6.1 Reversible & Irreversible Changes
Irreversible Reactions
- Complete conversion of reactants to products
- Forward direction only
- Products cannot reform reactants
- Example: Precipitation reactions
Reversible Reactions
- Never go to completion
- Both forward and backward directions
- Reactants ⇌ Products
- Example: Haber process for ammonia
6.2 Dynamic Equilibrium
What is Dynamic Equilibrium?
- Forward rate = Reverse rate
- Concentrations remain constant
- Reactions continue in both directions
- Macroscopic properties stable
Water Equilibrium Example
All three phases coexist at equilibrium in a closed system at 0°C
Color Change Equilibrium
At 100°C in sealed flask, mixture maintains constant brown color
Factors Affecting Equilibrium
Concentration Changes
- Add reactant → Shift to products
- Remove product → Shift to products
- Add product → Shift to reactants
- Remove reactant → Shift to reactants
Temperature Effects
- Increase temperature → Favors endothermic direction
- Decrease temperature → Favors exothermic direction
Pressure Effects (Gases)
- Increase pressure → Favors fewer gas moles
- Decrease pressure → Favors more gas moles
- No effect if equal moles on both sides
Catalyst Role
- Speeds up both forward and reverse equally
- No effect on equilibrium position
- Reduces time to reach equilibrium
Exercise Questions
Multiple Choice Questions:
i) What will happen if the rates of forward and reverse reactions are very high?
Answer: (a) The equilibrium point will reach very soon.
Explanation: High reaction rates mean the equilibrium is established quickly as both reactions proceed rapidly.
ii) Predict which components of the atmosphere react in the presence of lightning.
Answer: (d) N₂ and O₂
Explanation: Lightning provides energy for: N₂ + O₂ → 2NO (endothermic reaction)
v) What condition should be met for reversible reaction to achieve equilibrium?
Answer: (c) The concentrations of all reactants and products should become constant.
Explanation: At equilibrium, concentrations remain constant even though reactions continue.
vi) Why does gas come out when you open a can of fizzy drink?
Answer: (c) Because gas is dissolved under pressure and comes out when pressure decreases
Explanation: CO₂ is dissolved under high pressure. Opening decreases pressure, shifting equilibrium: CO₂(dissolved) ⇌ CO₂(gas)
Short Answer Questions:
i. How is dynamic equilibrium different from static equilibrium?
Answer: Dynamic equilibrium has continuous reactions in both directions with equal rates. Static equilibrium has no reactions at all – everything is stationary.
iii. How can you get maximum yield in a reversible reaction?
Answer: Remove products as they form, use optimal temperature (low for exothermic, high for endothermic), use high pressure if fewer gas moles on product side, use excess reactants.
Constructed Response Questions:
i. Why are some reactions irreversible while others are reversible?
Answer: Irreversible reactions produce very stable products (precipitates, gases that escape, highly exothermic reactions). Reversible reactions occur in closed systems where products can readily reform reactants.
ii. Why are combustion reactions generally irreversible?
Answer: Combustion produces very stable products (CO₂, H₂O) with strong bonds. The large energy release (highly exothermic) makes reverse reaction energetically unfavorable. Also, CO₂ escapes as gas.
Descriptive Questions:
i. How can you drive reversible reaction at equilibrium?
Answer:
(a) Forward: Add reactants, remove products, decrease temperature (exothermic), increase pressure (if fewer gas moles on product side)
(b) Backward: Add products, remove reactants, increase temperature (exothermic), decrease pressure (if fewer gas moles on product side)