One glucose molecule, C6 H12 O6(s), is converted to two...

One glucose molecule, $\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(\mathrm{s}),$ is converted to two lactic acid molecules, $\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}(\mathrm{s})$ during glycolysis. Given the combustion reactions of glucose and lactic acid, determine the standard enthalpy for glycolysis. $$\begin{aligned} \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(\mathrm{s})+6 \mathrm{O}_{2}(\mathrm{g}) \longrightarrow & 6 \mathrm{CO}_{2}(\mathrm{g})+6 \mathrm{H}_{2} \mathrm{O}(1) \\ \Delta_{\mathrm{r}} H^{\circ} &=-2808 \mathrm{k} \mathrm{J} \mathrm{mol}^{-1} \\ \mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}(\mathrm{s})+3 \mathrm{O}_{2}(\mathrm{g}) & \longrightarrow \\ 3 \mathrm{CO}_{2}(\mathrm{g})+3 \mathrm{H}_{2} \mathrm{O}(1) & \Delta_{\mathrm{r}} H^{\circ}=-1344 \mathrm{kJ} \mathrm{mol}^{-1} \end{aligned}$$

One glucose molecule, $\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(\mathrm{s}),$ is converted to two lactic acid molecules, $\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}(\mathrm{s})$
during glycolysis. Given the combustion reactions of glucose and lactic acid, determine the standard enthalpy for glycolysis.
$$\begin{aligned}
\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(\mathrm{s})+6 \mathrm{O}_{2}(\mathrm{g}) \longrightarrow & 6 \mathrm{CO}_{2}(\mathrm{g})+6 \mathrm{H}_{2} \mathrm{O}(1) \\
\Delta_{\mathrm{r}} H^{\circ} &=-2808 \mathrm{k} \mathrm{J} \mathrm{mol}^{-1} \\
\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}(\mathrm{s})+3 \mathrm{O}_{2}(\mathrm{g}) & \longrightarrow \\
3 \mathrm{CO}_{2}(\mathrm{g})+3 \mathrm{H}_{2} \mathrm{O}(1) & \Delta_{\mathrm{r}} H^{\circ}=-1344 \mathrm{kJ} \mathrm{mol}^{-1}
\end{aligned}$$

Key Concepts

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is independent of the pathway taken, depending only on the initial and final states. This principle allows for the calculation of unknown reaction enthalpies by algebraically combining other reactions whose enthalpy changes are known.

Standard Enthalpy Change of Reaction

The standard enthalpy change of reaction is the heat absorbed or released when a reaction occurs under standard conditions (typically 1 atm pressure and 298 K). It is a measure of the energy change associated with a reaction, calculated from the heats of formation or combustion of reactants and products.

Combustion Reactions

Combustion reactions involve the rapid oxidation of a substance in the presence of oxygen, typically producing carbon dioxide and water. These reactions are important in thermochemistry because their enthalpy changes are well-characterized and can be used as reference reactions to determine the energy changes in other processes.

Glycolysis

Glycolysis is a central metabolic pathway that converts glucose into simpler molecules (such as lactic acid under anaerobic conditions), releasing energy in the process. In a thermochemical context, the enthalpy change of glycolysis can be estimated by comparing the energies involved in the combustion of the reactants and products.

Transcript

00:01 In this question, they are wanting us to find the delta h for this balanced chemical reaction for glycolysis.

00:08 So we have one molecule of glucose going to two molecules of lactic acid.

00:15 So they gave us two other equations to try and figure out the delta h for this reaction.

00:23 And they gave us the delta h's for those reactions.

00:26 So what we're going to do is we're going to add reaction 1 and reaction 2 together.

00:30 So the first reaction is of glucose and it's combustion with oxygen to produce co2 and water.

00:38 The second reaction is the combustion of lactic acid with o2 to create co2 and water.

00:45 If we look at this balanced equation that they want us to find, we see that lactic acid is on the product side and that glucose is on the reactant side.

00:57 So we have lactic acid on the product, glucose on the reactant.

01:01 We see that for reaction one, we have glucose on the reactant side, which is what we would like.

01:07 But for reaction two, we have lactic acid on the reactant side, and we would want that on the product side.

01:14 So to start off this reaction, i'm going to reverse reaction two.

01:25 All right.

01:27 And we are also going to want to times reaction two by two, because for every one molecule of glucose, we have two molecules of lactic acid.

01:34 So when i reverse reaction 2, it'll look like 6 co2 gas plus 6 water going to 2 lactic acid plus 6 .02.

02:03 And when we reverse a reaction and we times that reaction by a certain number, we also have to do the exact same thing for its delta h.

02:11 So we see that the delta h for reaction 2 is negative 1 ,344 kilojoules per mole...

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