5.1.6: Equilibrium

Question 1

What is a homogeneous equilibrium?

Answer 1

An equilibrium in which all the species making up the reactants and products are in the same physical state.

Question 2

What is a heterogeneous equilibrium?

Answer 2

An equilibrium in which species making up the reactants and products are in different physical states.

Question 3

When is a reaction in equilibrium?

Answer 3

-When the rates of the forward and reverse reactions are equal.

Question 4

What is the expression for Kc?

Answer 4

For the reaction:

aA + bB ↔ cC +dD

K_c=[C]^c[D]^d / [A]^a[B]^b

Question 5

Once an equilibrium is established, Kc remains constant unless…

Answer 5

The temperature of the reaction changes.

Question 6

In order to determine a value for Kc, what needs to be known?

Answer 6

The concentrations of the reactants and products at equilibrium.

Question 7

What are the most common ways to determine the conc. of reactants and products?

Answer 7

• Titration
• Using a colorimeter

Question 8

How can titration be used to find out the concentrations of products and reactants at equilibrium?

Answer 8

Titrating reactants or products against a substance with a known concentration lets us know how much of the reactant or product is present.

Question 9

What are the drawbacks of using titration to find out the concentrations of products and reactants at equilibrium?

Answer 9

• Reactant or product has to be removed from the reaction mixture and this will alter the position of the equilibrium.
• Other substances may be present that can affect the results of titrations
• e.g. if an alkali used to titrate against eqm. substance and acid catalyst is also used, both the eq. substance and catalyst will react with the alkali.

Question 10

How can a colorimeter be used to monitor the concentrations of products and reactants throughout?

Answer 10

• Coloured substances alter the amount of light that can pass through a substance.
• More intense colours mean the substance has a higher concentration and absorbs more light.

Question 11

Often, only the equilibrium concentration of one substance is determined and compared with the initial concentration. How can the other concentrations be found?

Answer 11

Other concentrations can then be calculated using the balanced equation.

Question 12

When a heterogeneous equilibrium is present, why do the molar concentrations of solids and pure liquids not change?

Answer 12

Their volume remains constant.

Question 13

How does a heterogeneous equilibrium affect the expression for K_c?

Answer 13

• concentrations of solid substances are not included in the expression.
• concentrations of pure liquids are not included in the expression.