Problem 7 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

For the following Assertion and Reason, the correct option is: Assertion: The \(\mathrm{pH}\) of water increases with increase in temperature. Reason: The dissociation of water into \(\mathrm{H}^{+}\)and \(\mathrm{OH}^{-}\)is an exothermic reaction. (a) Both assertion and reason are true, and the reason is the correct explanation for the assertion. (b) Both assertion and reason are false. (c) Both assertion and reason are true, but the reason is not the correct explanation for the assertion. (d) Assertion is not true, but reason is true.

Step-by-Step Solution

Answer

Option (b): Both assertion and reason are false.

Step 1: Evaluate the Assertion

The assertion states that the \(\mathrm{pH}\) of water increases with an increase in temperature. Normally, as temperature rises, the ionization of water increases, leading to more \(\mathrm{H}^{+}\) and \(\mathrm{OH}^{-}\) ions in solution. This typically causes the \(\mathrm{pH}\) value to actually decrease, not increase, because \(\mathrm{pH}\) is a measure of hydrogen ion concentration.

Step 2: Evaluate the Reason

The reason provided is that the dissociation of water into \(\mathrm{H}^{+}\) and \(\mathrm{OH}^{-}\) is an exothermic reaction. The dissociation of water is actually an endothermic process, meaning it absorbs heat. Thus, increasing temperature favors the dissociation, not the process being exothermic.

Step 3: Determine the Relationship between Assertion and Reason

Both the assertion and reason are independently false as explained in the previous steps. The increase in temperature decreases the \(\mathrm{pH}\), and the reaction is endothermic, not exothermic. Therefore, the reason does not serve as a valid explanation for the assertion.

Step 4: Select the Correct Option

Given both the assertion and reason are invalid, the correct option is (b), indicating both statements are false.

Key Concepts

pH and Temperature

Understanding the relationship between pH and temperature is crucial in chemistry. pH is a scale that measures the acidity or basicity of a substance, defined as the negative logarithm of the hydrogen ion concentration. An increase in temperature generally affects the ionization of water. This ionization results in more hydrogen ions (\(\mathrm{H}^{+}\)) and hydroxide ions (\(\mathrm{OH}^{-}\)) being present in the solution. As a result, higher temperatures typically lead to a decrease in pH, not an increase. This is because pH, as a measure, is inversely related to hydrogen ion concentration. In essence: - Higher temperature = More (\(\mathrm{H}^{+}\)) + (\(\mathrm{OH}^{-}\)) ions - More ions = Lower pH.Remember that pH changes can affect chemical reactions and biological processes significantly. Careful management of temperature is crucial in scenarios where control of pH is necessary.

Endothermic Reactions

Endothermic reactions are processes that absorb energy from their surroundings, typically in the form of heat. The dissociation of water into its ions (\(\mathrm{H}^{+}\) and \(\mathrm{OH}^{-}\)) is a primary example of an endothermic reaction because it requires energy to break the bonds in water molecules. In these reactions: - Energy flows into the system. - The surroundings typically become cooler as energy is absorbed. Understanding this concept is crucial, as it directly opposes exothermic reactions, where energy is released. For students studying chemistry, it's important to remember that the direction of energy flow determines whether a reaction is endothermic or exothermic. Recognizing this can help you predict the behavior of chemical reactions under varying conditions.

Water Ionization

Water ionization is a fundamental concept in chemistry that refers to the process by which water molecules (\(\mathrm{H}_{2}\mathrm{O}\)) split into hydrogen ions (\(\mathrm{H}^{+}\)) and hydroxide ions (\(\mathrm{OH}^{-}\)). This process has a profound impact on both pH levels and the chemical characteristics of solutions.Key points to consider: - Ionization is temperature dependent; increased heat generally promotes more ionization. - Ionized water is essential for acid-base balance in nature and many industrial processes.The understanding of water ionization helps students grasp how water can act as both an acid and a base (amphoteric nature). It's fundamental to many chemical equations and reactions. By understanding water ionization deeply, students can predict changes in reaction dynamics as conditions (like temperature) change.