Reversible Reactions & Equilibrium

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Notes

Reversible Reactions

In **reversible reactions**, products can react to reform reactants; the reaction occurs in both directions.
The symbol **⇌** is used to indicate a reversible reaction.
If the forward reaction is **exothermic**, the reverse reaction is **endothermic** (same amount of heat transferred).
Example: N₂ + 3H₂ ⇌ 2NH₃ (forward exothermic, reverse endothermic).
**Hydrated salts** contain water of crystallisation; heating removes water to form **anhydrous salts**.
Copper(II) sulfate: CuSO₄·5H₂O (blue) ⇌ CuSO₄ (white) + 5H₂O; forward endothermic, reverse exothermic.
Cobalt(II) chloride: CoCl₂·6H₂O (pink) ⇌ CoCl₂ (blue) + 6H₂O; forward endothermic, reverse exothermic.
These colour changes are used as **tests for water** (blue to pink for CoCl₂, white to blue for CuSO₄).

Equilibrium (Extended Tier)

A reversible reaction reaches **equilibrium** in a **closed system** (no reactants or products can escape).
At equilibrium: **rate of forward reaction = rate of reverse reaction**; concentrations of reactants and products remain **constant**.
Equilibrium is **dynamic** – molecules continuously interconvert at equal rates.
Example: In the Haber process, initially forward rate is high; as reactants decrease and products increase, rates become equal.

Le Chatelier's Principle (Extended Tier)

**Le Chatelier's Principle**: If a change is made to a system at equilibrium, the system moves to **oppose the change**.
**Temperature**: Increase temperature favours the **endothermic** direction; decrease favours the **exothermic** direction.
**Pressure**: Increase pressure shifts equilibrium to the side with **fewer gas molecules**; decrease shifts to side with more gas molecules.
**Concentration**: Increase reactant concentration shifts equilibrium to the **right** (more products); increase product concentration shifts to the **left**.
**Catalyst** does **not** affect the position of equilibrium – it speeds up both forward and reverse reactions equally, reaching equilibrium faster.

The Haber Process (Extended Tier)

Manufactures **ammonia** via N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (forward exothermic).
**Raw materials**: Nitrogen from **fractional distillation of liquid air**; Hydrogen from **natural gas (methane)**.
**Conditions**: **450 °C**, **200 atm**, **iron catalyst**.
**Compromise conditions**: Lower temperature favours yield but slows rate; higher pressure favours yield but is expensive and dangerous.
**Catalyst** (iron) allows equilibrium to be reached faster, enabling a lower operating temperature.
**Process stages**: Compress gases → react over catalyst → cool to liquefy ammonia → recycle unreacted N₂ and H₂.
**Yield increases** with increasing pressure and decreasing temperature (see data tables).

The Contact Process (Extended Tier)

Manufactures **sulfuric acid** via main stage: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) (forward exothermic).
**Catalyst**: **Vanadium(V) oxide (V₂O₅)**.
**Conditions**: **450 °C**, **2 atm (200 kPa)**.
**Compromise**: Low temperature favours yield but slows rate; pressure is kept low (≈1 atm) because equilibrium already gives ~96% yield and high pressure would liquefy SO₂.
SO₃ is absorbed into **98% sulfuric acid** to form **oleum (H₂S₂O₇)**, then diluted to give concentrated H₂SO₄.
SO₃ is **not** absorbed directly into water to avoid forming a dangerous mist of sulfuric acid.

Particle arrangement in solids, liquids, and gases. Reversible reactions often involve changes of state (e.g., hydrated salt dehydration).

Practice questions

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1.What does the symbol ⇌ mean in a chemical equation?
Easy
- AThe reaction is reversible
- BThe reaction goes to completion
- CThe reaction is exothermic
- DThe reaction is at equilibrium
2.When anhydrous cobalt(II) chloride is added to water, what colour change is observed?
Easy
- ABlue to pink
- BPink to blue
- CWhite to blue
- DBlue to white
3.Which of the following is a condition needed for a reversible reaction to reach equilibrium?
Easy
- AOpen system
- BClosed system
- CHigh temperature
- DPresence of a catalyst
4.In the Haber process, what is the source of nitrogen?
Easy
- ANatural gas
- BAir
- CWater
- DAmmonia
5.For the reversible reaction N2(g) + 3H2(g) ⇌ 2NH3(g), the forward reaction is exothermic. Which change would increase the yield of ammonia at equilibrium?
Medium
- AIncreasing the temperature
- BDecreasing the pressure
- CAdding a catalyst
- DDecreasing the temperature
6.In the Contact process, the main stage reaction is 2SO2(g) + O2(g) ⇌ 2SO3(g). The forward reaction is exothermic. What is the approximate temperature used for this stage?
Medium
- A200 °C
- B450 °C
- C800 °C
- D1000 °C
7.Which statement is correct about a catalyst in a reversible reaction at equilibrium?
Medium
- AIt increases the yield of products
- BIt speeds up the forward reaction only
- CIt increases the rate of both forward and reverse reactions equally
- DIt shifts the equilibrium to the right
8.Hydrated copper(II) sulfate is heated. Which statement describes this reaction?
Medium
- AEndothermic, blue to white
- BExothermic, white to blue
- CEndothermic, white to blue
- DExothermic, blue to white

More questions

9.Consider the equilibrium: ICl(l) + Cl2(g) ⇌ ICl3(s) (dark brown) (yellow). The forward reaction is exothermic. What is observed when the temperature is increased?
Hard
- AThe mixture becomes darker brown
- BThe mixture becomes yellow
- CNo colour change
- DThe mixture becomes colourless
10.For the Haber process at 450 °C and 200 atm, the percentage of ammonia in the equilibrium mixture is about 15%. If the pressure is increased to 400 atm while keeping temperature constant, what happens to the percentage of ammonia?
Hard
- AIt decreases
- BIt increases
- CIt stays the same
- DIt becomes zero
11.In the Contact process, sulfur trioxide is not absorbed directly into water because:
Hard
- AIt would produce a dangerous mist of sulfuric acid
- BThe reaction is too slow
- CSulfur trioxide is insoluble in water
- DThe temperature would become too high
12.For the reaction 2NO2(g) ⇌ N2O4(g) (brown) (colourless), what colour change occurs if the pressure is increased?
Hard
- ABecomes more brown
- BBecomes colourless
- CNo change
- DBecomes yellow
13.In the Haber process, the forward reaction is exothermic. The graph below shows how the percentage of ammonia in the equilibrium mixture varies with temperature at a fixed pressure. Which statement best explains the shape of the graph? [Diagram: a graph with temperature on x-axis and % ammonia on y-axis, a curve decreasing as temperature increases]
Medium
- AAs temperature increases, the equilibrium shifts to the left because the forward reaction is exothermic.
- BAs temperature increases, the equilibrium shifts to the right because the forward reaction is exothermic.
- CAs temperature increases, the equilibrium shifts to the left because the forward reaction is endothermic.
- DAs temperature increases, the equilibrium shifts to the right because the forward reaction is endothermic.
14.The table shows the percentage of ammonia in the equilibrium mixture at different temperatures and pressures for the Haber process. Pressure / atm | % ammonia at 300 °C | % ammonia at 500 °C 100 | 45 | 9 200 | 65 | 18 300 | 72 | 25 400 | 78 | 31 What is the effect of increasing pressure on the percentage yield of ammonia?
Medium
- AIncreasing pressure increases the percentage yield at both temperatures.
- BIncreasing pressure decreases the percentage yield at both temperatures.
- CIncreasing pressure increases the percentage yield at 300 °C but decreases it at 500 °C.
- DIncreasing pressure has no effect on the percentage yield.
15.For the reversible reaction: 2SO2(g) + O2(g) ⇌ 2SO3(g). The forward reaction is exothermic. Which combination of conditions gives the highest equilibrium yield of SO3?
Hard
- ALow temperature and high pressure
- BHigh temperature and low pressure
- CLow temperature and low pressure
- DHigh temperature and high pressure
16.In the Contact process, the main reaction is: 2SO2(g) + O2(g) ⇌ 2SO3(g). The reaction is carried out at a pressure of about 2 atm instead of a much higher pressure. Which is the best reason for using this relatively low pressure?
Medium
- AThe equilibrium already gives a very high yield of SO3 at low pressure, and higher pressure would be dangerous and expensive.
- BHigh pressure favours the reverse reaction, so a low pressure is needed to obtain a good yield.
- CThe catalyst works only at low pressure.
- DHigh pressure would cause the reactants to liquefy.
17.Which statement about a catalyst in a reversible reaction at equilibrium is correct?
Easy
- AIt increases the rate of both the forward and reverse reactions equally.
- BIt increases the rate of only the forward reaction.
- CIt shifts the equilibrium position to the right.
- DIt increases the yield of products at equilibrium.
18.Consider the equilibrium: ICl(l) + Cl2(g) ⇌ ICl3(s) (dark brown) (yellow). The forward reaction is exothermic. The mixture is heated. What is observed and why?
Hard
- AThe mixture becomes darker brown because the equilibrium shifts to the left.
- BThe mixture becomes yellow because the equilibrium shifts to the right.
- CThe mixture becomes darker brown because the equilibrium shifts to the right.
- DThe mixture becomes yellow because the equilibrium shifts to the left.
19.For the reaction: 2NO2(g) ⇌ N2O4(g) (brown) (colourless). What colour change occurs if the pressure is increased?
Medium
- AThe mixture becomes less brown (more colourless) because equilibrium shifts to the right.
- BThe mixture becomes more brown because equilibrium shifts to the right.
- CThe mixture becomes less brown because equilibrium shifts to the left.
- DThere is no colour change because pressure does not affect equilibrium.
20.A student adds anhydrous cobalt(II) chloride (blue) to water. Which colour change occurs and what does this indicate?
Medium
- ABlue to pink; water is present.
- BPink to blue; water is present.
- CBlue to white; water is present.
- DWhite to blue; water is present.
21.In the Haber process, the conditions used are 200 atm, 450 °C, and an iron catalyst. The percentage of ammonia in the equilibrium mixture under these conditions is about 15%. If a new catalyst allowed the same yield to be achieved at 400 °C, what would be the main advantage?
Hard
- ALower temperature would increase the equilibrium yield of ammonia.
- BLower temperature would decrease the rate of reaction, so more catalyst is needed.
- CLower temperature would reduce energy costs and increase the yield.
- DLower temperature would shift equilibrium to the left, producing more ammonia.
22.For the equilibrium: N2(g) + 3H2(g) ⇌ 2NH3(g), what happens to the unreacted nitrogen and hydrogen after the product ammonia is removed?
Medium
- AThey are recycled back into the reactor.
- BThey are released into the atmosphere.
- CThey are burned off.
- DThey are stored for later use.
23.What is the catalyst used in the Haber process?
Easy
- AIron
- BVanadium(V) oxide
- CPlatinum
- DNickel
24.In an experiment to demonstrate the reversibility of the hydration of cobalt(II) chloride, a student places anhydrous cobalt(II) chloride (blue) in a sealed container with a few drops of water. After some time, the solid turns pink. Which statement explains this observation?
Medium
- AThe forward reaction CoCl2 + 6H2O ⇌ CoCl2·6H2O is exothermic, so equilibrium favours the hydrated salt at room temperature.
- BThe reverse reaction is exothermic, so equilibrium favours the anhydrous salt.
- CThe forward reaction is endothermic, so equilibrium favours the hydrated salt only at high temperature.
- DWater acts as a catalyst, speeding up the forward reaction only.
25.Which of the following is the correct colour change when hydrated copper(II) sulfate is heated?
Easy
- ABlue to white
- BWhite to blue
- CBlue to pink
- DPink to blue
26.Consider the equilibrium: 2SO2(g) + O2(g) ⇌ 2SO3(g). The forward reaction is exothermic. In the Contact process, the main stage is carried out at about 450 °C and 2 atm pressure. Which statement explains why a pressure much higher than 2 atm is not used?
Hard
- AHigher pressure would favour the reverse reaction and decrease the yield of SO3.
- BHigher pressure would cause the SO2 to liquefy, making it difficult to maintain the gaseous equilibrium.
- CThe equilibrium already gives about 96% yield of SO3 at 2 atm, so higher pressure is unnecessary and expensive.
- DHigher pressure would increase the rate of the reverse reaction more than the forward reaction.
27.In the Haber process, the gases leaving the reactor are cooled. Which substance liquefies and is removed?
Medium
- ANitrogen
- BHydrogen
- CAmmonia
- DIron
28.A student investigates the equilibrium: CoCl2·6H2O(s) ⇌ CoCl2(s) + 6H2O(g). The forward reaction is endothermic. In a closed system at equilibrium, the colour is a mixture of pink and blue. Which change would cause the mixture to become more pink?
Hard
- AIncreasing the temperature
- BAdding a catalyst
- CDecreasing the temperature
- DAdding more water vapour
29.In the Haber process, nitrogen is obtained from which source?
Medium
- ANatural gas
- BThe air
- CWater
- DAmmonia
30.A reversible reaction is at equilibrium in a closed system. Which statement is always true?
Easy
- AThe concentrations of reactants and products are equal.
- BThe forward reaction has stopped.
- CThe rate of the forward reaction equals the rate of the reverse reaction.
- DThe reaction has finished.
31.For the equilibrium: N2(g) + 3H2(g) ⇌ 2NH3(g), the forward reaction is exothermic. The table shows the percentage of ammonia in the equilibrium mixture at different temperatures and pressures. Which combination gives the highest yield of ammonia?
Hard
- A100 atm, 500 °C
- B200 atm, 300 °C
- C300 atm, 500 °C
- D400 atm, 300 °C
32.In an experiment to determine the effect of concentration on equilibrium, a student adds a few drops of concentrated hydrochloric acid to a green solution containing [Co(H2O)6]2+ ions and Cl- ions. The equilibrium is: [Co(H2O)6]2+(aq) + 4Cl-(aq) ⇌ [CoCl4]2-(aq) + 6H2O(l). The solution turns blue. Which statement explains this?
Hard
- AIncreasing Cl- concentration shifts equilibrium to the left, forming more [Co(H2O)6]2+.
- BIncreasing Cl- concentration shifts equilibrium to the right, forming more [CoCl4]2-.
- CHCl acts as a catalyst, speeding up the forward reaction.
- DWater is removed, shifting equilibrium to the left.
33.In the Contact process, sulfur trioxide is absorbed into concentrated sulfuric acid rather than water because:
Easy
- ASulfur trioxide does not react with water.
- BAbsorbing into water produces a dangerous mist of sulfuric acid.
- CWater would dilute the product too much.
- DConcentrated sulfuric acid acts as a catalyst.
34.Which diagram correctly shows the relative amounts of reactants and products at equilibrium for the reaction N2(g) + 3H2(g) ⇌ 2NH3(g) under typical Haber process conditions (200 atm, 450 °C)? (Assume the initial mixture contained only N2 and H2 in a 1:3 ratio.)
Medium
- AA bar chart with N2 and H2 bars twice as high as NH3 bar.
- BA bar chart with NH3 bar about 15% of the total height, and N2 and H2 bars occupying the rest.
- CA bar chart with equal heights for all three gases.
- DA bar chart with NH3 bar higher than N2 and H2 bars.
35.For the Haber process at 450 °C and 200 atm, the percentage of ammonia in the equilibrium mixture is about 15%. If the pressure is increased to 400 atm while keeping temperature constant, what will happen to the percentage of ammonia?
Hard
- AIncrease
- BDecrease
- CStay the same
- DBecome zero
36.In the Haber process, the forward reaction is exothermic. The graph shows how the percentage of ammonia in the equilibrium mixture varies with temperature at two different pressures. At 300 °C and 200 atm, the percentage of ammonia is approximately 65%. If the temperature is increased to 500 °C while keeping the pressure at 200 atm, what is the approximate percentage of ammonia?
Medium
- A18%
- B45%
- C65%
- D72%
37.For the equilibrium: 2SO2(g) + O2(g) ⇌ 2SO3(g) (forward reaction exothermic), the equilibrium mixture at a certain temperature contains 96% sulfur trioxide. If the pressure is increased, what will happen to the percentage of sulfur trioxide?
Hard
- AIt will increase
- BIt will decrease
- CIt will remain at 96%
- DIt will first increase then decrease
38.In an experiment, a student adds a few drops of concentrated hydrochloric acid to a green solution containing [CoCl4]2- (blue) and [Co(H2O)6]2+ (pink) at equilibrium. The solution turns blue. What does this indicate about the effect of chloride ions on the equilibrium?
Medium
- AThe equilibrium shifts to the left
- BThe equilibrium shifts to the right
- CThe equilibrium is unaffected
- DThe catalyst is consumed
39.For the reversible reaction: CH4(g) + H2O(g) ⇌ 3H2(g) + CO(g) (forward reaction endothermic), which combination of temperature and pressure gives the highest yield of hydrogen?
Hard
- ALow temperature, high pressure
- BLow temperature, low pressure
- CHigh temperature, high pressure
- DHigh temperature, low pressure
40.In the Contact process, sulfur trioxide is absorbed into concentrated sulfuric acid rather than water. What is the reason for this?
Medium
- ATo prevent the formation of a dangerous mist of sulfuric acid
- BTo increase the rate of reaction
- CTo shift the equilibrium to the right
- DTo reduce the temperature required

Slides

Reversible Reactions

In reversible reactions, products can react to form reactants again.
The forward and reverse reactions occur simultaneously.
If the forward reaction is exothermic, the reverse is endothermic.
The symbol ⇌ is used to show reversibility.

Which symbol represents a reversible reaction?

Hydrated and Anhydrous Salts

Hydrated salts contain water of crystallisation (e.g., CuSO₄·5H₂O, blue).
Heating removes water to form anhydrous salt (e.g., CuSO₄, white).
Adding water reverses the change (exothermic).
Cobalt(II) chloride: CoCl₂·6H₂O (pink) ⇌ CoCl₂ (blue) + 6H₂O.

Anhydrous cobalt(II) chloride is ____ and turns ____ when water is added.

Dynamic Equilibrium

Equilibrium occurs in a closed system where nothing can enter or leave.
At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction.
Concentrations of reactants and products remain constant.
Equilibrium is dynamic: reactions continue at equal rates.

Which of these are true about dynamic equilibrium?

The forward and reverse reactions stop.
The rates of forward and reverse reactions are equal.
Concentrations of reactants and products are constant.
Equilibrium can be reached in an open system.

Le Chatelier's Principle

If a change is made to a system at equilibrium, the system opposes the change.
Changes include temperature, pressure, and concentration.
The 'position of equilibrium' shifts to favour either forward or reverse reaction.

According to Le Chatelier's principle, increasing the temperature of an equilibrium system will favour the reaction that is:

exothermic
endothermic
neither
both

Effect of Pressure on Equilibrium

Only affects reactions with gases.
Increasing pressure shifts equilibrium to the side with fewer gas molecules.
Decreasing pressure shifts to the side with more gas molecules.

For the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), increasing pressure shifts equilibrium to the ____ because the ____ side has fewer gas molecules.

Effect of Catalysts

A catalyst speeds up both forward and reverse reactions equally.
It does NOT change the position of equilibrium.
It helps the system reach equilibrium faster.

Which statements about catalysts in equilibrium are true?

A catalyst increases the yield of products.
A catalyst speeds up the attainment of equilibrium.
A catalyst lowers the activation energy for both reactions.

The Haber Process

Manufactures ammonia from nitrogen and hydrogen.
Equation: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (exothermic forward).
Conditions: 200 atm, 450°C, iron catalyst.
Nitrogen from air, hydrogen from natural gas (methane).

What is the catalyst used in the Haber process?

Vanadium(V) oxide
Iron
Nickel
Platinum

Compromise Conditions in the Haber Process

High pressure (200 atm) favours product yield but is expensive and dangerous.
Low temperature favours yield but slows the reaction.
450°C is a compromise: reasonable yield at a fast enough rate.
Iron catalyst allows lower temperature by reducing activation energy.

The temperature of 450°C in the Haber process is a ____ between yield and ____.

The Contact Process

Manufactures sulfuric acid.
Main stage: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) (exothermic forward).
Conditions: 450°C, 2 atm, vanadium(V) oxide catalyst.
SO₃ is absorbed into 98% H₂SO₄ to form oleum, then diluted.

What catalyst is used in the Contact process?

Iron
Vanadium(V) oxide
Nickel
Manganese dioxide

Conditions in the Contact Process

2 atm pressure: high yield (96% SO₃) without need for extreme pressure.
450°C: compromise between yield and rate.
Higher pressure would liquefy SO₂ and increase costs.

Which are true about the Contact process conditions?

The pressure used is 200 atm.
The temperature is 450°C.
The catalyst is vanadium(V) oxide.
The forward reaction is endothermic.

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