Chemical kinetics and equilibrium system

(A) Rates of chemical reactions

The rate of a chemical reaction is the number of moles of reactant converted or product formed per unit time.i.e 

Rate of reaction= Change in concentration of reactant or product / time taken for the change .e.g. Zn_(s) + 2HCl_(aq) > ZnCl_2(s) + H_2(g)

Rate of reaction can be measured by:

1. Taking a given mass of zinc 
2. Adding an excess of hydrochloric acid to it.
3. Noting the time taken for all the zinc to react.

The reaction is given by: rate of reaction

Rate of reaction= amount of zinc (moldm^-3)/Time taken (sec)

The rate of reaction is the speed at which reactants are used up or products are formed.

*Rate curve for chemical reaction

Source: www.commons.wikimedia.org 

Collision theory

Collision theory states that for a chemical reaction to occur, the reacting particles must collide with one another. Not every collision between reactant particles results in a reaction. For a reaction to occur, the colliding reactant particles must possess a certain minimum amount of energy called activation energy. Thus, reaction can only occur if the energy of the colliding reactant particles is equal to or more than the activation energy.

*Exothermic reaction

Source: www.commons.wikimedia.org 

*Endothermic reaction

Source: www.commons.wikimedia.org 

Factors affecting rates of chemical reactions

1. Nature of reactants
2. Concentration and pressure (for gases) of reactants
3. Temperature of the reaction mixture
4. Surface areas of reactants
5. Presence of light
6. Presence of catalyst

*Reaction rate increases with increasing concentration of a reactants

Source: www.chem.libretexts.org 

*Reaction rate increases with increasing temperature

Source: www.chem.libretexts.org 

(B) Meaning of chemical equilibrium

Chemical equilibrium can be defined as the state where there is no observable change in the properties of a system with respect to time. Equilibrium systems may be static or dynamic. An example of static equilibrium is the balanced see-saw because it is stationary, while the saturated solution is an example of dynamic equilibrium.

Equilibrium in reversible reactions

A reversible reaction is one which can be made to proceed in either direction under suitable conditions. 

Note that all reversible reactions can proceed in either direction, depending on the condition of the reactions. In some cases, both forward and backward reactions operate under the same conditions, while in others they operate under different conditions.

Le chatelier’s principle

Le chatelier’s principle states that if a chemical system is in equilibrium and one of the factors involved in the equilibrium is altered, the equilibrium will shift, so as to neutralize the effect of the change.

Factors affecting equilibrium

1. Effect of temperature changes on equilibrium mixture

2. Effect of pressure change on equilibrium mixture

Equilibrium constant

The equilibrium constant, K, may be derived by the use of law of mass action.

This law by Goldberg and wage states that at constant temperature, the rate of a chemical reaction is directly proportional to the active mass of the reacting substances.

Equilibrium constant is a measure of the ratio of the equilibrium concentration of the products of a reaction to the equilibrium concentration of the reactants; with each concentration raised to the power corresponding to the coefficient in the balanced equation of the reaction.

In the general reaction

aA + bB > cC + dD

The equilibrium constant

 K= [C]^c [D]^d / [A]^a [B]^b

Where [ ] stands for equilibrium constant.

Past questions

1. Which of the following reactions will be affected by change in pressure at equilibrium? (Wassce 2004)

A. X_(g) + Y_(g) > W _(g) + Z_(g)

B. X_(g) + 2Y_(g)  > W_(g) + Z_(g)

C. 2X_(g) > W_(g) + Z_(g)

D. 2Y_(g) > W_(g) + Z_(g)

Answer: B

2. Consider the following equilibrium reaction: 3Fe_(s) + 4H₂O_(g) > Fe₃O_4(s) + 4H_2(g), ∆H=-150KJmol^-1. Explain the effect of the following factors on the position of equilibrium: (I) decrease in temperature (ii) increase in pressure (iii) removal of hydrogen. (Wassce 2004)

Answer: 

(i) the forward reaction is exothermic, decrease in temperature will favour the forward reaction or the position of equilibrium shifts to the right

(ii) there will be no effect on the position of equilibrium as the number of gaseous molecules are the same on both sides.

3.  What will happen if more heat is applied to the following system at equilibrium? X_2(g) + 3Y_2(g) > 2XY_3(g); ∆H=-xkjmol^-1. (Wassce 2002)

A. The yield of XY₃ will increase

B. More of XY₃ will decompose

C. More of X₂ will react

D. The forward reaction will go to completion

Answer: B

4. What is the expression for the equilibrium constant (Kc) for the following reaction? N_2(g) + O_2(g) < > 2NO_(g). (Wassce 2002)

A. [NO]²/ [N₂] + [O₂]

B. [2NO]²/[N₂][O₂]

C. [N₂] + [O₂]/2[NO]² 

D. [NO]²/[N₂][O²]

Answer: D

5. 2H₂O_(I) + 2F_2(g) < > 4HF_(aq) + O_2(g)

In the reaction above, the substance that is being reduced is? (Jamb 2013)

A. F_2(g)

B. HF_(aq)

C. O_2(g)

D. H₂O_(l)

Answer: D

6. Z in the diagram represents? (Jamb2014)

A. Heat of reaction

B. Activation energy

C. Free energy

D. Entropy of reaction

Answer: B

7. PCl_5(l) < > PCl_3(l) + Cl_2(g) ∆H=+ve

In the reaction above, the forward reaction is favored by? (Jamb 2016)

A. Increasing the size of the containing vessel

B. Increasing the pressure

C. Adding a catalyst

D. Increasing the temperature

Answer: D

8. 2H_2(g) + O_2(g) < > 2H₂O_(g).         ∆H=-ve 

When the temperature of the system above is increased, the position of the equilibrium will shift to the? (Jamb 2017)

A. Right and the value of the equilibrium constant decreases

B. Right and the value of the equilibrium constant remains unchanged

C. Left and the value of the equilibrium constant decreases

D. Left and the value of the equilibrium constant remains unchanged.

Answer: C