Video Transcript
Which of the following hydrogen halides has the smallest bond enthalpy? (A) HI, (B) HF, (C) HBr, (D) HCl.
All of the answer choices are hydrogen halides. Hydrogen halides are a type of chemical compound composed of a hydrogen atom joined to a halogen atom with a single bond. Recall that halogens are found in group 17 or seven A on the periodic table and include fluorine, chlorine, bromine, and iodine. This question is asking us to determine which of the hydrogen halides has the smallest bond enthalpy.
Bond enthalpy, also called bond energy, is the average amount of energy required to break a specific bond in one mole of molecules. In order to break chemical bonds, a system must absorb energy from the surroundings. The amount of energy needed depends on a variety of factors, one of which is the atomic radius of each atom in the chemical bond.
Atomic radius is a measure of the size of an atom. If we were to look at the relative sizes of the halogen atoms, we’d see that atomic size increases as we move down the group. So, why do we see this trend? Let’s take a look at the electron shells of a fluorine atom. Fluorine contains two electron shells. Chlorine, which is lower than fluorine in the group, has more electrons, and these electrons are organized into three electron shells. As we continue to move down the group, atomic radius gets larger because the number of electron shells increases.
Because a chlorine atom is larger than a fluorine atom, the length of a chemical bond between the chlorine atom and a hydrogen atom is longer than the bond between the fluorine atom and a hydrogen atom. This trend continues as we move down the group. When comparing all of the hydrogen halides, the bond between an iodine atom and a hydrogen atom is the longest. Longer bonds are weaker and require less energy to break, while shorter bonds are stronger and require more energy to break. This means that the shorter the bond is between the hydrogen atom and the halogen atom, the greater the bond enthalpy.
Because the bond in hydrogen iodide, HI, is the longest when compared to the other hydrogen halides, it has the smallest bond enthalpy. Therefore, the correct answer is answer choice (A) HI.
