Question

Define reaction rate. Assuming constant temperature and a closed reaction vessel, why does the rate change with time?

Short answer

The reaction rate changes with time because the concentrations of reactants decrease as they are converted to products.

Step-by-step solution

- Define Reaction Rate

The reaction rate refers to the speed at which reactants are converted into products in a chemical reaction. It is typically measured by the change in concentration of a reactant or product per unit time.

- Explain Reaction Rate Dependency on Concentration

The reaction rate depends on the concentration of the reactants. According to the rate law, the reaction rate is proportional to the product of the reactants' concentrations raised to their respective powers, which are determined by the reaction order.

- Discuss the Effect of Time on Reaction Rate

As the reaction progresses, the concentrations of the reactants decrease. Since the reaction rate is dependent on these concentrations, a decrease in reactant concentrations results in a decrease in the reaction rate over time.

- Summarize Why Rate Changes with Time

With constant temperature and a closed reaction vessel, the rate changes with time because the reactants are being consumed and their concentrations are decreasing. This leads to a reduced rate as the reaction proceeds.

Key concepts

rate law

The rate law is a mathematical expression that explains the relationship between the rate of a chemical reaction and the concentration of its reactants. It is often written in the form:
\(Rate = k [A]^m [B]^n \)
Here, \(k\) is the rate constant, \[A\] and \[B\] are the reactant concentrations, and \(m\) and \(n\) are the reaction orders with respect to each reactant.

The rate law is crucial because it helps predict how the reaction rate will change when the concentrations of the reactants are varied. This understanding is key to controlling and optimizing reactions in both laboratory and industrial settings.

reaction order

The reaction order of a chemical reaction refers to the powers to which the concentration terms are raised in the rate law equation. It indicates how the concentration of each reactant affects the overall reaction rate.

For a reaction given by the formula: \(aA + bB \rightarrow products\), the rate law might look like: \(Rate = k [A]^m [B]^n \)
The reaction order is \(m\) for reactant A and \(n\) for reactant B. The overall reaction order is the sum of \(m+n\).

Understanding reaction order is essential for predicting how changes in reactant concentrations impact the reaction rate. For instance, if a reaction is first-order with respect to a reactant, doubling its concentration will also double the reaction rate.

reactant concentration

Reactant concentration plays a vital role in determining the rate of a chemical reaction. At the start of a reaction, the concentrations of reactants are highest, leading to a higher reaction rate.

As the reaction progresses, reactants are converted to products, thus decreasing their concentrations. With lower concentrations, the likelihood of reactant molecules colliding decreases, leading to a slower reaction rate.

This explains why reaction rates generally decrease over time if conditions remain constant. Monitoring and adjusting reactant concentrations help manage reaction rates, which is important in both experimental chemistry and industrial processes.

chemical kinetics

Chemical kinetics is the study of the rates of chemical reactions and the factors that affect these rates. It explores how different conditions, such as temperature, pressure, and concentration, influence the speed of a reaction.

Through chemical kinetics, scientists can determine the rate law and reaction order, helping to predict how fast reactions will proceed under various conditions. This field also involves studying the reaction mechanisms, which are the step-by-step sequences of elementary reactions by which overall chemical change occurs.

Understanding chemical kinetics is essential for reacting control and optimization, whether in a lab setting or large-scale manufacturing. It aids in developing new reactions, improving existing processes, and enhancing safety by preventing uncontrolled reaction rates.