Problem 14 - Chemistry: The Central Science

Question

(a) What are the units usually used to express the rates of reactions occurring in solution? (b) From your everyday experience, give two examples of the effects of temperature on the rates of reactions. (c) What is the difference between average rate and instantaneous rate?

Step-by-Step Solution

Answer

The rates of reactions occurring in solutions are usually expressed in units of concentration per time, such as moles per liter per second (mol/L·s) or molarity per second (M/s). Two everyday examples of temperature effects on reaction rates are food spoilage, where higher temperatures lead to faster spoilage, and dissolving solutes in water, where higher temperatures make solutes dissolve more rapidly. The average rate of a reaction represents the overall rate at which the reaction progresses over a specified time interval, while the instantaneous rate is the rate of the reaction at a specific point in time, providing a more precise measure of the reaction rate at that specific moment.

Step 1: (a) Units for expressing reaction rates in solutions

The rates of reactions occurring in solutions are usually expressed in units of concentration per time, such as moles per liter per second (mol/L·s) or molarity per second (M/s). These units describe how the concentration of a reactant or product changes over time.

Step 2: (b) Everyday examples of temperature effects on reaction rates

1. Food Spoilage: The rate at which food goes bad, or spoils, is influenced by temperature. At higher temperatures, the chemical reactions that cause food to spoil occur more rapidly, leading to faster expiration. This is why perishable foods like fruits, vegetables, and meats are often stored at colder temperatures in refrigerators to slow down this process and avoid spoilage. 2. Dissolving solutes in water: When dissolving sugar, salt, or other solutes in water, increasing the temperature of the water makes the solute dissolve faster. The reason is higher temperatures cause particles to move more rapidly, which leads to increased frequency and energy of collisions between solvent and solute molecules. This, in turn, accelerates the process of dissolving.

Step 3: (c) Difference between average rate and instantaneous rate

The average rate of a reaction is the change in concentration of a reactant or product over a specified time interval. It is calculated by dividing the total change in concentration by the total time interval during which the change occurred. The average rate represents the overall rate at which the reaction progresses over the time interval. On the other hand, the instantaneous rate is the rate of the reaction at a specific point in time. It is obtained by considering an infinitesimally small time interval around that point, and calculating the rate of change in concentration within that interval. Instantaneous rate provides a more precise measure of the reaction rate at that specific moment, while the average rate gives a broader view of the reaction progress over a longer time period.