Reversible Reactions & Equilibrium

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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).

Hydrated copper(II) sulfate (blue) heated to anhydrous copper(II) sulfate (white); adding water reverses the change

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.

Le Chatelier's Principle: effect of conditions

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 production of ammonia by the Haber process

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.

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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?
Easy
- 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?
Easy
- AEndothermic, blue to white
- BExothermic, white to blue
- CEndothermic, white to blue
- DExothermic, blue to white

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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?
Medium
- AThe mixture becomes darker brown
- BThe mixture becomes yellow
- CNo colour change
- DThe mixture becomes colourless
10.In the Haber process (450 °C, 200 atm), if the pressure is increased to 400 atm while keeping the temperature constant, what happens to the percentage of ammonia in the equilibrium mixture?
Medium
- 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:
Easy
- 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?
Medium
- ABecomes more brown
- BBecomes colourless
- CNo change
- DBecomes yellow
13.In the Haber process, the forward reaction is exothermic. The graph below shows the percentage yield of ammonia against pressure, plotted for five different reaction temperatures. At any given pressure, the lower-temperature curves give a higher yield than the higher-temperature curves. Which statement best explains this?
Hard
- 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.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?
Medium
- ALow temperature and high pressure
- BHigh temperature and low pressure
- CLow temperature and low pressure
- DHigh temperature and high pressure
15.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.
16.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.
17.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.
18.A student adds anhydrous cobalt(II) chloride (blue) to water. Which colour change occurs and what does this indicate?
Easy
- ABlue to pink; water is present.
- BPink to blue; water is present.
- CBlue to white; water is present.
- DWhite to blue; water is present.
19.In the Haber process, the conditions used are 200 atm, 450 °C, and an iron catalyst. If a new catalyst allowed the same yield of ammonia to be achieved at a lower temperature of 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.
20.For the equilibrium: N2(g) + 3H2(g) ⇌ 2NH3(g), what happens to the unreacted nitrogen and hydrogen after the product ammonia is removed?
Easy
- AThey are recycled back into the reactor.
- BThey are released into the atmosphere.
- CThey are burned off.
- DThey are stored for later use.
21.What is the catalyst used in the Haber process?
Easy
- AIron
- BVanadium(V) oxide
- CPlatinum
- DNickel
22.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.
23.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
24.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?
Medium
- 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.
25.In the Haber process, the gases leaving the reactor are cooled. Which substance liquefies and is removed?
Easy
- ANitrogen
- BHydrogen
- CAmmonia
- DIron
26.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?
Medium
- AIncreasing the temperature
- BAdding a catalyst
- CDecreasing the temperature
- DAdding more water vapour
27.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.
28.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?
Medium
- 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.
29.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?
Medium
- AIt will increase
- BIt will decrease
- CIt will remain at 96%
- DIt will first increase then decrease
30.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
31.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?
Medium
- ALow temperature, high pressure
- BLow temperature, low pressure
- CHigh temperature, high pressure
- DHigh temperature, low pressure
32.In the Contact process, sulfur trioxide is absorbed into concentrated sulfuric acid rather than water. What is the reason for this?
Easy
- 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
33.Which symbol is used in chemical equations to indicate a reversible reaction?
Easy
- A⇌
- B→
- C←
- D⟷
34.In a reversible reaction, if the forward reaction is exothermic, then the reverse reaction is endothermic.
Easy
True or false?
35.What is the colour of anhydrous cobalt(II) chloride?
Easy
- ABlue
- BPink
- CWhite
- DGreen
36.Which statement about dynamic equilibrium is correct?
Easy
- AThe forward and reverse reactions have stopped.
- BThe concentrations of reactants and products are equal.
- CThe rates of the forward and reverse reactions are equal.
- DThe system is open to the surroundings.
37.A catalyst changes the position of equilibrium in a reversible reaction.
Easy
True or false?
38.In the Haber process, what is the source of hydrogen?
Easy
- AAir
- BNatural gas (methane)
- CWater
- DAmmonia
39.For the equilibrium 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), the forward reaction is exothermic. Which change would shift the equilibrium to the left?
Medium
- AIncreasing the pressure
- BDecreasing the temperature
- CIncreasing the concentration of SO₂
- DIncreasing the temperature
40.What is the catalyst used in the Contact process for the oxidation of sulfur dioxide to sulfur trioxide?
Easy
- AIron
- BVanadium(V) oxide
- CPlatinum
- DNickel
41.In a closed system at equilibrium, the concentration of reactants and products remains constant.
Easy
True or false?
42.In the Haber process, the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) is exothermic. The conditions used are 200 atm, 450 °C, and an iron catalyst. Which statement best explains why a temperature of 450 °C is used instead of a lower temperature?
Medium
- AA lower temperature would give a lower yield of ammonia.
- BA lower temperature would make the reaction too slow.
- CA lower temperature would require a different catalyst.
- DA lower temperature would increase the pressure too much.
43.In the Contact process, the main reaction occurs at a temperature of 450 °C and a pressure of 2 atm. At this pressure, the equilibrium mixture contains approximately what percentage of sulfur trioxide?
Easy
44.Complete the sentence about reversible reactions.
Easy
If the forward reaction is exothermic, then the reverse reaction is ____.
45.In the Contact process, the main reaction occurs at a temperature of 450 °C and a pressure of 2 atm. At this pressure, the equilibrium mixture contains approximately what percentage of sulfur trioxide?
Easy
- A116 %
- B96 %
- C86 %
- D106 %

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