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Electron Gain Enthalpy - Definition, Positive and Negative Electron Gain Enthalpies, Practice Problems and FAQ
Sam’s mother baked a delicious chocolate cake and offered it separately to Sam and his grandpa. Sam was super excited and was willing to eat it up without his mother's help. On the other hand, his grandpa was reluctant to have the cake owing to his health and age-related issues. So, Sam's mother had to help him eat the cake.
Well, This yummy chocolate cake is our beloved ‘electron’, whereas Sam and his grandpa are neutral gaseous atoms, who are to be fed with electrons!
For grandpa, energy has to be supplied from outside for having the cake (electron), and for Sam, he was happily willing to have the cake (electrons) and get happy (or stable).
This energy change on accepting electrons is electron gain enthalpy!
The enthalpy change associated with the process of addition of an electron to a neutral gaseous atom (let’s say X) in order to convert it into an anion (X-), is defined as “Electron Gain Enthalpy’’ (∆egH).
- It is measured in terms of kJ mol-1
- Depending on the nature of the element, the process of accepting electrons in a neutral gaseous atom can either be exothermic or endothermic.
X (g)+ e- X- (g)
TABLE OF CONTENTS
- Positive and Negative Electron Gain Enthalpies.
- Difference Between Positive Electron Gain Enthalpy and Negative Electron Gain Enthalpy
- Practice Problems
- Frequently Asked Questions - FAQ
Positive and Negative Electron Gain Enthalpies
- When energy is released while adding an electron to a gaseous atom, ∆egH is negative (exothermic). This usually happens when an atom gains stability (or stable electronic configuration) upon accepting electrons.
- When energy is supplied to a gaseous atom while adding an electron, ∆egH is positive (endothermic). This is the case when generally an atom/ion is reluctant to accept electrons on account of interelectronic repulsion.
Difference Between Positive Electron Gain Enthalpy and Negative Electron Gain Enthalpy
|
S. No. |
Positive electron gain enthalpy |
Negative electron gain enthalpy |
|
1. |
This is the amount of energy that needs to be supplied to a gaseous atom for the addition of an electron, making ∆egH positive in this case. |
This is the amount of energy released from a gaseous atom for the addition of an electron, making ∆egH negative in this case. |
|
2. |
When an atom is reluctant for the addition of a new electron in its valence shell and this drives it towards instability, its electron gain enthalpy tends to be positive. |
When an atom in order to gain stability tends to gain an electron, its electron gain enthalpy is negative. |
|
3. |
Noble gases have high positive ∆egH. |
Halogens have very high negative ∆egH. |
|
4. |
Metals tend to have positive electron gain enthalpy |
Non-metals generally have negative |
|
5. |
The second electron gain enthalpy of an atom is always positive. |
In general, the first electron gain enthalpy of an atom (non-metals) is negative. |
Practice Problems
Q1. On comparing the electron gain enthalpy of alkali metals, the electron gain enthalpy of halogens is ____.
A. Very high negative
B. Very low negative
C. Nearly the same
D. Very high positive
Answer: Halogens on gaining one electron attain stable octet configuration of the next nearest noble gas. Hence, they obtain stability by gaining electrons. So, they release energy on accepting electrons and hence, electron gain enthalpy is very high negative. Unlike them, alkali metals are more willing to release their valence electrons and they will need huge energy to be supplied for accepting electrons. Hence, option A) is the correct answer.
Q2. The family of which elements have the highest negative electron gain enthalpy?
A. Carbon family
B. Halogens
C. Oxygen family
D. Nitrogen family
Answer: Halogens have a very strong tendency to accept an electron as gaining one electron lets them attain a stable octet configuration of the next nearest noble gas. Hence, they have the highest negative electron gain enthalpy. Hence, option B) is the correct answer
Q3. Which family of elements have highly positive electron gain enthalpy?
A. Halogens
B. Noble gases
C. Chalcogens
D. Transition metals
Answer: Noble gases are highly stable owing to their octet configuration. Disturbing their electronic configuration by adding electrons will disturb their stability. So, their E.G.E is highly positive. Hence, option B) is the correct answer
Q4. Mention the term for the enthalpy change associated with an isolated gaseous atom (X) when it gains an electron to form its corresponding anion.
A. Ionisation enthalpy
B. Electron gain enthalpy
C. Electron affinity
D. Electronegativity
Answer: The enthalpy change associated with the process of addition of an electron to a neutral gaseous atom in order to convert it into an anion (X-), is defined as “Electron Gain Enthalpy’’(∆egH).
Frequently Asked Questions:
Question 1. Why does nitrogen being a non-metal have a positive electron gain enthalpy value?
Answer: The electronic configuration of nitrogen is1s22s22p3. It has a half-filled 2p orbital which is extremely stable. Thus, it has to be supplied with energy in order to make it gain electrons. Hence, its electron gain enthalpy is positive.
Question 2. Are electron gain enthalpy and electron affinity the same?
Answer: Electron gain enthalpy is at times referred to as electron affinity although there is a slight difference between the two.
Electron affinity is defined as the energy released when an electron is added to a neutral isolated gaseous atom. The order of electron gain enthalpy will be opposite of electron affinity.
Electron gain enthalpy is the enthalpy change involved in a process of addition of an electron to a neutral gaseous atom.
Question 3. Why is the electron gain enthalpy of noble gases positive?
Answer: Atoms of noble gases have a completely filled outer-electronic configuration. Hence, there is no space in their valence orbitals to accommodate incoming electrons. And the additional electron has to be placed in an orbital of the next higher shell. As a result, energy has to be supplied to add on additional electrons in order to break their extreme stable configurations. Hence, electron gain enthalpy is positive. In Fact, they have the maximum possible positive values of electron gain enthalpy.
Question 4. Which has a greater value of electron gain enthalpy? Fluorine or Chlorine?
Answer: Electron gain enthalpy of fluorine should have been more negative than chlorine considering the periodic trends of electron gain enthalpy down the group. But instead, the electron gain enthalpy of chlorine is greater (more negative) than fluorine.
This is on account of the small size of fluorine, which makes the incoming electron in 2p face extreme interelectronic repulsion unlike the addition of electron in 3p orbital of chlorine. Consequently, the incoming electron does not feel much attraction from the nucleus and therefore, the electron gain enthalpy of F is less negative than that of Cl.
Related Topics
| Factors affecting Electron Gain Enthalpy | Ionic Radii |
| Electron Affinity and Electron Gain Enthalpy | Successive electron gain enthalpy |
| Ionisation Enthalpy | Electronegativity |
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