Reversible Chemical Reactions

Introduction

Most chemical reactions go one way. You cannot "unburn" a match or "uncook" an egg. But some reactions can run forwards and backwards: the products can react together to form the starting substances again. These are called reversible reactions . They are important in industry, in living things and in everyday chemistry, from indicators that change colour to the manufacture of fertilisers. This page introduces them.

Core explanation

What makes a reaction reversible? In a reversible reaction, new substances really do form, so it is a chemical change. But under suitable conditions, those new substances can react to give back the originals. Chemists show this with a special double arrow, ⇌ , instead of the usual →.

A + B ⇌ C + D

The forward reaction goes from left to right; the reverse reaction goes from right to left. Changing the conditions, such as the temperature or the concentration, decides which direction is favoured.

Example 1: ammonium chloride. When the white solid ammonium chloride is heated, it decomposes into two gases, ammonia and hydrogen chloride. When these gases cool, for example near the cooler top of a test tube, they react together and white ammonium chloride forms again.

ammonium chloride ⇌ ammonia + hydrogen chloride NH₄Cl ⇌ NH₃ + HCl

Example 2: hydrated copper(II) sulfate. Heating blue hydrated copper(II) sulfate drives off water and leaves the white anhydrous salt. Adding water to the white solid turns it blue again.

hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

Example 3: indicators. Litmus is red in acid and blue in alkali. Adding alkali to red litmus turns it blue; adding acid turns it back to red. Each colour is a slightly different form of the indicator molecule, and the reaction between them is reversible.

Energy in reversible reactions. If a reaction is exothermic in one direction, it is endothermic by exactly the same amount in the other direction. Decomposing ammonium chloride and dehydrating copper(II) sulfate both take in energy, so they need heating. The reverse reactions release the same amount of energy: adding water to white anhydrous copper(II) sulfate makes it noticeably warm.

Reaction Forward direction Reverse direction --- --- --- Ammonium chloride Heat: solid → gases (endothermic) Cool: gases → solid (exothermic) Copper(II) sulfate Heat: blue → white (endothermic) Add water: white → blue (exothermic) Litmus Add alkali: red → blue Add acid: blue → red

Step-by-step reasoning

To recognise a reversible reaction:

1. Check that a new substance forms, so that it is a chemical change. 2. Look for a way to make the products turn back into the reactants by changing conditions. 3. If this works, write the equation with ⇌. 4. If the forward reaction needs heat, expect the reverse reaction to release heat.

Visual explanation

Picture a see-saw with reactants on one side and products on the other. Heating tips it towards products; cooling tips it back towards reactants. Above it, the ⇌ symbol shows one arrow pointing each way, representing the forward and reverse reactions.

Real-world analogy

A reversible reaction is like a revolving door. People can go through in either direction, and which way most people go depends on conditions, such as whether it is raining outside or it is the end of the working day.

Real-world example

The Haber process makes ammonia for fertilisers from nitrogen and hydrogen:

nitrogen + hydrogen ⇌ ammonia

Because the reaction is reversible, not all the nitrogen and hydrogen turn into ammonia at once. Engineers choose the temperature and pressure carefully, and recycle unreacted gases, to produce ammonia economically. It is one of the most important reactions in the world, supplying fertiliser for much of the global food supply.

Why?

Why is the energy change the same size in both directions? Energy is conserved. If turning A into B releases a certain amount of energy, then turning B back into A must take in exactly that amount, or energy would be created or destroyed on a round trip, which never happens.

Common misconception

"Reversible reactions are physical changes because they can be undone." They are chemical changes: new substances with different properties really do form. Ammonia and hydrogen chloride are gases, completely different from solid ammonium chloride.

Worked example

Question: Blue cobalt(II) chloride paper turns pink in damp air. Gently warming the pink paper turns it blue again. Write a word equation using the correct symbol and state which direction is endothermic.

Reasoning: Water turns the blue anhydrous salt into the pink hydrated form, and heating removes the water again. Removing water needs energy.

Answer: anhydrous cobalt(II) chloride (blue) + water ⇌ hydrated cobalt(II) chloride (pink). The reverse direction, removing water by heating, is endothermic.

Quick check

1. What does the symbol ⇌ mean in a chemical equation? Answer: The reaction is reversible and can go in both directions.

Exam focus

Learn the ⇌ symbol and at least two examples, especially ammonium chloride and hydrated copper(II) sulfate. A common exam question asks: "if the forward reaction is endothermic, what can you say about the reverse reaction?" The answer is that it is exothermic, transferring the same amount of energy.

Advanced insight

In a closed container, a reversible reaction reaches a state called dynamic equilibrium . The forward and reverse reactions continue, but at equal rates, so the amounts of reactants and products stay constant. Changing the conditions shifts the balance; this behaviour is described by Le Chatelier's principle, met in later chemistry.

Summary

A reversible reaction is a chemical change in which the products can react to reform the reactants, shown by ⇌. Examples include the decomposition of ammonium chloride, the dehydration of hydrated copper(II) sulfate and colour changes of indicators. If the forward reaction is endothermic, the reverse is exothermic by the same amount. Conditions such as temperature decide which direction is favoured.

Practice questions

1. Define a reversible reaction. Answer: A reaction in which the products can react together to form the original reactants again. 2. Write a word equation for the reversible decomposition of ammonium chloride. Answer: ammonium chloride ⇌ ammonia + hydrogen chloride. 3. Adding water to white anhydrous copper(II) sulfate makes it warm. What does this tell you about the reaction when blue copper(II) sulfate is heated? Answer: Heating blue copper(II) sulfate is endothermic, taking in the same amount of energy that is released when water is added. 4. Why is a reversible reaction still classed as a chemical change? Answer: New substances with different properties are formed, even though they can be turned back.