Problem 18 - 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

(a) The units usually used to express rates of reactions in solutions are moles per liter per second (M/s) or moles per liter per minute (M/min). (b) Two examples of temperature effects on reaction rates are food spoilage and the activation of cold packs and hot packs. Higher temperatures speed up food spoilage, while lower temperatures slow it down. Cold packs become cold due to an endothermic reaction, while hot packs become hot due to an exothermic reaction. (c) The average rate is the change in concentration over a specified period, while the instantaneous rate refers to the rate of a reaction at a specific point in time.

Step 1: (a) Units for Rates of Reactions in Solution

The units typically used to express the rates of reactions in solutions are moles per liter per unit time (Molarity/time). Usually, it is expressed as moles per liter per second (M/s) or moles per liter per minute (M/min). To better understand reaction rates, we measure the change in the concentration of reaction participants over time.

Step 2: (b) Two Examples of Temperature Effects on Reaction Rates

1. Food Spoilage: At higher temperatures, microorganisms and enzymes in food react faster, causing food to spoil more quickly. On the other hand, when food is kept in the fridge, the low temperature slows down the reaction rate, preserving the food for a longer period. 2. Cold Packs and Hot Packs: Chemical reactions within cold packs and hot packs are activated by changing temperatures. When a cold pack is activated, an endothermic reaction takes place, which means that energy (in the form of heat) is absorbed from the surroundings, making the pack feel cold. On the other hand, when a hot pack is activated, an exothermic reaction takes place, releasing energy in the form of heat and making the pack feel hot.

Step 3: (c) Difference Between Average Rate and Instantaneous Rate

The average rate is the change in concentration of reactants or products over a specified period. It is calculated by taking the difference in concentrations at two different time points and dividing by the change in time. The average rate gives an indication of the overall reaction rate over a time interval. Instantaneous rate, on the other hand, refers to the rate of a reaction at a specific point in time. It is calculated by taking the derivative of concentration with respect to time or by using the tangent of the concentration-time graph at that particular instant. The instantaneous rate gives us information about how the reaction is proceeding at that specific moment. In summary, while the average rate provides a general idea of the reaction rate over a specific time interval, the instantaneous rate offers more detailed information on the rate of the reaction at a specific point in time.

Key Concepts

Units for reaction rates

Understanding the units used for expressing reaction rates is essential in chemistry. When reactions take place in a solution, the rate is typically communicated through the change in concentration of reactants or products over time.
The most common unit for reaction rates in solutions is 'moles per liter per second' (M/s), often referred to as molarity per time unit.
This unit signifies how many moles of a reactant are being consumed, or how many moles of a product are being formed, per liter of solution every second.
Expressing reaction rates in these units allows scientists to effectively communicate and compare the speed of different reactions, regardless of their nature or environment.
In some cases, depending on the reaction speed being measured, different time units such as minutes or hours might be used, e.g., moles per liter per minute (M/min).

Temperature effects on reactions

Temperature dramatically influences the speed of chemical reactions. When the temperature increases, molecules move faster, leading to more frequent and energetic collisions among reactant particles. This generally makes reactions occur more quickly.
For example:

• Food Spoilage: Higher temperatures accelerate reactions in food, causing it to spoil. Refrigerators slow this process by maintaining low temperatures.
• Cold and Hot Packs: These packs demonstrate chemical reactions that either absorb or release heat. Cold packs absorb heat due to endothermic reactions, cooling their surroundings, while hot packs release heat through exothermic reactions.

Such examples show how critical temperature control can be in practical situations, from food preservation to camping equipment. Altering temperature is a useful method for controlling the rate of reactions in many real-world applications.

Average vs instantaneous rate

The rate of a chemical reaction can be understood in two ways: average rate and instantaneous rate.

• Average Rate: This is the change in concentration of reactants or products over a defined time interval. You calculate this by subtracting the initial concentration from the final concentration and dividing by the time elapsed. It gives a general overview of how quickly the reaction proceeds over that time.
• Instantaneous Rate: This is the rate at a specific moment during the reaction. It's determined by the slope of the tangent line to the concentration-time graph at that particular point. This requires calculus, specifically taking the derivative, to determine. Instantaneous rate offers a more precise snapshot of a reaction's speed at any moment.

Understanding both rates is crucial for analyzing reaction kinetics effectively. While average rate is useful for a broad view, instantaneous rate provides insightful details, especially in reactions with changing speeds.