1. Reversible Reactions
Noadswood Science, 2012

2. Friday, April 21, 2017
Reversible Reactions
To be able to explain and give examples of reversible reactions A B A B
These combine
These decompose

3. Reversible Reactions
Many reactions, such as burning fuel, are irreversible - they go to completion and cannot be reversed easily
Reversible reactions are different – in a reversible reaction, the products can react to produce the original reactants again…
If the reaction releases energy in one direction (exothermic), it will take in same amount of energy in the other (endothermic)… A B A B
These combine
These decompose

4. ammonium chloride ammonia + hydrogen chloride
Reversible Reactions
When writing chemical equations for reversible reactions two arrows are used, each with just half an arrowhead – the top one pointing right, and the bottom one pointing left: -
ammonium chloride ammonia + hydrogen chloride
The equation shows that ammonium chloride (a white solid) can break down to form ammonia and hydrogen chloride
It also shows that ammonia and hydrogen chloride (colourless gases) can react to form ammonium chloride again

5. copper(II) sulfate + water hydrated copper(II) sulfate
Reversible
The animation below shows a reversible reaction involving white anhydrous copper(II) sulfate and blue hydrated copper(II) sulfate
copper(II) sulfate + water hydrated copper(II) sulfate
The reaction between anhydrous copper(II) sulfate and water is used as a test for water – the white solid turns blue in the presence of water…

6. Experiment
Ammonium salts are made by reacting ammonia with an acid but some of these salts will decompose back into reactants when heated
Heat
Heat makes the solid disappear as it changes into gases
Solid reappears as it changes back again in the cool part of the tube
ammonium chloride ammonia + hydrogen chloride
NH4Cl NH3 + HCl

7. Equilibrium
A reversible reaction is where products can, under appropriate conditions, turn back into reactants
There will be a range of conditions over which both the forward and backward reaction will take place and this can lead to a state of balance with both reactants and products present in unchanging amounts – this is dynamic equilibrium
Equilibrium – because the amounts do not change
Dynamic – because the reaction is still occurring
It is rather like the situation where a man is walking the wrong way along a moving pavement or escalator – neither have stopped but the man could remain in the same place for ever!

8. Equilibrium
In reversible reactions equilibrium means balance but this balance does not have to be at the half-way point – we may have mostly reactants with just a little product or vice versa
There are 2 factors that we can change that influence the position of an equilibrium: -
Temperature
Concentration (or pressure in gas reactions)
Finding the conditions that gives the most product is really important in industrial chemical reactions…