Chemistry · 08 / 10
Kinetics: rate and reaction time
Final-year concepts · Worked example and 4 exercises with solutions, at your pace.
The principle, simply
A rate measures change per unit time. A quantity or concentration curve gives a slope whose sign depends on whether a reactant or product is tracked. Temperature, concentration and catalysts can affect rate. A catalyst provides a different reaction pathway; it does not itself shift thermodynamic equilibrium.
- 01Tracked quantity
- 02Time interval
- 03Slope and sign
- 04Model conditions
WORKED EXAMPLE
From principle to reasoning
A product rises from 0.10 to 0.16 mol/L in 30 s. Its average formation rate is 0.002 mol/L/s. An instantaneous rate requires local slope.
Solve a problem in this subject
- Identify species
Write formulas and useful states. Distinguish atoms, ions, molecules and amount of substance. Total charge is conserved.
- Connect quantities
Use m = nM and C = n/V with consistent units. V is the final solution volume.
- Use coefficients
Balance before building an extent table. Compare initial amounts divided by coefficients to identify the limiting reactant.
- Check context
Remaining amounts cannot be negative. Check conservation, units and assumptions: dilution, medium, completion or equilibrium.
YOUR TURN
01 · Try before checking
A reactant falls from 0.50 to 0.38 mol/L in 60 s. Find its average rate of change.
One hint
Final minus initial concentration, divided by elapsed time.
Reveal and understand the solution
−0.12/60 = −0.002 mol/L/s. Negative means disappearance. A positive reaction rate also depends on convention and stoichiometric coefficient.
Compare method, conditions and answer; identify the exact part to revisit.
IMPROVE THROUGH PRACTICE
Three additional challenges with solutions
Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.
Product concentration rises from 0.02 to 0.08 mol/L in 30 s. Find average appearance rate.
A hint to get started
Use the change, not just final concentration.
Step-by-step solution
- Δc = 0.08 − 0.02 = 0.06 mol/L.
- Average rate = 0.06/30.
- Rate = 0.002 mol/L/s.
Which step did you miss? Retry without help, then explain why the method works.
How can c(t) estimate instantaneous rate at 20 s?
A hint to get started
Instantaneous rate is a local slope.
Step-by-step solution
- Draw the tangent at 20 s.
- Use two points on that tangent to calculate Δc/Δt.
- A distant secant gives only an average rate.
Which step did you miss? Retry without help, then explain why the method works.
Does a catalyst increase the final equilibrium amount by speeding a reaction?
A hint to get started
Distinguish reaction speed from equilibrium position.
Step-by-step solution
- It provides a faster reaction pathway.
- It speeds the approach to equilibrium in both directions.
- It does not change the equilibrium constant at fixed temperature.
Which step did you miss? Retry without help, then explain why the method works.
The trap to avoid
Average and instantaneous slopes differ; a catalyst does not increase equilibrium yield.
Without notes, can you…
- Explain the principle in your own words?
- Rebuild the diagram and its connections?
- Solve the exercise and justify the method?
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