Video: Applying Knowledge of the Energetics and Bond Energy Changes of the Formation of Ammonia from Hydrogen and Nitrogen

For statements (I) and (II), state for each if they are true or false. (I) The reaction of hydrogen with nitrogen to form ammonia is an exothermic reaction. (II) When hydrogen and nitrogen react to form ammonia, more energy is released when the bonds are formed in the ammonia molecule than is used to break the bonds in the hydrogen and nitrogen molecules. If both are true, state if (II) is a correct explanation for (I).

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Video Transcript

For statements (I) and (II), state for each if they are true or false. (I) The reaction of hydrogen with nitrogen to form ammonia is an exothermic reaction. (II) When hydrogen and nitrogen react to form ammonia, more energy is released when the bonds are formed in the ammonia molecule than is used to break the bonds in the hydrogen and nitrogen molecules. If both are true, state if (II) is a correct explanation for (I).

The ammonia molecule is made up of one atom of nitrogen and three atoms of hydrogen, while hydrogen and nitrogen are normally diatomic gases, H2 and N2. Ammonia is typically made on an industrial scale using the Haber process, where nitrogen and hydrogen gases are reacted under high pressure and at high temperature to produce ammonia molecules. The first statement suggests that this reaction is exothermic, which means that more energy is given out because of the reaction than is taken in. This is commonly indicated by a negative 𝛥H. A negative enthlapy change indicates that the system has lost energy and the surroundings have gained energy.

At this point, you could just remember that the reaction in the Haber process is exothermic. But to back that up, you might recall that in the Haber process, the higher the reaction temperature, the lower the yield. This is indicative of the forward reaction being exothermic. But before declaring statement (I) to be true, I’m going to prove it’s true using bond energies. Statement (II) says that when hydrogen and nitrogen react to form ammonia, more energy is released when the bonds are formed in the ammonia product than is used to break the bonds in the hydrogen and nitrogen reactants. It takes 942 kilojoules to break one mole’s worth of nitrogen-nitrogen triple bonds. But it only takes 432 kilojoules to break one mole of hydrogen-hydrogen single bonds.

Meanwhile, the bonds in our product ammonia are nitrogen-hydrogen single bonds, which require 386 kilojoules per mole to break. But we’re breaking the bonds in the reactants, nitrogen and hydrogen, and we’re forming the bonds in ammonia. So rather than requiring energy, the formation of the bonds in the products will release 386 kilojoules per mole of bonds. If we imagine we’re going to use one mole of nitrogen molecules, we’ll need 942 kilojoules to break the nitrogen-nitrogen triple bonds. For each nitrogen, we’ll need three hydrogen molecules. So we’ll need 1296 kilojoules to break the hydrogen-hydrogen bonds in three moles of hydrogen. And for each molecule of nitrogen, we’ll get two molecules of ammonia, meaning we’ll have a total of six NH single bonds. The formation of six moles of single NH bonds will release 2316 kilojoules of energy.

To get the total energy, we just need to sum these together. We have to put in 2238 kilojoules to break up the reactants. And we get 2316 kilojoules out when our products form, giving us an energy change of negative 78 kilojoules. 78 kilojoules released to the surroundings because of the reaction of one mole of nitrogen with three moles of hydrogen to form two moles of ammonia. So we’ve demonstrated that more energy is released when the bonds are formed in ammonia than is used to break the bonds in hydrogen and nitrogen. And the reaction is exothermic.

Both statements are true. Therefore, the last part of the question should be addressed, and we need to state if statement (II) is a correct explanation for statement (I). It is indeed true. The reaction of hydrogen with nitrogen to form ammonia is an exothermic reaction for the reasons we’ve laid out here. More energy is released when the bonds are formed in our product ammonia than is used to break the bonds in the hydrogen and nitrogen molecules.

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