Write Lewis structures for \(\mathrm{CO}_{3}^{2-}, \mathrm{HCO}_{3}^{-}\), and \(\mathrm{H}_{2} \mathrm{CO}_{3}\). When acid is added to an aqueous solution containing carbonate or bicarbonate ions, carbon dioxide gas is formed. We generally say that carbonic acid \(\left(\mathrm{H}_{2} \mathrm{CO}_{3}\right)\) is unstable. Use bond energies to estimate \(\Delta H\) for the reaction (in the gas phase) $$ \mathrm{H}_{2} \mathrm{CO}_{3} \longrightarrow \mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O} $$ Specify a possible cause for the instability of carbonic acid.

To draw the Lewis structure, first, count the total number of valence electrons. Carbon has 4, each oxygen atom has 6, and there are 2 extra electrons due to the 2- charge. So, the total valence electrons are: \[4+(3 \times 6)+2=24\] Create the Lewis structure by placing the carbon atom at the center since it is the least electronegative and the oxygen atoms around it. Then, connect each oxygen with a single bond to the carbon atom to create a total of 8 electrons used. Distribute the remaining 16 electrons among the oxygen atoms (5 electrons in each oxygen atom and one oxygen atom with 6 electrons). The Lewis structure of CO32- looks like this: ``` O \\ 𝚺 C - O- // O ```

Calculating the valence electrons: Carbon has 4, each oxygen atom has 6, hydrogen has 1, and there is 1 extra electron due to the 1- charge. So, the total valence electrons are: \[4+(3 \times 6)+1+1=24\] Follow the same logic as before, place the carbon atom at the center and distribute these around it: 3 oxygen atoms and 1 hydrogen atom. Then, distribute the remaining electrons among the oxygen atoms (5 electrons in each oxygen atom and one oxygen atom with 6 electrons) and connect the hydrogen atom to one of the oxygens. The Lewis structure of HCO3- is: ``` H | O - C - O- \\ O ```

Calculating the valence electrons: Carbon has 4, each oxygen atom has 6, and hydrogen has 1. So, the total valence electrons are: \[4+(3 \times 6)+(2 \times 1)=24\] Connect one hydrogen atom with each of the spare electrons on two oxygen atoms, resulting in the following Lewis structure of H2CO3: ``` H O C - O- \\ O ``` #Step 2: Estimating ΔH using Bond Energies#

To calculate ΔH for the reaction: H2CO3 → CO2 + H2O, we need to find the energy change involved in breaking the bonds in H2CO3 and forming the bonds in CO2 and H2O. ΔH = (Σ bond energies of products) - (Σ bond energies of reactants) Bond energies are as follows: \[C=O\: double\: bond: 799 \:kJ/mol\] \[C-O\: single\: bond: 358\: kJ/mol\] \[O-H\: bond\: in\: water: 467\: kJ/mol\] For H2CO3, we have 1 C=O double bond, 2 C-O single bonds, and 2 O-H bonds: Energies in H2CO3: 1(799) + 2(358) + 2(467) = 2449 kJ/mol. For the products, CO2 has 2 C=O double bonds, and H2O has 2 O-H bonds: Energies in CO2 and H2O: 2(799) + 2(467) = 2532 kJ/mol. Now, we can calculate ΔH: ΔH = 2532 - 2449 = 83 kJ/mol #Step 3: Explaining the Instability of Carbonic Acid#

Carbonic acid's instability is attributed to the ease with which it breaks down into carbon dioxide and water. This breakdown occurs due to the weak O-H bond and its relatively high energy, making it easier for the molecule to release CO2 gas and form water. The positive ΔH value indicates that the reaction is endothermic, which means that it absorbs energy from its environment during the breakdown process, making it more favorable under certain conditions.