Question

When acetic acid is treated with aqueous sodium bicarbonate, \(\mathrm{CO}_{2}\) gas is evolved. The 'Carbon' of \(\mathrm{CO}_{2}\) comes from (a) carboxyl group (b) methyl group (c) bicarbonate (d) both carboxyl group and bicarbonate

Short answer

Answer: (c) bicarbonate

Step-by-step solution

Identify the structure of reactants

Acetic acid has the formula \(\mathrm{CH}_{3}\mathrm{COOH}\), which has a carboxyl group (-\(\mathrm{COOH}\)) and a methyl group (-\(\mathrm{CH}_{3}\)). Aqueous sodium bicarbonate has the formula \(\mathrm{NaHCO}_{3}\) and has a bicarbonate ion (\(\mathrm{HCO}_{3}^{-}\)).

Write down the balanced chemical equation

The balanced chemical equation for the reaction can be written as: $$\mathrm{CH}_{3}\mathrm{COOH} + \mathrm{NaHCO}_{3} \rightarrow \mathrm{CH}_{3}\mathrm{COONa} + \mathrm{H}_{2}\mathrm{O} + \mathrm{CO}_{2}$$ This equation shows the products formed during the reaction: sodium acetate (\(\mathrm{CH}_{3}\mathrm{COONa}\)), water (\(\mathrm{H}_{2}\mathrm{O}\)), and carbon dioxide (\(\mathrm{CO}_{2}\)).

Identify the origin of the Carbon

In this step, we will track the origin of the carbon atom in the \(\mathrm{CO}_{2}\) molecule. The carbon dioxide molecule has one carbon atom, which can be traced back to the molecules react from the reactants. From the balanced chemical equation, it is evident that the carbon atom in \(\mathrm{CO}_{2}\) comes from the bicarbonate ion. The bicarbonate ion loses hydrogen and oxygen in the form of water, leaving behind the carbon atom to form a molecule of carbon dioxide. The remaining atoms from the carboxyl group and the methyl group form the sodium acetate. So, the 'Carbon' in \(\mathrm{CO}_{2}\) comes from: (c) bicarbonate

Key concepts

Acetic Acid

Acetic acid is a simple carboxylic acid with the chemical formula \(\text{CH}_3\text{COOH}\). It is a colorless liquid and has a distinctive sour smell. Acetic acid is a vital component of vinegar, which typically contains around 5% acetic acid by volume. The structure of acetic acid consists of two parts:

• Carboxyl group: \(-\text{COOH}\), responsible for the acidic properties of the molecule.
• Methyl group: \(-\text{CH}_3\), which is a simple alkyl group attached to the carboxyl unit.

Overall, the acetic acid molecule is characterized by its ability to donate a proton \((\text{H}^+)\) due to the carboxyl group, making it an acidic compound. It participates in various chemical reactions, of which one common reaction is with sodium bicarbonate.

Sodium Bicarbonate

Sodium bicarbonate, also known as baking soda, bears the chemical formula \(\text{NaHCO}_3\). It appears as a white, powdery solid and is known for its alkaline nature. The bicarbonate ion \((\text{HCO}_3^-)\) in sodium bicarbonate plays a crucial role in reactions with acids.
Sodium bicarbonate is composed of:

• Sodium ion \((\text{Na}^+)\)
• Bicarbonate ion \((\text{HCO}_3^-)\)

In reactions, particularly with acetic acid, the bicarbonate ion is pivotal because it reacts with the hydrogen ions from the acid. This reaction releases water and carbon dioxide gas, making sodium bicarbonate a popular choice for leavening in baking due to the gas expansion that occurs.

Carbon Dioxide Evolution

The term "Carbon Dioxide Evolution" refers to the production of carbon dioxide gas \((\text{CO}_2)\) during a chemical reaction. In the given reaction between acetic acid and sodium bicarbonate, carbon dioxide evolution is a key observable outcome.
When acetic acid \((\text{CH}_3\text{COOH})\) reacts with sodium bicarbonate \((\text{NaHCO}_3)\), the bicarbonate ion \((\text{HCO}_3^-)\) reacts with hydrogen ions from acetic acid, releasing carbon dioxide gas. The balanced chemical equation showcases this process:

• \(\text{CH}_3\text{COOH} + \text{NaHCO}_3 \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2\)

The release of \(\text{CO}_2\) is an exothermic event, meaning it releases energy, and it's this effervescence (bubbling) that is visually observed when the reaction takes place.

Chemical Equation Balancing

Balancing chemical equations is an essential skill in chemistry that ensures the law of conservation of mass is upheld. This law states that matter cannot be created or destroyed in a chemical reaction, meaning the same number of atoms for each element must appear on both reactant and product sides of the equation.
In the reaction between acetic acid and sodium bicarbonate, the equation is balanced as follows:

• Reactants: 1 acetic acid molecule \((\text{CH}_3\text{COOH})\) and 1 sodium bicarbonate molecule \((\text{NaHCO}_3)\)
• Products: 1 sodium acetate molecule \((\text{CH}_3\text{COONa})\), 1 water molecule \((\text{H}_2\text{O})\), and 1 carbon dioxide molecule \((\text{CO}_2)\)

To balance a chemical equation, one must ensure that the number of atoms for each element is conserved across reactants and products. This often involves adjusting coefficients before the chemical formulas. Balanced equations are crucial for quantifying the reactants and products in chemical reactions accurately.