Most cars in the United States use gasohol for fuel. Gasohol...

Most cars in the United States use gasohol for fuel. Gasohol is a mixture consisting of about $10 \%$ ethanol and $90 \%$ gasoline. The enthalpy of combustion per gram of gasoline is $-47.8 \mathrm{~kJ} / \mathrm{g}$. Using the bond energies in Table $8.4$, estimate the enthalpy of combustion per gram of ethanol. How do the two enthalpies of combustion compare with each other? The combustion reaction for ethanol is $$ \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(l)+3 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{CO}_{2}(g)+3 \mathrm{H}_{2} \mathrm{O}(g) $$

Most cars in the United States use gasohol for fuel. Gasohol is a mixture consisting of about $10 \%$ ethanol and $90 \%$ gasoline. The enthalpy of combustion per gram of gasoline is $-47.8 \mathrm{~kJ} / \mathrm{g}$. Using the bond energies in Table $8.4$, estimate the enthalpy of combustion per gram of ethanol. How do the two enthalpies of combustion compare with each other? The combustion reaction for ethanol is
$$
\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(l)+3 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{CO}_{2}(g)+3 \mathrm{H}_{2} \mathrm{O}(g)
$$

Key Concepts

Enthalpy of Combustion

Enthalpy of combustion is the heat released when one mole or one gram of a substance completely reacts with oxygen under standard conditions. It serves as a measure of the energy content of fuels and is fundamental in understanding fuel efficiency and performance in energy generation and engine operation.

Bond Energies

Bond energies, or bond dissociation energies, quantify the strength of chemical bonds by representing the energy required to break a bond in a molecule. They are used to estimate the energy changes in chemical reactions by considering the energy required to break bonds in the reactants versus the energy released upon formation of bonds in the products.

Thermochemical Calculations Using Bond Energies

This concept involves calculating the enthalpy change of a reaction by summing the energies needed to break bonds in the reactants and subtracting the energies released from the formation of bonds in the products. It is a fundamental approach in thermochemistry that allows for estimating reaction energetics even when direct experimental measurements are not available.

Fuel Energy Density and Comparison

The comparison of enthalpy values per unit mass for different fuels, such as gasoline and ethanol, is essential for evaluating their relative effectiveness as energy sources. This concept, often referred to as fuel energy density, helps in determining which fuel provides more energy for a given mass, a key factor in fuel selection and engine design.

Transcript

00:01 So in this video, we're going to go over question 170 from chapter 8.

00:05 Most cars in the united states use gasahole for fuel.

00:09 Gasol is a mixture consisting of about 10 % ethanol and 90 % gasoline.

00:13 The enthalpy of combustion per gram of gasoline is negative 47 .8 kilojoules per gram.

00:18 Using bond energies from table 8 .5, estimate the enthalpy of combustion per gram of ethanol.

00:25 How do the two enthletes of combustion compare with each other? assume the combustion reaction for ethanol is ch3 -ch -20h plus 302 yields 2 -c -o2 plus 3h2 -2 -2 -2 -3h -2.

00:38 So if we're determining the amplity of combustion by using bond energies, let's take inventory of what bonds we're breaking and what bonds we're making.

00:49 So we're breaking our carbon -carbon bond, and we're breaking all of our carbon -hydrogen bonds, as well as our bond from carbon -to -oxygen bonds.

00:59 We're also breaking our three bonds in o2, and we're forming four carbon -oxygen double bonds in co2, and we're forming five oh bonds in h -2.

01:12 You might say, why not six? and that's because we never broke this oh bond, so we only need five.

01:18 So let's write all that out.

01:20 So we have our five carbon -hydrogen bonds.

01:23 We have our carbon -oxygen bond, our carbon -carbon bond, and our oxygen bond, times 3.

01:29 And all of these bond energies are positive because it takes energy to break a bond...

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