Reversible Reactions and Equilibrium

In many reactions the products cannot turn back into the reactants. But in a reversible reaction the products can react together to reform the original reactants. The reaction can go both ways.

A reversible reaction is written with a special double arrow, ⇌, instead of a single arrow:

$A+B\rightleftharpoons C+D$

The forward reaction (left to right) makes C and D. The reverse reaction (right to left) remakes A and B. Both directions are possible.

The ⇌ symbol shows a reversible reaction. A single arrow → shows a reaction that only goes one way.

The direction that dominates can be changed by changing the conditions, such as temperature. This is what makes reversible reactions so useful in industry, where conditions are chosen to favour the product you want.

A clear example of a reversible reaction is the breakdown of ammonium chloride. When heated, solid ammonium chloride splits into two gases, ammonia and hydrogen chloride:

$N{H}_{4}Cl\rightleftharpoons N{H}_3+HCl$

Heating drives the reaction to the right, forming the gases. As the gases move to a cooler part of the tube, the reverse reaction happens: the ammonia and hydrogen chloride recombine to form solid ammonium chloride again.

Condition	Direction favoured	What you see
Heating	Forward (splits apart)	White solid turns to colourless gases
Cooling	Reverse (recombines)	White solid reforms on the cooler glass

This shows the defining feature of a reversible reaction: changing the condition (here, temperature) changes which direction the reaction goes.

Because a reversible reaction can go in either direction, its energy change also works both ways.

If a reversible reaction is exothermic in one direction, it is endothermic in the other direction, and the same amount of energy is transferred each way.

If the forward reaction releases 50 kJ of energy (exothermic), then the reverse reaction absorbs exactly 50 kJ (endothermic). The energy is not lost; it flows in the opposite direction when the reaction reverses.

Direction	Energy change	Amount of energy
Forward (say, exothermic)	Energy released to surroundings	e.g. 50 kJ
Reverse (then endothermic)	Same energy absorbed from surroundings	e.g. 50 kJ

This symmetry is a direct consequence of energy conservation: reversing the reaction reverses the energy flow, but the size of the transfer stays the same.

The classic example of this energy symmetry is copper sulfate.

Hydrated copper sulfate is blue and contains water within its crystals. Heating it drives off the water, leaving white anhydrous copper sulfate. This forward reaction is endothermic because energy must be supplied to remove the water:

$\underset{\text{blue}}{CuS{O}_4\cdot 5H_{2}O}\rightleftharpoons \underset{\text{white}}{CuS{O}_4}+5H_{2}O$

Adding water to the white anhydrous solid reverses the change. It turns blue again, and this reverse reaction is exothermic, releasing the same amount of energy that was needed to drive the water off.

Change	Direction	Colour	Energy
Heat the blue crystals	Forward	Blue → white	Endothermic
Add water to the white solid	Reverse	White → blue	Exothermic

The return to blue on adding water is used as a chemical test for water: white anhydrous copper sulfate turns blue in the presence of water.

A reversible reaction can reach a balance point called equilibrium, but only in a closed system, where no reactants or products can escape or be added.

At the start, only reactants are present, so the forward reaction is fast. As products build up, the reverse reaction speeds up while the forward reaction slows down. Eventually the two rates become equal.

Equilibrium is reached when the forward and reverse reactions occur at exactly the same rate. This can only happen in a closed system.

At equilibrium, the amounts of reactants and products stay constant. This does not mean the reaction has stopped. Both the forward and reverse reactions are still happening, but because they proceed at the same rate, there is no overall change in the amounts present. This is why it is called dynamic equilibrium.

The word "dynamic" is the key idea examiners test. At equilibrium the concentrations stay the same, yet reactions are still occurring on both sides.

At equilibrium	True or false	Why
The forward and reverse rates are equal	True	This is the definition of equilibrium
The reaction has stopped	False	Both reactions continue; only the amounts stay constant
The amounts of reactants and products are equal	False	The amounts are constant, but not necessarily equal to each other
It only happens in a closed system	True	An open system loses substances, so no balance can be reached

A useful picture: imagine an escalator where people walk up as fast as it moves down. Everyone is moving, yet no one makes net progress. The amounts stay fixed while both reactions keep running.

Equilibrium means constant amounts, not equal amounts, and certainly not a stopped reaction.

1. Saying the reaction stops at equilibrium

At equilibrium both the forward and reverse reactions continue; they just happen at the same rate. State that the amounts stay constant, not that the reactions stop.

2. Thinking equilibrium means equal amounts of reactants and products

Equilibrium means the amounts are constant, not that reactants and products are present in equal quantities. The actual proportions depend on the conditions.

3. Forgetting the closed system condition

Equilibrium can only be reached if nothing escapes. In an open flask, gases such as ammonia would leave and no balance could form. Always state that a closed system is needed.

4. Getting the copper sulfate energy directions the wrong way round

Removing water (heating, blue to white) is endothermic. Adding water (white to blue) is exothermic. The energy transferred is the same size in both directions.

5. Using a single arrow for a reversible reaction

A reversible reaction must be written with ⇌, not →. Using a single arrow suggests the reaction only goes one way and can lose the mark.