IGCSE Chemistry Revision Notes
Topic:Chemical reactions
Sub Topic: 6.3 Reversible reactions and equilibrium
Syllabus Objectives:
CORE:
EXTENDED:
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Definition: Some chemical reactions are reversible, indicated by the symbol
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·5H2O(s) --->CuSO4(s) + 5H2O(g)
Adding water to anhydrous compounds rehydrates them, forming the hydrated form.
Examples:
Anhydrous copper(II) sulfate (white) turns blue upon addition of water.
CuSO4(s) + 5H2O(l) ---> CuSO4·5H2O(s)
Anhydrous cobalt(II) chloride (blue) turns pink upon addition of water.
CoCl2(s) + 6H2O(l) ----> CoCl2·6H2O(s)
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.
Increasing temperature favors the endothermic reaction.
Decreasing temperature favors the exothermic reaction.
Increasing pressure favors the side with fewer gas molecules.
Decreasing pressure favors the side with more gas molecules.
Increasing concentration of reactants shifts the equilibrium to the products side.
Increasing concentration of products shifts the equilibrium to the reactants side.
Catalysts speed up both the forward and reverse reactions equally, reaching equilibrium faster but not changing the position of equilibrium.
Symbol Equation: N2(g) + 3H2(g) -> 2NH3(g)
Sources:
Hydrogen: Methane (CH4) from natural gas.
Nitrogen: Air.
Typical Conditions: 450°C, 20000 kPa (200 atm), iron catalyst.
Symbol Equation: 2SO2(g) +O2(g) ->2SO3(g)
Sources:
Sulfur Dioxide: Burning sulfur or roasting sulfide ores.
Oxygen: Air.
Typical Conditions: 450°C, 200 kPa (2 atm), vanadium(V) oxide (V2O5) catalyst.
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.
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.
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