Chemistry 9701/51 — May/June 2021

Background Concept

For an experiment to be a fair test, only the independent variable should affect the dependent variable. All other factors that could influence the rate of reaction must be kept constant; these are controlled variables.

Factors affecting the rate of a heterogeneous catalysed reaction (solid catalyst, liquid reactant) include:

• Concentration of the reactant (more particles = more frequent collisions).
• Temperature (higher kinetic energy = more frequent and more energetic collisions).
• Surface area of the solid catalyst (more exposed particles = more active sites for reaction).
• Volume of the reactant solution (must be consistent to ensure comparable conditions, though technically concentration is the key factor, volume is often controlled to keep the total system volume constant).

Understanding the Question

The student is testing different metal oxides. To ensure that any difference in the initial rate is only due to the identity of the metal oxide, all other conditions must be identical.

Approach

List the standard factors that affect reaction rates and select two that are relevant to this specific experimental setup.

Step-by-Step Reasoning

• Concentration of H₂O₂: If one trial uses a more concentrated solution, the rate will naturally be higher, ruining the comparison. Must be controlled.
• Volume of H₂O₂: While concentration is the primary factor, keeping the volume constant ensures the total amount of reactant and the depth of the liquid (affecting pressure/temperature distribution) are consistent.
• Temperature: Reaction rates are highly temperature-dependent. The experiment must be conducted at a constant temperature (e.g., room temperature, or in a water bath).
• Particle size of the metal oxide: The reaction occurs on the surface of the solid catalyst. A finer powder has a larger surface area and will react faster than a large lump, even if the mass is the same. Particle size must be controlled.
• Selecting any two of these provides the required answer.

Key Takeaways

When planning a kinetics experiment, always consider the collision theory factors: concentration, temperature, surface area, and catalyst mass/type. All must be controlled except the one being investigated.

Common Mistakes

• Suggesting 'the mass of the metal oxide' as a controlled variable without also mentioning 'particle size'. If you change the particle size but keep the mass the same, the surface area changes, altering the rate. (Though mass is often controlled, surface area/particle size is the more scientifically precise answer for heterogeneous catalysis).
• Suggesting 'the time' as a controlled variable. Time is what you measure or set as an interval; it is not a condition of the reaction environment.

Things to Be Careful About

The mark scheme accepts 'particle size'. 'Surface area' is also acceptable and is the underlying physical reason why particle size must be controlled. Ensure you specify which concentration or volume (e.g., 'concentration of hydrogen peroxide', not just 'concentration').

Background Concept

To measure the rate of a reaction that produces a gas, the gas must be collected and its volume measured over time. Two common methods are:

1. Over water: The gas is collected in an inverted measuring cylinder or burette filled with water, submerged in a water trough. The gas displaces the water, and the volume is read from the graduations.
2. Gas syringe: The gas is collected directly in a calibrated gas syringe, avoiding the solubility issues of some gases in water (though O₂ is only slightly soluble, making the water method acceptable here).

The apparatus must be gas-tight (sealed) so that all generated gas is directed into the collection device and not lost to the atmosphere.

Understanding the Question

The student needs to collect oxygen gas accurately to measure its volume over time. The question asks for a labelled diagram of the assembled apparatus.

Approach

Draw a standard gas-collection-over-water setup or a gas syringe setup. Ensure all key components are labelled: the reaction vessel, the seal, the delivery tube, and the gas collection device.

Step-by-Step Reasoning

• Reaction vessel: A conical flask is ideal as it can be stoppered and allows for easy addition of the solid catalyst to the liquid. It must contain the reactants: hydrogen peroxide (liquid) and the metal oxide (solid).
• Seal: The flask must be sealed with a rubber bung (stopper) to prevent gas from escaping.
• Delivery tube: A delivery tube (or glass tube with rubber tubing) must pass through the bung to transport the gas from the flask to the collection device.
• Collection device: An inverted measuring cylinder (with graduation marks) filled with water and placed in a trough of water is a standard, accurate method. Alternatively, a gas syringe is acceptable and often more accurate as it doesn't rely on water displacement.
• Labels: The diagram must clearly label the contents of the flask ('hydrogen peroxide', 'metal oxide') and the collection device ('measuring cylinder' or 'gas syringe').

Key Takeaways

Gas collection apparatus must be a closed system from the reaction vessel to the measuring device. Always label the contents of the reaction vessel and the measuring device.

Common Mistakes

• Forgetting to draw a stopper/bung in the flask. Without a seal, the gas will escape through the top of the flask, and the volume collected will be inaccurate.
• Drawing the delivery tube extending into the liquid in the flask. The delivery tube should just pass through the bung; if it is in the liquid, the gas will have to bubble through the liquid, which is unnecessary and could cause pressure buildup or liquid to be pushed out.
• Not labelling the contents. The mark scheme specifically rewards identifying 'contents of flask'.

Things to Be Careful About

• If drawing the water displacement method, ensure the measuring cylinder is inverted and submerged in water. The delivery tube should be positioned under the mouth of the inverted cylinder.
• State symbols are not required in an apparatus diagram, but clear labels are essential.
• A gas syringe is a perfectly valid alternative and is often preferred in modern practicals to avoid the slight solubility of oxygen in water and the difficulty of reading a meniscus in a small cylinder.

Background Concept

Reliability refers to the consistency of results. If an experiment is repeated and gives similar results, it is reliable. Reliability is improved by repeating measurements and identifying/correcting anomalies.

Accuracy refers to how close a result is to the true value. This is improved by using better apparatus or correcting systematic errors.

Understanding the Question

The question asks how to ensure the results are reliable. This is a standard practical skills question about experimental design.

Approach

State the standard method for improving reliability in any quantitative experiment.

Step-by-Step Reasoning

• To check if the results are consistent, the student should repeat the experiment for each metal oxide.
• This means running the reaction with the same metal oxide multiple times (e.g., 3 times).
• The results can then be compared. If they are close (concordant), the results are reliable. Anomalies can be spotted and excluded, and a mean can be calculated.

Key Takeaways

'Repeat and calculate a mean' or 'repeat until concordant results are obtained' are the standard answers for improving reliability.

Common Mistakes

• Saying 'use better apparatus'. This improves accuracy, not necessarily reliability.
• Saying 'do it more carefully'. This is too vague and not a specific scientific method.
• Confusing reliability with validity. Validity is about whether the experiment tests what it is supposed to test (controlled variables); reliability is about consistency (repeats).

Things to Be Careful About

Be specific. 'Repeat the experiments' is good. 'Repeat until consistent results' is better. Mentioning 'calculate a mean' or 'identify anomalies' adds depth and is often rewarded.

Background Concept

The rate of a reaction can be monitored by measuring the change in any physical property that is proportional to the amount of reactant used or product formed. For a reaction producing a gas:

1. Volume of gas (collected over water or in a gas syringe) — increases over time.
2. Mass of the system (reaction vessel + contents) — decreases over time as gas escapes into the atmosphere.

Understanding the Question

The question asks for an alternative method that does not involve the collection of gas (i.e., not measuring volume). How else can we monitor the production of oxygen gas?

Approach

Think about what happens to the system when a gas is produced and escapes. The total mass decreases.

Step-by-Step Reasoning

• The reaction is: $2\text{H}_2\text{O}_2(\text{aq}) \rightarrow 2\text{H}_2\text{O}(\text{l}) + \text{O}_2(\text{g})$.
• If the reaction vessel is left open to the atmosphere (e.g., a conical flask on a balance), the oxygen gas will escape.
• The total mass of the flask and its contents will decrease as the oxygen leaves.
• By placing the reaction vessel on a balance and recording the mass at regular time intervals, the student can plot mass vs. time. The gradient of this graph (or the rate of mass loss) gives the rate of reaction.

Key Takeaways

Mass loss is a standard alternative to gas volume collection for reactions that produce a gas that escapes the system.

Common Mistakes

• Suggesting 'measuring the concentration of hydrogen peroxide'. This is difficult and slow to do (e.g., via titration) and not practical for monitoring initial rates in real-time.
• Suggesting 'measuring the temperature'. The reaction is slightly exothermic, but the temperature change is small and slow to measure, making it a poor method for initial rates.

Things to Be Careful About

Be specific. 'Measuring the mass' is not enough; you must specify 'measuring the decrease in mass' or 'mass loss' over time. Mentioning the use of a 'balance' or 'digital balance' is good practice.

Background Concept

A catalyst is a substance that increases the rate of a reaction without being used up or chemically changed at the end of the reaction. To demonstrate this experimentally, you must show that the catalyst's mass and chemical identity (and thus its catalytic activity) remain unchanged after the reaction.

Understanding the Question

The student has finished the reaction. How can they prove the metal oxide is still the same catalyst and hasn't been consumed or altered?

Approach

Provide two valid experimental methods to verify the catalyst's properties post-reaction: one based on mass conservation, one based on reactivity.

Step-by-Step Reasoning

• Method 1 (Mass check):
  1. The metal oxide is a solid in a liquid mixture. Separate it by filtering (or centrifuging).
  2. Wash the residue to remove any adhering hydrogen peroxide or water.
  3. Dry the solid (e.g., in an oven or between filter papers).
  4. Weigh the dried solid. If the mass is the same as the mass weighed before the reaction, it has not been consumed.
• Method 2 (Reactivity check):
  1. Take the recovered solid metal oxide.
  2. Add it to a fresh sample of hydrogen peroxide (of the same concentration and volume as before).
  3. Measure the rate of reaction (e.g., volume of gas in a set time).
  4. If the rate is the same as in the original experiment, the catalyst has not been deactivated or chemically changed.

Key Takeaways

To prove a catalyst is unchanged, show that its mass is conserved and its catalytic activity is retained.

Common Mistakes

• Saying 'it looks the same'. Visual appearance is not proof of chemical identity or mass conservation.
• Forgetting to dry the catalyst before weighing. Wet catalyst will have a higher mass due to water, giving a false result.
• Not specifying 'fresh' hydrogen peroxide in Method 2. If you reuse the same mixture, the hydrogen peroxide is already decomposed, so no reaction will occur.

Things to Be Careful About

The mark scheme accepts either method (mass or reactivity). Both require two clear steps (e.g., filter+dry+weigh, or reuse+compare rate). Ensure the steps are logically ordered.

Background Concept

Hydrogen peroxide ($\text{H}_2\text{O}_2$) is unstable and slowly decomposes at room temperature into water and oxygen gas:
$2\text{H}_2\text{O}_2(\text{aq}) \rightarrow 2\text{H}_2\text{O}(\text{l}) + \text{O}_2(\text{g})$
This decomposition is accelerated by light, heat, and impurities (including the metal ions in the bottle cap or dust).

Understanding the Question

A bottle of aqueous hydrogen peroxide has a small hole in the lid. Why is this necessary for storage?

Approach

Consider the physical consequences of a gas being produced in a sealed container.

Step-by-Step Reasoning

• As hydrogen peroxide slowly decomposes, oxygen gas is produced.
• If the bottle were completely sealed, the oxygen gas would have nowhere to go.
• The gas would build up, increasing the pressure inside the bottle.
• Over time, the pressure could become high enough to deform or burst the plastic bottle, or blow the lid off, creating a safety hazard.
• The small hole allows the oxygen to diffuse out (escape) slowly, keeping the internal pressure equal to atmospheric pressure.

Key Takeaways

Containers for substances that slowly release gas must have a vent to prevent pressure build-up and potential explosion.

Common Mistakes

• Saying 'to let the air in'. The issue is gas escaping, not air entering.
• Saying 'to prevent it from going bad'. While true that decomposition degrades the peroxide, the hole specifically addresses the physical pressure hazard, not the chemical degradation (which is why peroxide is stored in dark, opaque bottles).

Things to Be Careful About

Focus on the pressure aspect. 'To allow gas to escape' is the core answer. Adding 'to prevent pressure build-up' or 'to prevent the bottle from bursting' demonstrates a deeper understanding of the physical chemistry involved.