Chemical Equilibrium, Le Chatelier’s Principle

CONTENT

• Reversible Reactions
• Le Chatelier’s Principle
• Factors Affecting Chemical Equilibrium
• Equilibrium Constant

EQUILIBRIUM

Equilibrium can be defined as the state of a system where there is no observable or detectable change in the properties of the system with respect to time. Examples of system in equilibrium

1. a balanced see – saw
2. a saturated solution of NaCl

STATIC AND DYNAMIC EQUILIBRIUM

Static equilibrium is attained when a system in equilibrium is in a state of rest or stationary state.

Example of this is balanced see – saw

Dynamic equilibrium can occur during a physical change or a chemical change that is reversible.  Dynamic equilibrium involving a physical change is referred to as physical equilibrium while that involving a chemical change is a chemical equilibrium.

A system is said to be in a state of dynamic equilibrium when both the forward and backward reactions take place at the same rate. Example

            N₂O_4(g)    =  2NO_2(g)

EQUILIBRIUM IN REVERSIBLE REACTION

A reversible reaction is one which proceeds in both forward and backward directions under suitable conditions.

For example: NH₄Cl_(s)  —-> NH_3(g)  +  HCl_(g)

A reversible reaction is said to be in dynamic equilibrium when both the forward and backward reaction are occurring at the same rate, thereby producing no net change in concentrations of the reactants or products.

Properties of system in equilibrium

1.       A chemical reaction at equilibrium is in dynamic state with the rates of the forward and backward reactions are equal.

2.       The equilibrium position can be reached from reactants alone or form product alone

3.       For equilibrium to be established, the system must be closed

EVALUATION

1. When is a reversible reaction said to be in dynamic equilibrium
2. List three properties of a system in a state of equilibrium

FACTORS AFFECTING THE POSITION OF EQUILIBRIUM

The position of equilibrium depends on the following set of condition

1. Temperature
2. Concentration
3. Pressure (for gases)

Variation in any of these factors will upset the balance of the system and results in a shift in the position of the equilibrium.  These factors and their effect on chemical systems in equilibrium were studied by Le Chatelier’s who formed the 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 position will shift so as to neutralize the effect of the change.

Importance of Le Chatelier’s principle in chemical industry:  It helps to

1. Define optimum conditions for chemical process
2. Reduce undesirable reversibility
3. Predict the effect of an altered factors on the equilibrium

EFFECT OF TEMPERATURE

For an endothermic reaction, increase in temperature shifts the position of equilibrium to the right. That is, the forward reaction is favoured while a decrease in temperature favours the backward reaction.  The reverse is the case for exothermic reactions.

EFFECT OF CONCENTRATION

In a system of equilibrium, if the concentration of reactants is increased or product decreased, the forward reaction will be favoured, that is; equilibrium position shifts to the right.  Also, if the concentration of the products is increased or reactant decreased, the backward reaction will be favoured, equilibrium position shifts to the left.

EFFECT OF PRESSURE

Pressure of gas is proportional to the number of moles.  Increase in number of moles implies increase in pressure and vice visa.  If the forward reaction proceeds by decreases in number of moles (pressure), an imposed increase pressure will favour forward reaction.  Reverse is the case if it proceeds by increase in number of moles (pressure). In other words, when high pressure is applied, equilibrium will shift to the side with less number of moles while when low pressure is applied, equilibrium will shift to the side with greater number of moles.

N_2(g)                 +          3H_2(g)                —–> 2NH_3(g)

1 mole                         3 moles                    2 moles

Increase in pressure favours forward reaction while decrease in pressure favours backward reaction

EFFECT OF CATALYST

A catalyst does not change (affect) the position of equilibrium.  Addition of catalyst, however increase the rate of both forward and backward reaction by lowering the activation of energy of the reaction.

Equilibrium position is attained at faster rate when a catalyst is present.

EVALUATION

1.         State Le Chatelier’s principle

2.         Explain the effect of

            i.          Increase in temperature

            ii.         decrease in pressure

on the following reaction

2SO_2(g) +          O_2(g)     ↔       2SO_3(g)  H = ∆ -3957KJmol^-1     

HABER PROCESS

Haber process is an industrial process for the manufacture of ammonia.

Ammonia, NH₃, is an important chemical for the manufacture of fertilizers. During the process, the following equilibrium is established

N_2(g)     +          3H_2(g)                2NH_3(g)     H =   ∆-96KJmol^-1

1 mole          3 moles         2 moles

CHOICE OF TEMPERATURE

Since forward reaction is exothermic, Le Chatelier’s principle shows that the greatest yield of ammonia, NH₃, would be obtained at low temperature.  However, if the temperature is too low, time taken to reach equilibrium would be much.  A temperature of 500⁰C gives reasonable yield of ammonia.

CHOICE OF PRESSURE

Since forward reaction leads to decrease in number of moles (pressure), Le Chatelier’s principle shows that high yield of ammonia would be obtained at high pressure.  The pressure of 25atm gives a satisfactory yield.

CHOICE OF CATALYST

Iron is used as catalyst in Haber process.  Iron is found to be effective in speeding up the rate of which equilibrium is reached.  It is used in form of small pellets which increased the surface area available for contact.

EQUILIBRIUM CONSTANT (K) AND LAW OF MASS ACTION

The law of mass action states that at constant temperature, the rate of reaction is proportional to the active masses of each of the reactants;

The active mass of the reactants is the concentration of the substance raised to the power of coefficient (number of mole) of that substance in the equation of reaction under study.

NOTE: Active mass = molar concentration x activity coefficient

In most reactions, the active masses of the reactant may be taken as their molar concentrations.

Solving problems relating to Chemical Equilibrium

GENERAL EVALUATION/REVISION

1. State the conditions for the optimum yield of ammonia in Haber process.
2. State Le Chatelier’s principle
3. Define the following terms and state how they vary across a period and down a group: Ionization energy, Electronegativity, Electron affinity
4. Define oxidation, reduction, oxidizing agent and reducing agent in terms of electron transfer
5. An organic compound contains the following by mass: 0.188g of C, 0.062g of H and 0.25g of O. If the vapour density of the compound is 16, determine its molecular formula.

READING ASSIGNMENT

New School Chemistry for Senior Secondary School by O.Y. Ababio (6^th edition) Pages 265-275

WEEKEND ASSIGNMENT

SECTION A: Write the correct option ONLY.

1.       When the concentration of both reactants and product are equal

a.  K = 1       b. K > 1                    c.  K ≥1 d. K = 0

2.       In the reaction

2SO_2(g)    +          O_2(g)                 =          2SO_3(g)             H =  ∆-xkJ/mole

          increase in temperature favours the

          a. backward reaction      b. forward reaction     c. yield of SO₃

3.       How is the equilibrium constant for the forward reaction (K_f) related to that of the reverse reaction (K_r)? a. K_r is the additive inverse of K_f b. K_r is the multiplicative inverse of K_f c. K_r is same as K_f d. The product of K_r and K_f is zero

4.       The equilibrium constant expression involves a. solid, liquid and gaseous species b. gaseous species only c. species in aqueous solution only d. species in aqueous solution and gases only

5.       Increase in pressure favours the forward reaction for an equilibrium reaction that proceeds with a. increase in pressure b. decrease in number of moles c. no change in pressure d. increase in number of moles

SECTION B

1.       State the Le Chatelier’s principle

2.       Calculate (a) the equilibrium constant (Kp) for the reaction below at 45^oC

          (b) the equilibrium constant for the backward reaction below:

                        H_2(g)  +  I_2(g)  →  2HI_(g)

Given that the partial pressure for H₂, I₂ and HI are 0.065atm, 0.45atm and 0.245atm respectively. From your answer, deduce which of the reaction is favoured.