Adaptive practice for this skillA separate, individually graded pool that adapts to your answers: 11 validated questions, plus generated practice on the same skill.

Picture it first

\ln[A] = \ln[A]_0 - kt

First order: every half-life is the same length (dashed steps) — that is the fingerprint of first-order decay.

Grade 12 · Practice problems

Reaction rate: definition and measurement

53 written problems across two levels — 48 foundational · 5 intermediate. Work through them on paper, then reveal every answer to check. These are not graded and are counted separately from adaptive practice.

Foundational

1. FoundationalDefine reaction rate and state its usual units.

2. Foundational $[\mathrm{A}]$ falls from $1.20\ \text{mol/L}$ to $0.60\ \text{mol/L}$ in $10\ \text{s}$ . Find the average rate.

3. FoundationalA product's concentration rises from $0$ to $0.36\ \text{mol/L}$ in $9.0\ \text{s}$ . Find the average rate of formation.

4. FoundationalA reaction gives off $45\ \text{mL}$ of gas in $9.0\ \text{s}$ . Find the average rate of gas production.

5. FoundationalAs a gas escapes, a flask's mass falls from $100.0\ \text{g}$ to $91.0\ \text{g}$ in $3.0\ \text{s}$ . Find the rate of mass loss.

6. FoundationalA reactant is measured at $[\mathrm{A}]=0.80,\ 0.50,\ 0.30\ \text{mol/L}$ at $t=0,\ 10,\ 20\ \text{s}$ . Find the average rate over the first $10\ \text{s}$ .

7. FoundationalUsing the same data ( $0.80,0.50,0.30$ at $0,10,20\ \text{s}$ ), find the average rate over the interval $10$ – $20\ \text{s}$ .

8. FoundationalFrom the previous two answers ( $0.030$ then $0.020$ ), is the reaction speeding up or slowing down, and why?

9. FoundationalOn a concentration–time graph, the instantaneous rate at a point equals the slope of the ___.

10. FoundationalWhy do chemists often quote the initial rate of a reaction rather than a single overall rate?

11. FoundationalFor $\mathrm{N_2 + 3H_2 \rightarrow 2NH_3}$ , $\mathrm{N_2}$ is consumed at $0.10\ \mathrm{mol\,L^{-1}\,s^{-1}}$ . How fast is $\mathrm{H_2}$ consumed?

12. FoundationalFor $\mathrm{N_2 + 3H_2 \rightarrow 2NH_3}$ , $\mathrm{NH_3}$ is forming at $0.50\ \mathrm{mol\,L^{-1}\,s^{-1}}$ . How fast is $\mathrm{H_2}$ being consumed?

13. FoundationalFor $\mathrm{2H_2O_2 \rightarrow 2H_2O + O_2}$ , $\mathrm{O_2}$ forms at $0.20\ \mathrm{mol\,L^{-1}\,s^{-1}}$ . How fast does $\mathrm{H_2O_2}$ decompose?

14. FoundationalFor $\mathrm{2SO_2 + O_2 \rightarrow 2SO_3}$ , $\mathrm{SO_3}$ forms at $0.30\ \mathrm{mol\,L^{-1}\,s^{-1}}$ . How fast is $\mathrm{O_2}$ consumed?

15. FoundationalFor $\mathrm{4Fe + 3O_2 \rightarrow 2Fe_2O_3}$ , $\mathrm{O_2}$ is consumed at $0.030\ \mathrm{mol\,s^{-1}}$ . How fast is $\mathrm{Fe}$ consumed, in $\mathrm{mol\,s^{-1}}$ ?

16. FoundationalWrite the relationship between the rate of appearance of $\mathrm{NH_3}$ and the rate of disappearance of $\mathrm{N_2}$ for $\mathrm{N_2 + 3H_2 \rightarrow 2NH_3}$ .

17. FoundationalWhich measurable property would you follow for a reaction whose product is coloured?

18. FoundationalWhich property would you monitor for a reaction that changes the number of dissolved ions?

19. FoundationalIn a rigid closed vessel at constant temperature, why can the total pressure be used to follow a reaction that changes the number of moles of gas?

20. FoundationalIsothermal compression of a fixed amount of reacting gas increases which concentration-related quantity?

21. FoundationalA reaction makes $0.24\ \text{mol/L}$ of product in the first $6.0\ \text{s}$ but only $0.06\ \text{mol/L}$ more in the next $6.0\ \text{s}$ . What does this show?

22. FoundationalFor a reaction whose rate falls as its reactant is consumed, where is the reactant's concentration–time curve steepest?

23. FoundationalA reactant halves in the first $20\ \text{s}$ and halves again over the next $40\ \text{s}$ . Is the reaction rate constant over these two intervals?

24. FoundationalFor $\mathrm{2A \rightarrow B}$ , $\mathrm{B}$ forms at $0.15\ \mathrm{mol\,L^{-1}\,s^{-1}}$ . How fast is $\mathrm{A}$ consumed?

25. FoundationalTwo reactions are plotted on the same axes, following the same kind of species in the same concentration and time units. Reaction 1's curve is steeper at the start. Which has the greater initial rate?

26. FoundationalAt constant temperature a gas reaction follows $r=k[\text{A}][\text{B}]$ . The volume is suddenly halved while the amounts of A and B are unchanged. By what factor does the rate change immediately?

27. FoundationalA tangent drawn to a concentration–time curve at $t=15\ \text{s}$ passes through the points $(10\ \text{s},\ 0.60\ \text{mol/L})$ and $(20\ \text{s},\ 0.40\ \text{mol/L})$ . Find the instantaneous rate at $t=15\ \text{s}$ .

28. FoundationalFor a reaction that starts with no product and whose rate falls as reactant is consumed, explain why the initial rate is the largest instantaneous rate.

29. FoundationalA rate is reported as $1.2\ \text{mol}\,\text{L}^{-1}\,\text{min}^{-1}$ . Convert it to $\text{mol}\,\text{L}^{-1}\,\text{s}^{-1}$ .

30. FoundationalDistinguish a continuous method from a sampling method for following a reaction, giving one example of each.

31. FoundationalIn a clock reaction the sharp colour change appears after $12\ \text{s}$ in a warm run and after $36\ \text{s}$ in a cold run. Using rate $\propto 1/t$ , how many times faster is the warm run?

32. FoundationalFor $2\text{A}+\text{B}\rightarrow 3\text{C}$ , $\text{C}$ forms at $0.90\ \text{mol}\,\text{L}^{-1}\,\text{s}^{-1}$ . Find the unique rate of reaction.

33. FoundationalFor that same reaction $2\text{A}+\text{B}\rightarrow 3\text{C}$ with unique rate $0.30\ \text{mol}\,\text{L}^{-1}\,\text{s}^{-1}$ , how fast is $\text{A}$ being consumed?

34. FoundationalWhy can the orders in a rate law not be read off the coefficients of the balanced equation?

35. FoundationalIn a rate study, tripling $[\text{A}]$ (all else fixed) makes the initial rate nine times larger. What is the order with respect to $\text{A}$ ?

36. FoundationalIn a rate study, doubling $[\text{B}]$ leaves the initial rate unchanged. What is the order with respect to $\text{B}$ ?

37. FoundationalA reaction starts with no product present and its rate falls as reactant is consumed. Describe the shape of its product-concentration curve and state where the slope is greatest.

38. FoundationalA mixture is prepared containing no product. Why is the reverse reaction negligible at the very start?

39. FoundationalA reaction is monitored by conductivity because the number of ions changes. Give a reaction feature that would make conductivity a poor choice instead.

40. FoundationalFor $4\text{NH}_3+5\text{O}_2\rightarrow 4\text{NO}+6\text{H}_2\text{O}$ , $\text{O}_2$ is consumed at $0.50\ \text{mol}\,\text{L}^{-1}\,\text{s}^{-1}$ . How fast is $\text{NO}$ formed?

41. FoundationalFor a reaction with a positive order in its reactant, why is the reactant curve steepest at the start rather than at the end?

42. FoundationalFor $\text{N}_2+3\text{H}_2\rightarrow 2\text{NH}_3$ , write the unique rate of reaction in terms of all three species.

43. FoundationalA colorimeter follows a reaction because a product is deeply coloured. What quantity is actually being measured, and how is it linked to concentration?

44. FoundationalGive one reason the rate of a positive-order reaction falls as it proceeds, and state a case where a rate stays approximately constant even though reactant is being consumed.

45. FoundationalWhy does dividing each species' rate by its coefficient let two experimenters agree on a single rate value?

46. FoundationalThe concentration of a reactant falls from 0.80 mol/L to 0.50 mol/L in 30 s. Find the average rate of reaction (in terms of that reactant).

47. FoundationalFor N₂ + 3H₂ → 2NH₃, if hydrogen is consumed at 0.60 mol/(L·s), how fast is ammonia formed?

48. FoundationalIn 2N₂O₅ → 4NO₂ + O₂, oxygen forms at 0.024 mol/(L·s). At what rate is N₂O₅ decomposing?

Intermediate

49. IntermediateA reaction releases $72\ \text{mL}$ of gas in $12\ \text{s}$ . Find the average rate of gas production in $\text{mL}\,\text{s}^{-1}$ , and explain why this is not yet a reaction rate in $\text{mol}\,\text{L}^{-1}\,\text{s}^{-1}$ .

50. IntermediateA reactant falls from $0.500$ to $0.250\ \text{mol/L}$ in $30\ \text{s}$ , then from $0.250$ to $0.125\ \text{mol/L}$ in the next $30\ \text{s}$ . Compare the two average rates and comment.

51. IntermediateA student says the average rate over a whole reaction equals the instantaneous rate at the midpoint time. Is this always true?

52. IntermediateIn two experiments at the same temperature, each changing one concentration while the other is held fixed, doubling $[\text{A}]$ multiplies the initial rate by $2$ and doubling $[\text{B}]$ multiplies it by $4$ . Write the form of the rate law.

53. IntermediateTripling $[\text{A}]$ multiplies the rate by $27$ . Determine the order in $\text{A}$ and predict the factor if $[\text{A}]$ is instead quadrupled.