Chemistry 9701/52 — May/June 2012

Background Concept

When decomposing a solid that produces both a condensable vapour and a permanent gas, the apparatus must: (1) safely heat the solid, (2) condense and collect the vapour to prevent it from interfering with gas volume measurement, and (3) accurately measure the volume of the permanent gas.

Understanding the Question

Draw a labelled diagram showing how to heat the solid, condense the water vapour using ice, and collect the nitrogen(I) oxide gas. Use standard school laboratory equipment.

Approach

1. Heating: Use a hard glass boiling tube containing the solid, clamped and heated by a Bunsen burner. Seal with a rubber bung and delivery tube. (No water baths or hot plates as per mark scheme).
2. Condensation: Route the delivery tube into a U-tube or test tube immersed in crushed ice. This cools the water vapour to liquid, which is collected here.
3. Gas Collection: Route the remaining gas (now only $\text{N}_2\text{O}$) into a calibrated gas collecting device. A gas syringe is ideal. It must be labelled with its capacity (e.g., '$50\text{ cm}^3$ gas syringe').

Step-by-Step Reasoning

• Heating apparatus: A hard glass boiling tube is essential as it can withstand direct flame heating without cracking. It must be closed (bunged) with a delivery tube to direct the products. Label: 'hard glass boiling tube', 'Bunsen burner'.
• Water condenser: The delivery tube from the boiling tube leads into a U-tube or a test tube placed in a beaker containing crushed ice. The ice cools the water vapour to liquid water ($25\text{ }^{\circ}\text{C}$), which condenses and is trapped. Label: 'U-tube / test tube', 'crushed ice', 'beaker'. The connection to the gas collector must be gas-tight.
• Gas collector: A delivery tube from the ice-cooled condenser leads into a gas syringe. The syringe must be calibrated and labelled with its size (minimum $10\text{ cm}^3$, e.g., '$50\text{ cm}^3$ gas syringe'). Alternatively, an inverted burette or measuring cylinder could be used, but a syringe is cleaner and avoids water vapour issues if the condenser is effective.

Key Takeaways

Apparatus diagrams for multi-product decomposition must clearly show the sequence: reaction -> condensation -> gas collection. All key components must be labelled with names and sizes.

Common Mistakes

• Using a water bath or hot plate to heat the solid (mark scheme explicitly rejects this).
• Forgetting to label the gas collector with its capacity (e.g., just writing 'gas syringe' without '$50\text{ cm}^3$').
• Not making the connection between the condenser and gas collector gas-tight (if gas collection is attempted).
• Drawing a Liebig condenser without ensuring it is connected correctly and gas-tight to the collector.

Things to Be Careful About

The mark scheme requires 'minimum $10\text{ cm}^3$' for the gas collector. A $10\text{ cm}^3$ syringe is too small for practical experiments with multiple data points; a $50\text{ cm}^3$ or $100\text{ cm}^3$ syringe is more realistic. Ensure the ice is explicitly mentioned as the cooling agent for the water condenser.

Background Concept

To establish a linear relationship experimentally, you need multiple data points (at least 5) covering a reasonable range of the independent variable. You must be able to predict the expected outcomes (volumes) and calculate the required starting materials (masses) to achieve those outcomes. The experiment must have a clear, observable end-point to ensure the reaction is complete before recording data.

Understanding the Question

Design the experiment to test the prediction in (a)(i). This involves: (i) planning enough data points for a graph, (ii) stating target gas volumes, (iii) calculating the mass of reactant needed for one target volume, and (iv) defining how you know the reaction is finished.

Approach

1. Data points: Plan 5+ experiments with varying masses of $\text{NH}_4\text{NO}_3$.
2. Volumes: Choose a range of gas volumes that fit within the gas syringe (e.g., $10-30\text{ cm}^3$ in a $50\text{ cm}^3$ syringe).
3. Calculation: Use the molar volume ($24.0\text{ dm}^3\text{ mol}^{-1}$ at $25\text{ }^{\circ}\text{C}$) and stoichiometry to find the mass of $\text{NH}_4\text{NO}_3$ needed for one of the target volumes.
4. End-point: Define an observable sign that decomposition is complete (constant gas volume or solid gone).

Step-by-Step Reasoning

(i) Sufficient data:
To plot a meaningful graph as in (a)(iii), you need at least 5 data points. This means performing the experiment 5 times with 5 different masses of $\text{NH}_4\text{NO}_3$. Record the volume of gas for each.

(ii) Intended gas volumes:
Assume a $50\text{ cm}^3$ gas syringe. A good range is $10\text{ cm}^3$ to $30\text{ cm}^3$. The minimum should be $\ge 10\text{ cm}^3$ to reduce percentage reading errors. The maximum should be $\le 30\text{ cm}^3$ to leave room in the syringe and avoid exceeding its capacity. State these volumes explicitly.

(iii) Calculation:
Let's calculate the mass needed to produce $30\text{ cm}^3$ ($0.030\text{ dm}^3$) of $\text{N}_2\text{O}$.

• Moles of $\text{N}_2\text{O} = \frac{\text{volume in dm}^3}{\text{molar volume}} = \frac{0.030}{24.0} = 0.00125\text{ mol}$.
• From the equation $\text{NH}_4\text{NO}_3 \rightarrow \text{N}_2\text{O} + 2\text{H}_2\text{O}$, the ratio is 1:1. So, moles of $\text{NH}_4\text{NO}_3 = 0.00125\text{ mol}$.
• $M_r(\text{NH}_4\text{NO}_3) = 14.0 + 4.0 + 14.0 + 48.0 = 80.0\text{ g mol}^{-1}$.
• Mass = moles $\times M_r = 0.00125 \times 80.0 = 0.100\text{ g}$.
  Show this working clearly. The mark scheme allows a volume from a stated mass, but calculating mass from volume is the standard approach.

(iv) Ensuring complete decomposition:
The mark scheme specifically requires an observation, not a deduction. You cannot just say 'wait until it's all decomposed'. You must say: 'heat until the volume reading on the gas syringe stops increasing (becomes constant)' OR 'heat until all the solid white powder has disappeared'.

Key Takeaways

Experimental design requires planning the range of data, calculating required quantities, and defining clear observable end-points.

Common Mistakes

• Planning only 2 or 3 experiments (need at least 5 for a reliable graph).
• Stating volumes outside the range (e.g., $5\text{ cm}^3$ is too small for accurate reading; $40\text{ cm}^3$ might exceed a small syringe).
• Calculating mass without showing the mole conversion from volume.
• Stating 'wait until no more gas is produced' as the end-point (this is a deduction/assumption, not an observation. The observation is 'the syringe plunger stops moving' or 'the volume reading is constant').

Things to Be Careful About

• Significant figures: The molar volume is given as $24.0$ (3 s.f.), so your mass should be to 3 s.f. ($0.100\text{ g}$, not $0.1\text{ g}$).
• Units: Convert $\text{cm}^3$ to $\text{dm}^3$ when using the molar volume ($24.0\text{ dm}^3\text{ mol}^{-1}$). $30\text{ cm}^3 = 0.030\text{ dm}^3$.
• The calculation is not restricted to the maximum capacity of the collector; you can calculate for any volume in your planned range.

Background Concept

Ammonium nitrate is a well-known oxidising agent used in fertilisers and explosives. When heated, it can decompose rapidly. The key hazard is its reactivity with combustible (organic) materials, which can lead to fire or explosion. Additionally, the apparatus becomes very hot during the experiment.

Understanding the Question

Identify one hazard from the provided information and state a practical precaution to minimise the risk.

Approach

Read the 'hazcard' information in the question stem. Extract the hazard (oxidising, explosive with organics) and match it with a standard laboratory safety procedure.

Step-by-Step Reasoning

• Hazard: The text states 'Oxidising: Contact with combustible material may cause fire. Explosive when mixed with combustible material. Do not allow the salt to become contaminated with organic matter'.
• Precaution 1 (Chemical hazard): Keep the ammonium nitrate away from any combustible materials (e.g., paper, wood, solvents). Do not grind it (as grinding can generate heat/sparks). Wear chemical-resistant gloves to prevent skin contact and contamination.
• Precaution 2 (Thermal hazard): The boiling tube will be very hot. Use heat-resistant gloves or tongs to handle the apparatus after heating.

Key Takeaways

Always read the provided hazard information. Match the specific hazard (oxidising, corrosive, flammable) with the correct PPE or handling procedure.

Common Mistakes

• Stating 'ammonium nitrate is flammable' (it is an oxidiser, not combustible itself).
• Giving a vague precaution like 'be careful' or 'use safety goggles' (must be specific to the hazard identified).
• Not linking the precaution to the hazard (e.g., saying 'wear gloves' without specifying chemical-resistant, or not mentioning keeping away from combustibles).

Things to Be Careful About

The mark scheme accepts 'hot apparatus' as a hazard with 'heat resistant gloves/tongs' as the precaution. However, the primary hazard highlighted in the text is the oxidising/explosive nature. Addressing the chemical hazard is safer.

Background Concept

A data table for plotting a graph must include the raw measurements (with units) and the calculated values needed for the axes (without units, as axes are typically labelled with the quantity, e.g., 'moles'). The table should have clear headings with quantities and units.

Understanding the Question

Draw a table showing the data to be recorded and the values to be calculated to plot the graph from (a)(iii) (moles of gas vs moles of reactant).

Approach

1. Raw data columns: Mass of $\text{NH}_4\text{NO}_3$ (measured) and Volume of $\text{N}_2\text{O}$ (measured). Both need units.
2. Calculated columns: Moles of $\text{NH}_4\text{NO}_3$ (from mass) and Moles of $\text{N}_2\text{O}$ (from volume). These are dimensionless numbers (no units in the column heading).
3. Format: Use a standard table with 4 columns and at least 5 rows of data (or placeholders).

Step-by-Step Reasoning

• Column 1: 'Mass of $\text{NH}_4\text{NO}_3$ / g' or 'Mass of ammonium nitrate (g)'. This is the independent variable.
• Column 2: 'Volume of $\text{N}_2\text{O}$ / $\text{cm}^3$' or 'Volume of nitrogen(I) oxide ($\text{dm}^3$)'. This is the raw dependent variable.
• Column 3: 'Number of moles of $\text{NH}_4\text{NO}_3$' (no unit). Calculated as mass / 80.0.
• Column 4: 'Number of moles of $\text{N}_2\text{O}$' (no unit). Calculated as volume (in $\text{dm}^3$) / 24.0.
• The graph will plot Column 4 (y-axis) against Column 3 (x-axis).
• Ensure the table has at least 5 rows to show sufficient data points.

Key Takeaways

Data tables must clearly separate raw measurements (with units) from calculated values (without units in the heading). The columns must directly correspond to the axes of the final graph.

Common Mistakes

• Including units in the calculated mole columns (e.g., 'moles / mol'). The heading should just be 'Number of moles'.
• Forgetting units in the raw data columns (e.g., 'Mass of NH4NO3' without '/ g').
• Using 'Amount of NH4NO3' instead of 'Mass' (amount is ambiguous; mass or moles is required, but mass is the raw measurement).

Things to Be Careful About

The mark scheme requires exactly four fully correct columns for 2 marks. Three correct columns give 1 mark. Ensure all four are present and correctly formatted. The unit for volume can be $\text{cm}^3$ or $\text{dm}^3$, but if $\text{cm}^3$ is used, the calculation to moles must convert to $\text{dm}^3$ first (or use $24000\text{ cm}^3\text{ mol}^{-1}$).