Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

4.1.3Unit 4. Rates, Equilibria and Further Organic Chemistry

Reaction Orders – Graphs — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 12 of 56 Unit quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
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.
0%read

Part 1 of 6Concentration–Time Graphs

The shape of the graph indicates the order of the reaction with respect to the reactant.

Zero order
  • Zero order The graph is a straight line sloping downwards, because the rate stays constant as the concentration falls.
First order
  • First order The graph is a curve that falls with a decreasing gradient, since the rate drops in proportion to the concentration.
Second order
  • Second order The curve falls more steeply at the start and then flattens more slowly, so the half-life gets longer.
Gradient
  • Gradient At any time the gradient of the tangent equals the rate of reaction at that moment.

Part 2 of 6Rate–Concentration Graphs

Rate–Concentration Graphs
Zero order
  • Zero order A horizontal line shows that the rate does not change when the concentration changes.
First order
  • First order A straight line through the origin shows that rate is directly proportional to concentration.
Second order
  • Second order A curve through the origin that gets steeper with concentration, showing rate ∝ [A]².
Gradient of first order
  • Gradient of first order The gradient of the straight line equals the rate constant k.
OrderConcentration–time shapeRate–concentration shape
0Straight line, downwardHorizontal line
1Exponential decay curveStraight line through origin
2Steeper curve, longer half-livesUpward curve through origin

Part 3 of 6Half-Life

1
The half-life t½ is the time for the concentration of a reactant to fall to half its value.
2
Read the time for the concentration to fall from one value to half of that value, and repeat from other starting points.
3
If successive half-lives are constant, the reaction is first order.
4
If successive half-lives increase, the reaction is probably second order; for zero order they decrease.
5
For a first-order reaction k = ln 2 ÷ t½, which is about 0.693 ÷ t½.

Part 4 of 6Using Graphs in Practice

Draw tangents
  • Draw tangents Draw tangents at several concentrations, find their gradients, and then plot the rate against concentration.
Check data
  • Check data Compare the shape from your data with the standard shapes, taking care over experimental scatter.
Limit
  • Limit A graph shows the order for one reactant only when others are in large excess or held constant.
Rate constant
  • Rate constant For first order, the gradient of the rate–concentration graph gives k directly.

Part 5 of 6Worked Example

Question

  • The concentration of a reactant is 0.80 mol dm⁻³ at 0 s, 0.40 mol dm⁻³ at 30 s and 0.20 mol dm⁻³ at 60 s.
  • Find the order and k.

  1. The concentration halves from 0.80 to 0.40 in 30 s, and from 0.40 to 0.20 in a further 30 s.
  2. The half-life is constant at 30 s, so the reaction is first order.
  3. k = ln 2 ÷ t½ = 0.693 ÷ 30 = 0.0231 s⁻¹.

Answer First order; k = 0.0231 s⁻¹

Part 6 of 6Exam Checklist & Tips

4.1.3 – Key Points for the Exam

  • Half-life The time taken for the concentration of a reactant to fall to half its initial value.
  • Tangent A straight line touching a curve at one point and having the same gradient there.
  • First order A reaction whose rate is directly proportional to the concentration of one reactant.
  • Rate–concentration graph A plot of rate against concentration used to find the order.
  • Exponential decay A decrease in which each equal time interval multiplies the quantity by the same fraction.

Common Mistakes

  • Drawing a first-order concentration–time graph as a straight line.
  • Saying that a constant half-life means zero order, when it indicates first order.
  • Reading the half-life only once, rather than checking that two successive half-lives are equal.
  • Mixing up rate–concentration and concentration–time graphs when deciding the order.

Last updated:

\n