Question

Which of the following chemical equations is connected to the definitions of (a) the first ionization energy of oxygen, (b) the second ionization energy of oxygen, and (c) the electron affinity of oxygen? (i) $\mathrm{O}(g)+{\mathrm{e}}^{-}⟶{\mathrm{O}}^{-}(g)$ (ii) $\mathrm{O}(g)⟶{\mathrm{O}}^{+}(g)+{\mathrm{e}}^{-}$ (iii) $\mathrm{O}(g)+2{\mathrm{e}}^{-}⟶{\mathrm{O}}^{2-}(g)$ (iv) $\mathrm{O}(g)⟶{\mathrm{O}}^{2+}(g)+2{\mathrm{e}}^{-}$ (v) ${\mathrm{O}}^{+}(g)⟶{\mathrm{O}}^{2+}(g)+{\mathrm{e}}^{-}$

Short answer

(a) The first ionization energy of oxygen corresponds to equation (ii): $$\mathrm{O}(g)⟶{\mathrm{O}}^{+}(g)+{\mathrm{e}}^{-}$$ (b) The second ionization energy of oxygen corresponds to equation (v): $${\mathrm{O}}^{+}(g)⟶{\mathrm{O}}^{2+}(g)+{\mathrm{e}}^{-}$$ (c) The electron affinity of oxygen corresponds to equation (i): $$\mathrm{O}(g)+{\mathrm{e}}^{-}⟶{\mathrm{O}}^{-}(g)$$

Step-by-step solution

The first ionization energy is the energy required to remove the first electron from a neutral atom to form a positively charged ion. In the case of oxygen, it refers to the energy needed to remove one electron from an oxygen atom, represented as: $$\mathrm{O}(g)⟶{\mathrm{O}}^{+}(g)+{\mathrm{e}}^{-}$$ #Step 2: Define second ionization energy#

The second ionization energy is the energy required to remove a second electron from an already positively charged ion, which already had one electron removed. In the case of oxygen, it refers to the energy needed to remove another electron from an O⁺ ion, represented as: $${\mathrm{O}}^{+}(g)⟶{\mathrm{O}}^{2+}(g)+{\mathrm{e}}^{-}$$ #Step 3: Define electron affinity#

Electron affinity is the energy change associated with the addition of an electron to a neutral atom. In the case of oxygen, it refers to the energy change when an electron is added to an oxygen atom, represented as: $$\mathrm{O}(g)+{\mathrm{e}}^{-}⟶{\mathrm{O}}^{-}(g)$$ #Step 4: Match definitions with chemical equations#

(a) The first ionization energy of oxygen is represented by the process of removing one electron from an oxygen atom. The chemical equation that corresponds to this definition is: (i) $\mathrm{O}(g)⟶{\mathrm{O}}^{+}(g)+{\mathrm{e}}^{-}$ (b) The second ionization energy of oxygen is represented by the process of removing another electron from an O⁺ ion. The chemical equation that corresponds to this definition is: (v) ${\mathrm{O}}^{+}(g)⟶{\mathrm{O}}^{2+}(g)+{\mathrm{e}}^{-}$ (c) The electron affinity of oxygen is represented by the process of adding an electron to an oxygen atom. The chemical equation that corresponds to this definition is: (iii) $\mathrm{O}(g)+{\mathrm{e}}^{-}⟶{\mathrm{O}}^{-}(g)$

Key concepts

First Ionization Energy

Ionization energy is an intrinsic property of atoms that measures the difficulty of removing an electron from a neutral atom or ion. First ionization energy refers specifically to the energy required to remove the first electron from a neutral atom. In gaseous oxygen, for instance, we can illustrate this process with the chemical equation $$\mathrm{O}(g)⟶{\mathrm{O}}^{+}(g)+{\mathrm{e}}^{-}$$Imagine an oxygen atom floating freely in the gas phase. The first ionization energy is the energy input necessary for this atom to release its least tightly bound electron, resulting in a positively charged ion, or cation, denoted as O⁺, plus a free electron. This process requires a significant amount of energy because it's overcoming the electrostatic forces holding the electron within the atom's electron cloud. Elements further to the right and up on the periodic table typically have higher first ionization energies because their electrons are more closely held by a stronger nuclear charge.

Second Ionization Energy

Once an atom has lost one electron and has become a positively charged ion, the energy required to remove a second electron is known as the second ionization energy. For oxygen, the equation representing this process is $${\mathrm{O}}^{+}(g)⟶{\mathrm{O}}^{2+}(g)+{\mathrm{e}}^{-}$$In this scenario, the oxygen ion (O⁺) has already parted with an electron thanks to the first ionization energy. As you might expect, the second ionization energy is always higher than the first for a given element. This is because once an electron is removed, the remaining electrons experience a greater net positive charge from the nucleus, thus requiring more energy to overcome this increased attraction. This demonstrates the general trend that ionization energies increase with the sequential removal of electrons.

Electron Affinity

In contrast to ionization energy, electron affinity deals with the addition of an electron to a neutral atom, which releases energy. Oxygen's electron affinity, for example, is demonstrated by the following reaction: $$\mathrm{O}(g)+{\mathrm{e}}^{-}⟶{\mathrm{O}}^{-}(g)$$This process describes how much affinity, or liking, an atom has for an extra electron. When a neutral oxygen atom in its gaseous state captures an electron, it turns into a negatively charged ion (O⁻), releasing a quantifiable amount of energy in the process. Electron affinity values can vary widely across the periodic table, but they tend to increase across a period (left to right) as the atoms are closer to completing their valence shell, making the addition of an extra electron more energetically favorable.

Chemical Equations

Chemical equations are symbolic representations of chemical reactions, depicting the substances involved, their physical states, and the transformation occurring between reactants and products. An equation must follow the law of conservation of mass, meaning that atoms are neither created nor destroyed, merely rearranged. A basic example would be the reaction of hydrogen and oxygen to form water, expressed as $$2H_2(g)+O_2(g)⟶2H_{2}O(l)$$Here, the reactants (hydrogen and oxygen in their gaseous forms) are on the left side, and the product (liquid water) is on the right side. The numbers before the chemical formulas are coefficients used to balance the equation, ensuring that the number of each type of atom is the same on each side of the equation. In the context of ionization energies and electron affinity, chemical equations provide a clear and concise way to show the changes happening to an atom or ion during these processes.