Ionization Enthalpy Overview
Ionization enthalpy, also known as ionization energy, is an essential concept in chemistry and physics. It refers to the amount of energy required to remove an electron from an atom or ion in its gaseous state. In this article, we will explore the definition of ionization enthalpy, the trends observed in the periodic table, and the various applications in different fields of science.
What is Enthalpy?
In thermochemistry, enthalpy is the heat content of a system. It is the sum of the internal energy of the system plus the product of its pressure and volume. Enthalpy is a state function. The SI unit of enthalpy is the joule.
A quantity related to a thermodynamic system that is equal to the heat transmitted during an isobaric process and is expressed as the system's internal energy plus the product of the system's pressure and volume. h is the symbol for enthalpy.
What is Ionization Energy?
Ionization energy is the amount of energy required to remove an electron from an atom or molecule. The higher the ionization energy, the more difficult it is to remove an electron. The first ionization energy is always greater than the second ionization energy, which is always greater than the third ionization energy, and so on.
In a liquid solution, ionization is common. For example, at the surface of a piece of metallic zinc in contact with an acidic solution, neutral hydrogen chloride gas molecules, HCl, react with similarly polar water molecules, H2O, to produce positive hydronium ions, H3O+, and negative chloride ions, Cl-; zinc atoms, Zn, lose electrons to hydrogen ions and become colorless zinc ions, Zn2+.
What is Ionization Enthalpy?
Ionization enthalpy is a measure of the amount of energy required to remove an electron from an atom or ion in its gaseous state. The ionization energy is measured in electron volts (eV) or kilojoules per mole (kJ/mol). It is an essential property of an atom or ion that is influenced by various factors such as the atomic radius, electron shielding, and nuclear charge.
Ionization Enthalpy | Definition | Sign | Trend |
| Ionization Enthalpy | The amount of energy required to remove a single electron from the ground state of a gaseous atom (X) | Always positive | Increases across periods, decreases down groups. |
The Relationship between Enthalpy and Ionization Energy
The table below explains the relationship between Enthalpy and Ionization energy.
Property | Enthalpy | Ionization Energy |
| Definition | Enthalpy is the measure of the heat energy exchanged between a system and its surroundings during a constant-pressure process. | Ionization energy is the minimum amount of energy required to remove an electron from a gaseous atom or ion in its ground state. |
| Formula | ∆H = H_final - H_initial | IE = energy required to remove an electron |
| Unit | Joules (J) or kilojoules per mole (kJ/mol) | Electronvolts (eV) or kilojoules per mole (kJ/mol) |
| Sign | ∆H can be positive or negative, depending on whether the reaction is exothermic or endothermic. A positive ∆H indicates an endothermic reaction, where the system absorbs heat from the surroundings. A negative ∆H indicates an exothermic reaction, where the system releases heat to the surroundings. | Ionization energy is always positive, indicating that energy must be added to the system to remove an electron. |
| Relationship | Enthalpy and ionization energy are indirectly related. | The ionization energy of an atom or ion is related to its electron affinity and electronegativity. |
| Effect of Enthalpy on Ionization Energy | Exothermic reactions release energy, resulting in more stable products with lower potential energy. These stable products require more energy to remove an electron, resulting in higher ionization energy. Endothermic reactions absorb energy, resulting in less stable products with a higher potential energy. These unstable products require less energy to remove an electron, resulting in lower ionization energy. | Enthalpy does not directly affect ionization energy, but it is indirectly related through the atom or ion's electron affinity and electronegativity. Higher electron affinity and electronegativity lead to a stronger attraction between the nucleus and electrons, resulting in higher ionization energy. |
| Examples | The combustion of methane (CH4) is an exothermic reaction that releases energy in the form of heat. The products of this reaction, carbon dioxide and water, are more stable than the reactant methane. Therefore, they require more energy to remove an electron, resulting in higher ionization energy. | The ionization energy of fluorine (F) is higher than that of chlorine (Cl), even though they are in the same group. This is because fluorine has a higher electron affinity and electronegativity, resulting in a stronger attraction between the nucleus and electrons and higher ionization energy. |
Trends in Ionization Enthalpy
The ionization enthalpy of an element is dependent on its position in the periodic table. There is a general trend in the periodic table, whereby the ionization enthalpy increases from left to right across a period and decreases from top to bottom down a group. The increase in the ionization enthalpy across a period is due to the increase in the nuclear charge and the decrease in the atomic radius. The decrease in the ionization enthalpy down a group is due to the increase in the atomic radius and electron shielding.
Table 1: Trends in Ionization Enthalpy in the Periodic Table
| Period | Element | Ionization Enthalpy (kJ/mol) |
| 2 | Li | 520 |
| 2 | Be | 899 |
| 2 | B | 801 |
| 2 | C | 1086 |
| 2 | N | 1402 |
| 2 | O | 1314 |
| 2 | F | 1681 |
| 2 | Ne | 2081 |
Read more about the Enthalpy of Atomization.
Applications of Ionization Enthalpy
Ionization enthalpy plays a vital role in various fields of science. In analytical chemistry, it is used to identify the presence of elements in a sample. The ionization energy of an element can be used to distinguish it from other elements in the sample. In addition, ionization enthalpy is used in the design of electronic devices such as transistors and diodes. It is also an essential parameter in studying chemical reactions and bonding.
Table 2: Ionization Enthalpy of Selected Elements
| Element | Ionization Enthalpy (kJ/mol) |
| Li | 520 |
| Na | 496 |
| K | 418 |
| Mg | 738 |
| Ca | 590 |
| Al | 578 |
| Si | 786 |
| P | 1012 |
| S | 1000 |
Factors Influencing Ionization Energy
- The size of a positive nuclear charge:
It takes more energy to remove an electron because the nuclear charge's pull on the topmost electron grows as the nuclear charge does. As a consequence, the energy of ionization increases.
- Atomic weight (distance of the outermost electron from the nucleus):
As the atomic size increases, the negative electron's pull on the positive nucleus weakens, requiring less energy to eliminate one electron. As a consequence, the ionization energy decreases.
- The screening (shielding) impact of electrons in the inner sphere:
The outermost electron is protected from the nucleus' pull by the repelling effect of the interior electrons. The positive nucleus's attraction to the negative electron weakens with increasing shielding, needing less energy to extract an electron. As a consequence, the ionization energy decreases.
Read more about the Enthalpy Formula.
Individual distinctions within the group
When moving down a group of elements, the ionization enthalpy decreases for the following reasons:
- The atomic size increases down the group due to the addition of one more energy shell with each subsequent element, which causes the valence electron's distance from the nucleus to increase. The nucleus's force of attraction for the valence electron decreases as a result, which reduces the ionization enthalpy.
- The number of inner electron shells increases with each successive element, which shields the valence electron from the nucleus's force of attraction. This further weakens the nucleus's force of attraction for valence electrons and lowers the ionization enthalpy.
- The charge on the nucleus increases as the atomic number increases. Therefore, the nucleus's force of attraction for the valence electron increases, but the effect is offset by the increasing distance between the valence electron and the nucleus. As a result, the ionization enthalpy decreases.
- As we move down the group, the valence electrons become less tightly bound to the nucleus due to the increasing distance between the nucleus and the valence electrons. This further reduces the ionization enthalpy.
How to Measure Ionization Enthalpy
In order to measure ionization enthalpy, one must first understand what enthalpy is. Enthalpy is a measure of the heat content in a system. In order to calculate enthalpy, one must use the equation:
Enthalpy = Heat Capacity x Temperature
where heat capacity is the amount of heat required to raise the temperature of a substance by one degree Celsius. Once the heat capacity has been determined, the temperature can be plugged in and the enthalpy calculated.
Ionization enthalpy is the amount of heat required to remove an electron from an atom or molecule. This type of enthalpy is always positive, as it takes energy to remove an electron from a system. The equation for ionization enthalpy is:
Ionization Enthalpy = (Heat Capacity of Atom or Molecule) x (Temperature)
To calculate ionization enthalpy, one must determine the heat capacity of the atom or molecule in question. This can be done through experimentation or by using data from a reliable source. Once the heat capacity has been determined, the temperature can be plugged in, and the ionization enthalpy calculated.
Things to Remember about Ionization Enthalpy
Here are some key points to remember about "ionization enthalpy":
- Ionization enthalpy is the energy required to remove one mole of electrons from one mole of gaseous atoms or ions in the ground state.
- Ionization enthalpy increases across a period and decreases down a group in the periodic table.
- The first ionization enthalpy is always less than the second ionization enthalpy, which is less than the third, and so on.
- Ionization enthalpy is affected by factors such as atomic size, effective nuclear charge, and shielding effect.
- Elements with high ionization enthalpy are less likely to lose electrons and tend to form negative ions.
- The noble gasses have the highest ionization enthalpy values among all elements because they have a stable electronic configuration.
- Ionization enthalpy is an important factor in determining the reactivity of elements, and elements with low ionization enthalpy are more reactive than those with high ionization enthalpy.
- The trend of ionization enthalpy can be explained by the increase in the nuclear charge across a period and the increase in the number of electron shells down a group.
Read More About-
Ionization Enthalpy: Previous Year Questions
- Define ionization enthalpy. Explain the trend in the first ionization enthalpy across period 3. (NEET 2021)
- Write the general electronic configuration of the p-block elements. Which element has the highest second ionization energy in the p-block? (NEET 2020)
- Explain the periodic trends in the ionization enthalpy of the elements. (NEET 2019)
- Which of the following elements has the highest first ionization energy? Na, K, Rb, or Cs? Explain. (NEET 2018)
- Which one of the following pairs of elements has the larger value of the first ionization energy? (a) N or O, (b) C or N, (c) Be or Mg, (d) Li or Na. (NEET 2017)
- In the periodic table, as one moves from left to right in a period, the ionization energy of the elements (a) increases gradually, (b) remains constant, (c) decreases gradually, (d) first increases and then decreases. (NEET 2016)
- The ionization energy of the hydrogen atom in the ground state is 13.6 eV. The energy required to excite the electron in a hydrogen atom from the ground state to the first excited state is (a) 3.4 eV, (b) 6.8 eV, (c) 10.2 eV, (d) 13.6 eV. (NEET 2015)
- What is meant by ionization energy? Explain the factors affecting ionization energy. (CBSE 2020)
- Define the term ionization enthalpy. Why does the first ionization enthalpy of potassium less than that of sodium? (CBSE 2019)
- Write the trend of ionization enthalpy across a period and down a group in the modern periodic table. Explain the reason for the trend.(CBSE 2018)
- Explain the difference between the first and second ionization enthalpy with examples. (CBSE 2017)
- The first ionization energy of sodium is lower than that of magnesium, but the second ionization energy of magnesium is higher than that of sodium. Explain. (CBSE 2016)
- What is the relationship between ionization enthalpy and reactivity? Give an example to support your answer. (CBSE 2015)
- Define ionization energy. Why is the first ionization energy of magnesium less than that of aluminum? (DUET 2020)
- Explain the factors affecting the ionization energy of an atom. What is the trend in the first ionization energy down a group in the periodic table? (DUET 2019)
- Which element in the second period has the highest first ionization energy? Explain. (DUET 2018)
- Explain the trends in the first ionization energies of the elements in the periodic table. (DUET 2017)