Reaction Rate and Concentration
# Introduction
Chemical kinetics is the study of reaction rates, the changes in the concentrations of reactants and products with time. Reaction rate is often expressed in terms of the rate of change of the concentration of a reactant or product. The concentration of a substance is the amount of that substance present in a given volume.
Basic Concepts
Reaction rate:The rate of change of the concentration of a reactant or product with time. Concentration: The amount of a substance present in a given volume.
Rate law:An equation that expresses the relationship between the reaction rate and the concentrations of the reactants. Rate constant: A constant that appears in the rate law and reflects the intrinsic reactivity of the reactants.
Equipment and Techniques
The following equipment and techniques are commonly used to study reaction rates:
Spectrophotometer:A device that measures the amount of light absorbed by a substance. Gas chromatograph: A device that separates and analyzes the components of a gas mixture.
HPLC:A device that separates and analyzes the components of a liquid mixture. Stopped-flow spectrophotometer: A device that allows the rapid mixing of reactants and the measurement of the reaction rate.
Types of Experiments
The following are some common types of experiments used to study reaction rates:
Initial rate method:The reaction rate is measured at the beginning of the reaction, when the concentration of the reactants are still relatively high. Half-life method: The half-life of a reaction is the time it takes for the concentration of a reactant to decrease by half.
Integrated rate law method:* The rate law is integrated and the rate constant is determined from the slope of a plot of the concentration of a reactant or product versus time.
Data Analysis
The data from reaction rate experiments can be analyzed to determine the rate law and the rate constant. The rate law is an equation that expresses the relationship between the reaction rate and the concentrations of the reactants. The rate constant is a constant that appears in the rate law and reflects the intrinsic reactivity of the reactants.
Applications
The study of reaction rates has a wide range of applications, including:
Chemical engineering:The design of chemical reactors. Environmental chemistry: The study of the rates of environmental reactions.
Pharmacology:The study of the rates of drug metabolism. Biochemistry: The study of the rates of biochemical reactions.
Conclusion
The study of reaction rates is a complex and challenging field. However, it is also a fascinating and rewarding field that has a wide range of applications.Reaction Rate and Concentration
# Key Points
- Reaction rate: The change in concentration of reactants or products over time.
- Factors affecting reaction rate:
- Concentration of reactants
- Temperature
- Surface area
- Presence of a catalyst
- Rate law: An equation that expresses the relationship between the reaction rate and the concentrations of the reactants.
- Order of reaction: The sum of the exponents of the concentrations in the rate law.
- Integrated rate law: An equation that expresses the concentration of reactants or products as a function of time.
Main Concepts
Relationship between Reaction Rate and Concentration
The reaction rate is directly proportional to the concentration of each reactant to the power of its order. For a reaction with the general form:
aA + bB → cC + dD
The rate law is:
rate = k[A]^m[B]^n
Where:
- k is the rate constant
- m and n are the orders of reaction with respect to A and B, respectively
Determining the Rate Law
The rate law can be determined experimentally by measuring the reaction rate at different concentrations of the reactants. The orders of reaction can be determined by comparing the slopes of the plots of the rate versus the concentration of each reactant.
Integrated Rate Laws
Integrated rate laws can be used to predict the concentration of reactants or products at a given time. The integrated rate law for a first-order reaction is:
[A] = [A]0e^(-kt)
Where:
- [A] is the concentration of A at time t
- [A]0 is the initial concentration of A
- k is the rate constant
Experiment: Reaction Rate and Concentration
Objective:
To demonstrate the effect of concentration on the rate of a chemical reaction.
Materials:
- 2 beakers (100 mL)
- Graduated cylinder (10 mL)
- Stopwatch
- Sodium thiosulfate solution (0.1 M)
- Hydrochloric acid (0.1 M)
- Starch solution (1%)
- Iodine solution (0.1 M)
Procedure:
- Fill each beaker with 50 mL of sodium thiosulfate solution.
- Add 10 mL of hydrochloric acid to one beaker (Beaker A).
- Add 10 mL of water to the other beaker (Beaker B).
- Swirl both beakers to mix the solutions.
- Add a few drops of starch solution to each beaker.
- Start the stopwatch.
- Add a few drops of iodine solution to each beaker.
- Stop the stopwatch when the solution in Beaker A turns from colorless to dark blue.
- Record the time it took for the reaction to complete.
- Repeat steps 6-9 with different concentrations of hydrochloric acid (0.05 M, 0.025 M, and 0.0125 M).
Observations:
The reaction in Beaker A (with hydrochloric acid) will complete faster than the reaction in Beaker B (with water). This is because the concentration of hydrochloric acid affects the rate of the reaction. As the concentration of hydrochloric acid increases, the rate of the reaction also increases.
Conclusion:
The experiment demonstrates that the rate of a chemical reaction is directly proportional to the concentration of the reactants. This means that the more concentrated the reactants, the faster the reaction will occur.