IGCSE Reversible reactions and equilibrium

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Syllabus Objectives

IGCSE Chemistry Revision Notes

Topic:Chemical reactions

Sub Topic: 6.3 Reversible reactions and equilibrium

Syllabus Objectives:

CORE:

State that some chemical reactions are reversible as shown by the symbol .
Describe how changing the conditions can change the direction of a reversible reaction for:
(a) the effect of heat on hydrated compounds
(b) the addition of water to anhydrous compounds limited to copper(II) sulfate and cobalt(II) chloride

EXTENDED:

State that a reversible reaction in a closed system is at equilibrium when:
(a) the rate of the forward reaction is equal to the rate of the reverse reaction
(b) the concentrations of reactants and products are no longer changing
Predict and explain, for a reversible reaction, how the position of equilibrium is affected by:
(a) changing temperature
(b) changing pressure
(c) changing concentration
(d) using a catalyst using information provided
State the symbol equation for the production of ammonia in the Haber process, N₂(g) + 3H₂(g) -> 2NH₃(g)
State the sources of the hydrogen (methane) and nitrogen (air) in the Haber process
State the typical conditions in the Haber process as 450°C, 20000kPa /200atm and an iron catalyst
State the symbol equation for the conversion of sulfur dioxide to sulfur trioxide in the Contact process, 2SO2(g) + O2(g) -> 2SO3(g)
State the sources of the sulfur dioxide (burning sulfur or roasting sulfide ores) and oxygen (air) in the Contact process
State the typical conditions for the conversion of sulfur dioxide to sulfur trioxide in the Contact process as 450°C, 200kPa /2atm and a vanadium(V) oxide catalyst
Explain, in terms of rate of reaction and position of equilibrium, why the typical conditions stated are used in the Haber process and in the Contact process, including safety considerations and economics

Revision Notes

Free IGCSE Chemistry Notes Reversible reactions and equilibrium

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Free IGCSE Chemistry Summary Notes Reversible Reactions and Equilibrium

Reversible Reactions:
- Definition: Some chemical reactions are reversible, indicated by the symbol ?.
Changing Conditions and Reversible Reactions:
- Effect of Heat on Hydrated Compounds:
  - Heating hydrated compounds can remove water, forming anhydrous compounds.
  - Example: Heating hydrated copper(II) sulfate (blue) forms anhydrous copper(II) sulfate (white) and water.
    - CuSO?·5H?O(s) ? CuSO?(s) + 5H?O(g)
- Addition of Water to Anhydrous Compounds:
  - Adding water to anhydrous compounds rehydrates them, forming the hydrated form.
  - Examples:
    - Copper(II) sulfate:
      - Anhydrous copper(II) sulfate (white) turns blue upon addition of water.
      - CuSO?(s) + 5H?O(l) ? CuSO?·5H?O(s)
    - Cobalt(II) chloride:
      - Anhydrous cobalt(II) chloride (blue) turns pink upon addition of water.
      - CoCl?(s) + 6H?O(l) ? CoCl?·6H?O(s)

Extended Objectives

Equilibrium in a Closed System:
- Definition: A reversible reaction in a closed system is at equilibrium when:
  - (a) The rate of the forward reaction is equal to the rate of the reverse reaction.
  - (b) The concentrations of reactants and products are no longer changing.
Factors Affecting the Position of Equilibrium:
- Changing Temperature:
  - Increasing temperature favors the endothermic reaction.
  - Decreasing temperature favors the exothermic reaction.
- Changing Pressure:
  - Increasing pressure favors the side with fewer gas molecules.
  - Decreasing pressure favors the side with more gas molecules.
- Changing Concentration:
  - Increasing concentration of reactants shifts the equilibrium to the products side.
  - Increasing concentration of products shifts the equilibrium to the reactants side.
- Using a Catalyst:
  - Catalysts speed up both the forward and reverse reactions equally, reaching equilibrium faster but not changing the position of equilibrium.
Haber Process:
- Symbol Equation: N?(g) + 3H?(g) ? 2NH?(g)
- Sources:
  - Hydrogen: Methane (CH?) from natural gas.
  - Nitrogen: Air.
- Typical Conditions: 450°C, 20000 kPa (200 atm), iron catalyst.
Contact Process:
- Symbol Equation: 2SO?(g) + O?(g) ? 2SO?(g)
- Sources:
  - Sulfur Dioxide: Burning sulfur or roasting sulfide ores.
  - Oxygen: Air.
- Typical Conditions: 450°C, 200 kPa (2 atm), vanadium(V) oxide (V?O?) catalyst.
Explanation of Typical Conditions:
- Haber Process:
  - Rate of Reaction: High temperature increases reaction rate, but too high temperatures reduce ammonia yield due to favoring the reverse reaction. A compromise temperature of 450°C is used.
  - Position of Equilibrium: High pressure favors ammonia production but is costly and requires strong equipment.
  - Catalyst: Iron catalyst speeds up the reaction, reducing time and energy costs.
  - Safety and Economics: Moderate temperature and pressure balance yield, safety, and cost.
- Contact Process:
  - Rate of Reaction: High temperature increases the reaction rate, but too high temperatures favor the reverse reaction.
  - Position of Equilibrium: Moderate pressure (2 atm) and temperature (450°C) maximize sulfur trioxide yield.
  - Catalyst: Vanadium(V) oxide catalyst increases reaction rate without being consumed.
  - Safety and Economics: Conditions are chosen to maximize yield while considering safety and cost-effectiveness.