Topic 10 - Equilibrium I

Question 1

What is the definition of a reversible reaction?

Answer 1

One where both: the reactants react to form the products whilst the products simultaneously break down to reform the reactants

Question 2

What is used to show reversible reaction?

Answer 2

2 half arrows pointing to the products and the reactants

Question 3

The formation of products is called the…

Answer 3

The forwards reaction

Question 4

The formation of reactants is called the…

Answer 4

The backwards reaction

Question 5

What conditions must there be for dynamic equilibrium?

Answer 5

A closed system

Question 6

What features are characteristic of dynamic equilibrium?

Answer 6

• Rate of the forwards reactions = Rate of the backwards reaction
• Both the forwards and backwards reaction are proceeding
• The concentration of both the reactants and products remains constant

Question 7

Are all equilibrium mixtures the same?

Answer 7

No

Question 8

What is the definition of an equilibrium mixture?

Answer 8

The amounts of products and reactants present when dynamic equilibrium is reached

Question 9

State Le Chateliers Principle

Answer 9

If a dynamic equilibrium is subject to a change in conditions, the position of equilibrium movea to counteract that change

Question 10

What are the 3 factors that change the position of equilibrium?

Answer 10

• Temperature (reversible reactions have an exothermic and endothermic direction)
• Concentration (if you feed off your products or change the concentrations of one of your products or reactants
• Pressure (to do with gases and shifting to the side with less/more moles)

Question 11

Does the addition of a catalyst change the position of equilibrium?

Answer 11

No

Question 12

What happens when a catalyst is added to a reversible reaction?

Answer 12

• The rate of both the forwards and backwards reaction is increased
• Dynamic equilibrium is reached more quickly

Question 13

What is the definition of a homogenous equilibrium?

Answer 13

An equilibrium where everything is present in the same phase/state

Question 14

What is the definition of a heterogenous equilibrium?

Answer 14

An equilibrium where compounds are present in more than one phase

Question 15

What is Kc?

Answer 15

The equilibrium constant

Question 16

What is the definition of Kc?

Answer 16

A numerical value associated with the equilibrium concentrations of each product and reactant in a reaction AT A GIVEN TEMPERATURE

Question 17

What is the only condition that changes the value of Kc?

Answer 17

Temperature

Question 18

How do you find the units for Kc?

Answer 18

• Each species has the units moldm(-3)
• Use indices rules in order to determine the units of Kc for each individual equilibria

NB: sometimes an equilibria will have no units

Question 19

What is the Kc expression for this equilibrium:

mA + nB <=> pC + qD

Answer 19

[A]^m[B]^n / [C]^p[D]^q

Question 20

What do the reacting mole numbers become in Kc expressions?

Answer 20

The powers in front of the species

Question 21

Write the equation for the contact process and, therefore, its Kc expression and units

Answer 21

2SO2 (g) + O2 (g) <=> 2SO3 (g)

Kc = [SO3]^2 / [SO2]^2[O2]

units - dm^3mol^-1

Question 22

Why is it important that you write down the equation next to your Kc expression?

Answer 22

As Kc expression can be written either way round as they describe reversible reactions. Therefore, you need to write down the reaction direction you are taking as forward (and therefore the backwards reaction too)

Question 23

Write the equation for the contact process and, therefore, its Kc expression and units

Answer 23

N2 (g) + 3H2 (g) <=> 2NH3 (g)

Kc = [NH3]^2 / [N2][H2}^3

units - dm^6mol^-2

Question 24

What is the effect on Kc if the POE is in the centre?

Answer 24

Kc = 1

Question 25

What is the effect on Kc if the POE lies to the right?

Answer 25

Kc > 1

Question 26

What is the effect on Kc if the POE lies to the left?

Answer 26

Kc < 1

Question 27

Which state is not included in any Kc expressions? Why?

Answer 27

Solids - they do not have a concentration

Question 28

If solids are present in an equilibria what do you do to them in the Kc expression?

Answer 28

get rid of them

Question 29

If you are given the moles in a Kc question what do you need to do?

Answer 29

• Determine starting moles and then equilibrium moles of all species in the reaction
• Use the volume provided to determine the concentrations of each of the species at equilibrium
• Plug the concentrations into a Kc expression to determine Kc

Question 30

If gases are reacted in a container that is 5dm^3 what is the volume you use when determining concentration of gases at equilibrium?

Answer 30

5 dm^3 - as gases fill the volume of whatever container they are in

Question 31

What is the main factor you need to consider when determining equilibrium moles?

Answer 31

Reacting moles in the equation

Question 32

What does a titration of a sample of the equilibrium solution help to determine?

Answer 32

The equilibrium concentration of a species - form that you can determine equilibrium moles of all your species and, therefore Kc