Stearic acid is a solid at room temperature.
The diagram below shows the apparatus used for finding the melting point of stearic acid.
The apparatus was heated at a steady rate and the temperature recorded every minute.
(a) State the name of the piece of apparatus labelled
(b) (i) Suggest why the water needs to be kept stirred during this experiment.
(ii) Describe a chemical test for water.
(c) A graph of temperature of stearic acid against time of heating is shown below.
(i) What was the temperature of the stearic acid after 3 minutes heating?
(ii) Use the information on the graph to determine the melting point of stearic acid.
(d) Describe the arrangement and motion of the particles in liquid stearic acid.
(e) A sample of stearic acid contained 1% of another compound with a higher relative molecular mass.
(i) Which one of the following statements about this sample of stearic acid is correct?
Tick one box.
(ii) Describe one area of everyday life where the purity of substances is important.
▶️ Answer/Explanation
(a) The apparatus labelled:
– A: Thermometer (measures temperature of stearic acid).
– B: Beaker (holds water for heating).
(b) (i) Stirring ensures uniform heat distribution, preventing localized hot spots and ensuring accurate temperature readings.
(ii) Test: Anhydrous copper sulfate (white). Result: Turns blue in the presence of water.
(c) (i) From the graph, the temperature after 3 minutes is 48°C.
(ii) The melting point is where the temperature plateaus: 72°C.
(d) In liquid stearic acid:
– Arrangement: Particles are close but disordered.
– Motion: Particles slide past each other (fluid motion).
(e) (i) Correct statement: The melting point is different (impurities lower/alter melting point).
(ii) Purity is critical in medicine/drugs (ensures safety and efficacy).
Zinc and copper are elements next to each other in the Periodic Table.
(a) Zinc is obtained from zinc blende in a two-step process.
- In step 1, zinc blende is converted into zinc oxide.
- In step 2, zinc oxide is converted into zinc in a blast furnace.
Outline how each of these steps are done.
In your answer:
- give one chemical equation for each step
- describe how zinc is removed from the blast furnace in step 2.
(b) Name the alloy formed when zinc is mixed with copper.
(c) Copper is a transition element. It can have variable oxidation states.
State two other chemical properties of transition elements which make them different from Group I elements.
(d) A compound of copper can be used to test for water.
(i) State the full name of this compound of copper.
(ii) State the colour change that occurs when water is added to this compound of copper.
(e) Aqueous potassium iodide reacts with aqueous copper(II) sulfate to produce iodine.
(i) Balance the chemical equation for this reaction.
(ii) Deduce the charge on the copper ion in CuI.
(iii) In terms of electron transfer, explain why copper is reduced in this reaction.
(iv) Identify the reducing agent.
▶️ Answer/Explanation
(a)
Step 1: Zinc blende (ZnS) is roasted in air to form zinc oxide (ZnO) and sulfur dioxide (SO₂).
Equation: \(2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2\)
Step 2: Zinc oxide is reduced by carbon (coke) in a blast furnace to produce zinc vapor and carbon monoxide (CO).
Equation: \(ZnO + C \rightarrow Zn + CO\)
Removal of zinc: Zinc vapor is distilled and condensed to obtain pure zinc.
(b) The alloy formed is brass (a mixture of zinc and copper).
(c) Two properties of transition elements:
1. Form colored compounds (unlike Group I metals, which form colorless compounds).
2. Act as catalysts (e.g., copper in the Haber process).
(d)
(i) The compound is anhydrous copper(II) sulfate.
(ii) Color change: from white to blue (due to hydration).
(e)
(i) Balanced equation: \(4KI + 2CuSO_4 \rightarrow 2CuI + I_2 + 2K_2SO_4\)
(ii) Charge on copper in CuI: 1+ (since iodine has a 1- charge).
(iii) Copper is reduced because it gains an electron (Cu²⁺ → Cu⁺).
(iv) The reducing agent is potassium iodide (KI) or iodide ions (I⁻).