Problem 14 - A Complete Resource Book in Chemistry for JEE Main

Question

The \(\mathrm{pH}\) of pure water at \(80^{\circ} \mathrm{C}\) will be \((\mathrm{a})=7\) (b) \(<7\) (c) \(>7\) (d) none of these

Step-by-Step Solution

Answer

pH of pure water at 80°C is less than 7.

Step 1: Understanding pH and its Dependence on Temperature

The pH of a solution is a measure of its acidity or alkalinity. Pure water at 25°C has a pH of 7, which is neutral. However, the pH value changes with temperature because the ionization constant of water ( Kw ) is temperature-dependent. As the temperature increases, Kw increases, leading to a change in [H^+] concentration and thus a change in pH.

Step 2: Determining the Effect of Increased Temperature

When temperature increases, the autoionization of water increases, resulting in a higher Kw . At 80°C, Kw is greater than it is at 25°C. This means that both [H^+] and [OH^-] concentrations will increase, but because Kw is greater, the neutral point (where [H^+] = [OH^-] ) occurs at a pH less than 7.

Step 3: Conclusion on pH of Pure Water at 80°C

Since increased temperature causes the ionization of water to increase, the pH of pure water at 80°C will be less than 7. Therefore, the correct answer is option (b) \(<7\).

Key Concepts

Temperature Effect on pH

The pH of water is influenced by its temperature. You might know that at 25°C, pure water has a neutral pH of 7. However, this value changes when the temperature changes. As temperature increases, so does the ionization of water, causing the ionization constant, or Kw, to also increase. This increase in Kw means more hydrogen ions \([H^+]\) and hydroxide ions \([OH^-]\) are produced. As a result, the pH decreases. This concept allows us to understand why pure water at 80°C has a pH lower than 7.

Ionization Constant (Kw)

The ionization constant of water, Kw, is a crucial element in understanding the pH behavior of water. Kw is defined as the product of the concentrations of hydrogen ions \([H^+]\) and hydroxide ions \([OH^-]\) in water. At standard room temperature (25°C), Kw is approximately \[1.0 \times 10^{-14}\]. However, it's not a fixed value and varies with temperature. What happens when temperature rises is that Kw increases, leading to greater ionization of water. More \([H^+]\) and \([OH^-]\) ions form, which in turn, impacts the pH level, making it lower than the neutral value at higher temperatures.

Autoionization of Water

Autoionization is a process where water molecules spontaneously dissociate into ions. In this reaction, two water molecules interact to produce a hydroxide ion \([OH^-]\) and a hydronium ion \([H_3O^+]\). This process is dynamic and influences the characteristics of water, particularly its pH. As the temperature increases, autoionization happens more readily, leading to a higher ionization constant Kw. This explains why hotter water exhibits a different pH than room temperature water. More ions mean altered pH levels, specifically a decrease when temperature is higher.

Acidity and Alkalinity

Acidity and alkalinity of solutions are measured by the pH scale, ranging from 0 to 14. A pH of 7 is deemed neutral, like pure water at 25°C. Any pH value below 7 is considered acidic, while above 7 is alkaline. As seen, with rising temperatures, even though water remains chemically neutral (due to equal quantities of \([H^+]\) and \([OH^-]\) increasing), the actual pH reading falls below 7. This shift doesn't mean the water becomes acidic in the literal sense; rather, the definition of neutral shifts with Kw as temperature changes. This adjustment showcases how acidity and alkalinity can change with circumstances like temperature.