Problem 95 - Chemistry and Chemical Reactivity

Question

Describe each of the following statements as true or false. If false, rewrite the sentence to make it correct. (a) The rate-determining elementary step in a reaction is the slowest step in a mechanism. (b) It is possible to change the rate constant by changing the temperature. (c) As a reaction proceeds at constant temperature, the rate remains constant. (d) A reaction that is third-order overall must involve more than one step.

Step-by-Step Solution

Answer

(a) True. (b) True. (c) False; rate decreases. (d) False; single step possible.

Step 1: Identify the slowest step

The statement in part (a) states: 'The rate-determining elementary step in a reaction is the slowest step in a mechanism.' In chemical kinetics, the rate-determining step is indeed the slowest step because it limits the overall reaction rate. If the slowest step is 'clogged,' subsequent steps cannot proceed at a faster rate due to this bottleneck.

Step 2: Temperature affects rate constant

The statement in part (b) is: 'It is possible to change the rate constant by changing the temperature.' This statement is true because, according to the Arrhenius equation, the rate constant, denoted as 'k,' is affected by temperature; an increase in temperature generally leads to an increase in the rate constant and vice versa.

Step 3: Rate of reaction and concentration

For part (c), the statement reads: 'As a reaction proceeds at constant temperature, the rate remains constant.' This is false. In most reactions, as the reactants are consumed, their concentrations decrease, leading to a decrease in reaction rate, even if the temperature is kept constant. The corrected statement should be: 'As a reaction proceeds at constant temperature, the rate typically decreases as reactants are consumed.'

Step 4: Order of reaction and number of steps

Part (d) declares: 'A reaction that is third-order overall must involve more than one step.' This statement is false. Reactions can be third-order involving a single step if they have three molecules collide simultaneously, although this is less common due to the low probability. A corrected statement could be: 'A reaction that is third-order overall can occur in a single step or multiple steps, depending on the mechanism.'

Key Concepts

Rate-Determining Step

In the world of chemical kinetics, the rate-determining step is a crucial concept. It’s the slowest step within a reaction mechanism and acts like a major traffic jam on a highway. This step controls the overall pace of the chemical reaction. Imagine a series of reactions where everything hinges on this one slow step. Just like the slowest car on a highway sets the pace for others, in a chemical reaction, the slowest step dictates how fast the whole process goes.

- **Key Points**: - The slowest step limits the reaction rate. - Acts as a bottleneck in the process. - Determining the rate-determining step is crucial for understanding the reaction mechanism.

Recognizing this step helps chemists and researchers predict how changes in conditions or substances might affect the entire reaction speed.

Reaction Rate

The reaction rate refers to how quickly or slowly a reaction proceeds. It is typically expressed in terms of the concentration of reactants consumed or products formed over time. Many factors influence reaction rates, including the nature of the reactants, concentration, and especially the temperature.

It's common to see reaction rates decrease over time. As reactants are used up, their lower concentration leads to a dip in speed, even if the temperature stays the same. So, keeping track of concentrations is vital to predict how the reaction will proceed. Scientists study reaction rates to control industrial processes and make them more efficient.

Temperature and Rate Constant

Temperature plays a significant role in chemical reactions, particularly affecting the rate constant, denoted as 'k'. According to the Arrhenius equation, as the temperature increases, the rate constant typically increases too. This is why reactions often proceed faster at higher temperatures.

- **Factors Influencing Rate Constant**: - Higher temperature provides more energy. - Increased energy helps reactant molecules overcome activation energy barriers. - Leads to more frequent and effective collisions between molecules.

Understanding this relationship helps scientists manipulate reaction conditions to control reaction speed, whether it’s making a reaction faster or slower as needed for different applications. By adjusting temperatures, chemists can alter how a reaction proceeds, a crucial component in creating efficient and sustainable chemical processes.

Reaction Order

Reaction order gives insight into the relationship between the concentration of reactants and the reaction rate. It determines how the rate is affected by the concentration of reactants. The sum of the powers of the concentration terms in the rate law equation defines the order. Reactions can be first-order, second-order, or even third-order, depending on how the concentration effectively doubles, triples, etc., the rate. However, the idea that a third-order reaction must involve multiple steps is a misconception. While having multiple steps can occur, it’s also possible for three molecules to collide in a single step, although that's rare due to the low probability of three molecules meeting perfectly.

Understanding the reaction order helps predict how a reaction will unfold under different conditions and is fundamental in designing chemical processes and optimizing reaction conditions.