Tag: fuel cells

Questions Related to fuel cells

The overall reaction of a hydrogen-oxygen fuel cell is:

chemistry energy fuel cell fuel cells electrochemistry, rechargeable batteries, and fuel cells

  1. $2H _{2(g)}+O _{2(g)}\rightarrow 2H _2O _{(l)}$

  2. $2H _{2(g)}+4OH^- _{(aq)}\rightarrow 4H _2O _{(l)}+4e^-$

  3. $O _{2(g)}+2H _2O _{(l)}+4e^-\rightarrow 4OH^- _{(aq)}$

  4. $4OH^- _{(aq)}+4e^-\rightarrow 2H _2O _{(l)}$

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Correct Option: A
Explanation:
A fuel cell is an electrochemical cell that converts the chemical energy from a fuel into electricity through an electrochemical reaction of hydrogen fuelwith oxygen or anotheroxidizing agent.
The reactions which takes place in hydrogen-oxygen fuel cell are,
At cathode:$2H _{2(g)}+4OH^{-} _{(aq)}\rightarrow 4H _2O _{(l)}+4e^{-}$
At cathode:$O _{2(g)}+2H _2O _{(l)}+4e^{-}\rightarrow 4OH^{-} _{(aq)}$
Overall reaction: $2H _{2(g)}+O _{2(g)}\rightarrow 2H _2O _{(l)}$

$H _{2}-O _{2}$ fuel cell is:

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  1. primary cell

  2. secondary cell

  3. both

  4. cant be predicted

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Correct Option: B
Explanation:

The hydrogen-oxygen fuel cell is the secondary voltaic cell. When the direction of current is reversed, the original reactants can be regenerated and the cell can be recharged.

Hydrogen -Oxygen fuel cells are used in space craft to supply:

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  1. power for heat and light

  2. electrical power

  3. oxygen

  4. water

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Correct Option: B
Explanation:

In spacecrafts, the hydrogen oxygen fuel cell is a source of electrical power. 

Hence, option $B$ is correct.

In which reactants are not contained within the cell but are continuously supplied from external source?

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  1. Fuel cell

  2. Dry cell

  3. Lithium battery

  4. Lead storage battery

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Correct Option: A
Explanation:

In fuel cell ( such as hydrogen oxygen fuel cell), the reactants are continuously supplied from external source.

Hence, option A is correct.

Which one of the following cells can convert chemical energy of $O _{2}$ and $H _{2}$ directly into electrical energy:

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  1. mercury cell

  2. danicell cell

  3. fuel cell

  4. lead storage cell

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Correct Option: C
Explanation:

In hydrogen- oxygen fuel cell, the hydrogen is fuel and oxygen is the oxidizing agent. In this cell, the energy of combustion of hydrogen is converted into electrical energy.
Hence, option C is correct.

Which of the following statements is true for fuel cells ?

chemistry energy fuel cell fuel cells electrochemistry, rechargeable batteries, and fuel cells

  1. They are more efficient

  2. They are free from pollution

  3. They run till reactants are active

  4. All of the above

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Correct Option: D
Explanation:

The fuel cells are more efficient. The energy efficiency of fuel cells is around 70 %. The only reaction product is water. Hence the fuel cells are free from pollution. The reactants are continuously supplied from external source. Hence fuel cells run till reactants are active.

In fuel cell oxidants used are:

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  1. $O _{2}$

  2. $H _{2}O _{2}$

  3. $HNO _{3}$

  4. All

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Correct Option: D

The efficiency of a fuel cell is given by:

chemistry energy fuel cell fuel cells electrochemistry, rechargeable batteries, and fuel cells

  1. $\frac{\Delta S}{\Delta G}$

  2. $\frac{\Delta H}{\Delta G}$

  3. $\frac{\Delta G}{\Delta S}$

  4. $\frac{\Delta G}{\Delta H}$

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Correct Option: D
Explanation:
The fuel cell thermodynamic efficiency is given by the ratio of the Gibbs function change to the Enthalpy change in the overall cell reaction. The Gibbs function change measures the electrical work and the enthalpy change is a measure of the heating value of the fuel. 
Efficiency = $ \frac{dG}{dH} $$=\frac{\Delta G}{\Delta H}$

If a fuel cell methanol is used as fuel and oxygen gas is used as an oxidiser. The reaction is $CH _3OH _{(l)} +\frac{3}{2}O _{2(g)} \rightarrow CO _{2(g)}+2H _2O _{(g)}$ at 298 K standard Gibb's energies of formation for $CH _3OH(l)H _2O(l)$ and $CO _2(g)$ are $-166.2$, $-237.2$ and $394.4$ $KJ \ mol^{-1}$ respectively. If standard enthalpy of combustion of methanol is -726 KJ $mol^{-1}$, efficieny of the fuel cell will be 

chemistry energy fuel cell fuel cells electrochemistry, rechargeable batteries, and fuel cells

  1. 80%

  2. 87%

  3. 90%

  4. 97%

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Correct Option: A

What is the approximate value of $ \Delta S^{\circ} $ for the fuel cell reaction at $ 25^{\circ}$? 

chemistry energy fuel cell fuel cells electrochemistry, rechargeable batteries, and fuel cells

  1. $ -0.1624\,JK^{-1}$

  2. $ -162.4\,JK^{-1} $

  3. $ +162.4\,JK^{-1} $

  4. $ +0.1624\,JK^{-1} $

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Correct Option: B