.6 Equilibria

Question 1

Le Chat’s Principle

Answer 1

If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance.

So in other words, if any factor is changed which affects the equilibrium mixture, the position of equilibrium will shift so as to oppose the change.

Question 2

At equilibrium, both forward and backward reactions occur … so the concentrations of all the reactants and products …

Answer 2

at the same rate

remain constant.

Question 3

Define the following
 Reactant
Product
Completion
Closed system
Reversible
Equilibrium mixture
Dynamic equilibrium
Backwards reaction
Forwards reaction
Dynamic
Le Chatelier's Principle

Answer 3

a substance used up in a reaction
a substance made in a reaction

reaction continues until one of the reactants is completely used up and then it stops

one where the reactants and products cannot escape

one which does not go to completion and that can occur in either direction

a mixture in which the proportions of all components remains constant

the point in a chemical reaction, where the forwards and backwards reactions are happening at the same rate, with the concentrations of all reactants and products remaining constant

one that goes from right to left in an equation

one that goes from left to right in an equation

one which proceeds in both directions at the same time

a system at equilibrium will respond to oppose any change imposed upon it

Question 4

Why change proportion of reactants to products in equilibrium mixture, where does equilibrium move if products/reactants increased, and what conditons effect position of equilibrium

Answer 4

In this way you are able to obtain a greater yield of the products. This is called changing the position of equilibrium.

• If the proportion of products in the equilibrium mixture is increased, the equilibrium is moved to the right, or in the forward direction.
• If the proportion of reactants in the equilibrium mixture is increased, the equilibrium is moved to the left, or in the backward direction.

You can often move the equilibrium position to the left or right by varying conditions like temperature, the concentration or species involved, or the pressure (in the case of reactions involving gases).

Question 5

Changing the overall pressure- affects reactions involving what, effect in increase/decrease in pressure, effect when number of moles on either side are the same.

Answer 5

Pressure changes only affect reactions involving gases.

Only change the position of equilibrium if there are a different number of molecules on either side of the equation.

Increase the pressure- Position of equilibrium will move to decrease the pressure- move to side with fewer gas molecules- reduce pressure
vice versa for Decrease in pressure (move to side with ore gas molecules, raise pressure)

Same number of moles of gases on both sides of the equation, then pressure has no effect on the equilibrium position

Question 6

Effects of temperature

Answer 6

Reversible reactions that are exothermic (give out heat) in one direction are endothermic (take in heat) in the other direction.

Increase the temperature- equilibrium moves in the direction that cools the system down. To do this it will move in the direction which absorbs heat (is endothermic)

Decrease the temperature- equilibrium moves in direction to warm up system. Moves in direction to produce heat (exothermic)

Question 7

If equilibrium moves to the RIGHT you get more…

If equilibrium moves to the LEFT you get more…

Answer 7

Products

Reactants

Question 8

Four conditions for equilibrium

Answer 8

• Equilibrium can only be reached in a closed system (one where the reactants and products can’t escape). The system does not have to be scaled. For example, a beaker may be a closed system for a reaction that takes place in a solvent, as long as the reactants, products, and solvent do not evaporate.
• Equilibrium can be approached from either direction (in figure l, from liquid or from vapour) and the final equilibrium position will be the same (as long as conditions, such as temperature and pressure, stay the same).
• Equilibrium is a dynamic process. It is reached when the rates of two opposing processes, which are going on all the time (in figure 1, evaporation and condensation ), are the same.
• You know that equilibrium has been reached when the macroscopic properties of the system do not change with time. These are properties like density, concentration, colour, and pressure - properties that do not depend on the total quantity of matter

Question 9

Catalysts:
Effect do they have on the position of equilibrium?

What effect does this have on the composition of the equilibrium mixture?

How do they work and what effect does this have on the forwards and backwards reaction rates?

Why are catalysts very important in industry?

Answer 9

None

None

Producing an alternative route for the reaction, which has a lower activation energy
Forward and back reactions speed up equally

Allow equilibrium to be reached more quickly- increase the frequency of successful collisions (not overall collisions) because a greater proportion of collisions now exceeds this lower activation energy

Question 10

Write expression for Kc and units for

A + B — C + 2D

Answer 10

expression- check in booklet

units=mol dm-1

Question 11

Expression for Kc and units for

N2(g) + 3H2(g) —- 2NH3(g)

Answer 11

expression- check in booklet

units= mol-2 dm6

Question 12

What is Kc? How to write Kc expression? Write one for an example equation. Equations need to be… and… what does homogenous mean?

Answer 12

Calculated as a numerical value
measure of how far to the RIGHT an equilibrium is

HIGHER Kc the FURTHER RIGHT equilibrium is

Expression always as a fraction using concentrations, PRODUCTS/REACTANTS e.g

All substances in same state/phase

Question 13

What is Kc for an equilibrium affected by? In what way?

Answer 13

ONLY temperature

Increasing the temperature decreases the value of the equilibrium constant.

Where the forward reaction is endothermic, increasing the temperature increases the value of the equilibrium constant. (very rare as most reaction exothermic)

The position of equilibrium also changes if you change the temperature

Question 14

Use the information below to answer this question.

A saturated solution of magnesium hydroxide, Mg(OH)2, contains 0.1166 g of Mg(OH)2 in 10.00 dm3 of solution. In this solution, the magnesium hydroxide is fully dissociated into ions.

The equilibrium constant expression for the dissolving of magnesium hydroxide is K = [Mg2+] [OH−]2.

In a saturated solution of Mg(OH)2 at a different temperature, the concentration of hydroxide ions is 1.0 × 10−3 mol dm−3.

Which one of the following has the correct value and units for K under these conditions?

A 1.0 × 10−6 mol2 dm−6

B 5.0 × 10−7 mol2 dm−6

C 1.0 × 10−9 mol3 dm−9

D 5.0 × 10−10 mol3 dm−9

Answer 14

D

Question 15

A sample of chlorine gas was sealed in a tube, heated and an equilibrium was established.

Cl2(g) ⇌ 2Cl(g)

Which one of the following is not true?

A The concentration of chlorine atoms remains the same when a catalyst is added to the tube.

B Increase in temperature causes an increase in the concentration of chlorine atoms.

C Increase in pressure causes an increase in the concentration of chlorine atoms relative to chlorine molecules.

D Addition of more chlorine gas to the tube causes an increase in the concentration of chlorine atoms.

Answer 15

C

Question 16

The ester methyl ethanoate is hydrolysed as shown in the following equation.

CH3COOCH3(l) + H2O(l) CH3COOH(l) + CH3OH(l) ΔH = +3 kJ mol−1

The equilibrium yield of ethanoic acid could be increased by

A lowering the temperature.

B adding a catalyst.

C adding more water to the reaction mixture.

D adding more methanol to the reaction mixture.

Answer 16

C