The first ionization enthalpy of Na, Mg and Si, respectively, are : 496, 737 and 786 kJ mol–1. The first ionization enthalpy (kJ mol–1) of Al is:
- 487
- 768
- 577
- 856
The Correct Option is C
Solution and Explanation
The question asks us to determine the first ionization enthalpy of aluminum (Al) given the ionization enthalpies of sodium (Na), magnesium (Mg), and silicon (Si). The options provided are 487, 768, 577, and 856 kJ mol–1. The correct answer is 577 kJ mol–1.
Here's a step-by-step explanation:
- First ionization enthalpy is the energy required to remove the outermost electron from a neutral atom in the gaseous state to form a cation.
- The trend in the periodic table generally shows that ionization energy increases across a period from left to right due to increased nuclear charge, which holds the electrons tighter.
- Let's consider the periodic table's trend for elements in the given period (3rd period): Na (Sodium), Mg (Magnesium), Al (Aluminum), and Si (Silicon). These elements follow this progression: Na < Mg < Al < Si in terms of ionization energy.
- Given values:
- Na: 496 kJ mol–1
- Mg: 737 kJ mol–1
- Si: 786 kJ mol–1
- Al is between Mg and Si in the periodic table. We can deduce that its ionization energy should logically fall between those of Mg and Si.
- The provided answer options are: 487, 768, 577, 856 kJ mol–1.
- Among these, 577 kJ mol–1 is the only value that fits the description of falling between the ionization energies of Na (737 kJ mol–1) and Si (786 kJ mol–1).
Conclusion: Therefore, by understanding the periodic trend and the context of these values, we identify the correct first ionization enthalpy of Aluminum to be \(577 \text{ kJ mol}^{–1}\).
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Concepts Used:
Enthalpy change
Enthalpy Change refers to the difference between the heat content of the initial and final state of the reaction. Change in enthalpy can prove to be of great importance to find whether the reaction is exothermic or endothermic.
Formula for change in enthalpy is:-
dH = dU + d(PV)
The above equation can be written in the terms of initial and final states of the system which is defined below:
UF – UI = qP –p(VF – VI)
Or qP = (UF + pVF) – (UI + pVI)
Enthalpy (H) can be written as H= U + PV. Putting the value in the above equation, we obtained:
qP = HF – HI = ∆H
Hence, change in enthalpy ∆H = qP, referred to as the heat consumed at a constant pressure by the system. At constant pressure, we can also write,
∆H = ∆U + p∆V
Standard Enthalpy of Reaction
To specify the standard enthalpy of any reaction, it is calculated when all the components participating in the reaction i.e., the reactants and the products are in their standard form. Therefore the standard enthalpy of reaction is the enthalpy change that occurs in a system when a matter is transformed by a chemical reaction under standard conditions.