AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2021: Question 5

• Gas collection apparatus selection (gas syringe vs. inverted cylinder)
• Critical evaluation of apparatus setup to prevent hazardous errors
• Graph plotting rules: drawing a smooth curve through the origin (0, 0)

• Accurate data reading from a grid scale (time vs volume)
• Mean rate calculation using the standard formula and selecting correct units ( cm³/s )
• Predicting the effect of concentration on reaction rate (initial gradient)

✅ Accepted Answers [1 Mark]

Complete the diagram by connecting either of the following at point X:

• A gas syringe connected securely to the end of the delivery tube.
• An inverted measuring cylinder (or graduated boiling tube/burette) filled with water resting in a water trough.

🧠 Diagram Instructions

❌ Common Errors

• Drawing a regular open beaker or conical flask (these cannot measure gas volume as the gas simply escapes).
• Drawing an upright test tube without measurement graduations.

✅ Mark Scheme Breakdown [2 Marks]

Error made [1 Mark] :

• The delivery tube is submerged into / dipping into the sulfuric acid.

Problem caused [1 Mark] :

• The sulfuric acid will be pushed up the delivery tube (liquid shoots into the syringe/water).
• OR no hydrogen gas will be collected / gas cannot escape through the tube.

💡 Key Knowledge

In any gas collection setup, the bottom of the delivery tube inside the reaction flask must remain well above the surface of the liquid.

As gas is generated, pressure increases inside the sealed flask. If the tube is in the liquid, the increased pressure forces the acid liquid into the delivery tube instead of the gas.

✅ Awarding the Mark [1 Mark]

A single, smooth curved line of best fit that passes through the plotted points and must start at the origin (0, 0).

🧠 Exam Technique: Drawing Curves of Best Fit

• Always check (0, 0): At time = 0 seconds, no reaction has occurred yet, so exactly 0 cm³ of gas has formed. The line must touch (0, 0).
• Draw in one smooth stroke: Turn your paper so your wrist acts as a natural pivot. Avoid "feathery" or sketched multi-stroke lines.
• Level off: The curve should level off horizontally around 250–300 s where the reaction finishes (volume reaches 60 cm³).

❌ Common Errors

• Joining the points dot-to-dot with a ruler (costs the mark immediately).
• Starting the line at the first data point (30 s, 30 cm³) instead of (0, 0).

✅ Final Answers Summary

Mean rate of reaction: 0.75

Unit: cm³/s

❌ Calculation Traps & Common Errors

• Inverted division: Calculating 60 / 45 = 1.33 . Always remember: rate = amount ÷ time!
• Wrong unit choice: Picking s/cm³ (time per volume) or g/s (used when measuring mass lost on a balance, not gas volume).
• Scale reading mistakes: Each small square on the y-axis is 2 cm³ (10 small squares = 20 cm³). Pay close attention when reading intermediate points!

✅ Correct Answer [1 Mark]

Tick box 2:

☑ "The line of best fit for higher concentration would have a steeper slope."

💡 Collision Theory Behind This

• Higher concentration means there are more acid particles per unit volume.
• Particles collide more frequently (greater frequency of successful collisions).
• A faster rate of reaction produces gas more quickly, resulting in a steeper initial gradient/slope on the graph.

🧠 Examiner Tip

Notice the question only asks about the steepness of the slope (the rate), not the final volume. The reaction starts faster, so the line rises more steeply from (0, 0).