Reversible Reactions & Directions
Chemistry: IB Diploma Higher Level, topic 5.5.3
Reversible Reactions
Some chemical reactions are reversible, meaning they can proceed in both forwards and backwards directions.
Reversible reactions
- A + B ⇌ C + D
- The reactants (A + B) can combine to give the products (C + D).
- The products (C + D) can combine to give the reactants (A + B).
Exothermic and endothermic reactions
- If the forward reaction is exothermic (transfers energy from reactants to the environment), the backward reaction will be endothermic (transfers energy from the environment to the reactants).
Conservation of energy
- Energy is conserved during chemical reactions.
- The energy released/absorbed by the forward reaction will be exactly equal to the energy absorbed/released by the backward reaction.
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Reactants to products
- There is a net conversion of reactants to products if:
- The rate of the forward reaction is greater than the rate of the backwards reaction.
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Products to reactants
- There is a net conversion of products to reactants if:
- The rate of the reverse reaction is greater than the rate of the forward reaction.
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Equilibrium
- The reaction reaches equilibrium when the rates of the forward and reverse reactions are equal.
- No changes are observed in a system at equilibrium.
Test yourself on Reversible Reactions & Directions
In a reversible reaction, energy cannot be lost or gained overall. Why must the amount of energy absorbed by the backward reaction equal the amount of energy released by the forward reaction?
The equation NH4Cl ⇌ NH3 + HCl shows a reversible reaction. What forms when ammonia gas reacts with hydrogen chloride gas?