Video: Identifying the Label Corresponding to the ΔH of the Reaction That Forms CO₂(g) from CO(g) and O₂(g) on an Enthalpy Diagram

Which label corresponds to the Δ𝐻 of the reaction that forms CO₂(g) from CO(g) and O₂(g)?

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Video Transcript

Which label corresponds to the Δ𝐻 of the reaction that forms CO2 gas from CO gas and O2 gas?

In this question, we’re being asked to find the Δ𝐻, that is, the enthalpy change, of a reaction where carbon dioxide is formed as a product from carbon monoxide and oxygen as reactants. The enthalpy change for a reaction is defined with the symbol Δ𝐻. And it represents the change in enthalpy between the products and reactants in a reaction at constant pressure.

Notice that the enthalpy is represented as a vertical scale on the left side of this diagram. The diagram shows three separate reactions occurring. These have been labeled number one, two, and three.

In reaction one, we see one mole of methane gas reacting with two moles of oxygen gas to produce one mole of carbon dioxide gas and two moles of liquid water. This equation represents the complete oxidation or complete combustion of one mole of methane. Because the products are at a lower enthalpy relative to the reactants that we started with here, we would say this reaction is exothermic. This reaction would release 890 kilojoules of heat to the surroundings.

Although carbon dioxide is formed as a product in this reaction, the reactants are not carbon monoxide and oxygen as specified in the question. Since we started with methane and not carbon monoxide as specified in the question, this enthalpy change is not the correct answer. We need to look for a reaction where carbon monoxide is a reactant and carbon dioxide is in fact a product. The enthalpy change we’re looking for, therefore, could be the enthalpy change for reaction two or three.

In reaction two, we start with one mole of methane gas and two moles of oxygen gas as our reactants. According to the process in the diagram, we form one mole of carbon monoxide gas, two moles of liquid water, and half a mole of oxygen gas. Notice that the products of this process are also at a lower enthalpy level than the reactants that we started with. So this reaction is also exothermic. In fact, it releases 607 kilojoules of heat energy to the surroundings.

In this reaction, carbon monoxide gas is one of the products. This is a lethally toxic gas. And it’s formed from the incomplete combustion of methane in this reaction. Incomplete combustion occurs when fuels are burned in a limited supply of oxygen. It’s clear to see from the equation that although we started with two moles of oxygen gas, there’s half a mole of oxygen gas left over after the process is completed, this is the unreacted oxygen. In this reaction, the reactants do not match the reactants described in the question. We started with methane gas and not carbon monoxide gas. Although oxygen gas is a reactant in both situations, negative 607 kilojoules is not the enthalpy change for the reaction that we are seeking.

In the process shown by reaction three, we are taking one mole of carbon monoxide and reacting it with the unreacted oxygen from process two, that is, half a mole of oxygen, to from a mole of carbon dioxide gas. Carbon dioxide gas is the only new product in this process. Although there are two moles of liquid water on both sides of the equation, this remains unchanged. And we could ignore it as far as the reaction is concerned.

Carbon monoxide gas does burn quite nicely when you light it with a lighted splint. And it burns with a pale-blue flame. Extra heat energy is released during this process. So this reaction is also exothermic, and its Δ𝐻 value is negative 283 kilojoules.

Notice at this stage that the diagram represents two alternative reaction pathways to arrive at the same products from the same reactants. Energetically, the two reaction pathways must be just as exothermic as each other. In fact, we find that the enthalpy change for pathway number one equates to the enthalpy change for pathway number two plus the enthalpy change for pathway number three.

Since the reactants and products are the same in both pathways, the enthalpy change is independent of the route taken. If we look closely at reaction three, we can see that the reactants and the only new product formed matches those in the question. So negative 283 kilojoules is the label that corresponds to the Δ𝐻 of the reaction that forms carbon dioxide from carbon monoxide and oxygen gas. The other two labels in the diagram are simply labels for the reactants and the products. They’re not labels for the enthalpy change or a Δ𝐻 value. They’re not the correct answers. So the correct Δ𝐻 for the reaction in the question is negative 283 kilojoules.

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