Views: 28

CHEMICAL EQUILIBRIUM

1.a. Define chemical equilibrium

Chemical equilibrium refers to the state of a system in which the concentration of the reactant and the concentration of the products do not change with time and the system does not display any further change in properties.

2.Explain types of chemical equilibrium

There are two types of chemical equilibrium:

1. Homogenous Chemical Equilibrium

In this type, the reactants and the products of chemical equilibrium are all in the same phase. Homogenous equilibrium can be further divided into two types: Reactions in which the number of molecules of the products is equal to the number of molecules of the reactants. For example,

H₂ (g) + I₂ (g) ⇌ 2HI (g)

N₂ (g) + O₂ (g) ⇌ 2NO (g)

Reactions in which the number of molecules of the products is not equal to the total number of reactant molecules. For example,

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)

COCl₂ (g) ⇌ CO (g) + Cl₂ (g)

Heterogeneous Chemical Equilibrium

In this type, the reactants and the products of chemical equilibrium are present in different phases. A few examples of heterogeneous equilibrium are listed below.

CO₂ (g) + C (s) ⇌ 2CO (g) 

CaCO₃ (s) ⇌ CaO (s) + CO₂ (g)

Thus, the different types of chemical equilibrium are based on the phase of the reactants and products.

1. For an equilibrium reaction, what effect does reversing the reactants and products have on the value of the equilibrium constant?

By reversing the reactants and products for an equilibrium reaction, the equilibrium constant becomes: K’=1K K′=1K.

2. (a) State the meaning of the following

• Endothermic reactions

Endothermic reactions are chemical reactions in which the reactants absorb heat energy from the surroundings to form products. These reactions lower the temperature of their surrounding area, thereby creating a cooling effect.

• Exothermic reactions

An exothermic reaction is a reaction in which energy is released in the form of light or heat. Thus in an exothermic reaction, energy is transferred into the surroundings rather than taking energy from the surroundings as in an endothermic reaction.

• State the effects of an increase in temperature  on a reversible
• Endothermic reaction

If temperature is increased, the position of equilibrium moves in the exothermic direction to increase the temperature i.e., the reaction will move backwards.

• Exothermic reaction

If temperature is increased, the position of equilibrium moves in the endothermic direction to reduce the temperature i.e., the reaction will move forward.

3. State the law of mass action  as applied in a state of chemical equilibrium

States  that the rate of any chemical reaction is proportional to the product of the masses of the reacting substances, with each mass raised to a power equal to the coefficient that occurs in the chemical equation.

4. The decomposition of calcium carbonate  is represented by the following equation

CaCO₃  ⇌      CaO + CO₂

• Explain how the increase in pressure would affect the equilibrium

According to Le Chatelier’s principle a reaction tends to oppose any change in it’s state of equilibrium. If we increase the pressure then volume of CO₂ will be decreased so the volume of CaCO₃ will be decreased as well.

• Other than pressure , state other factors  which affect the position of the equilibrium in a reversible reaction

Changes in concentration, temperature, and pressure can affect the position of equilibrium of a reversible reaction.

5. List three factors which affect the chemical equilibriam in a reversible reaction

Changes in concentration, temperature, and pressure can affect the position of equilibrium of a reversible reaction.

6. State the effects of a catalyst on the position of chemical equlibrium

Catalyst does not affect the position of equilibrium and hence it does not have any effect on the value of equilibrium constant. only change in temperature can change the equilibrium constant. catalyst only affects the forward and reverse reaction equally.

7. Which of the following equilibrium are homogeneous and which are heterogeneous?
   1. 2HF(g)⇌H2(g)+F2(g)

This equilibrium is homogenous as all substances are in the same state.

1. C(s)+2H2(g)⇌CH4(g)

This equilibrium is heterogeneous as not all substances are in the same state.

1. H2C=CH2(g)+H2(g)⇌C2H6(g)
2. This equilibrium is homogeneous as all substances are in the same state.
3. 2Hg(l)+O2(g)⇌2HgO(s)

This equilibrium is heterogeneous as not all substances are in the same state.

8. Classify each equilibrium system as either homogeneous or heterogeneous.

NH4CO2NH2(s)⇌2NH3(g)+CO2(g) This equilibrium is heterogeneous as not all substances are in the same state.

C(s)+O2(g)⇌CO2(g)

This equilibrium is heterogeneous as not all substances are in the same state.

2Mg(s)+O2(g)⇌2MgO(s)

 This equilibrium is heterogeneous as not all substances are in the same state.

AgCl(s)⇌Ag⁺(aq)+Cl⁻(aq)

This equilibrium is heterogeneous as not all substances are in the same state.

9. If an equilibrium reaction is endothermic, what happens to the equilibrium constant if the temperature of the reaction is increased? if the temperature is decreased?

. According to Le Chatelier’s principle, equilibrium will shift in the direction to counteract the effect of a constraint (such as concentration of a reactant, pressure, and temperature). Thus, in an endothermic reaction, the equilibrium shifts to the right-hand side when the temperature is increased which increases the equilibrium constant and the equilibrium shifts to the left-hand side when the temperature is decreased which decreases the equilibrium constant.

10. Industrial production of NO by the reaction

 N2(g)+O2(g)⇌2NO(g)

 is carried out at elevated temperatures to drive the reaction toward the formation of the product. After sufficient product has formed, the reaction mixture is quickly cooled. Why?

After sufficient industrial production of NO by the reaction of N2(g)+O2(g)⇌2NO(g)  at elevated temperatures to drive the reaction toward the formation of the product, the reaction mixture is cooled quickly because it quenches the reaction and prevents the system from reverting to the low-temperature equilibrium composition that favors the reactants.

11. How would you differentiate between a system that has reached chemical equilibrium and one that is reacting so slowly that changes in concentration are difficult to observe?

To differentiate between a system that has reached equilibrium and one that is reacting slowly that changes in concentrations are difficult to observe we can use Le Chatelier’s principle to observe any shifts in the reaction upon addition of a constraint (such as concentration, pressure, or temperature).

12. What is the relationship between the equilibrium constant, the concentration of each component of the system, and the rate constants for the forward and reverse reactions?

The relationship between the equilibrium constant, the concentration of each component of a system, and the rate constants for the forward and reverse reactions considering a reaction of a general form:

          aA+bB⇌cC+dD      is    K=[C]^c[D]^d        =kf

                                                      [A]^a[B]^b              Kr

13. Consider the reaction 2H₂+ O₂ 2H₂ State the  correct the expression for the equilibrium constant for this reaction?

14. Nitrogen oxides are air pollutants produced by the reaction of nitrogen and oxygen at high temperatures. At 2000°C, the value of the equilibrium constant K_Cfor the reaction,

             N₂ (g) + O₂ (g) ⇌ 2 NO (g), is 4.1 × 10⁻⁴.

Calculate the equilibrium concentration of NO (g) in air at 1.00 atm pressure and 2000°C. The equilibrium concentrations of N₂ and O₂ at this pressure and temperature are 0.036 M and 0.0089 M, respectively.

We are given all of the equilibrium concentrations except that of NO. Thus, we can solve for the missing equilibrium concentration by rearranging the equation for the equilibrium constant.

Thus [NO] is 3.6 × 10⁻⁴ mol/L at equilibrium under these conditions.

We can check our answer by substituting all equilibrium concentrations into the expression for the reaction quotient, Q_C, to see whether it is equal to the equilibrium constant, and thus confirm that the system is indeed at equilibrium.

The answer checks; our calculated value gives the equilibrium constant within the error associated with the significant figures in the problem.

15. Why is Chemical Equilibrium called Dynamic Equilibrium

The stage at which the rate of the forward reaction is equal to the rate of backward reaction is called an equilibrium stage. At this point, the number of reactant molecules converting into products and product molecules into reactants are the same. The same equilibrium can be carried out with the same reactants anywhere with similar conditions with continuous interchanging of molecules hence chemical equilibrium is dynamic.

16. State the following as used in chemical equilibria
    1. Henry’s law

Was first formulated in 1803 by the English physician and chemist William Henry . It states that the weight of a gas dissolved by a liquid is proportional to the pressure of the gas upon the liquid. The law, holds true  only for dilute solutions and low gas pressures.

1. Le chateliers’s principle

Le chatelier’s principle states that changes in the temperature, pressure, volume, or concentration of a system will result in predictable and opposing changes in the system in order to achieve a new equilibrium state.

Le Chatelier’s principle is an observation about chemical equilibria of reactions. It States that If the concentration of a reaction species is increased (at constant T and V), the equilibrium system will shift in the direction that reduces the concentration of that species. Le Chȃtelier’s principle can be used to predict the effect that a stress like changing concentration has on a reaction system at equilibrium.

17. State four factor which affect chemical equilibrium

According to Le-Chatelier’s principle, if there is any change in the factors affecting the equilibrium conditions, the system will counteract or reduce the effect of the overall transformation. This principle applies to both chemical and physical equilibrium.

There are several factors like temperature, pressure and concentration of the system which affect equilibrium. Some important factors affecting chemical equilibrium are discussed below.

1. Change in Concentration:

The concentration of the reactants or products added is relieved by the reaction which consumes the substance which is added. The concentration of reactants or products removed is relieved by the reaction which is in the direction that replenishes the substance which is removed. When the concentration of the reactant or product is changed, there is a change in the composition of the mixture in chemical equilibrium.

1. Change in Pressure:

Change in pressure happens due to the change in the volume. If there is a change in pressure it can affect the gaseous reaction as the total number of gaseous reactants and products are now different. According to Le Chatelier’s principle, in heterogeneous chemical equilibrium, the change of pressure in both liquids and solids can be ignored because the volume is independent of pressure.

1. Change in Temperature:

The effect of temperature on chemical equilibrium depends upon the sign of ΔH of the reaction and follows Le-Chatelier’s Principle. As temperature increases the equilibrium constant of an exothermic reaction decreases. In an endothermic reaction the equilibrium constant increases with an increase in temperature.

Along with the equilibrium constant, the rate of reaction is also affected by the change in temperature. As per Le Chatelier’s principle, the equilibrium shifts towards the reactant side when the temperature increases in case of exothermic reactions, for endothermic reactions the equilibrium shifts towards the product side with an increase in temperature.

Effect of a Catalyst:

A catalyst does not affect the chemical equilibrium. It only speeds up a reaction. In fact, catalyst equally speeds up the forward as well as the reverse reaction. This results in the reaction reaching its equilibrium faster.

The same amount of reactants and products will be present at equilibrium in a catalysed or a non-catalysed reaction. The presence of a catalyst only facilitates the reaction to proceed through a lower-energy transition state of reactants to products.

1. Effect of Addition of an Inert Gas:

When an inert gas like argon is added to a constant volume it does not take part in the reaction so the equilibrium remains undisturbed. If the gas added is a reactant or product involved in the reaction then the reaction quotient will change.

18. Ammonia is manufactured according to the following equation

  N² + 3H²  ⇌   2NH³            ΔH=      -92KJMol^-1

          (g)    (g)         (g)

Explain what happens to the yield of ammonia when; a) Temp increases, b) Press increases c) Catalyst changes

Ammonia is made in the Haber Process (3H2(g) + N2(g)<-> 2NH3(g)). Using Le Chetelier’s Principal, what happens to the equilibrium yield of ammonia when…:

Le Chetelier’s Principal gives us a guide to work out what will happen when we change the conditons of a reaction in dynamic equilibrium. It states that if the dynamic equilibiurm is disturbed, the equilibrium position will change in order to counteract the change that has occured.

1. A) When we increase the temperature of the system, the reaction with the greatest surface area, or the one which produces the most moles of gas, is more favourable. Hence the equilibrium position will shift towards the reverse reaction in the Haber process and the equilibrium yield of ammonia will decrease.
2. B) Increasing the pressure has an opposite effect to an increase in the temperature in that it favours the reaction which produces the least moles of gas. Since the forward reaction produces 2 moles from 4 moles, the number of moles of gas is decreasing hence the forward reaction is favoured when the pressure increases. The equilibrium yield of ammonia will therefore increase.
3. C) A catalyst is a compound, usually a metal such as Fe or Pd, that provides the reaction with an alternative reaction pathway with a lower activation energy. Hence it increases the rate of the reaction. A catalyst has NO EFFECT on the equilibrium position hence the equilibrium yield of ammonia does not change (though the rate at which the reaction reaches equilibrium might).