Measuring the rate of chemical reactions

1. Define reaction rate and state its usual units.

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

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

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

5. As 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. A 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. Using 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. From the previous two answers (0.030 then 0.020), is the reaction speeding up or slowing down, and why?

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

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

11. For \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. For \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. For \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. For \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. For \mathrm{4Fe + 3O_2 \rightarrow 2Fe_2O_3}, \mathrm{O_2} is consumed at 0.030\ \mathrm{mol\,L^{-1}\,s^{-1}}. How fast is \mathrm{Fe} consumed?

16. Write 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. Which measurable property would you follow for a reaction whose product is coloured?

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

19. Why does following the pressure work for a gas-producing reaction in a closed vessel?

20. Increasing the pressure of a gaseous reaction has the same effect on rate as increasing which quantity?

21. A 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. On a concentration–time graph, where is the curve steepest for a typical reaction?

23. A reactant halves in the first 20\ \text{s} and halves again over the next 40\ \text{s}. Is the rate constant?

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

25. Two reactions are followed on the same graph; reaction 1's curve is steeper at the start. Which has the greater initial rate?

26. A gas reaction is compressed to half its original volume. Qualitatively, what happens to its rate?

27. A 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. Explain why the initial rate is always the largest instantaneous rate for a typical reaction.

29. A 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. Distinguish a continuous method from a sampling method for following a reaction, giving one example of each.

31. Sketch-describe the shape of a product-concentration curve over time and state where its slope is greatest.

32. Why is a reactant curve steepest at the start rather than at the end?

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

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

35. Give one reason a reaction rate cannot stay constant throughout a typical reaction.

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