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While electric and hybrid electric cars grow in popularity, two-thirds of all new private cars licensed in Ireland between January and June 2021 were powered only by petrol or diesel - Leaving Cert Chemistry - Question 6 - 2022

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While electric and hybrid electric cars grow in popularity, two-thirds of all new private cars licensed in Ireland between January and June 2021 were powered only by... show full transcript

Worked Solution & Example Answer:While electric and hybrid electric cars grow in popularity, two-thirds of all new private cars licensed in Ireland between January and June 2021 were powered only by petrol or diesel - Leaving Cert Chemistry - Question 6 - 2022

Step 1

Compare petrol (a mixture of light gasoline and naphtha) and diesel (gas oil) in terms of (i) average molecular mass of the hydrocarbons present

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Answer

Petrol has a lower average molecular mass compared to diesel. This is primarily because petrol consists of lighter hydrocarbons, generally in the range of C4 to C12, while diesel contains heavier hydrocarbons, typically in the C12 to C20 range. Thus, the average molecular mass of petrol is smaller than that of diesel oil.

Step 2

Compare petrol (a mixture of light gasoline and naphtha) and diesel (gas oil) in terms of (ii) boiling point

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Answer

Petrol has a lower boiling point than diesel. The boiling point of petrol ranges approximately from 30°C to 200°C whereas diesel boils at a higher temperature, generally between 180°C to 360°C. This difference is due to the larger molecular size and greater intermolecular forces present in diesel, which require more energy to break.

Step 3

Explain the term octane number of a fuel.

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Answer

The octane number of a fuel is a measure of its resistance to engine knocking during combustion. It is defined as the percentage of isooctane (C8H18) in a fuel blend, which resists knocking, compared to n-heptane (C7H16) that is more prone to knocking. A higher octane number indicates better performance in high compression engines.

Step 4

Give the systematic IUPAC name and draw the structure of a molecule of the alkane fuel assigned an octane number of zero.

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Answer

The alkane assigned an octane number of zero is n-heptane, with the systematic IUPAC name being heptane. Its structure can be drawn as:

   H    H    H    H    H    H
   |    |    |    |    |    |
H--C--C--C--C--C--C--H
   |    |    |    |    |    |
   H    H    H    H    H    H

Step 5

Give the systematic IUPAC name and draw the structure of a molecule of the alkane fuel assigned an octane number of 100.

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Answer

The alkane assigned an octane number of 100 is 2,2,4-trimethylpentane. Its structure can be represented as follows:

       CH3
        | 
  CH3-C-CH-CH2-CH3
        | 
       CH3

Step 6

What structural feature of the molecules of the alkane fuel assigned octane number 100 accounts for its high octane number?

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Answer

2,2,4-trimethylpentane has a branched structure which prevents the molecules from packing closely together. This leads to a reduced chance of auto-ignition under compression, thus accounting for its high octane number compared to straight-chain hydrocarbons.

Step 7

Suggest a reason why elemental hydrogen (H₂) does not occur in nature on Earth as a fuel source.

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Answer

Elemental hydrogen (H₂) does not occur naturally on Earth due to its high reactivity. It readily forms compounds with other elements, such as water (H₂O) and hydrocarbons, making it scarce in its elemental form. Moreover, hydrogen gas is a light gas that escapes Earth's gravity and dissipates into space.

Step 8

Give a chemical source of hydrogen gas, other than steam reforming.

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Answer

A chemical source of hydrogen gas other than steam reforming is the electrolysis of water, where water is split into hydrogen and oxygen gases using an electric current.

Step 9

Define heat of reaction.

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Answer

The heat of reaction is defined as the amount of energy transferred during a chemical reaction when the reaction proceeds to completion. It is calculated as the difference between the total enthalpy of the products and the reactants.

Step 10

Explain why the same amount of energy is released when one mole of hydrogen is used up in a fuel-cell as when one mole of hydrogen is burned in excess oxygen.

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Answer

The same amount of energy is released in both processes because they are fundamentally the same chemical reaction (combination of hydrogen with oxygen to form water), whether occurring in a fuel cell or during combustion. The energy released corresponds to the amount of bond energy released when forming water, regardless of the reaction pathway.

Step 11

Use the heat of reaction for the steam reforming of methane above and the heats of formation of steam and carbon monoxide to calculate the heat of formation of methane.

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Answer

To calculate the heat of formation of methane (ΔHf for CH₄), we can use Hess's law:

ΔHreaction=∑ΔHproducts−∑ΔHreactantsΔH_{reaction} = \sum \Delta H_{products} - \sum \Delta H_{reactants}

From the balanced equation:

CH4(g)+H2O(g)⇌CO(g)+3H2(g)CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g)

Where:

  • ΔH for the reaction = 206.2 kJ
  • ΔHf for steam = -241.8 kJ/mol
  • ΔHf for CO = -110.5 kJ/mol

Putting the values together:

ΔHf(chen)=ΔHproducts−ΔHreactants ΔHf(chen)=((−110.5)+(3×0))−((206.2)+(−241.8)) ΔHf(chen)=−74.9kJ/molΔH_{f(chen)} = ΔH_{products} - ΔH_{reactants}\ ΔH_{f(chen)} = ((-110.5) + (3 × 0)) - ((206.2) + (-241.8))\ ΔH_{f(chen)} = -74.9 kJ/mol

This shows the heat of formation of methane is -74.9 kJ/mol.

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