Answer
The instability of carbonic acid is primarily due to the weak O-H bonds and the thermodynamically favorable decomposition into carbon dioxide and water, which is a more stable configuration.
Work Step by Step
Let's go through the requested tasks step by step.
1. Lewis structures for $CO_{3}^{2-}, HCO_{3}^{-}$, and $H_{2}CO_{3}$:
a) $CO_{3}^{2-}$ (Carbonate ion):
Lewis structure: O=C=O
The carbonate ion has a double bond between the carbon and each oxygen atom, and the overall charge is -2.
b) $HCO_{3}^{-}$ (Bicarbonate ion):
Lewis structure: H-O-C=O
The bicarbonate ion has a single bond between the hydrogen and one of the oxygen atoms, a double bond between the carbon and the other oxygen atom, and the overall charge is -1.
c) $H_{2}CO_{3}$ (Carbonic acid):
Lewis structure: H-O-C=O
H
Carbonic acid has a single bond between the hydrogen and one of the oxygen atoms, a double bond between the carbon and the other oxygen atom, and two hydrogen atoms bonded to the oxygen atoms.
2. Reaction: $H_{2}CO_{3}\rightarrow CO_{2}+H_{2}O$
To estimate the change in energy ($\Delta E$) for this reaction in the gas phase, we can use bond energies.
Bond energies:
- C=O: 805 kJ/mol
- O-H: 463 kJ/mol
Bonds broken:
- 2 × O-H (2 × 463 kJ/mol = 926 kJ/mol)
- 1 × C=O (805 kJ/mol)
Bonds formed:
- 2 × C=O (2 × 805 kJ/mol = 1610 kJ/mol)
- 2 × O-H (2 × 463 kJ/mol = 926 kJ/mol)
$\Delta E = \text{Bonds formed} - \text{Bonds broken}$
$\Delta E = 1610 \text{ kJ/mol} - 926 \text{ kJ/mol} - 805 \text{ kJ/mol} = -121 \text{ kJ/mol}$
The negative value of $\Delta E$ indicates that the reaction is exothermic, meaning it releases energy.
3. Possible cause for the instability of carbonic acid:
Carbonic acid is considered unstable because it readily decomposes into carbon dioxide and water. This is due to the weak nature of the O-H bond in carbonic acid, which can easily break, allowing the carbon dioxide and water to form.
The instability of carbonic acid can be attributed to the following factors:
- The C=O double bond in carbonic acid is relatively strong, but the O-H bonds are relatively weak.
- The decomposition of carbonic acid into carbon dioxide and water is thermodynamically favored, as it results in a more stable configuration with stronger bonds.
- The presence of the H-O-C=O structure in carbonic acid creates a strained and unstable arrangement, further contributing to its instability.
