The standard enthalpy of combustion of ethene gas, C2 H4(g), is -1411.1 kJ / mol at 298 K. Giv

The standard enthalpy of combustion of ethene gas, C2 H4(g),...

The standard enthalpy of combustion of ethene gas, $\mathrm{C}_{2} \mathrm{H}_{4}(g)$, is $-1411.1 \mathrm{~kJ} / \mathrm{mol}$ at $298 \mathrm{~K}$. Given the following enthalpies of formation, calculate $\Delta H_{\mathrm{f}}^{\circ}$ for $\mathrm{C}_{2} \mathrm{H}_{4}(g)$. $$ \begin{array}{ll} \mathrm{CO}_{2}(g) & -393.5 \mathrm{~kJ} / \mathrm{mol} \\ \mathrm{H}_{2} \mathrm{O}(l) & -285.8 \mathrm{~kJ} / \mathrm{mol} \end{array} $$

The standard enthalpy of combustion of ethene gas, $\mathrm{C}_{2} \mathrm{H}_{4}(g)$, is $-1411.1 \mathrm{~kJ} / \mathrm{mol}$ at $298 \mathrm{~K}$. Given the following enthalpies of formation, calculate $\Delta H_{\mathrm{f}}^{\circ}$ for $\mathrm{C}_{2} \mathrm{H}_{4}(g)$.
$$
\begin{array}{ll}
\mathrm{CO}_{2}(g) & -393.5 \mathrm{~kJ} / \mathrm{mol} \\
\mathrm{H}_{2} \mathrm{O}(l) & -285.8 \mathrm{~kJ} / \mathrm{mol}
\end{array}
$$

Key Concepts

Standard Enthalpy of Formation

The standard enthalpy of formation is the heat change that occurs when one mole of a compound is formed from its elements in their standard states. It serves as a reference point for calculating the enthalpy change of reactions, making it an essential concept in thermochemistry.

Standard Enthalpy of Combustion

The standard enthalpy of combustion is the amount of heat released when one mole of a substance is completely burned in oxygen under standard conditions. Determining this value helps in understanding the energy content of fuels and allows for the application of Hess' Law to find unknown formation enthalpies.

Hess' Law

Hess' Law states that the total enthalpy change of a reaction is independent of the pathway taken, meaning that the enthalpy changes for individual steps in a reaction can be added together to yield the overall reaction enthalpy. This principle is fundamental in calculating unknown enthalpy values by relating them to known standard enthalpies of formation or combustion.

Reaction Stoichiometry

Reaction stoichiometry involves the quantitative relationships and mole ratios of reactants and products as depicted in a balanced chemical equation. It ensures that all components are correctly accounted for in thermodynamic calculations, such as those using Hess' Law for determining overall reaction enthalpies.

Transcript

00:01 In this video, we're going to discuss how to find the heat of formation of a particular substance in a given chemical equation.

00:10 So they give us the combustion of ethene gas.

00:14 Ethene gas is c2h4.

00:16 And i went ahead and wrote out the balanced equation on the very top of the first screen.

00:23 And i marked h2o as liquid.

00:25 Everything else is gas.

00:26 Just saving a little bit of room on the screen.

00:28 And again, we need to find the heat of formation in the standard state or in standard conditions of the ethene gas.

00:37 And they give us the heat of formation of carbon dioxide at negative 393 .5 and water at negative 285 .8.

00:47 Those are both in kilojoules per mole.

00:50 So remember that in order to find the heat of formation of any substance, we need to be given the total.

00:57 They do give us a total in the problem.

01:00 It's the delta h for the entire equation, the heat of formation for the entire equation is negative 1411.

01:11 So i forgot to put that in there.

01:13 I'll do that right now.

01:14 Delta h, which is everybody combined, is negative 14 .1.

01:23 Okay.

01:24 So we need to use the equation.

01:27 We just sum up all the products on the right hand side.

01:30 And then sum up all the heats of formation on the left -hand side for the reactants and find the difference.

01:38 And then keep in mind that we do have an unknown on the reactant side.

01:42 So i'll go ahead and start a third screen to start the math.

01:45 So we know the total is negative 1411...

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