Chemical Equilibrium

Question 1

Gibbs energy is a spontaneous reaction?

Answer 1

In a spontaneous reaction at constant temperate and pressure Gibbs energy of a reaction < 0 so the system moves to a lower Gibbs energy, the Gibbs energy of the system decreases as the reaction proceeds

Question 2

When is the lowest Gibbs energy change reached?

Answer 2

The lowest Gibbs energy is achieved when the reactants are completely converted to products

Question 3

What happens when a reaction comes to equilibrium?

Answer 3

Starting with the reactants the system can move to lower Gibbs energy by forming products and proceeding in the forward direction, however once the system reaches the equilibrium point the Gibbs energy has reached a minimum. If the reaction continued the Gibbs energy would increase and this process would not be spontaneous, so the composition of the reaction mixture does not change beyond point equilibrium which represent the equilibrium composition of the reaction mixture

Question 4

When does a system come to equilibrium?

Answer 4

A system comes to equilibrium when it reaches its minimum Gibbs energy

Question 5

What happens when an equilibrium is reached?

Answer 5

Reactions do not stop when equilibrium is reached although the overall proportion of reactants and products does not change individual molecules may still react but not every molecule that undergoes the forward reaction another molecule will undergo the reverse reaction, there is a dynamic equilibrium

Question 6

What is the equilibrium constant K?

Answer 6

The composition of a reaction mixture when it comes to equilibrium is described by an equilibrium constant K

Question 7

What is Kp?

Answer 7

For gas phase reactions an equilibrium constant Kp calculated in terms of the equilibrium partial pressure Peqm of the component is usually used, the units of Kp depend on the stoichometry of the reaction

Question 8

What is Kp?

Answer 8

For reactions in solution Kc calculated in terms of the molar concentrations is more useful, in dilute solutions the water term is essentially constant and a new equilibrium constant Ka is defined that incorporates Ka, this is called the acidity constant and is a measure of the strength of an acid

Question 9

What is K?

Answer 9

Defined as the ratio of the thermodynamic activities of the products at equilibrium to those of the reactants

Question 10

What is the activity of a gas?

Answer 10

The ratio of its partial pressure measured in bar to the standard state pressure

Question 11

What is the activity of a solution?

Answer 11

The ratio of its molar concentration to the standard

Question 12

What does the activity of a gas and solution being measured mean?

Answer 12

K has the same numerical value as Kp if partial pressures are measured in bar or as Kc if concentration are measured in moldm-3

Question 13

Units of K?

Answer 13

It is important to remember that activities have no units so that K has no units and is dimensionless

Question 14

Summary of Kc?

Answer 14

Equilibrium constant in terms of concentrations units in terms of concentrations moldm-3 used for reactions in solution and sometimes for reactions in the gas phase

Question 15

Summary of Kp?

Answer 15

Equilibrium in terms of pressure units in terms of pressure used for reactions in the gas phase

Question 16

Summary of K?

Answer 16

Equilibrium constant in terms of activities no units since activities are dimensionless used for all reactions

Question 17

What is Ksp?

Answer 17

Thermodynamic equilibrium constant sometimes defined in concentrations rather than activity, useful for calculating the solubility of sparingly soluble compounds, the molar solubility S is measured in moldm-3 and gives the number of moles of solute dissolved in 1dm3 of a saturated solution, K is an equilibrium constant and does not change at a fixed temperature therefore addition of one of these ions will reduce the concentration of the other

Question 18

What does the reaction quotient Q do?

Answer 18

If a reaction is not at equilibrium the amounts of products and reactants will change until they reach equilibrium composition. The Gibbs energy of the system changes as the reaction proceeds since the proportions of reactant and product change. To investigate how G changes you need a measure of the composition of the reaction mixture as it changes during the reaction, the reaction quotient can do this

Question 19

What is the value of Q at equilibrium?

Answer 19

When the system reaches equilibrium the value of Q is the same as the value of the equilibrium constant K

Question 20

When can Q can be measured?

Answer 20

The value of Q can be calculated at any stage during the reaction and changes as the reaction proceeds towards equilibrium, if Q does not equal K the reaction moves so as to bing it to equilibrium so that Q=K

Question 21

How can the reaction quotient provide a way to predict the direction of a reaction?

Answer 21

If the Gibbs energy of reaction < 0 or Q < K then the forward reaction proceeds
If the Gibbs energy of reaction > 0 or Q > K then the reverse reaction proceeds
If the Gibbs energy of reaction is equal to zero or Q = K the system is at equilibrium

Question 22

How does the value of K give some information about the composition of the reaction mixture at equilibrium?

Answer 22

If K is large K&raquo_space; 1 then the equilibrium mixture will contain mainly product, if K is small K &laquo_space;1 then the reaction does not proceed vert far and little product forms. If K=1 then the equilibrium mixture will contain substantial amount of both products and reactants

Question 23

Le Chateliers principle?

Answer 23

When a change is made to a system in dynamic equilibrium the system responds to minimise the effect of the change

Question 24

Why does pressure and temperature have an effect on the reactions?

Answer 24

Because of the way they influence of the Gibbs energy change of reaction and the equilibrium constant and hence the position of equilibrium

Question 25

Changing the pressure?

Answer 25

Change in Gibbs reaction energy is the value at 1 bar so by definition does not change with changing pressure therefore the equilibrium constant does not vary with pressure at constant temperature, this however does not mean that pressure has no effect on the position of equilibrium and the composition of the equilibrium

Question 26

What is the partial pressure given by?

Answer 26

PA = XA x Ptotal

where P is the partial pressure of a component A in a mixture XA is the mole fraction of component A and p total is the total pressure of the system

Question 27

How will the composition change if p total changes?

Answer 27

K is a constant at constant temperature so if p total changes the mole fractions must change to compensate, if it increases the mole fractions must change to keep k constant, however changing the pressure does not always change the position of equilibrium if there are the same number of moles on each side the p total term will therefore cancel out in the expression for K

Question 28

Changing the amount of a component?

Answer 28

Adding or removing one of the components from a reaction mixture will affect the concentration or pressure of the others in the equilibrium mixture

Question 29

Changing the temperature?

Answer 29

Le Chateliers principle states that the equilibrium position for an exothermic reaction will move to favour the reactants if the temperature is increase, for an endothermic reaction an increased temperature favours the product

Question 30

Using the Van’t Hoff equation in a straight line?

Answer 30

A graph of lnK vs 1/T gives a straight line with the gradient of the enthalpy change of the reactions divided by the gas constant, change of enthalpy and change of entropy don’t vary much with temperature so for an endothermic reaction the gradient term will be negative and as the temperature increases the magnitude of this terms gets smaller, subtracting a smaller value means that lnK and hence K will be bigger than at a lower temperature, if K is bigger a larger promotion of products will form. This agrees with Le Chateliers principle which states the position of equilibrium moves in the direction of the endothermic change when the temperature is raised because this lowers the temperature and minimises the effect of the change

Question 31

Exothermic reaction value of K temperature change and equilibrium composition?

Answer 31

Exothermic, temperature increase K gets smaller and favours reactants
Exothermic, temperature decrease K gets bigger and favours products

Question 32

Endothermic reaction value of K temperature change and equilibrium composition?

Answer 32

Endothermic, temperature decrease K gets smaller and favours reactants
Exothermic, temperature increase K gets bigger and favours products

Question 33

What is a catalyst?

Answer 33

A substance that increases the rate of a reaction without being used up, it works by providing a different reaction path with a lower activation energy but do not change the initial and final states

Question 34

How does a catalyst affect equilibrium?

Answer 34

The rates of both the forward and reverse reaction are increased so that a catalyst has no effect on the position of the equilibrium, since K depends on Gibbs energy change of reaction it depends only on the energies of the products and reactions and not on the path by which a reaction occurs and so is a state function