Question

What is meant by "pH"? True or false: A strong acid always has a lower pH than a weak acid does. Explain.

Short answer

pH is a measure of the acidity or alkalinity of a solution, defined as the negative logarithm of the hydrogen ion concentration: \(pH = -\log_{10} [H^+]\). The pH scale ranges from 0 to 14. Strong acids fully dissociate in water, releasing more hydrogen ions than weak acids. It is true that a strong acid always has a lower pH than a weak acid, given the same initial concentration, due to the higher hydrogen ion concentration in strong acids.

Step-by-step solution

Define pH

pH is a measure of the acidity or alkalinity of a solution. It is defined as the negative logarithm of the hydrogen ion (H+) concentration in a solution. Mathematically, pH is given by: \[pH = -\log_{10} [H^+]\] where [H+] represents the concentration of hydrogen ions in moles per liter. The pH scale ranges from 0 to 14, with 7 being neutral, values less than 7 being acidic, and values greater than 7 being alkaline (or basic).

Understand Strong and Weak Acids

An acid is a substance that releases hydrogen ions (H+) into a solution when dissolved in water. The strength of an acid is determined by the extent to which it releases these hydrogen ions. A strong acid is one that completely ionizes (dissociates) in water, releasing all of its hydrogen ions. A weak acid only partially ionizes in water, meaning not all of its hydrogen ions are released into the solution.

Comparing pH values of Strong and Weak Acids

Since pH is a measure of hydrogen ion concentration, a lower pH value indicates a higher concentration of hydrogen ions (H+). As we know, strong acids release more hydrogen ions than weak acids given the same initial concentration. Therefore, a strong acid will generally have a lower pH than a weak acid.

Answering the True or False Question

It is true that a strong acid always has a lower pH than a weak acid does, given the same initial concentration. This is because a strong acid fully dissociates and releases more hydrogen ions (H+) into the solution, resulting in a higher H+ concentration and a lower pH value compared to a weak acid.

Key concepts

Strong and Weak Acids

Acids are substances that can donate protons (hydrogen ions, H+) to other substances. Their strength is defined by their ability to dissociate in water. Strong acids completely dissociate in water, meaning they release all available protons into the solution. Examples include hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3). On the pH scale, these acids typically have a value that is much less than 7.

Weak acids, in contrast, only partially dissociate in water. This means that in a solution, only a fraction of the acid molecules lose their proton, resulting in fewer hydrogen ions in the solution. Acetic acid (CH3COOH) and citric acid (C6H8O7) are common examples of weak acids. Even when these acids are in a concentrated form, their pH values tend to be higher compared to strong acids of similar concentration. The partial dissociation is essential in biological systems where over-accumulation of ions can be harmful.

Importance in Chemistry

Understanding the difference between strong and weak acids is fundamental in predicting reaction outcomes, calculating pH, and in titration studies where neutralization reactions occur. It's crucial when we want to control the pH in various products or chemical reactions.

Hydrogen Ion Concentration

The concentration of hydrogen ions in a solution is a key metric in deciphering the solution's acidity. This concentration is commonly expressed in moles per liter (M). In an aqueous solution, when an acid is added, it releases hydrogen ions (H+), which are responsible for the acidic properties of the solution.

To measure this, we turn to the concept of pH, the scale that quantifies acidity or basicity. The pH is the negative logarithm to base 10 of the hydrogen ion concentration, as in \(pH = -\log_{10} [H^+]\). A higher concentration of hydrogen ions leads to a lower pH, indicating a stronger acid. Conversely, a lower concentration signals a higher pH and thus a weaker acidic solution or a basic one.

Calculating Hydrogen Ion Concentration

Inversely, knowing the pH can tell us about the hydrogen ion concentration using the mathematical relationship \[ [H^+] = 10^{-pH} \]. This calculation is especially pertinent when assessing chemical reactions, biological functions, or industrial processes that are pH-dependent.

Acid Dissociation Constant

Every acid has an associated acid dissociation constant (Ka), which is a measure of its ability to donate hydrogen ions in a solution. It's defined by the equilibrium concentration of the products (hydrogen ions and the conjugate base) over the concentration of the undissociated acid, typically written as \(Ka = \frac{[A^-][H^+]}{[HA]}\).

The larger the Ka, the stronger the acid because it implies a greater ratio of dissociated ions to undissociated acid molecules. Strong acids have large Ka values, sometimes so large that they are not often used in calculations because these acids completely dissociate. Conversely, weak acids have smaller Ka values, signaling only partial dissociation.

Relevance of Ka

The Ka value is central when determining the pH of a solution and predicting the direction of chemical equilibrium in acid-base reactions. In pharmacology and chemistry, it helps to predict how an acid will behave in solution hence influencing drug design, buffer solutions, and much more.