1. What determines pH? Write the chemical equation that shows what happens when carbon dioxide reacts with water and name all of the reactants and products. Explain why carbon dioxide in the body is considered an acid despite the fact that carbon dioxide does not contain hydrogen. Include all of the key terms in your explanation.

Answer 1

What Determines pH?

pH is a measure of the hydrogen ion concentration \([H^+]\) in a solution. It is calculated using the formula:

\[ \text{pH} = -\log[H^+] \]

A lower pH indicates a higher concentration of hydrogen ions, which means the solution is more acidic, while a higher pH indicates a lower concentration of hydrogen ions, making the solution more basic or alkaline.

Chemical Equation for the Reaction of Carbon Dioxide with Water

When carbon dioxide \((\text{CO}_2)\) reacts with water \((\text{H}_2\text{O})\), a chemical equilibrium is established, producing carbonic acid \((\text{H}_2\text{CO}_3)\). The reaction can be represented by the following chemical equation:

\[ \text{CO}_2 (g) + \text{H}_2\text{O} (l) \rightleftharpoons \text{H}_2\text{CO}_3 (aq) \]

Reactants and Products:

• Reactants: Carbon dioxide (\( \text{CO}_2 \)) and water (\( \text{H}_2\text{O} \)).
• Product: Carbonic acid (\( \text{H}_2\text{CO}_3 \)).

Explanation of Carbon Dioxide as an Acid in the Body

Carbon dioxide is considered an acid in the body even though it does not contain \(\text{H}^+\) ions inherently. This is because of the following reasons:

1. Acid Definition: According to the Brønsted-Lowry definition of acids, an acid is a substance that can donate a proton (\(\text{H}^+\)). When \(\text{CO}_2\) dissolves in water, it forms carbonic acid, which can dissociate to release \(\text{H}^+\) ions: \[ \text{H}_2\text{CO}_3 (aq) \rightleftharpoons \text{H}^+ (aq) + \text{HCO}_3^- (aq) \] This dissociation increases the concentration of \(\text{H}^+\), thereby lowering the pH of the solution, making it acidic.

2. Buffer System: In the context of the human body, this reaction is crucial for maintaining acid-base balance, particularly in the blood. The bicarbonate buffer system (involving \( \text{HCO}_3^{-} \) and \( \text{H}_2\text{CO}_3 \)) helps regulate pH through a reversible reaction that either donates or accepts protons based on the body's needs.

3. Physiological Implications: When \(\text{CO}_2\) levels increase in the body (for example, during respiration), it can lead to a decrease in blood pH, resulting in a state known as respiratory acidosis. This physiological response highlights the role of carbon dioxide as an acid-forming compound in biochemical processes.

In summary, despite carbon dioxide lacking hydrogen in its molecular structure, its conversion to carbonic acid in the presence of water allows it to act as an acid by contributing to the \(\text{H}^+\) concentration in biological fluids, thus affecting the pH.