This question is about zinc and magnesium - AQA - GCSE Chemistry - Question 4 - 2014 - Paper 1
Question 4
This question is about zinc and magnesium.
Zinc is produced by electrolysis of molten zinc chloride, as shown in Figure 4.
Figure 4
Negative electrode
Positive el... show full transcript
Worked Solution & Example Answer:This question is about zinc and magnesium - AQA - GCSE Chemistry - Question 4 - 2014 - Paper 1
Step 1
Why must the zinc chloride be molten for electrolysis?
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Answer
Zinc chloride must be molten for electrolysis because it allows for the movement of ions, ensuring that an electric current can flow. In a solid state, the ionic structure prevents ions from moving freely, which is essential for the process.
Step 2
Describe what happens at the negative electrode.
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At the negative electrode, reduction occurs as zinc ions (extZn2+) gain electrons. The half-equation can be represented as:
ightarrow ext{Zn}$$
This results in the formation of solid zinc.
Step 3
Complete the half equation for the reaction at the positive electrode.
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The completed half equation for the reaction at the positive electrode is:
ightarrow ext{Cl}_2 + 2e^-$$
Step 4
How can you tell from the equation that the reaction is done at a high temperature?
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The presence of magnesium as a product indicates that the reaction is likely performed at high temperatures, as magnesium is typically produced through reduction of its oxide requiring high energy conditions.
Step 5
What is meant by an endothermic reaction?
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An endothermic reaction is one that absorbs energy from the surroundings, usually in the form of heat. This energy is absorbed to break bonds in the reactants, allowing the reaction to proceed.
Step 6
Calculate the mass of magnesium oxide needed to produce 1.2 tonnes of magnesium.
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To calculate the mass of magnesium oxide (MgO) required, we start with the reaction stoichiometry:
For the equation:
ightarrow 2 ext{Mg} + ext{SiO}_2$$
1 mole of MgO produces 1 mole of Mg. Given:
- Molecular mass of Mg = 24 g/mol
- Molecular mass of O = 16 g/mol
- Molecular mass of MgO = 40 g/mol
To produce 1.2 tonnes (or 1200 kg = 1,200,000 g) of magnesium:
Mass of MgO required =
$$ rac{1,200,000 ext{ g}}{2} imes 40 ext{ g/mol} = 2,400,000 ext{ g} = 2.4 ext{ tonnes}$$
Step 7
Calculate the percentage yield.
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The percentage yield can be calculated using the formula:
Give one reason why the calculated yield of magnesium might not be obtained.
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One reason for not obtaining the calculated yield is that some of the magnesium may have escaped or reacted in an unintended way during the production process.
