4.4.3Unit 4. Rates, Equilibria and Further Organic Chemistry
Equilibrium Constant and Conditions — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 31 of 56 Unit quiz
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Section 1 of 4Changing Concentration
Adding or removing a species temporarily changes the ratio of concentrations away from the value of K.
Add a reactant
- Add a reactant The ratio of products to reactants falls below K, so the forward reaction speeds up until the ratio returns to K.
Remove a product
- Remove a product The ratio again falls below K, so more product forms and the position of equilibrium moves to the right.
Add a product
- Add a product The ratio rises above K, so the reverse reaction dominates until the original value of K is restored.
Value of K
- Value of K K itself does not change;
- only the equilibrium concentrations adjust until the expression returns to the same value.
Section 2 of 4Changing Pressure
Pressure matters only when the number of moles of gas differs on the two sides.
Changing Pressure
Fewer gas moles on right
- Fewer gas moles on right Raising pressure shifts the position to the side with fewer gas molecules, as in N₂ + 3H₂ ⇌ 2NH₃.
Equal gas moles
- Equal gas moles If both sides have the same number of gas moles, as in H₂ + I₂ ⇌ 2HI, pressure has no effect on the position.
Why K is constant
- Why K is constant Partial pressures change so that the Kp expression returns to the same value, so K is unchanged by pressure.
Rate effect
- Rate effect Higher pressure also raises the rate of reaching equilibrium because gas molecules collide more often.
Section 3 of 4Catalysts and Overall Conditions
A catalyst does something different from other changes.
1
A catalyst provides an alternative pathway with a lower activation energy for both the forward and the reverse reactions.
2
It speeds up the forward and reverse rates by the same factor, so the equilibrium mixture is reached sooner.
3
The position of equilibrium and the value of K are therefore unchanged by adding a catalyst.
4
Industrial processes choose conditions as a compromise between yield, rate and cost, often using a catalyst to allow a lower temperature.
Section 4 of 4Exam Checklist & Tips
4.4.3 – Key Points for the Exam
- Position of equilibrium The relative amounts of reactants and products in the equilibrium mixture.
- Le Chatelier principle If a system at equilibrium is disturbed it shifts to oppose the change.
- Catalyst A substance that speeds up a reaction without being used up.
- Reaction quotient The ratio of concentrations at any moment, which equals K only at equilibrium.
Common Mistakes
- Saying a pressure change alters Kc or Kp; only temperature does that.
- Thinking a catalyst increases the yield of product at equilibrium.
- Forgetting that equal numbers of gas moles on each side mean pressure has no effect on position.
- Using concentration changes to explain a change in K.
Last updated:
3 & IGCSE —
Kc
- : Kc! (ΔH < 0) Kc. (ΔH > 0) Kc.
- : ( ).
- : Kc .