The absolute enthalpy of neutralization of the reaction,
`MgO(s)+2HCl(aq.)+H_(2)O(l)` will be

Options

A. less that 57 kJ `" mol"^(-1)`
B. `-57 " kJ mol"^(-1)`
C. greater than 57 kJ `" mol"^(-1)`
D. 57 kJ `" mol"^(-1)`

Explanation

To find the absolute enthalpy of neutralization for the reaction:

\[ \text{MgO(s)} + 2\text{HCl(aq)} + \text{H}_2\text{O(l)} \rightarrow \text{MgCl}_2 + 2\text{H}_2\text{O} \]

we need to consider the enthalpy changes associated with the reaction.

### Step 1: Identify the Reaction and Components
The reaction involves magnesium oxide (MgO), hydrochloric acid (HCl), and water (H2O). When MgO reacts with HCl, it forms magnesium chloride (MgCl2) and water.

**Hint:** Identify all reactants and products in the reaction to understand what is being formed.

### Step 2: Understand the Enthalpy of Neutralization
The enthalpy of neutralization is the heat change that occurs when an acid and a base react to form water. For strong acids and strong bases, this value is typically around -57 kJ/mol for the formation of one mole of water.

**Hint:** Recall that the enthalpy of neutralization for strong acid-strong base reactions is generally a fixed value.

### Step 3: Consider the Nature of MgO
Magnesium oxide is a weak base and does not completely dissociate in solution like strong bases do. Therefore, it requires some energy (let's denote it as X) to dissociate into its ions (Mg²⁺ and O²⁻).

**Hint:** Recognize that weak bases do not fully dissociate in solution, which affects the overall enthalpy change.

### Step 4: Set Up the Energy Equation
When MgO reacts with HCl, the reaction can be broken down into two parts:
1. The energy required to dissociate MgO (X).
2. The energy released during the formation of water from H⁺ and OH⁻ ions.

Since there are two moles of water formed, the total energy released from the neutralization will be \(2 \times (-57 \text{ kJ}) = -114 \text{ kJ}\).

Thus, the overall enthalpy change can be expressed as:
\[ \Delta H = X - 114 \text{ kJ} \]

**Hint:** Write down the enthalpy change equation based on the energy required and released during the reaction.

### Step 5: Estimate the Value of X
Since the energy required to dissociate MgO is relatively small (let's assume it is around 4-10 kJ), we can estimate that X will be a small positive number.

**Hint:** Consider that the energy required to break bonds in weak bases is much smaller compared to the energy released during the formation of water.

### Step 6: Conclusion on Enthalpy of Neutralization
Since X is a small positive value, the overall enthalpy change will be slightly less negative than -114 kJ. This means the absolute enthalpy of neutralization will be greater than -57 kJ/mol.

**Hint:** Conclude that the enthalpy of neutralization for this reaction is greater than the typical value for strong acid-strong base reactions due to the weak base nature of MgO.

### Final Answer
The absolute enthalpy of neutralization for the reaction is greater than -57 kJ/mol.