Topic 6.3: Reversible Reactions and Equilibrium Revision Notes
1. Overview
In many chemical reactions, reactants are completely converted into products. However, some reactions are reversible, meaning the products can react together to reform the original reactants. Understanding how to control these reactions is essential for large-scale industrial processes like the production of fertilizers and acids.
Key Definitions
- Reversible Reaction: A reaction that can go both forwards (left to right) and backwards (right to left).
- Dynamic Equilibrium: A state in a closed system where the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products remain constant.
- Closed System: A reaction vessel where no reactants or products can escape, but energy can be transferred.
- Anhydrous: A substance containing no water.
- Hydrated: A substance chemically combined with water (water of crystallisation).
Core Content
The Reversible Reaction Symbol
Reversible reactions are represented by the symbol: $\rightleftharpoons$
Water and Hydrated Salts
You can change the direction of certain reversible reactions by adding or removing water or heat.
1. Copper(II) sulfate
- Word Equation: hydrated copper(II) sulfate $\rightleftharpoons$ anhydrous copper(II) sulfate + water
- Symbol Equation: $CuSO_4 \cdot 5H_2O(s) \rightleftharpoons CuSO_4(s) + 5H_2O(l)$
- Observation:
- Forward reaction (Endothermic): Heating blue hydrated copper(II) sulfate crystals turns them into a white anhydrous powder.
- Reverse reaction (Exothermic): Adding water to white anhydrous copper(II) sulfate turns it back to blue and releases heat.
2. Cobalt(II) chloride
- Word Equation: hydrated cobalt(II) chloride $\rightleftharpoons$ anhydrous cobalt(II) chloride + water
- Symbol Equation: $CoCl_2 \cdot 6H_2O(s) \rightleftharpoons CoCl_2(s) + 6H_2O(l)$
- Observation:
- Forward reaction: Heating pink hydrated cobalt(II) chloride turns it into blue anhydrous cobalt(II) chloride.
- Reverse reaction: Adding water to blue anhydrous cobalt(II) chloride turns it pink.
Two test tubes; one showing the addition of water to white powder turning blue, the other showing heating of blue crystals turning white.
Extended Content (Extended Curriculum Only)
Characteristics of Equilibrium
In a closed system, a reversible reaction reaches equilibrium when:
- The rate of the forward reaction is equal to the rate of the reverse reaction.
- The concentrations of reactants and products remain constant (though not necessarily equal).
Predicting the Position of Equilibrium (Le Chatelier’s Principle)
If a change is made to the conditions of a system at equilibrium, the system shifts to counteract that change.
| Change | Effect on Equilibrium Position |
|---|---|
| Increase Temperature | Shifts in the direction of the endothermic reaction. |
| Decrease Temperature | Shifts in the direction of the exothermic reaction. |
| Increase Pressure | Shifts to the side with the fewer moles of gas. |
| Decrease Pressure | Shifts to the side with the more moles of gas. |
| Increase Concentration | Shifts to consume the added substance. |
| Add Catalyst | No effect on position; it increases the rate of both reactions equally. |
The Haber Process (Ammonia Production)
- Equation: $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ (Forward reaction is exothermic)
- Sources:
- Nitrogen: Obtained from the air.
- Hydrogen: Obtained from methane (natural gas) reacted with steam.
- Conditions:
- Temperature: 450°C
- Pressure: 20,000 kPa / 200 atm
- Catalyst: Iron (Fe)
- Explanation of Conditions:
- Pressure: High pressure favors the right side (4 moles gas → 2 moles gas), increasing yield. 200 atm is a compromise between high yield and the high cost/safety risk of equipment.
- Temperature: Low temperature favors the exothermic forward reaction (higher yield), but the rate would be too slow. 450°C is a compromise temperature to get a reasonable yield at a fast enough rate.
The Contact Process (Sulfuric Acid Production)
- Equation: $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$ (Forward reaction is exothermic)
- Sources:
- Sulfur Dioxide: Burning sulfur or roasting sulfide ores (e.g., zinc blende).
- Oxygen: Obtained from the air.
- Conditions:
- Temperature: 450°C
- Pressure: 200 kPa / 2 atm
- Catalyst: Vanadium(V) oxide ($V_2O_5$)
- Explanation of Conditions:
- Pressure: High pressure favors the right side (3 moles gas → 2 moles gas), but the yield is already so high (~98%) at 2 atm that higher pressure is not economically necessary.
- Temperature: 450°C is used to ensure a fast reaction rate without shifting the equilibrium too far to the left.
Key Equations
| Process | Balanced Symbol Equation |
|---|---|
| Hydrated Copper Sulfate | $CuSO_4 \cdot 5H_2O(s) \rightleftharpoons CuSO_4(s) + 5H_2O(l)$ |
| Hydrated Cobalt Chloride | $CoCl_2 \cdot 6H_2O(s) \rightleftharpoons CoCl_2(s) + 6H_2O(l)$ |
| Haber Process | $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ |
| Contact Process | $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$ |
Common Mistakes to Avoid
- ❌ Wrong: Thinking equilibrium means the concentrations of reactants and products are equal.
- ✓ Right: Equilibrium means the concentrations are constant and the rates are equal.
- ❌ Wrong: Saying a catalyst increases the yield of a reaction.
- ✓ Right: A catalyst only increases the rate at which equilibrium is reached; it does not change the position of equilibrium.
- ❌ Wrong: Forgetting state symbols in equations for the Haber or Contact process.
- ✓ Right: Always include $(g)$ for all reactants and products in these industrial gas-phase reactions.
Exam Tips
- Command Word: Explain: When asked to explain the effect of pressure, always mention the number of moles of gas on each side of the equation.
- Command Word: Describe: When describing the test for water using cobalt(II) chloride, mention the color change from blue to pink.
- Compromise Conditions: If asked why 450°C is used in the Haber process, emphasize it is a compromise between rate (faster at high temp) and yield (higher at low temp for exothermic reactions).
- Real-World Contexts: Expect questions on the Haber process regarding fertilizer production and the Contact process regarding sulfuric acid.
- State Symbols: Always check if the question asks for state symbols. In equilibrium questions, the state $(g)$ is vital for pressure calculations.