Chemical Equilibria Flashcards
Reversible reactions
- Reactions that proceed in both the forward and backward directions (⇌)
- Not complete → state of dynamic equilibrium → mixture of both reactants and products
Dynamic equilibrium
A state in a reversible system in which the rates of the forward and backward reactions are continuing at the same rate, resulting in no net change in the macroscopic properties (concentrations, partial pressure) of the reactants and products
Features of a system at dynamic equilibrium
- Forward rate = backward rate ≠ 0
- Microscopic processes continue
- Only in a closed system
- Can be attained from either direction if T is constant
Reaction quotient, Qc
- At any given time, Qc is the ratio of concentrations of the reactants and products raised to their stoichiometric ratios
- aA + bB ⇌ cC + dD
- Qc = ([C]^c x [D]^d)/([A]^a x [B]^b)
Equilibrium constant, Kc
- At a given temperature
- [A], [B], [C] and [D] remain constant
- Qc becomes constant → Kc
Position of equilibrium
Relative composition (concentration) of the products and reactants present in a reaction mixture at equilibrium
Equilibrium constant for gaseous system, Kp
Gaseous system → use partial pressures
Rate constants and equilibrium constants
- Rate of forward reaction, ratef = kf [A][B]
- Rate of backward reaction, rateb = kb [C][D]
- ratef = rateb, kf [A][B] = kb [C][D]
- Kc = kf/kb at a given temperature
Variations in the forms of K
Kforward = 1/Kbackward
Form of K for an overall reaction
Overall = K1 x K2 x …
Form of K for a reaction with coefficients multiplied by a common factor, n
K’ = Kⁿ
Factors affecting equilibrium constants, Kc
- Since rate constants kf and kb are independent of concentration, but dependent on T, Kc is constant at a specific T and it varies with T
- Only affected by temperature changes
- Not affected by concentration, partial pressures, catalysts
Significance of equilibrium constants
- K is a measure of the extent of a reaction
- Indicates how far a reaction proceeds towards product side at a given temperature
- Small Kc: position of eqm to the left → “no rxn”
- Large Kc: position of eqm to the right → “almost complete”
- Intermediate Kc: significant amts of reactants and products present
Homogeneous equilibrium
All substances involved in the same phase
Heterogeneous equilibrium
- Substances not in the same phase
- Concentrations and vapour pressure of solids are constant → exclude in Kc
- Concentration of water when present in large amount (solvent) is approximately constant → exclude
Calculation of Kc from concentrations
- I.C.E table
- Balanced chemical equation
- Initial, change and equilibrium amt
- Make sure you use concentration not volume when calculating
Calculations of Kp from partial pressures
Don’t need to divide by total pressure, just plug and play
Calculations involving degree of dissociation
- 0 ≤ Degree of dissociation, α = amount of reactant dissociated/total initial amount of reactant ≤ 1
- Increases as T increases for endothermic rxns
Gibbs free energy
- Reactions tend towards a lower Gibbs free energy (G)
- Mixture of reactants and products → ↑entropy → ↓G
- Reaction mixture adjust composition such that G is at a minimum
- Chemical eqm under constant pressure and temp → ΔG = 0
Position of equilibrium
- Tends towards species with lower G
- ΔG⦵ = GB⦵ - GA⦵
- Value of equilibrium constant, K depends only on ΔG⦵
- ΔG⦵ = -RTlnK
ΔG⦵ = -RTlnK
- For ΔG⦵<0, K>1 → position of eqm lies more to the right side
- For ΔG⦵>0, K<1 → position of eqm lies more to the left side
Changes to equilibrium (4)
- Concentration
- Pressure/volume
- Temperature
- Catalyst
Le Chatelier’s Principle
- When a system in equilibrium is subjected to a change, the system will react to counteract the change imposed so as to re-establish the equilibrium
- Where (position of eqm), why (partially offset), how (produce)
Analysis of forward and backward reaction rates using collision theory
When conditions change → system no longer in eqm → rates of forward and backward reactions no longer equal
Comparing reaction quotient Q to equilibrium constant K
- Qc < Kc → forward reaction favoured
- Qc = Kc → system at eqm
- Qc > Kc → backward reaction favoured
Effect of concentration changes
- aA + bB ⇌ cC + dD
- When [A]↑ → eqm position shift right to partially offset ↑A by removing some A until new eqm reached
- Eqm mixture contain more A, C, D, less B
Effect of pressure changes (2)
- Changes in partial pressure of substances
2. Changes in total pressure of system
Changes in partial pressure of substances
- Add/remove gas
- Partial pressure ∝ concentration
- Similar to changing concentration
Changes in total pressure of system
- Increase by compression, decrease by expansion
- Effect depends on stoichiometry of reaction (if gas particles on each side of equation diff)
- When P↑, system try to counteract P↑ by favouring reaction that ↓P → reaction that produces fewer gas particles favoured → position of eqm shifts
Effect of volume changes
- Similar to changing total pressure
- Impact on rate → partial pressures of all gases ↓ → rate ↓ → rxn takes longer time to reach eqm
Effect of temperature changes
- Shift depends on whether forward reaction is endothermic/exothermic
- System tries to counteract T↑ by favouring forward endothermic reaction in order to absorb heat
- Eqm position shifts right
- New eqm mixture contains more C&D and less A&B
Effect of temperature on time taken to reach equilibrium
- When T↑, rate constants of both forward and backward reactions ↑
- No. particles with energy ≥ Ea ↑
- Time taken to reach eqm ↓
Effect of catalyst
- No effect on Kc and composition of eqm mixture
- Lowers Ea of both forward and backward reactions to the same extent
- Eqm reached more quickly
- Eqm position remains unchanged
Haber process
- Produce ammonia
- Take place quickly
- High yield of product
- Minimise cost (avoid very high T and P)
Condition of Haber process (5)
- Temperature
- Pressure
- Catalyst
- Continual removal of ammonia
- Molar ratio of N₂ : H₂ = 1:3
Temperature
- 450°C
- Forward reaction exothermic → lower T → higher yield
- Low T: rate of production too slow
- High T: lower yield and higher production cost
- Compromise → moderately high T
Pressure
- About 200atm
- Forward reaction reduces no. gas particles
- High P: ↑ yield
- Too high P: ↑ cost of production & safety concerns
- Moderate pressure
Catalyst
- Finely divided iron catalyst with aluminium oxide as promoter
- ↑ production rate
Continual removal of ammonia
- Shifts position of eqm right → ↑ yield
- Cool reaction mixture to -50°C → liquify NH₃
Molar ratio of N₂ : H₂ = 1:3
- Similar to that of stoichiometric ratio
- Minimise excess
