Hydrocarbon fuels can be produced from methanol by reactions...

Hydrocarbon fuels can be produced from methanol by reactions such as the following, which yields 1-hexene: $$ 6 \mathrm{CH}_3 \mathrm{OH}(\mathrm{g}) \rightarrow \mathrm{C}_6 \mathrm{H}_{12}(\mathrm{~g})+6 \mathrm{H}_2 \mathrm{O}(\mathrm{g}) $$ Compare the standard heat of combustion at $298.15 \mathrm{~K}\left(25^{\circ} \mathrm{C}\right)$ of $6 \mathrm{CH}_3 \mathrm{OH}(\mathrm{g})$ with the standard heat of combustion at $298.15 \mathrm{~K}\left(25^{\circ} \mathrm{C}\right)$ of $\mathrm{C}_6 \mathrm{H}_{12}(\mathrm{~g})$ for reaction products $\mathrm{CO}_2(\mathrm{~g})$ and $\mathrm{H}_2 \mathrm{O}(\mathrm{g})$.

Hydrocarbon fuels can be produced from methanol by reactions such as the following, which yields 1-hexene:
$$
6 \mathrm{CH}_3 \mathrm{OH}(\mathrm{g}) \rightarrow \mathrm{C}_6 \mathrm{H}_{12}(\mathrm{~g})+6 \mathrm{H}_2 \mathrm{O}(\mathrm{g})
$$

Compare the standard heat of combustion at $298.15 \mathrm{~K}\left(25^{\circ} \mathrm{C}\right)$ of $6 \mathrm{CH}_3 \mathrm{OH}(\mathrm{g})$ with the standard heat of combustion at $298.15 \mathrm{~K}\left(25^{\circ} \mathrm{C}\right)$ of $\mathrm{C}_6 \mathrm{H}_{12}(\mathrm{~g})$ for reaction products $\mathrm{CO}_2(\mathrm{~g})$ and $\mathrm{H}_2 \mathrm{O}(\mathrm{g})$.

Key Concepts

Hess's Law

Hess's Law states that the enthalpy change of a total reaction is the same, regardless of the pathway taken, if the initial and final conditions are identical. This principle allows one to calculate the standard heat of combustion by summing the enthalpy changes of individual steps or using tabulated standard enthalpies of formation.

Combustion Reaction

A combustion reaction involves a fuel reacting with oxygen to produce heat, usually resulting in products like carbon dioxide and water. An understanding of combustion reactions is fundamental in evaluating the energy yield and efficiency of hydrocarbon fuels, and it plays a critical role in energy engineering and environmental impact assessments.

Standard Heat of Combustion

This refers to the amount of heat released when one mole of a substance is completely burned in oxygen under standard conditions (typically 298 K and 1 atm), forming combustion products such as carbon dioxide and water. It is a measure of the energy content of a fuel, and comparing these values helps in understanding the energy efficiency or performance of different fuels.

Standard Enthalpy of Formation

This is the heat change that results when one mole of a compound is formed from its elements in their standard states. It serves as a foundation for calculating the overall enthalpy change for a reaction by using known formation values of the reactants and products, particularly in combustion reactions.

Transcript

00:01 Hydrogen can be produced using the reaction of steam or h2o with various hydrocarbons.

00:05 Compare the mass of h2 or hydrogen gas expected from the reaction with steam with 100 grams each of coal, petroleum, and methane.

00:14 We're going to assume a complete reaction in each case.

00:17 And for coal, we're going to use c or just carbon, petroleum, ch2, and methane is ch4.

00:26 So what we're going to do is we're going to start with 100 grams each of our carbon.

00:30 We're going to convert this to moles by saying one mole of carbon weighs 12 .01 grams.

00:39 From here, you can see our grams of carbon cancel out.

00:42 We're left with just moles.

00:44 Now, from here, we convert from one mole of carbon to one mole of h2 or hydrogen gas.

00:54 Here, your moles of carbon will cancel.

00:56 You'll be left with moles of hydrogen gas.

00:58 Now from here, we just have to say one mole of h2 weighs 2 .016 grams.

01:07 And now we can see moles of h2 cancel.

01:10 We're left with just grams.

01:12 So let's start with 100, and we're going to divide that by 12 .01, multiply that by 101, and then multiply that by 2 .016.

01:22 Now, if we started with 100 grams of carbon, we will end up with 16 .786.

01:29 0 grams of hydrogen gas.

01:33 So what happens if we use petroleum instead? so we'll set up the same thing.

01:39 Except this time we're going to say grams of ch2.

01:43 One mole of ch2 weighs 14 .026 grams.

01:51 From here we say one mole of ch2 yields 2 moles of h2...

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