Question

Which of the following is the expected product of the reaction of \(\mathrm{K}(s)\) and \(\mathrm{H}_{2}(g) ?(\mathbf{i}) \mathrm{KH}(s),(\mathbf{i} \mathbf{i}) \mathrm{K}_{2} \mathrm{H}(s),\) (iii) \(\mathrm{KH}_{2}(s),\) \((\mathbf{i} \mathbf{v}) \mathrm{K}_{2} \mathrm{H}_{2}(s), \mathrm{or}(\mathbf{v}) \mathrm{K}(s)\) and \(\mathrm{H}_{2}(g)\) will not react with one another.

Short answer

The expected product of the reaction between solid potassium (K(s)) and hydrogen gas (H₂(g)) is (i) \(\mathrm{KH}(s)\).

Step-by-step solution

Consider Reactivity of Potassium and Hydrogen

Potassium (K) is an alkali metal and has a strong tendency to lose one electron to form K⁺ ion. Hydrogen can gain an electron to form H⁻ ion (hydride) or form a covalent bond with another hydrogen atom (in molecular hydrogen, H₂).

Combine Ions Based on Their Charges

Determine the balanced charges of the ions in the product. K⁺ has a +1 charge and H⁻ has a -1 charge. When they combine, a 1:1 ratio of these ions will result in a neutral compound.

Identify the Correct Product Based on Ionic Charges

The compound formed in the reaction will have a 1:1 ratio of K⁺ ions and H⁻ ions resulting in a compound KH. Compare this to the given choices and find the correctly balanced compound. In this case, it is option (i) KH(s). So the correct answer is (i) \(\mathrm{KH}(s)\).

Key concepts

Reactivity of Potassium

Potassium is a highly reactive metal, classified within the alkali metals group of the periodic table. What makes potassium (K) very reactive is its willingness to part with the single electron in its outermost shell. In an effort to reach a stable electronic configuration, similar to the noble gases, potassium readily donates this electron when reacting with other elements, such as hydrogen.
Essentially, the reactivity of potassium is due to its position as the most electropositive metal of the periodic table after cesium. This character of potassium is key in understanding how it behaves in various chemical reactions, including its reaction with hydrogen to form a specific compound.

Formation of Hydride Ions

Hydride ions (H⁻) are formed when hydrogen atoms gain an electron, resulting in a negative charge. Since hydrogen is a non-metal, it is more inclined to achieve a helium-like noble gas configuration by gaining an electron rather than losing one. Therefore, when hydrogen gas, which exists as a diatomic molecule (H₂), comes into contact with highly reactive metals such as potassium (K), it can accept an electron donated by the metal.
Once the transfer of the electron is completed, the ion formed is known as a hydride ion. This process of forming a hydride ion is extremely important in reactions where hydrogen is combined with metals, effectively leading to compounds where hydrogen has a -1 oxidation state.

Balancing Ionic Charges

When it comes to the formation of compounds, balancing ionic charges is crucial. This concept is founded on the principle that the total charge in a compound must equal zero for the compound to be stable. In the case of alkali metals like potassium interacting with hydrogen to form a hydride, a neutral compound must result from the reaction.
Potassium loses one electron to become K⁺ with a +1 charge, whereas hydrogen gains an electron to become H⁻ with a -1 charge. When these two ions combine in a one-to-one ratio, they balance each other out: the positive charge of the potassium ion is neutralized by the negative charge of the hydride ion, resulting in the formation of a stable, neutral compound, which in this case is KH (potassium hydride). Understanding how to balance ionic charges is essential in predicting the products of a chemical reaction and ensuring the chemical equation is correctly balanced.