C5.3 Equilibria Flashcards
Reversible reactions
Products can react together to form original reactants
Example:
Reaction between copper(Il) sulfate and water is a reversible reaction
Blue crystals of copper(Il) sulfate are hydrated copper(lI) sulfate, CuSO4.5H2O
Each Cu2+ ion is surrounded by 5 water molecules
If you remove these water molecules by heating, you get white anhydrous copper(I) sulfate, CuSO4
Hydrated copper(Il) ⇌ Sulfate anhydrous copper(II) sulfate + water
⇌ symbol for reversible reactions
Used in equations for reversible reactions instead of an arrow
Combines the forward reaction and backward reaction:
Forward reaction: CuSO4.5H2O -> CuSO4 + 5H2O
Backward reaction: CuSO4 + 5H2O -> CuSO4.5H2O
Changing the conditions of a reversible reaction
Blue hydrated copper(II) sulfate decomposes when heated to form white anhydrous copper(Il) sulfate and water
Endothermic reaction because energy is transferred from surroundings to break bonds in reactants
The backward reaction is the opposite:
When water is added, white anhydrous copper(Il) sulfate becomes blue hydrated copper(II) sulfate
Energy is transferred to the surroundings (it is an exothermic reaction)
Enough energy is transferred by heating to produce steam in the reaction
Water of crystallisation
Hydrated copper(II) sulfate, CuSO4.5H2O is more accurately called copper(Il) sulfate-5-water
The water molecules in the ionic lattice are the “water of crystallisation”
Dynamic equilibrium
In a closed system no substances can enter or leave the reaction mixture
Example closed systems:
Stoppered flask is a closed system
Beaker of solution in which all the reacting substances remain in the solvent
Rate of the forward reaction = rate of backward reaction
This situation is called equilibrium:
Forward and backward reactions still happen, so it is a dynamic equilibrium
Concentrations of all the reacting substances remain constant
Concentrations of reacting substances are constant at equilibrium - they do not change, but are not necessarily equal
Equilibrium position
Gives you an idea of the ratio of equilibrium concentrations or procucts to reactants
Equilibrium position shifts right when concentration of reactants > concentration of products
Equilibrium position shifts left when the concentration of reactants < concentration of products
Equilibrium position may change if there are changes in conditions and this affects concentrations of reacting substances
What happens if you change the pressure of a reversible reaction
If pressure increased, equilibrium position moves in direction of the fewest moles of gas:
Example:
Explain the effect on the equilibrium position of increasing the pressure in this reaction:
2SO2(g) + O2(g) ⇌ 2SO3(g)
1) Count the total number of moles of gas on each side of the equation, depending on balancing constants
On the left: 2 + 1 = 3 mol
On the right: 2 mol
2) Determine which side has the fewest moles of gas
There are fewer moles of gas on right so equilibrium position moves to right if the pressure is increased
What happens if you change the concentration of a reversible reaction
If concentration of a substance is increased, equilibrium position moves in the direction away from that substance
Example:
Predict the effect on the equilibrium position of increasing the concentration of sulfuric acid, H2SO(aq), in this reaction:
2K2CrO4(aq) + H2SO4(aq) ⇌ K2Cr2O2(aq) + H20(l) + K2SO4(aq)
1) Determine which side of the equation contains sulfuric acid - sulfuric acid is on the left
2) The equilibrium position moves to the right if the concentration of sulfuric acid is increased
What happens if you change temperature of a reversible reaction
If the temperature is increased equilibrium position moves in direction of endothermic change to take in heat energy to react:
Temperature increases = equation favours the endothermic side - takes in heat energy
Temperature decreases = equation favours the exothermic side - releases heat energy
Example:
Explain the effect on the equilibrium position of increasing the temperature in this reaction:
2NO2(g) ⇌ N2O4(g) (Energy change = -58 kJ/mol)
Determine which direction represents the endothermic change - forward reaction is exothermic because energy change is negative - so heat is released to surroundings - so backward reaction is endothermic (energy change = +58 kJ/mol)
Equilibrium position moves to the left if temperature is increased
How is methanol made
Methanol is made by reacting carbon monoxide with hydrogen: CO(g) + 2H2(g) ⇌ CH3OH(g) (△H = -91 kJ/mol)
The equilibrium yield is amount of desired product present in a reaction at equilibrium depending on the:
Pressure
Temperature
Also depends on concentration of reactants
Increasing concentrations of carbon monoxide and hydrogen in the reaction mixture moves the equilibrium position to the right forming a higher equilibrium concentration of methanol
Basically you need to supply plenty of reactants to make plenty of product
Le Chatelier’s principle
Rule to predict what happens to reactions at equilibrium when conditions change:
When change is made to a reaction at equilibrium, the position of equilibrium shifts to oppose the change
When the concentration of a substance is decreased the equilibrium position shifts to increase its concentration again
How do you choose a suitable pressure for a reversible reaction
Higher pressure means that equilibrium shifts to side of the balanced equation with least number of moles (following balancing constants)
However, high pressures need expensive equipment to compress the gases, a lot of energy to run, and tough reaction vessels to withstand the pressure
In practice, chemical engineers choose a compromise pressure:
high enough to achieve reasonable equilibrium yield, but not so high to be expensive or hazardous
How do you choose a suitable temperature for a reversible reaction
Example:
Forward reaction in manufacture of methanol is exothermic, so backward reaction is endothermic
If temperature is increased, equilibrium position moves to the left and equilibrium yield of methanol decreases
However, at low temperatures rate of reaction is too low to produce methanol quickly enough to be profitable
In practice, chemical engineers choose a compromise temperature:
low enough to achieve reasonable equilibrium yield, but high enough to achieve reasonable rate of reaction
Why anhydrous copper(Il) sulfate is kept in a desiccator
Absorbs water - desiccators keep out water molecules so that the copper doesn’t get hydrated
