Reversible Equations and Equilibria - Topic 4 Flashcards
1
Q
What are chemical reactions?
A
- chemical reactions are reversible
- the use of the symbol ⇌ shows that the reaction is reversible
- the direction of some reversible reactions can be altered by changing the reaction conditions
2
Q
How can copper sulphate be used to show a reversible reaction?
A
- when you heat blue (hydrated) copper sulphate, it turns white
- this is because the water of crystallisation is evaporated, leaving anhydrous copper sulphate
- however, if water is added to this anhydrous (white) copper sulphate, it turns blue
- this is because the water is re-incorporated into the salt structure to reform the hydrated blue copper sulphate
3
Q
Dynamic equilibrium:
A
- reactants are continually making products and products are continually making reactants
- the amount of reactants and products do not appear to change so it looks like nothing is happening/no change in volume
4
Q
Why can’t reversible reactions be turned into equilibria in open environments?
A
- when the blue copper sulphate was heated, the water escaped into the atmosphere and so it could not react with the white copper sulphate to reform the reactants
- when the ammonium chloride was heated, the ammonia and hydrogen chloride gases rose up the tube and so were separated from the original reactants
- reactants can escape so reaction can’t reach equilibria
5
Q
How is equilibria reached in a closed environment?
A
- if reaction takes place in a closed environment then the products cannot ‘escape’ and so the forwards and reverse reactions occur at the same time and in the same place
- if the system is left long enough, then the forward and reverse reaction rates will become equal - at this point the reaction/system has reached equilibria
6
Q
4 conditions for equilibria:
A
- Equilibrium only takes place in a closed environment
- At equilibrium, the forward rate equals the reverse rate
- However, the amount of product does not equal the amount of reactant
- The amounts of products and reactants doesn’t change
7
Q
How does the rate of evaporation reach equilibrium in a closed environment?
A
- lots of water is evaporating (liquid → gas)
- only evaporation + no condensation
- vol of liquid water decreases
- vol of gas increases
- condensation starting to occur
- lots of evaporation still taking place at a greater rate than the rate of condensation
- rates of change decrease
- vol of liquid decreases
- vol of gas increases
- rate of evaporation = rate of condensation
- vol of liquid and gas no longer change therefore is in equilibrium
8
Q
Position of equilibrium:
A
the relative amounts of reactants and products
9
Q
How do we describe the position of equilibrium?
A
- REACTANTS ⇌ PRODUCTS
- if the position of equilibrium lies to the left, there are more reactants than products at equilibrium
- if the position of equilibrium lies to the right, there are more products than reactants at equilibrium
10
Q
Energy changes of reactions in equilibria:
A
- this is an exothermic reaction
- products have less chemical E than the reactants
- this E must have been released at heat
- the backwards reaction must be endothermic
- same energy taken in
11
Q
What are the energy changes for the formation of copper sulphate?
A
- heat blue hydrated copper sulphate to decompose it into water and anhydrous copper sulphate - endothermic reaction as energy taken in
- water is added to white anhydrous copper sulphate - must be exothermic, releases same amount of energy that was absorbed
12
Q
What is the formation of ammonia?
A
- formation of ammonia is a reversible reaction between nitrogen (extracted from the air) and hydrogen (obtained from natural gas)
- it can reach a dynamic equilibrium
13
Q
Properties of ammonia + explanations:
A
- low BP/MP - weak intermolecular forces between molecules → don’t require lot of E to overcome
- not conductive - no charge particles free to move and carry charge
14
Q
pH of ammonia:
A
alkaline > 7
15
Q
Reversible reaction between ammonia + water:
A
NH3(aq)+H2O(l) ⇌ NH4+(aq)+ OH-(aq)
