of nitrogen in the oxidation of atmospheric sulfur dioxide

1. Step 1: NO2(g) + SO2(g)—> SO3(g) + NO(g).

2. Step 2: NO(g) + 1/2O2(g)—> NO2(g).

3. Overall: SO2(g) + 1/2O2(g)—> SO3(g).

Describe the structure and function of a heterogenous catalyst

1. A heterogenous catalyst is one which is in a different phase from the reactants.

2. A heterogenous catalyst is usually in the solid phase and the reactants are usually liquids or gases.

3. A heterogenous catalyst provides a surface area for the adsorption of reactant molecules.

Describe the catalytic role of Fe in the Haber Process

1. N2(g) and H2(g) diffuse towards the surface of Fe.

2. The molecules are adsorbed onto the active sites at the surface.

3. The activation energy is lowered as intramolecular bonds of reactants are weakened by adsorption effects.

4. The concentration of reactant molecules at the surface of catalyst increases. The reactant molecules adjacent to one another react to form products. New bonds start to form.

5. NH3(g) molecules desorb and diffuse away from the surface of catalyst.

6. Overall: N2(g) + 3H2(g)—> NH3(g).

Describe the catalytic removal of oxides of nitrogen in the exhaust gases from car engines

1. Reduction of NO to N2: CO(g) + NO(g)—> CO2(g) + 1/2N2(g).

2. Oxidation of CO to CO2: CO(g) + 1/2O2(g)—> CO2(g).

3. Oxidation of unburnt hydrocarbons to carbon dioxide and water: CxHy + (x+y/4)O2–> xCO2 + (y/2)H2O.

Define autocatalytic reaction

1. An autocatalytic reaction is a reaction in which the reaction product catalyses the reaction.

2. 2MnO4-(aq) + 5C2O42-(aq) + 16H+(aq)—> 2Mn2+(aq) + 10CO2(g) + 8H2O(l). The product Mn2+, catalyses the reaction.