One type of ‘breathalyser’ instrument used by police for the measurement of the concentration of alcohol in a driver’s breath is a fuel cell. An acidic electrolyte is used. Ethanol is oxidised to ethanoic acid at one electrode and oxygen from the air is converted to water at the other. The overall equation for this reaction is $$C_2H_5OH(aq) + O_2(g) ightarrow CH_3COOH(aq) + H_2O(l)$$ a. Write the equation for the half reaction at the anode. b. A motorist who has consumed alcohol blows into the fuel cell. If the breath entering the cell provides alcohol at the rate of $3.0 imes 10^{-5} ext{ g}$ per second, calculate the maximum current, in amps, that the cell would produce. c. The nature of the electrodes in the cell is essential to the effective operation of the breathalyser. State two important functions that the electrodes must perform.

SSCE VCE Chemistry Primary Galvanic Cells and Fuel Cells as Sources of Energy: One type of ‘breathalyser’ instr

One type of ‘breathalyser’ instrument used by police for the measurement of the concentration of alcohol in a driver’s breath is a fuel cell - VCE - SSCE Chemistry - Question 8 - 2005 - Paper 1

Question 8

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One type of ‘breathalyser’ instrument used by police for the measurement of the concentration of alcohol in a driver’s breath is a fuel cell. An acidic electrolyte i... show full transcript

Worked Solution & Example Answer:One type of ‘breathalyser’ instrument used by police for the measurement of the concentration of alcohol in a driver’s breath is a fuel cell - VCE - SSCE Chemistry - Question 8 - 2005 - Paper 1

Step 1

a. Write the equation for the half reaction at the anode.

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Answer

The half reaction at the anode involves the oxidation of ethanol to ethanoic acid. The balanced half reaction can be represented as:

ightarrow CH_3COOH(aq) + 4H^+(aq) + 4e^-$$

Step 2

b. A motorist who has consumed alcohol blows into the fuel cell. If the breath entering the cell provides alcohol at the rate of $3.0 imes 10^{-5} ext{ g}$ per second, calculate the maximum current, in amps, that the cell would produce.

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Answer

To calculate the maximum current, we start by determining the moles of ethanol provided per second:

Calculate moles of ethanol: $ext{Molar mass of } C_2H_5OH = 46 ext{ g/mol}$ $ext{Moles of ethanol} = \frac{3.0 \times 10^{-5} g}{46 g/mol} = 6.52 \times 10^{-7} ext{ mol}$
Using Faraday's Law, the current (I) can be calculated: $I = n \times F$ where $F = 96485 ext{ C/mol}$ (Faraday's constant).
Calculate maximum current: $I = 6.52 \times 10^{-7} \text{ mol} \times 4 \times 96485 \text{ C/mol} = 0.25 \text{ A}$

Step 3

c. The nature of the electrodes in the cell is essential to the effective operation of the breathalyser. State two important functions that the electrodes must perform.

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Answer

The electrodes in the breathalyser fuel cell perform essential functions:

Carry Current: The electrodes must conduct the flow of electrons, allowing for the electrochemical reactions to occur efficiently.
Catalyze Electrode Reactions: They must facilitate the oxidation of ethanol and the reduction of oxygen, ensuring the reactions occur at a reasonable rate.
Porous to Oxygen/Air: The electrodes should allow the diffusion of oxygen from the air to participate in the reaction effectively.

One type of ‘breathalyser’ instrument used by police for the measurement of the concentration of alcohol in a driver’s breath is a fuel cell - VCE - SSCE Chemistry - Question 8 - 2005 - Paper 1

Worked Solution & Example Answer:One type of ‘breathalyser’ instrument used by police for the measurement of the concentration of alcohol in a driver’s breath is a fuel cell - VCE - SSCE Chemistry - Question 8 - 2005 - Paper 1

a. Write the equation for the half reaction at the anode.

b. A motorist who has consumed alcohol blows into the fuel cell. If the breath entering the cell provides alcohol at the rate of $3.0 imes 10^{-5} ext{ g}$ per second, calculate the maximum current, in amps, that the cell would produce.

c. The nature of the electrodes in the cell is essential to the effective operation of the breathalyser. State two important functions that the electrodes must perform.

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