Problem 18 - Chemistry: The Central Science in SI Units, Global Edition

Question

(a) What are the units usually used to express the rates of reactions occurring in solution? (b) As the temperature increases, does the reaction rate increase or decrease? (c) As a reaction proceeds, does the instantaneous reaction rate increase or decrease?

Step-by-Step Solution

Answer

(a) The units usually used to express rates of reactions in solutions are molarity per unit time (M/s or M/min). (b) As the temperature increases, the reaction rate generally increases due to higher kinetic energy and more frequent and effective collisions between reacting molecules. (c) As a reaction proceeds, the instantaneous reaction rate typically decreases because the concentration of reactants decreases, leading to less frequent collisions between reacting molecules or particles.

Step 1: Question (a): Units for Reaction Rates

The units most commonly used to express the rates of reactions occurring in solutions are molarity per unit time (M/s or M/min).

Step 2: Question (b): Effect of Temperature on Reaction Rate

As the temperature of the reaction system increases, the reaction rate usually increases. This is because as the temperature increases, the (average) kinetic energy of molecules or particles in the system also increases. This leads to more frequent collisions between them, and it also results in a higher probability of collisions having enough energy to overcome the activation energy barrier, leading to a faster reaction rate.

Step 3: Question (c): Instantaneous Reaction Rate as Reaction Proceeds

As a reaction proceeds, the instantaneous reaction rate typically decreases. This is primarily because the concentration of reactants decreases as the reaction progresses since the reactants get used up to form products. Lower reactant concentrations lead to less frequent collisions between reacting molecules or particles, lowering the overall rate of reaction over time.

Key Concepts

Molarity

When discussing reaction rates in solutions, we commonly use the unit of molarity per time, such as M/s or M/min. Molarity is a way to express concentration and is defined as the number of moles of solute per liter of solution. It helps us understand how concentrated a solution is.

• Moles of solute: This is the amount of substance present in the solution.
• Liters of solution: This is the total volume of the solution, including both the solute and the solvent.

This unit is useful because it provides a clear indication of how fast a reaction is happening in terms of how quickly the concentration of reactants decreases or how products are formed over time. In essence, higher molarity means more particles are available to react, which can influence how fast a reaction proceeds at any given moment.

Temperature Effect on Reaction Rate

Temperature plays a pivotal role in determining the speed of a chemical reaction. Typically, when the temperature increases, the reaction rate also increases.

• Increased kinetic energy: As temperature goes up, the molecules move faster, gaining kinetic energy.
• More frequent collisions: Faster-moving molecules means more collisions between them.
• Energy-rich collisions: The collisions have more energy, increasing the chance of overcoming activation energy barriers.

The activation energy is the minimum energy required for a reaction to occur. With increased temperature, more molecular collisions have enough energy to surpass this activation barrier, allowing the reaction to proceed more swiftly. Therefore, understanding the effect of temperature is essential in fields like chemistry and chemical engineering as it allows control over the rate at which reactions proceed.

Instantaneous Reaction Rate

As a reaction progresses, a common observation is the decrease in the instantaneous reaction rate. This is largely due to the diminished concentration of reactants over time.

• Lesser reactant concentration: As the reaction advances, reactants are converted to products, reducing their concentration.
• Fewer collisions: Lower concentrations mean that reactant particles collide less frequently.
• Slower reaction progression: With fewer particles available to react, the overall speed of the reaction goes down.

The instantaneous reaction rate provides a snapshot of the reaction speed at a specific moment in time. Crucially, as the reactants are used up, the instantaneous rate reflects the slowing pace of the reaction. This understanding helps chemists and engineers predict how long a reaction will take to reach completion, ensuring efficient planning and resource use in various chemical processes.