Video: Identifying the Label Corresponding to the ΔH of the Reaction That Forms CO(g) and H₂O(l) from CH₄(g) and O₂(g) on an Enthalpy Diagram

Which label corresponds to the ΔH of the reaction that forms CO₂(g) and H₂O(l) from CH₄(g) + O₂(g)?

05:08

Video Transcript

Which label corresponds to the ΔH of the reaction that forms CO2 gas and H2O liquid from CH4 gas plus O2 gas?

In this question, we’re being asked about the ΔH, that is, the enthalpy change of a chemical process that forms some products from some reactants. The reactants or the molecules that we start with in this chemical reaction are methane gas and oxygen gas. In this reaction, the products formed are carbon dioxide gas and liquid water. The chemical equation needs balancing. And we find that one mole of methane gas will react with two moles of oxygen gas to produce one mole of carbon dioxide gas and two moles of liquid water. Essentially, this reaction represents the complete combustion of one mole of methane gas. This will be an exothermic reaction, and heat energy will be released to the surroundings.

You’ll be familiar with the heat produced by this reaction if you used a Bunsen burner in a school or college lab. Bunsen burners mix natural gas, which is mostly methane, and oxygen from the air to produce combustion and plenty of heat. The question is asking us to find the ΔH of this reaction. ΔH is the enthalpy change for reaction, and this represents the change in enthalpy between the products and reactants in a reaction at constant pressure. In the diagram, the enthalpy is represented as a vertical axis on the left side of the diagram. The enthalpy is the relative heat content of the species shown. In the question, the reactants specified match those at the top of the diagram. Also in the question, the products specified match those at the very bottom of the diagram.

Energetically, the diagram represents two separate pathways to get from our reactants to our products. Pathway number one achieves this reaction in a single step. It represents the complete combustion of one mole of methane gas. The products are at a lower energy level relative to the reactants, and the reaction is exothermic. 890 kilojoules of heat is released to the surroundings. Pathway number two is the first reaction in an alternative route to get from our reactants to our products. Pathway number two generates one mole of carbon monoxide gas as a product. This is the incomplete combustion of methane, and carbon monoxide is a toxic gas produced when there’s limited oxygen available for combustion.

This reaction is still exothermic. But because carbon monoxide is a product formed and this is not one of the products specified in the question, it’s not the correct answer. As a result of this, negative 607 kilojoules is not the correct answer. The reaction occurring in pathway three completes the alternative route from our reactants to our products. One mole of carbon monoxide gas is reacting with the leftover oxygen from pathway number two to produce one mole of carbon dioxide gas. Notice that no new liquid water is produced in pathway number three. We could in fact remove it from each side of our equation to simplify things.

The reaction in pathway number three is exothermic by 283 kilojoules. However, because carbon dioxide gas is the only new product in this reaction, negative 283 kilojoules is not the correct answer either. We can also eliminate the labels for the reactants and products as correct answers in this diagram. These labels are not enthalpy changes or ΔH values. They simply represent the relative enthalpy levels of the reactants relative to the products.

Notice here that pathway one is just as exothermic, energetically, as the sum of pathway two plus pathway number three. This arises because the reactants and products are exactly the same whichever route is taken. The only label in the diagram that matches the enthalpy change for the reaction with the correct reactants and products as specified in the question is negative 890 kilojoules. The enthalpy change for the exothermic complete combustion of one mole of methane gas is therefore the correct answer here.

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