Question Video: Bond Energy Changes in the Water–Gas Shift Reaction Chemistry

The water–gas shift reaction is a major source of hydrogen gas for industrial processes. In this reaction, carbon monoxide is reacted with steam to produce hydrogen and carbon dioxide: CO + H₂O → CO₂ + H₂ The energies of selected bonds are listed in the table. Calculate the total change in bond energy for this reaction, per mole of hydrogen gas produced.

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Video Transcript

The water–gas shift reaction is a major source of hydrogen gas for industrial processes. In this reaction, carbon monoxide is reacted with steam to produce hydrogen and carbon dioxide. CO plus H2O react to form CO2 plus H2. The energies of selected bonds are listed in the table. Calculate the total change in bond energy for this reaction per mole of hydrogen gas produced.

Before solving the problem, let’s start by ensuring that the provided chemical equation is balanced. We need to count up the number of each type of atom on either side of the equation. On the reactant side of the equation, there is one carbon atom in the carbon monoxide molecule. And on the product side, there is one carbon atom in the carbon dioxide molecule. So, the carbon atoms are currently balanced.

Let’s take a look at the hydrogen atoms. On the reactant side of the equation, there are two hydrogen atoms in the water molecule, and on the product side there are also two hydrogen atoms in diatomic hydrogen. So, the hydrogen atoms are currently balanced.

Finally, let’s take a look at the oxygen atoms. On the left side of the equation, there is one oxygen atom in carbon monoxide and another oxygen atom in the water molecule for a total of two oxygen atoms. On the product side of the equation, there are two oxygen atoms in the carbon dioxide molecule for a total of two oxygen atoms. We have confirmed that this is a balanced chemical equation.

The provided table lists the bond energy for six types of bonds. In order to identify which types of bonds are in the reactant and product molecules, we’ll need to draw structural formulas. The carbon monoxide molecule contains one carbon-to-oxygen triple bond, while a water molecule contains two oxygen-to-hydrogen single bonds. As for the products, a carbon dioxide molecule contains two carbon-to-oxygen double bonds. And diatomic hydrogen contains one hydrogen-to-hydrogen single bond. None of the molecules contain carbon-to-hydrogen single bonds or carbon-to-oxygen single bonds. So, we won’t use those bond energy values in our calculation.

Bond energy, also referred to as bond enthalpy, is the average amount of energy required to break a bond in one mole of molecules, and the units used to measure bond enthalpy are kilojoules per mole. This question is asking us to calculate the total change in bond energy, which is also referred to as the change in bond enthalpy and is represented with the symbol ΔH. In order to calculate the change in bond enthalpy, we’ll need to find the sum of the bond energies for all the bonds in the reactant molecules and subtract off the sum of all the bond energies for all the bonds in the product molecules.

Recall that the system absorbs energy to break the bonds in the reactant molecules and that energy is released when new bonds are formed in the product molecules. Energy is absorbed to break one carbon-to-oxygen triple bond and two oxygen-to-hydrogen single bonds in the reactant molecules. Energy is released when two carbon-to-oxygen double bonds and one hydrogen-to-hydrogen single bond form in the product molecules.

After substituting in the bond energy values from the table into our equation, we get 1072 kilojoules per mole required to break the carbon-to-oxygen triple bond plus two times 459 kilojoules per mole to break the two oxygen-to-hydrogen single bonds. As for the products, two times 799 kilojoules of energy per mole are released when two carbon-to-oxygen double bonds form and 432 kilojoules of energy per mole are released when one hydrogen-to-hydrogen single bond forms. After simplification, we find that 1990 kilojoules per mole is the total amount of energy required to break the bonds in the reactant molecules. And 2030 kilojoules per mole is the total amount of energy released when the product molecules are formed.

After subtracting 2030 kilojoules per mole from 1990 kilojoules per mole, we get an answer of negative 40 kilojoules per mole. When the change in bond enthalpy is negative, this means that the reaction is exothermic. If we look at our calculation, we see that the energy released when the product molecules formed was greater than the energy required to break the bonds in the reactant molecules. The total change in bond energy for this reaction per mole of hydrogen gas produced is negative 40 kilojoules per mole.

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