R2.2Reactivity 2 — How much, how fast, how far?
How fast? The rate of chemical change — IB Chemistry SL Revision Notes & Topical Questions
20 min read 32 parts Lesson 2 of 3 1 quiz
Lesson summary
R2.2 How fast? The rate of chemical change — IB CHEMISTRY SL notes: Describe methods of measuring rate, calculate rate from graphs, and explain the effect of concentration, surface area, temperature and catalysts using collision theory.Definition = the change in concentration of …
- Graph reading
- Rate at a time
- Initial rate
- Equilibrium
- Disappearing cross
- Dissolving metal
Part 1 of 10Rate of reaction
Definition
Definition the change in concentration of a reactant or product per unit time, in mol dm−3 s−1.
Formula
- Formula rate = change in concentration ÷ time.
Graph reading
- Graph reading the steeper the gradient of a concentration-time graph, the faster the rate.
Rate at a time
- Rate at a time draw a tangent to the curve and calculate its gradient, Δy/Δx, using a large triangle that meets gridlines.
Initial rate
- Initial rate the gradient of the tangent at t = 0.
Sign
- Sign rate is always positive, so a negative reactant gradient is quoted as a positive rate.
Equilibrium
- Equilibrium in reversible reactions the concentrations stay constant once dynamic equilibrium is reached.
Part 2 of 10Measuring the rate of reaction
- Principle
- follow a property that changes with time and is proportional to the concentration of a reactant or product.
| Method | Property followed | Limitation |
|---|---|---|
| Mass loss | Gas escaping, e.g. CO2, with cotton wool in the neck | Gas must be dense; CO2 (M = 44) works but H2 (M = 2) does not |
| Gas volume | Gas syringe or inverted measuring cylinder over water | Gas must have low solubility, e.g. H2 |
| Colorimetry | Light passing through a coloured solution | Cannot follow precipitates, which scatter light |
| Conductivity or pH | Change in ion concentration | Needs ionic species that change |
| Titration of quenched samples | Concentration at fixed times | Sampling affects the mixture and is not continuous |
| Clock reaction | Time to a fixed visible point, e.g. disappearing cross | Gives only one value, an average rate (rate ∝ 1/t) |
- Plateau
- the reaction has finished when the gas volume levels off.
Part 3 of 10Sampling, quenching and clock reactions
1Quenching
- Quenching stopping the reaction in each sample by cooling or dilution so its concentration can be found by titration.
2Disappearing cross
- Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + SO2(g) + H2O(l) + S(s), where yellow sulfur obscures a cross.
3Dissolving metal
- Dissolving metal timing how long a piece of magnesium takes to dissolve in acid is another clock method.
4Method steps
- Method steps start the timer as the reactants mix, keep other variables constant and record at regular intervals.
Part 4 of 10Collision theory
Requirements
- Requirements particles must collide with energy at least equal to the activation energy and in the correct orientation.
Four factors
- Four factors collision frequency, collision energy, activation energy and collision geometry.
Successful collision
- Successful collision has sufficient energy and the correct orientation; otherwise the particles bounce apart.
Rate
- Rate depends on the number of successful collisions per unit time.
Why most collisions fail
- Why most collisions fail mainly because they lack the activation energy, not because of orientation.
Orientation
- Orientation matters most for large molecules such as proteins with active sites.
Part 5 of 10Factors affecting rate
| Factor | Change | Explanation |
|---|---|---|
| Concentration | Increase | More particles per volume, so more frequent collisions |
| Pressure of gases | Increase | Same particles in a smaller volume, so more frequent collisions |
| Surface area | Increase (powder) | More exposed particles, so more collisions |
| Temperature | Increase | More frequent collisions and a greater proportion with energy ≥ Ea; the energy effect dominates |
| Catalyst | Added | Alternative route with lower Ea, so more collisions succeed |
Rule of thumb
- Rule of thumb raising the temperature by 10 K often roughly doubles the rate.
Kinetic energy
- KE = ½mv2,
- so at the same temperature lighter particles move faster and diffuse more quickly.What the symbols mean: Ek = kinetic energy (J) · m = mass (kg) · v = speed (m s−1)
Part 6 of 10Activation energy and energy profiles
Activation energy Ea
Activation energy Ea the minimum energy colliding particles need for a successful reaction.
Exothermic
- Exothermic reactants are higher in energy than products; endothermic — reactants are lower.
Reverse reaction
- Ea(reverse) = Ea(forward) − ΔH, so it is larger for an exothermic reaction.
Calculation from data
- Calculation from data not required at SL.
Kinetic stability
- Kinetic stability a negative ΔH shows a reaction is favourable, but a high Ea can make it too slow to observe.
Part 7 of 10Catalysts
Action
- Action provide an alternative pathway with lower Ea and are unchanged at the end.
Effect on thermodynamics
- Effect on thermodynamics they do not change ΔH or the position of equilibrium.
Homogeneous
- Homogeneous in the same phase as the reactants.
Heterogeneous
- Heterogeneous in a different phase, e.g. solid iron in the Haber process.
Enzymes
- Enzymes biological catalysts that allow reactions at lower temperature and pressure.
Transition metals
- Transition metals often good catalysts because of variable oxidation states.
Sustainability
- Sustainability catalysts cut energy use, are reused in small amounts so give less waste, and increase selectivity.
Part 8 of 10Maxwell-Boltzmann distribution
Definition
Definition a curve showing the spread of kinetic energies of particles at a given temperature.
Shape
- Shape starts at the origin, peaks at the most probable energy and approaches but never touches the x-axis.
Area
- Area equals the total number of particles and stays constant when T changes.
Higher temperature
- Higher temperature the peak is lower and shifts right, the curves cross once and the high-energy tail is higher.
Catalyst
- Catalyst moves the Ea line to the left on the same curve, so the whole area to the right of the new Ea can react.
Part 9 of 10Why temperature raises the rate
1
Temperature increases.
2
Average kinetic energy increases.
3
The Maxwell-Boltzmann curve flattens and shifts right.
4
More particles have energy ≥ Ea.
5
More successful collisions occur per second.
6
The rate increases.
Part 10 of 10More from the specification
More from the specification
Nuclear magnetic resonance identification
- Nuclear magnetic resonance identification measuring chemical shifts and proton spin-spin splitting provides a definitive means to determine organic molecular connectivity.
Reaction rate definition
- The change in concentration of a reactant or product per unit time, measured in mol dm-3 s-1.
Maxwell-Boltzmann distribution curve
- A probability density distribution displaying the range of molecular kinetic energies present at a given absolute temperature.
Collision theory requirements
- Successful chemical reactions occur only when collisions result in particle kinetic energy exceeding activation energy (E >= Ea) with suitable geometry.
Reaction energy profiles
- Graphical diagrams showing potential energy versus reaction progress, illustrating activation energy barriers with and without catalysts.
Maxwell-Boltzmann distribution
- A graph showing how particle energies are spread at a given temperature.
Energy profile
- Diagram showing energy during a reaction
Key terms
| Term | Meaning |
|---|---|
| Rate of reaction | The change in concentration of a reactant or product per unit time. |
| Collision theory | Reactions occur when particles collide with energy ≥ Ea and the correct orientation. |
| Activation energy | The minimum energy for a collision to lead to reaction. |
| Catalyst | A substance that speeds a reaction by giving a lower-Ea route and is unchanged at the end. |
| Maxwell-Boltzmann distribution | The spread of kinetic energies of particles at a given temperature. |
| Quenching | Stopping a reaction in a sample so it can be analysed. |
Avoid these mistakes
1
Giving a negative rate — quote the rate as a positive value.
2
Saying a catalyst changes ΔH or the equilibrium position — it lowers Ea only.
3
Explaining the effect of temperature by collision frequency alone — the energy effect dominates.
4
Drawing Maxwell-Boltzmann curves that touch the x-axis or cross twice — they approach the axis and cross only once.
Topic quiz · 47 questionsQuiz for this lesson
Last updated:
\n