🔄 A reversible reaction is like a seesaw: the reactants can turn into products, but the products can also turn back into reactants. The reaction keeps going in both directions until a balance (equilibrium) is reached.
⚗️ Example: \$A \rightleftharpoons B\$ – the arrow shows that the reaction can go forward (A → B) and backward (B → A).
When asked to explain a reversible reaction, remember to mention the dynamic equilibrium and that the rates of the forward and reverse reactions become equal.
🚀 The Contact process is the industrial way to make sulfuric acid. It uses a catalyst to speed up the conversion of sulfur dioxide (\$SO2\$) to sulfur trioxide (\$SO3\$):
\$SO2 + \tfrac{1}{2}O2 \rightleftharpoons SO_3\$
Because the reaction is reversible, we need the right conditions to push the balance toward \$SO_3\$.
| Condition | Value | Why it matters |
|---|---|---|
| Temperature | \$450^{\circ}\text{C}\$ | Higher temperature speeds up the reaction but also favours the reverse reaction. 450 °C is a compromise that gives a good rate while still favouring \$SO_3\$. |
| Pressure | \$200\,\text{kPa}\$ (≈ 2 atm) | Increasing pressure pushes the equilibrium toward the side with fewer gas molecules (2 → 1). This helps produce more \$SO_3\$. |
| Catalyst | Vanadium(V) oxide (\$\text{V}2\text{O}5\$) | Catalysts lower the activation energy, allowing the reaction to reach equilibrium faster without changing the final equilibrium composition. |
📚 Remember: The equilibrium constant for the Contact process is:
\$Kc = \frac{[SO3]}{[SO2][O2]^{1/2}}\$
When the conditions above are met, \$Kc\$ is high enough that most of the \$SO2\$ is converted to \$SO_3\$ before the gas mixture leaves the reactor.
To answer questions about the Contact process, list the three key conditions (temperature, pressure, catalyst) and explain how each influences the position of equilibrium. Use the analogy of a seesaw to describe the reversible nature of the reaction.