Reversible & Irreversible Reactions
Irreversible reactions:
- Proceed in one direction only
- The reaction stops when at least one reactant is completely consumed
- Are denoted with a forward arrow (→)
- Example:
- Combustion CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)
Reversible reactions (and processes):
- Can proceed in both directions
- The reaction can continue in both directions because as the reactants react to form products, the products can react to reform the reactants
- Are denoted with two opposing half arrows (⇌)
- Examples:
- Evaporation/condensation H2O (l) ⇌ H2O (g)
- Absorption/desorption of a gas CO2 (g) ⇌ CO2 (aq)
- Dissolution/precipitation of a salt in a saturated solution NaCl (s) ⇌ Na+ (aq) + Cl- (aq)
- Acid-base proton transfer HCN (aq) + NH3 (aq) ⇌ NH4+ (aq) + CN- (aq)
- In the forward reaction, the proton (H+) is transferred from HCN to NH3
- Electron transfer in redox reactions Cd (s) + 2Ag+ (aq) ⇌ 2Ag (s) + Cd2+ (aq)
- In the forward reaction, two electrons are removed from one Cd atom and each electron is gained by an Ag+ ion
Exam Tip
Note that reactions involving a gas are only reversible if they occur in a closed system.