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
