(a) Catalysts increase the rate of chemical reactions. Name the type of metals often used as catalysts. [1]
(b) A student investigated the reaction of zinc powder with excess dilute hydrochloric acid in the absence of a catalyst.
\( \mathrm{Zn} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl}_2 + \mathrm{H}_2 \)
The student measured the volume of hydrogen gas produced at 10-second intervals. The graph shows the results.
(i) Deduce the volume of hydrogen produced in the first 25 seconds of the experiment.
volume = ………………………… cm³ [1]
(ii) Explain why no more hydrogen is produced after 50 seconds. [1]
(iii) The experiment is repeated using a catalyst. Draw a line on the grid to show how the volume of hydrogen changes with time when a catalyst is used. All other conditions stay the same. [2]
(iv) Describe what effect the following changes have on the rate of the reaction.
- The concentration of hydrochloric acid is decreased.
- Large pieces of zinc are used.
All other conditions stay the same. [5]
(c)
(i) Describe a test for hydrogen. [3]
(ii) State one use of hydrogen. [1]
▶️ Answer/Explanation
(a) Transition metals (e.g., platinum, nickel) are commonly used as catalysts due to their variable oxidation states. [1]
(b)(i) 70 cm³ (Read from the graph at t = 25 s). [1]
(b)(ii) The reaction stops because all the zinc has reacted (limiting reactant is completely consumed). [1]
(b)(iii) The new line should:
- Start at (0, 0) with a steeper gradient (faster initial rate due to catalyst). [1]
- Level off at 96 cm³ (same total volume of H₂) but reach completion before 50 seconds. [1]
(b)(iv)
- Decreased HCl concentration: Rate decreases (fewer collisions between reactant particles per unit time). [1]
- Large zinc pieces: Rate decreases (smaller surface area for reactions). [1]
(c)(i)
- Test: Apply a lighted splint to the gas. [1]
- Result: Hydrogen burns with a pop sound (or explodes). [1]
(c)(ii) Fuel (e.g., in rockets or fuel cells). [1]
A teacher passed hydrogen gas over hot copper(II) oxide.
\(CuO(s) + H_2(g) → Cu(s) + H_2O(g)\)
(a) Which substance is reduced in this reaction?
Explain your answer.
(b) The diagram shows the apparatus used.
The hydrogen was passed over the hot copper(II) oxide until the reaction was complete.
(i) As the experiment proceeds, suggest what happens to the mass of copper(II) oxide.
(ii) Suggest why electrical heating is used in this experiment and not a Bunsen burner.
(iii) Describe the chemical test for the presence of water.
▶️ Answer/Explanation
(a) Ans: Copper(II) oxide is reduced.
Reduction is the gain of electrons or loss of oxygen. Here, \(CuO\) loses oxygen to form \(Cu\), so it is reduced.
(b)(i) Ans: The mass of copper(II) oxide decreases.
As \(CuO\) is converted to \(Cu\) (which has a lower molar mass), the mass in the tube decreases.
(b)(ii) Ans: Electrical heating is used because hydrogen is flammable and could ignite with a Bunsen burner.
Using a Bunsen burner poses an explosion risk when working with hydrogen gas.
(b)(iii) Ans: Test: Add anhydrous copper(II) sulfate. Result: Turns from white to blue.
Alternatively, anhydrous cobalt(II) chloride turns from blue to pink in the presence of water.