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Explain the energy level diagram for an endothermic reaction. Define the heat of combustion. Calculate the mass of ethanol burnt.

Chemistry

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Answer

1.00 g\textbf{1.00 g}

-1366.2 kJ/mol\textbf{-1366.2 kJ/mol}

Step-by-step solution

4 steps

  1. Answer part a).

    An energy level diagram for an endothermic reaction shows the products at a higher energy level than the reactants, indicating that energy is absorbed from the surroundings. The enthalpy change (ΔH\Delta H) is positive.

    \begin{tikzpicture}[scale=0.8] % Axes \draw[->] (0,0) -- (0,5) node[above] {Energy}; \draw[->] (0,0) -- (6,0) node[right] {Reaction Progress}; % Reactants \draw[thick] (0.5,1) -- (1.5,1) node[left, midway, yshift=-0.5cm] {Reactants}; % Products \draw[thick] (4.5,4) -- (5.5,4) node[right, midway, yshift=-0.5cm] {Products}; % Activation energy curve \draw[thick] (1.5,1) .. controls (3,4.5) and (3,4.5) .. (4.5,4); % Delta H \draw[<->] (5.7,1) -- (5.7,4) node[midway, right] {$\Delta H > 0$}; % Activation energy arrow \draw[->] (1.5,1.2) -- (3,4.2); \node at (2.5,3) [above, xshift=-0.5cm] {Activation Energy}; \end{tikzpicture}

  2. Answer part b)i.

    The heat of combustion is the enthalpy change when one mole of a substance undergoes complete combustion in excess oxygen under standard conditions.

  3. Answer part b)ii.

    The mass of ethanol burnt is the difference between the initial and final mass of the lamp and ethanol.
    Initial mass of lamp and ethanol = 35.56 g35.56 \text{ g}
    Final mass of lamp and ethanol = 34.56 g34.56 \text{ g}
    Mass of ethanol burnt = Initial mass - Final mass

    Mass of ethanol burnt=35.56 g−34.56 g=1.00 g\text{Mass of ethanol burnt} = 35.56 \text{ g} - 34.56 \text{ g} = \boxed{\textbf{1.00 g}}

  4. Answer part b)iii.

    We need to calculate the heat of combustion per mole. We assume that the "29.7KJmol⁻¹ of energy was evolved" refers to 29.7 kJ as the total energy evolved for the mass of ethanol burnt, and "mol⁻¹" is a misplaced unit.

    First, calculate the number of moles of ethanol burnt.
    Mass of ethanol burnt = 1.00 g1.00 \text{ g}
    Molar mass of ethanol = 46 g/mol46 \text{ g/mol}

    Moles of ethanol=MassMolar mass=1.00 g46 g/mol≈0.021739 mol\text{Moles of ethanol} = \frac{\text{Mass}}{\text{Molar mass}} = \frac{1.00 \text{ g}}{46 \text{ g/mol}} \approx 0.021739 \text{ mol}

    Next, calculate the heat of combustion. Since energy is evolved, the reaction is exothermic, and the enthalpy change will be negative.
    Energy evolved = 29.7 kJ29.7 \text{ kJ}

    Heat of combustion=Energy evolvedMoles of ethanol\text{Heat of combustion} = \frac{\text{Energy evolved}}{\text{Moles of ethanol}} Heat of combustion=−29.7 kJ0.021739 mol\text{Heat of combustion} = \frac{-29.7 \text{ kJ}}{0.021739 \text{ mol}} Heat of combustion≈−1366.2 kJ/mol\text{Heat of combustion} \approx -1366.2 \text{ kJ/mol}

    Rounding to one decimal place:

    Heat of combustion=-1366.2 kJ/mol\text{Heat of combustion} = \boxed{\textbf{-1366.2 kJ/mol}}
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