Chemistry of Magnesium (Mg) - Practice Questions | Magnesium (Mg) Chemistry Guide

Chemistry of Magnesium (Mg) - Practice Questions

Multiple Choice Questions (MCQs)

Question 1

Which of the following statements about Magnesium (Mg) is incorrect? A) Magnesium reacts with steam to form magnesium oxide and hydrogen gas. B) Magnesium hydroxide is sparingly soluble in water. C) Magnesium does not react with nitrogen. D) Magnesium burns in air with a dazzling white flame.

Solution: The correct option is C.

Explanation: A) Magnesium reacts with steam ($\text{H}_2\text{O(g)}$) to form magnesium oxide ($\text{MgO}$) and hydrogen gas ($\text{H}_2$). $\text{Mg(s)} + \text{H}_2\text{O(g)} \rightarrow \text{MgO(s)} + \text{H}_2\text{(g)}$ B) Magnesium hydroxide ($\text{Mg(OH)}_2$) is sparingly soluble in water, making it a weak base (e.g., milk of magnesia). Its solubility is much lower than other alkaline earth metal hydroxides like $\text{Ca(OH)}_2$, $\text{Sr(OH)}_2$, or $\text{Ba(OH)}_2$. C) Magnesium reacts readily with nitrogen ($\text{N}_2$) when heated in air (or pure nitrogen) to form magnesium nitride ($\text{Mg}_3\text{N}_2$). This is a characteristic reaction for Group 2 metals, especially $\text{Mg}$ and $\text{Li}$ (diagonal relationship). $3\text{Mg(s)} + \text{N}_2\text{(g)} \xrightarrow{\text{heat}} \text{Mg}_3\text{N}_2\text{(s)}$ D) Magnesium is highly combustible and burns in air with a brilliant, dazzling white flame, producing both magnesium oxide and some magnesium nitride.

Question 2

Among the following compounds of Magnesium, which one is highly soluble in water? A) $\text{MgCO}_3$ B) $\text{Mg(OH)}_2$ C) $\text{MgSO}_4$ D) $\text{Mg}_3\text{N}_2$

Solution: The correct option is C.

Explanation: A) Magnesium carbonate ($\text{MgCO}3$) is practically insoluble in water. Its solubility product constant ($K{sp}$) is very low. B) Magnesium hydroxide ($\text{Mg(OH)}_2$) is sparingly soluble in water. C) Magnesium sulfate ($\text{MgSO}_4$) is highly soluble in water. This is an important distinction among alkaline earth metal sulfates, where solubility generally decreases down the group ($\text{BeSO}_4 > \text{MgSO}_4 > \text{CaSO}_4 > \text{SrSO}_4 > \text{BaSO}_4$). Epsom salt is $\text{MgSO}_4 \cdot 7\text{H}_2\text{O}$. D) Magnesium nitride ($\text{Mg}_3\text{N}_2$) reacts with water to produce magnesium hydroxide and ammonia, but it itself is not highly soluble. $\text{Mg}_3\text{N}_2\text{(s)} + 6\text{H}_2\text{O(l)} \rightarrow 3\text{Mg(OH)}_2\text{(s)} + 2\text{NH}_3\text{(g)}$

Question 3

When magnesium metal is heated in a limited supply of oxygen, the product formed is primarily: A) $\text{MgO}$ B) $\text{Mg}_3\text{N}_2$ C) A mixture of $\text{MgO}$ and $\text{Mg}_3\text{N}_2$ D) $\text{Mg}(\text{OH})_2$

Solution: The correct option is A.

Explanation: Magnesium burns vigorously in oxygen to form magnesium oxide ($\text{MgO}$). $2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)}$ When heated in air (which is ~78% nitrogen and ~21% oxygen), magnesium reacts with both oxygen and nitrogen. However, the reaction with oxygen is much more vigorous and exothermic than with nitrogen. If the supply of oxygen is limited, it indicates that oxygen is the primary reactant being considered and the main product will be magnesium oxide. While some $\text{Mg}_3\text{N}_2$ might form in air, if oxygen supply is limited (implying nitrogen is still present), $\text{MgO}$ is the primary product due to oxygen’s higher reactivity with Mg compared to nitrogen. The question specifies “limited supply of oxygen,” implying oxygen is the limiting reactant, leading to MgO formation as the primary product. If the question said “heated in air,” then C would be more appropriate. If it specified “heated in nitrogen atmosphere,” then B would be correct.

Assertion-Reason Questions

Question 1

Assertion (A): Magnesium burns in air with a dazzling white flame. Reason (R): Magnesium reacts with nitrogen to form magnesium nitride ($\text{Mg}_3\text{N}_2$).

A) Both A and R are true, and R is the correct explanation of A. B) Both A and R are true, but R is not the correct explanation of A. C) A is true but R is false. D) A is false but R is true. E) Both A and R are false.

Solution: The correct option is B.

Explanation: Assertion (A): Magnesium indeed burns in air with a dazzling white flame, primarily forming magnesium oxide ($\text{MgO}$). This statement is true. Reason (R): Magnesium reacts with nitrogen at high temperatures to form magnesium nitride ($\text{Mg}_3\text{N}_2$). This statement is also true. However, the reason for the dazzling white flame is primarily the intense exothermic reaction of magnesium with oxygen (present in air) to form magnesium oxide. While magnesium nitride also forms, it’s not the sole or primary reason for the “dazzling white flame.” The main product responsible for the brightness is $\text{MgO}$. Therefore, R is true, but it is not the correct explanation for A.

Question 2

Assertion (A): Magnesium hydroxide ($\text{Mg(OH)}_2$) is sparingly soluble in water. Reason (R): The lattice energy of magnesium hydroxide is very high, making it difficult to overcome by hydration energy.

Solution: The correct option is A.

Explanation: Assertion (A): Magnesium hydroxide ($\text{Mg(OH)}_2$) is known to be sparingly soluble in water. This statement is true. It is commonly used as an antacid (Milk of Magnesia) due to its low solubility and mild basicity. Reason (R): The solubility of ionic compounds depends on the balance between lattice energy (energy required to break the lattice) and hydration energy (energy released when ions are solvated by water molecules). For $\text{Mg(OH)}_2$, the $\text{Mg}^{2+}$ ion is relatively small, leading to a high charge density. The hydroxide ion ($\text{OH}^-$) is also small. This results in a very high lattice energy for $\text{Mg(OH)}_2$. Although hydration energy for $\text{Mg}^{2+}$ is also high due to its small size and high charge, it is not sufficient to overcome the exceptionally high lattice energy, especially when compared to the larger $\text{Ca}^{2+}$, $\text{Sr}^{2+}$, or $\text{Ba}^{2+}$ ions where lattice energy decreases and hydration energy also decreases, but the balance shifts. Therefore, $\text{Mg(OH)}_2$ has limited solubility. This statement is true and correctly explains the assertion.

Short Answer Questions

Question 1

Why does magnesium not impart any characteristic colour to the Bunsen flame (flame test), unlike other alkaline earth metals like Calcium and Barium?

Model Answer: Magnesium (Mg) does not exhibit a characteristic colour in the Bunsen flame test primarily due to its small atomic size and relatively high ionization energy compared to other alkaline earth metals such as Calcium (Ca), Strontium (Sr), and Barium (Ba). The valence electrons in magnesium are held more strongly by the nucleus. The energy provided by a typical Bunsen flame is insufficient to excite these strongly bound electrons to higher energy levels. Consequently, when these electrons de-excite, they do not emit radiation in the visible region of the electromagnetic spectrum, and thus no characteristic flame colour is observed. In contrast, for larger alkaline earth metal ions, the valence electrons are less strongly held, require less energy to excite, and upon de-excitation, emit visible light.

Question 2

Explain the chemical reactions of Magnesium with: i) Dilute Hydrochloric Acid ($\text{HCl}$) ii) Concentrated Sulphuric Acid ($\text{H}_2\text{SO}_4$)

Model Answer: i) Reaction with Dilute Hydrochloric Acid ($\text{HCl}$): Magnesium is an electropositive metal and readily reacts with dilute non-oxidizing acids like dilute $\text{HCl}$ to produce hydrogen gas and the corresponding magnesium salt. The reaction is quite vigorous. $\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$ In this reaction, magnesium gets oxidized from 0 to +2 oxidation state, while hydrogen from $\text{HCl}$ gets reduced from +1 to 0.

ii) Reaction with Concentrated Sulphuric Acid ($\text{H}_2\text{SO}_4$): Magnesium reacts with concentrated sulphuric acid. Concentrated $\text{H}_2\text{SO}_4$ acts as a strong oxidizing agent in addition to being an acid. The products formed depend on the concentration and temperature. Typically, it oxidizes magnesium to magnesium sulfate ($\text{MgSO}_4$) and itself gets reduced to sulfur dioxide ($\text{SO}_2$), along with water. $\text{Mg(s)} + 2\text{H}_2\text{SO}_4\text{(conc)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{SO}_2\text{(g)} + 2\text{H}_2\text{O(l)}$ In this reaction, magnesium is oxidized from 0 to +2, and sulfur in $\text{H}_2\text{SO}_4$ is reduced from +6 to +4. Hydrogen gas is generally not produced in significant amounts with concentrated oxidizing acids.

High-Order Thinking Skills (HOTS) Question

Question 1

Magnesium metal is considered relatively unreactive towards cold water, forming a thin protective layer of magnesium hydroxide that prevents further reaction. However, it reacts vigorously with steam. Explain this difference in reactivity based on thermodynamic and kinetic considerations.

Model Answer: The difference in reactivity of magnesium with cold water versus steam can be explained by considering both thermodynamic favorability and kinetic barriers.

Thermodynamic Consideration: The reaction of magnesium with water (both liquid and gaseous) to form magnesium hydroxide ($\text{Mg(OH)}_2$) or magnesium oxide ($\text{MgO}$) and hydrogen gas is thermodynamically favourable. $\text{Mg(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{Mg(OH)}_2\text{(s)} + \text{H}_2\text{(g)}$ $\text{Mg(s)} + \text{H}_2\text{O(g)} \rightarrow \text{MgO(s)} + \text{H}_2\text{(g)}$ (at higher temperatures) The standard Gibbs free energy change ($\Delta G^\circ$) for these reactions is negative, indicating that they are spontaneous under standard conditions. Thus, from a purely thermodynamic perspective, magnesium should react with water at all temperatures.
Kinetic Consideration:
- With Cold Water: At lower temperatures (cold water), the kinetic energy of the water molecules and magnesium atoms is low. The initial reaction forms a thin, insoluble, and relatively stable layer of magnesium hydroxide ($\text{Mg(OH)}_2$) on the surface of the magnesium metal. This layer acts as a protective barrier, preventing further direct contact between the magnesium metal and water molecules. The activation energy required to overcome this protective layer and for the reaction to proceed significantly is not met by the energy available at cold temperatures. Therefore, the reaction becomes kinetically very slow, making magnesium appear unreactive with cold water.
- With Steam: When magnesium is exposed to steam (water in gaseous state at high temperatures), the kinetic energy of the reactant molecules is significantly higher. This increased energy overcomes the activation barrier more easily. The high temperature causes the protective $\text{Mg(OH)}_2$ layer to dehydrate and convert into magnesium oxide ($\text{MgO}$). $\text{Mg(OH)}_2\text{(s)} \xrightarrow{\text{heat}} \text{MgO(s)} + \text{H}_2\text{O(g)}$ Unlike magnesium hydroxide, the magnesium oxide layer formed at high temperatures is porous and less protective, or it can even spall off, exposing fresh magnesium surface for reaction. The high temperature also facilitates the rapid diffusion of steam molecules to the magnesium surface. This combination of higher kinetic energy, breakdown/porosity of the protective layer, and rapid diffusion leads to a vigorous and rapid reaction with steam.

In summary, while the reaction of magnesium with water is thermodynamically feasible, its reactivity with cold water is kinetically hindered by the formation of a protective $\text{Mg(OH)}_2$ layer. With steam, the higher thermal energy overcomes this kinetic barrier, leading to a vigorous reaction and the formation of $\text{MgO}$.

Chemistry of Magnesium (Mg) - Practice Questions