4.1.1Unit 4. Rates, Equilibria and Further Organic Chemistry
Kinetic Rates — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 9 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 6Rate of Reaction
A reaction slows down as reactants are used up, so the rate is not constant during the reaction.
Definition
- Definition Rate is the change in concentration of a reactant or product per unit time.
Units
- Units Concentration is measured in mol dm⁻³ and time in seconds, so rate has units of mol dm⁻³ s⁻¹.
Average rate
- Average rate This is the total change in concentration divided by the total time taken over an interval.
Instantaneous rate
- Instantaneous rate This is the rate at one moment, equal to the gradient of the tangent on a concentration–time graph.
| Measure | Typical unit |
|---|---|
| Concentration change | mol dm⁻³ |
| Gas volume change | cm³ s⁻¹ |
| Mass change | g s⁻¹ |
| Rate of reaction | mol dm⁻³ s⁻¹ |
Section 2 of 6Collision Theory
Collision Theory
Collisions
- Collisions Particles must collide to react, but only a small fraction of collisions lead to reaction.
Activation energy
- Activation energy Colliding particles need at least the activation energy, which is the minimum energy needed to break bonds.
Orientation
- Orientation The particles must also meet in a suitable orientation for the reactive parts to interact.
Frequency
- Frequency Rate depends on the number of successful collisions per second, not just the total number of collisions.
Section 3 of 6Factors Affecting Rate
1
Raising concentration or the pressure of a gas puts more particles in a given volume, so collisions happen more often.
2
Raising temperature gives particles more kinetic energy, so a larger fraction exceed the activation energy and collisions are also more frequent.
3
Increasing the surface area of a solid exposes more particles to the other reactant, so more collisions occur each second.
4
A catalyst provides an alternative route with a lower activation energy, so more collisions are successful at the same temperature.
5
The catalyst is not used up, and it speeds up the forward and reverse reactions equally.
Section 4 of 6Maxwell–Boltzmann Distribution
Shape
- Shape The curve shows the spread of molecular energies, starting at the origin and with no molecules having zero energy.
Area
- Area The total area under the curve is equal to the total number of molecules and does not change with temperature.
Higher temperature
- Higher temperature The peak moves to a higher energy and is lower, with a larger area to the right of Eₐ.
Catalyst effect
- Catalyst effect Eₐ moves to a lower energy on the same curve, so more molecules have enough energy to react.
Section 5 of 6Worked Example
Question
- The concentration of a reactant falls from 0.500 mol dm⁻³ to 0.380 mol dm⁻³ in 60 s.
- Calculate the average rate.
- Change in concentration = 0.500 − 0.380 = 0.120 mol dm⁻³.
- Time interval = 60 s.
- Average rate = 0.120 ÷ 60 = 2.0 × 10⁻³ mol dm⁻³ s⁻¹.
Answer Rate = 2.0 × 10⁻³ mol dm⁻³ s⁻¹
Section 6 of 6Exam Checklist & Tips
4.1.1 – Key Points for the Exam
- Rate of reaction The change in concentration of a reactant or product per unit time.
- Activation energy The minimum energy that colliding particles need for a reaction to occur.
- Collision theory The idea that reactions occur when particles collide with enough energy and the right orientation.
- Catalyst A substance that increases the rate by providing a lower-energy pathway and is not used up.
- Maxwell–Boltzmann distribution A graph of the distribution of molecular energies in a gas or liquid.
Common Mistakes
- Saying that a higher temperature works only because collisions are more frequent, when the main effect is the larger fraction above Eₐ.
- Stating that a catalyst changes the position of equilibrium or raises the yield.
- Drawing the Maxwell–Boltzmann curve touching the y-axis or finishing at zero on the right.
- Using units of s⁻¹ or mol dm⁻³ for rate instead of mol dm⁻³ s⁻¹.
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