iGCSE Chemistry (0620) Theory (Core):12.1 Experimental design: Exam Style Questions Paper 3- New Syllabus
Question
- carries out the experiment at \(25^\circ\mathrm{C}\) using \(2.00 \, \mathrm{g}\) of magnesium ribbon and \(25 \, \mathrm{cm}^3\) of dilute sulfuric acid
- measures the total volume of gas produced at regular time intervals
- plots a graph of the results.
Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 7.1 — Characteristic properties of acids and bases (Part (a))
• Topic 12.1 — Experimental design (Part (b)(i))
• Topic 6.2 — Rate of reaction (Parts (b)(ii)–(vi))
• Topic 12.5 — Identification of ions and gases (Part (c))
▶️ Answer/Explanation
(a)
For the correct answer:
magnesium sulfate
The reaction is: magnesium + sulfuric acid → magnesium sulfate + hydrogen. The salt magnesium sulfate (\(\mathrm{MgSO_4}\)) is named from the metal (magnesium) and the acid (sulfuric acid → sulfate).
(b)(i)
For the correct diagram (3 marks):
• conical flask (1)
• gas syringe / syringe (1)
• connection from conical flask to gas syringe which will not allow any gas to escape (1)
The apparatus must be sealed to prevent gas loss. Magnesium ribbon is placed in the conical flask with dilute sulfuric acid. A delivery tube connects the flask to a gas syringe. As hydrogen gas is evolved, it pushes the syringe plunger, allowing the volume of gas to be measured directly at various time intervals.
(b)(ii)
\(30 \, (\mathrm{cm}^3)\)
Reading directly from the graph at \(t = 100 \, \mathrm{s}\), the interpolated volume on the y-axis corresponds to \(30 \, \mathrm{cm}^3\).
(b)(iii)
(one) reactant has been (completely) used up
The reaction stops because the limiting reactant — either all the magnesium ribbon has dissolved, or all the sulfuric acid molecules have been consumed — is no longer available to react. The graph plateauing at \(300 \, \mathrm{s}\) confirms no further gas production.
(b)(iv)
For the correct line (3 marks):
• steeper initial gradient starting at \((0,0)\) (1)
• levels off before \(300 \, \mathrm{s}\) (1)
• line levels off at \(80 \, \mathrm{cm}^3\) (1)
At a higher temperature, reactant particles possess greater kinetic energy. This results in a higher frequency of collisions and a greater proportion of collisions with energy exceeding the activation energy (\(E \geq E_\mathrm{a}\)). Consequently, the initial rate is faster (steeper curve slope). However, the same masses of reactants produce the same final volume of hydrogen; the reaction simply finishes sooner.
(b)(v)
decreasing the concentration: (rate) decreases / gets slower / slows down
Lower concentration means fewer acid particles per unit volume of solution. According to collision theory, this reduces the frequency of effective collisions between reactant particles per unit time, slowing the reaction rate.
(b)(vi)
catalyst: (rate) increases / gets faster / speeds up
A catalyst provides an alternative reaction pathway having a lower activation energy (\(E_\mathrm{a}\)). More colliding particles now possess the minimum energy required to react, increasing the frequency of successful collisions and thus the reaction rate. The catalyst itself remains chemically unchanged at the end.
(c)
test: lighted splint
observations: pops / explosion
Hydrogen gas is collected, e.g., in an inverted test tube. A burning splint applied to the mouth of the tube ignites the hydrogen, which reacts explosively with oxygen in the air to form water (\(2\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}\)), producing a characteristic squeaky pop sound.