5.2.1Additional lessons
Rate of Reaction — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 96 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Defining Rate
Rates change as a reaction proceeds, because the reactants become less concentrated.
Definition
- Definition Rate of reaction is the change in concentration of a reactant or product per unit time.
Units
- Units Concentration is in mol dm⁻³ and time in seconds, so rate is usually in mol dm⁻³ s⁻¹.
Reactants
- Reactants Reactant concentration falls during a reaction, so rate is quoted as a positive value for the decrease.
Products
- Products Product concentration rises, and the rate of formation is the increase in concentration per unit time.
Section 2 of 5Average and Instantaneous Rate
The curve is steepest at the start and flattens as reactants are used up.
1
Average rate is the total change in concentration divided by the total time interval chosen.
2
Plot a graph of concentration against time, which usually forms a curve whose steepness shows the rate.
3
For the instantaneous rate at time t, draw a tangent to the curve at that point.
4
Calculate the gradient of the tangent using a large triangle, since the gradient equals the rate.
5
Initial rate is the instantaneous rate at time zero, when the reactants are at their highest concentration.
Section 3 of 5Interpreting Rate Data
Interpreting Rate Data
Steep graph
- Steep graph A steep slope on a concentration-time graph means a fast reaction at that moment.
Flat graph
- Flat graph A horizontal line shows that the reaction has finished or reached equilibrium.
Comparing rates
- Comparing rates Compare rates using the same measured quantity and the same units.
Stoichiometry
- Stoichiometry Rates of change of different species differ if coefficients differ, so state which species is used.
Section 4 of 5Worked Example
Question
- The concentration of a reactant falls from 0.800 mol dm⁻³ to 0.500 mol dm⁻³ in 30.0 s.
- Calculate the average rate.
- Change in concentration = 0.800 − 0.500 = 0.300 mol dm⁻³.
- Time interval = 30.0 s.
- Average rate = 0.300 ÷ 30.0 = 0.0100 mol dm⁻³ s⁻¹.
Answer Average rate = 1.00 × 10⁻² mol dm⁻³ s⁻¹
Section 5 of 5Exam Checklist & Tips
5.2.1 – Key Points for the Exam
- Rate of reaction Change in concentration of a reactant or product per unit time.
- Instantaneous rate The rate at a specific moment, found from the tangent to a graph.
- Initial rate The instantaneous rate at the start of a reaction.
- Average rate Total change divided by the time interval over which it occurs.
Common Mistakes
- Giving a negative rate for a reactant instead of using the magnitude.
- Using a chord rather than a tangent to find instantaneous rate.
- Confusing rate of reaction with the extent or total yield.
- Forgetting units or leaving time in minutes while the answer is requested per second.
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