This question is about the development of lithium cells. The value of E° for lithium suggests that a lithium cell could have a large EMF. Table 9 shows some electrode potential data. Table 9 Li+(aq) + e- → Li(s) E° / V -3.04 2H2O(l) + 2e- → H2(g) + 2OH-(aq) -0.83 1/2 Cl2(g) + e- → Cl-(aq) +0.54 1) Use data in Table 9 to explain why an aqueous electrolyte is not used for a lithium cell. [2 marks] 2) In the 1970s lithium-iodine cells became a common power source for heart pacemakers. Lithium iodide is the final product of the cell reaction. Use data in Table 9 to calculate the cell EMF of a standard lithium-iodine cell. [1 mark] 3) An EMF value for a commercial lithium-iodine cell is 2.80 V. Suggest why this value is different from the value calculated in Question 09.2. [1 mark] 4) In some lithium cells, lithium perchlorate (LiClO4) is used as the electrolyte. Deduce the oxidation state of chlorine in LiClO4. [1 mark] 5) In other lithium cells, lithium cobalt oxide electrodes and lithium electrodes are used. Give an equation for the reaction that occurs at the positive lithium cobalt oxide electrode. [1 mark] 6) Give an equation for the reaction that occurs at the negative lithium electrode. [1 mark]

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This question is about the development of lithium cells - AQA - A-Level Chemistry - Question 9 - 2021 - Paper 1

Question 9

This question is about the development of lithium cells. The value of E° for lithium suggests that a lithium cell could have a large EMF. Table 9 shows some elect... show full transcript

Worked Solution & Example Answer:This question is about the development of lithium cells - AQA - A-Level Chemistry - Question 9 - 2021 - Paper 1

Step 1

Use data in Table 9 to explain why an aqueous electrolyte is not used for a lithium cell.

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Answer

Lithium is highly reactive and can potentially react with water to produce hydrogen gas, represented by the reaction:

ightarrow 2LiOH + H_2$$ Furthermore, the electrode potential for lithium is -3.04 V, which indicates it has a substantial tendency to reduce water, making an aqueous electrolyte unsuitable.

Step 2

Use data in Table 9 to calculate the cell EMF of a standard lithium-iodine cell.

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Answer

To calculate the cell EMF, we observe the half-reactions. Assuming that lithium iodide is formed from lithium and iodine, we use the electrode potential for lithium:

From Li+(aq) + e- → Li(s) : E° = -3.04 V
From 1/2 I2(g) + e- → I-(aq) : (not given but assumed from standard values) let's assign E° = +0.54 V for the iodine half-reaction.

The total cell EMF (E°cell) can be calculated as follows:

$E°_{cell} = E°_{cathode} - E°_{anode} = 0.54 - (-3.04) = 3.58 ext{ V}$

Step 3

Suggest why this value is different from the value calculated in Question 09.2.

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Answer

The EMF value of a commercial lithium-iodine cell is 2.80 V, which is lower than the theoretical value calculated. This discrepancy could be due to non-standard conditions such as concentration or temperature differences, or internal resistance within the cell that reduces its effective EMF.

Step 4

Deduce the oxidation state of chlorine in LiClO4.

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Answer

To deduce the oxidation state of chlorine in LiClO4, we consider that lithium (Li) has an oxidation state of +1, and each oxygen (O) has an oxidation state of -2. The oxidation state of chlorine (Cl) can be determined by evaluating the overall charge balance:

Let the oxidation state of Cl be x.

$+1 + x + 4(-2) = 0$
$x - 8 + 1 = 0$
$x = +7$

Thus, the oxidation state of chlorine in LiClO4 is +7.

Step 5

Give an equation for the reaction that occurs at the positive lithium cobalt oxide electrode.

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Answer

At the positive lithium cobalt oxide electrode, lithium ions are released into the electrolyte as cobalt oxide gets reduced. The half-reaction can be written as:

ightarrow CoO_2 + Li_2O$$

Step 6

Give an equation for the reaction that occurs at the negative lithium electrode.

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Answer

At the negative lithium electrode, lithium ions gain electrons to form solid lithium. The reaction can be expressed as:

ightarrow Li(s)$$

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This question is about the development of lithium cells - AQA - A-Level Chemistry - Question 9 - 2021 - Paper 1

Worked Solution & Example Answer:This question is about the development of lithium cells - AQA - A-Level Chemistry - Question 9 - 2021 - Paper 1

Use data in Table 9 to explain why an aqueous electrolyte is not used for a lithium cell.

Use data in Table 9 to calculate the cell EMF of a standard lithium-iodine cell.

Suggest why this value is different from the value calculated in Question 09.2.

Deduce the oxidation state of chlorine in LiClO4.

Give an equation for the reaction that occurs at the positive lithium cobalt oxide electrode.

Give an equation for the reaction that occurs at the negative lithium electrode.

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