Video: Applying Knowledge of the Energetics of Ammonia Production and the Polarity of Ammonia’s Bonds

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) Ammonia molecules have nonpolar covalent bonds. 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) Ammonia molecules have nonpolar covalent bonds. If both are true, state if II is a correct explanation for I.

Statement I refers to the reaction of hydrogen with nitrogen to form a molecule called ammonia. This reaction is generally referred to as the Haber process. It’s an important reaction where hydrogen is taken as a raw material, which is sourced from methane, which originates from crude oil. It’s reacted with nitrogen, which is found in air, to form the molecule ammonia, which is used in fertilizers to improve crop yield.

Written as a balanced equation, we see the reaction involves one molecule of nitrogen, which is a gas, three molecules of hydrogen, also a gas, forming two molecules of ammonia, which is also a gas. Industrially, it’s hard to get a high yield of ammonia as the reaction is reversible. In a closed system, it can reach dynamic equilibrium.

The reaction conditions of temperature and pressure have to be closely controlled in order to get the best possible yield of ammonia from this reversible process. We can see from the graph that a higher pressure at any given value of temperature gives a higher yield of ammonia. From the balanced equation, we see that four gas moles on the reactant side turns into two gas moles on the product side, if we’re looking at the forward process. If the reaction were in a closed system, and in fact we’re in equilibrium, then raising the pressure would shift the equilibrium position to the side with less gas moles. This is in accordance with Le Chatelier’s principle.

Super high pressures are dangerous and expensive. Industrially, a pressure of around 200 atmospheres proves to be the most economical. We can also see from the graph that raising the temperature favors the reverse reaction process as the yield of ammonia clearly drops. This means that the reverse reaction process is in fact endothermic. Adding heat to a system in equilibrium will encourage the system to oppose the change. It will shift in the endothermic direction and absorb the heat that was added in the first place. This again is in accordance with Le Chatelier’s principle.

If the reverse reaction process is endothermic, then the forward reaction process must be exothermic. You can’t go around this reaction cycle and create or destroy energy. In an exothermic process, heat is released from the reaction to the surroundings and the products end up in a lower energy level than the reaction started at. In the energy profile diagram for an exothermic process, we see the activation energy Eₐ labelled. This is actually very high for the Haber process. And industrially, we use catalysts to help speed the process up.

Statement I refers to the forward process for making ammonia, which is indeed an exothermic reaction. Statement I is, therefore, true. Statement II suggests that ammonia molecules have nonpolar covalent bonds. A covalent bond is a shared pair of electrons between atoms. It’s the type of bonding we usually find when no metals bond to other nonmetals. Since ammonia has the formula NH₃, one molecule contains one atom of nitrogen bonded to three atoms of hydrogen. Both of these are nonmetals. And, in fact, ammonia does contain covalent bonding.

To decide if ammonia molecules contain polar or nonpolar covalent bonds, we need to look at the idea of electronegativity in more detail. Electronegativity is defined as the ability of an atom to attract the bonding electrons in a covalent bond. In the periodic table, electron negativity increases as we move from left to right across any period. And it also increases as we move up any group. As we move across a period, we are adding protons to the nucleus. This increases the nuclear charge, which increases electronegativity. As we move down a group, we add more shells to the atoms. This increases their size, it increases the amount of shielding, and the electronegativity decreases as well.

In ammonia, we have hydrogen directly bonded to nitrogen, which is substantially more electronegative than hydrogen. Due to the substantial difference in electronegativity between the nitrogen atom and the hydrogen atoms, the sharing of the electron pairs is not perfect. There is a greater electron density at the nitrogen end of the bond. And we, therefore, say that bonds are polarized. We could say that ammonia contains polar bonds. This is indicated with a 𝛿⁻ and a 𝛿⁺ signs at the ends of each bond.

Statement II implies that ammonia contains nonpolar covalent bonds. This is clearly not the case. And statement II is false. Since statement II is false, it cannot be a correct explanation for statement I.

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