Chemical Equilibrium

A chemical reaction that does not go to completion is said to be in a state of

dynamic equilibrium
static equilibrium
neither of the other options

Which one of the following reactions is most unlikely to reach completion?

The reaction between nitrogen gas and hydrogen gas.
The burning of magnesium in oxygen gas.
A neutralisation reaction between hydrochloric acid and sodium hydroxide.
Sodium metal reacting with water.

When a chemical reaction has reached a dynamic state

the reactants are forming products while the products are forming reactants.
the reactants are no longer reacting.
the products are forming faster than they are being converted back into reactants.
there are no longer any reactants left.

Ammonia is important in the manufacture of fertilizers. It is made by reacting nitrogen and hydrogen. The process for this reaction was developed by

Fritz Haber
Justus Von Liebig
Wilhelm Bunsen
Erwin Schrödinger

A chemical reaction has reached dynamic equilibrium at a certain temperature.
Which one of the following statements is incorrect?

The reaction has stopped completely.
The rate of the forward reaction is equal to the rate of the reverse reaction.
Products are continuously being formed.
The concentrations of the reactants remains constant.

The diagram shows the concentrations of hydrogen, iodine and hydrogen iodide for the reaction between hydrogen and iodine.
Which one of the following statements is INCORRECT?

At point A on the time axis, the concentration of all three gases is zero.
The reaction between the gases reaches equilibrium at point B.
At point C, the system is in a state of dynamic equilibrium.
Adding more hydrogen at point D will alter the shape of the graph.

Le Châtelier's Principle states that if a _____________ is applied to at system at ____________, the system readjusts to minimise the stress causing it.
From the list below, select the respective words that are missing.

stress, equilibrium
stress, rest
pressure, equilibrium
pressure, rest

The reaction of nitrogen dioxide (NO₂) forming dinitrogen tetroxide (N₂O₄) is exothermic in the forward direction.
What two conditions will cause the equilibrium mixture to go dark brown?

Increase in temperature
Decrease in pressure
Increase in pressure
Adding more catalyst

When heat is applied to a system at equilibrium

the system absorbs the heat.
the system releases the heat.
more product(s) is/are formed.
the temperature of the system increases.

In the reaction
N₂ + 3H₂ ⇋ 2NH₃ ΔH = -92.4 kJ/mol
Select three ways of increasing the yield of ammonia (NH₃)

Add more hydrogen (H₂)
Lower the temperature
Remove the ammonia (NH₃) as it forms
Remove the nitrogen (N₂) as it forms
Increase the temperature
Add a catalyst
Lower the pressure

The reaction between hydrogen (H₂) and nitrogen (N₂) is described by the equation
N₂ + 3H₂ ⇋ 2NH₃ ΔH = -92.4 kJ/mol
What will be the effect of increasing the concentration of ammonia (NH₃)?

More hydrogen (H₂) is formed
More ammonia (NH₃) is formed
Less nitrogen (N₂) is formed
The temperature increases

Which one of the following equilibrium reactions is not affected by a change in pressure?

H_2(g) + I_2(g) ⇋ 2HI_(g)
PCl_3(g) + Cl_2(g) ⇋ PCI_5(g)
2NO_2(g) ⇋ N₂O_4(g)
N_2(g) + 3H_2(g) ⇋ 2NH_3(g)

The reaction
H_2(g) + I_2(g) ⇋ 2HI_(g) ΔH = +53 kJ/mol .
is not affected by a change in pressure.
Why is this?

There are equal numbers of molecules on both sides of the equation.
They are all in the gaseous state.
Because it is an endothermic reaction.
It is a catalysed reaction.

Consider the equilibrium reaction
C₂H₅OH + CH₃COOH ⇋ CH₃COOC₂H₅ + H₂O
What effect will adding a catalyst have on this reaction?

The system reaches equilibrium faster.
More CH₃COOC₂H₅ is formed.
More C₂H₅OH is formed.
Less H₂O is formed.

When cobolt (II) chloride is added to water, the following equilibrium is set up

with the red colour predominant.
On adding dilute HCl, the colour changes to blue.
Why does this happen?

Addition of chloride ions shifts the equilibrium to the left.
The equilibrium shifted to the right.
Because water changes the colour of the system to blue.
More Cl^- ions were formed.

The following equilibrium mixture is a purple colour

According to Le Châtelier's Principle, what will happen when water is added to the mixture?

It will go red as more Co(H₂O)₆²⁺ is formed.
It stays a violet colour as the water is neutral.
It will turn blue as CoCl₄^2- is being formed.
The water causes more Cl^- ions to form and these ions cause a change in the colour to red.

Consider the following equilibrium reaction

Which one of the following will cause a yellow colour to predominate?

Addition of sodium hydroxide (NaOH)
Addition of hydrochloric acid (HCl)
Removal of water
Addition of sodium chromate (Na₂CrO₄)

For the equilibrium reaction

dilute HCl is added to the mixture.
Select two statements to describe what is happening.

The mixture goes orange in colour.
The equilibrium shifts to the left.
The mixture goes yellow in colour.
More CrO₄^2- is formed.

Solutions of iron (III) chloride and potassium thiocyanate are mixed together and the following equilibrium mixture is set up:

The resulting mixture is red.
Which one of the following reagents must be added to change the colour of the mixture to yellow?

Dilute hydrochloric acid
Water
Iron (III) chloride solution (FeCl₃)
Potassium cyanide (KCNS)

What colour change will occur if concentrated sulfuric acid (H₂SO₄) is added to the following equilibrium mixture?

Yellow to orange
Orange to yellow
Orange to green
Green to orange

The forward reaction of the equilibrium system below is endothermic. Dilute HCl is added and the colour changes to blue.

What colour change occurs when the mixture is cooled?

Blue to pink
Pink to blue
No colour change

Henri Le Châtelier studied equilibrium reactions in industry in the late 19th century.
According to Le Châtelier’s principle, what effect would an increase in pressure have on the yield of ammonia at equilibrium?

There would be a greater yield of ammonia.
The yield of ammonia would be less.
It would have no effect on the yield of ammonia.
More hydrogen and nitrogen are formed.

Ammonia is formed in the Haber process according to the following balanced equation
N₂ + 3H₂ ⇋ 2NH₃ ΔH = -92.4 kJ/mol
The table shows the percentages of ammonia present at equilibrium under different conditions of temperature T and pressure P when hydrogen and nitrogen gases were mixed in a 3:1 molar ratio.
From the table, what conditions of temperature and pressure will give the highest yield of ammonia.

1000 atm and 573 K
1000 atm and 773 K
10 atm and 573 K
100 atm and 573 K

Ammonia is formed in the Haber process according to the following balanced equation:
N_2(g) + 3H_2(g) ⇋ 2NH_3(g) ΔH = -92.4 kJ/mol
The table shows the percentages of ammonia present at equilibrium under different conditions of temperature T and pressure P when hydrogen and nitrogen gases were mixed in a 3:1 molar ratio.
Is this an endothermic or exothermic reaction? Select the correct reason for your answer.

It is exothermic because as the temperature decreases, the yield increases. Lowering the temperature favours an exothermic reaction.
It is exothermic because as the pressure increases, the yield increases. Increasing the pressure favours an exothermic reaction.
It is endothermic because as the temperature decreases, the yield increases. Lowering the temperature favours an endothermic reaction.
It is exothermic because as the pressure increases, the yield increases. Increasing the pressure favours an exothermic reaction.

The equilibrium constant for the Haber Process is

If a catalyst is used in this process, what effect will it have on the value of K_c?

A catalyst will have no effect.
A catalyst will change equilibrium to the left.
A catalyst will change equilibrium to the right.

The equation
N_2(g) + 3H_2(g) ⇋ 2NH_3(g) ΔH = -92.4 kJ/mol
describes the Haber Process. According to Le Châtelier's Principle, a high yield of ammonia is favoured at a high pressure and low temperature.
However this is not practical.
Select two reasons from the list below why this is so.

The materials required to build a high pressure plant would be expensive.
At low temperatures, reactions are slower because less energy is available.
At low temperatures, other products besides ammonia are formed.
At high pressures, the particles of the two reacting gases are too far apart to react.

The Contact Process for making sulfuric acid involves two steps:
2SO_2(g) + O_2(g) ⇋ 2SO_3(g) ΔH = -196 kJ/mol
SO_3(g) + H₂O ⇋ H₂SO_4(l)
In the first stage, Le Châtelier's Principle suggests that high pressure and low temperature would produce more SO₃.
But this is not the case. Select two reasons why this is so.

Building high pressure plants is uneconomical.
Low temperatures would lower the rate of production.
High pressure would liquefy the oxygen.
It is more costly at low temperaturtes.

The reaction between SO₂ and O₂ may be described as
2SO_2(g) + O_2(g) ⇋ 2SO_3(g) ΔH = -196 kJ/mol
Select the two conditions that will favour a high yield of SO₃

Addition of more O₂
Lower the temperature
Lower the pressure
Removal of SO₂
Using a catalyst

For the reaction
H_2(g) + I_2(g) ⇋ 2HI_(g)
what is its correct equilibrium constant?

Consider the general equilibrium reaction
pA + qB ⇋ rC + sD
Which one of the following represents the correct equilibrium constant for this reaction?

The equilibrium constant for the reaction
2HI_(g) ⇋ H_2(g) + I_2(g)
is:

The Haber Process may be described by the equation
N_2(g) + 3H_2(g) ⇋ 2NH_3(g)
The equilibrium constant for this process is

If K_c = 2.5 for the reaction
N_2(g) + 3H_2(g) ⇋ 2NH_3(g)
then

the equilibrium lies predominantly to the right.
the equilibrium lies predominantly to the left.
there is 2.5 times as much ammonia as nitrogen.
the reaction has gone to completion.

Which one of the following statements is correct?

When K_c is very large, there are more products formed.
K_c does not depend on temperature.
K_c never has units.
The value of K_c gives us the rate of reaction.

For the reaction
H_2(g) + I_2(g) ⇋ 2HI_(g)
at equilibrium, in a 1 litre container, the amount of H₂ is 1 mole, the amount of I₂ is 2 moles & the amount of HI is 2 moles.
What is K_c for this reaction?

2
1
1.33
0.5

Three moles of ethanol and three moles of ethanoic acid were reacted together according to the equation
C₂H₅OH + CH₃COOH ⇋ CH₃COOC₂H₅ + H₂O
At equilibrium, there were 2 moles each of ethyl ethanoate (CH₃COOC₂H₅) and water formed.
What is the equilibrium constant for this reaction?

4
2
0.25
0.44

0.5 mole of PCl₅ was placed in a 4 litre sealed container. The PCl₅ decomposed according to the equation
PCl_3(g) + Cl_2(g) ⇋ PCI_5(g)
When the reaction had reached equilibrium, there were 0.2 mole of PCl₃ and Cl₂ present in the container.
Which one of the following is the correct dissociation constant for this reaction?

0.033
7.5
0.133
33

For the reaction
H_2(g) + I_2(g) ⇋ 2HI_(g)
the equilibrium constant, K_c is 49 at a fixed temperature.
Two moles of hydrogen and two moles of iodine are allowed to reach equilibrium at this temperature.
What is the concentration of hydrogen iodide at equilibrium?

1.55
9.2
0.77
1.75

Which change will increase the equilibrium concentration of sulfur trioxide in this reaction?
2SO_2(g) + O_2(g) ⇌ 2SO_3(g) 2SO_2(g)H = negative

Increasing the pressure
Decreasing the concentration of oxygen
Using a catalyst
Increasing the temperature

The equation below describes an equilibrium reaction occurring in a closed system.
A_(g) + B_(g) ⇌ 4C_{(g) ΔH = +58 kJ}
Under which set of the following conditions would the highest yield of C_(g) be obtained?

Temperature = 300^oC, Pressure = 100 kPa
Temperature = 50^oC, Pressure = 100 kPa
Temperature = 50^oC, Pressure = 200 kPa
Temperature = 300^oC, Pressure = 200 kPa

Consider the following equilibrium reaction in a closed container at 50 °C.
SO_2(g) + Cl_2(g) ⇌ SO₂Cl_2(g), ΔH = -85 kJ
Which of the following statements is correct?

Decreasing the temperature will increase the amount of SO₂Cl_2(g)
Increasing the volume of the container will increase the amount of SO₂Cl_2(g)
Increasing the temperature will increase the amount of SO₂Cl_2(g)
Adding a catalyst will increase the amount of SO₂Cl_2(g)

A 1.0 litre reaction vessel initially contains 6.0 mol of P and 6.0 mol of Q.
At equilibrium 4.0 mol of R is present.
What is the value of Kc for the following reaction?
P_(g)+ Q_(g) ⇌ R_(g)+ S_(g)

4.00
0.44
0.11
0.25

The equation for the Haber process is:
N_2(g) + H_{2 → 2NH_3(g) ΔH = − 92.2kJ}
Which conditions will favour the production of the greatest amount of ammonia at equilibrium?

Low temperature and high pressure
High temperature and high pressure
High temperature and low pressure
Low temperature and low pressure

Which changes cause an increase in the equilibrium yield of SO (g) in this reaction? 2SO_2(g) + O_2(g) ⇌ 2SO_3(g), ΔH = −196 kJ 1. Increasing the pressure 2. Decreasing the temperature 3. Adding oxygen

1, 2 and 3
1 and 2 only
2 and 3 only
1 and 3 only

The equation for a reversible reaction used in industry to convert methane to hydrogen is shown
below. CH_4(g) + H₂O_(g) → CO_(g) + 3H_2(g), ΔH = +210 kJ
Which statement is always correct about this reaction when equilibrium has been reached?

The value of ΔH for the reverse reaction is −210 kJ .
The concentrations of methane and carbon monoxide are equal.
The rate of the forward reaction is greater than the rate of the reverse reaction.
The amount of hydrogen is three times the amount of methane.

The expression for the equilibrium constant for a reaction is

At a certain temperature the values of [A], [B] and [C] are all 0.2 mol dm⁻¹.
What happens to the value of Kc when all three values are doubled to 0.4 mol dm⁻¹?

There is no change
It is halved.
It doubles.
It increases by a factor of four.

For the reaction below
H_2(g) + I_2(g) ⇌ 2HI_(g)
at a certain temperature, the equilibrium concentrations are (in mol dm⁻³)
[H₂] = 0.30, [I₂] = 0.30, [HI] = 3.0
What is the value of K?

100
5.0
15
10

The value of the equilibrium constant for the reaction2HI_(g) ⇌ H_2(g) + I_2(g)
is 0.25 at 440 °C .
What would the value of the equilibrium constant be for the following reaction at the same temperature?H_2(g) + I_2(g) ⇌ 2HI_(g)

4.0
0.5
0.25
2.0

A few drops of concentrated sulphuric acid were added to a mixture of 0·1 mol of methanol and 0·2 mol of ethanoic acid. Even after a considerable time, the reaction mixture was found to contain some of each reactant.
Which one of the following is the best explanation for the incomplete reaction?

An equilibrium mixture was formed.
The temperature was too low.
Insufficient methanol was used.
Insufficient ethanoic acid was used.

Which of the following applies correctly to the use of a catalyst in a chemical reaction?

No change in equilibrium, no change in ΔH
No change in equilibrium, increase in ΔH value
Equilibrium shifts to right, decrease in ΔH value
Equilibrium shifts to left, no change in ΔH

What will happen to the position of equilibrium and the value of the equilibrium constant when the temperature is increased in the following reaction:
Br_2(g) + Cl_2(g) ⇌ 2BrCl_(g), ΔH = +14 kJ

Equilibrium shifts towards the products and the equilibrium constant increases
Equilibrium shifts towards the reactants and the equilibrium constant decreases
Equilibrium shifts towards the reactants and the equilibrium constant increases
Equilibrium shifts towards the products and the equilibrium constant decreases

Carbonic acid is formed by dissolving carbon dioxide in water.
This equilibrium is represented by the following equation.
CO₂ + H₂O ⇌ H₂CO₃
If carbon dioxide is removed from this equilibrium mixture, the pH will

increase
decrease
remain approximately constant because the concentration of the hydrogencarbonate changes to compensate.
remain approximately constant because the concentration of the carbonic acid
changes to compensate.

Chemical Equilibrium

Multiple Choice Quiz