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Rate laws and rate constants
A topic from the subject of Kinetics in Chemistry.
Rate Laws and Rate Constants
Overview
Rate laws describe the relationship between the concentration of reactants and the rate of a chemical reaction. Rate constants are numerical values that quantify the rate of a reaction.Key Points
Reaction order:The sum of the exponents of the concentration terms in the rate law is the order of the reaction. Rate constant: The proportionality constant in the rate law that depends on temperature and other factors.Integrated rate laws:Express the relationship between concentration and time. Half-life: The time required for the concentration of a reactant to decrease by half.
Arrhenius equation:* Relates the rate constant to temperature and the activation energy.
Main Concepts
Concentration dependence: The rate of a reaction is proportional to the concentrations of the reactants raised to their respective orders. Temperature dependence: The rate constant increases with temperature, following the Arrhenius equation.Predicting reaction rates: Rate laws allow us to predict the rate of a reaction if the concentrations of the reactants are known. Applications: Rate laws are used in chemical engineering, medicine, and other fields to design processes and optimize reactions.
Experiment: Determining the Rate Law and Rate Constant for a Chemical Reaction
# ObjectiveTo experimentally determine the rate law and rate constant for the following reaction:
2A + B → C
Materials
Sodium thiosulfate solution (Na2S2O3) Iodine solution (I2)
Hydrochloric acid (HCl) Sodium hydroxide (NaOH)
Starch solution Burette
Pipette Stopwatch
Procedure
1. Prepare the reaction mixture. In a test tube, combine the following solutions:
10 mL of Na2S2O3 solution 5 mL of I2 solution
* 5 mL of HCl solution
2. Start the reaction. Add 2 drops of starch solution to the reaction mixture. This will turn the solution a deep blue color.
3. Titrate the reaction mixture. Using a burette, add NaOH solution dropwise to the reaction mixture until the blue color disappears. Record the volume of NaOH solution used.
4. Repeat steps 1-3 for different initial concentrations of A, B, and HCl. By varying the initial concentrations, you can determine the order of the reaction with respect to each reactant.
Key Procedures
Using a clean burette and pipette.This is essential for accurate measurements. Starch solution acts as an indicator for the reaction endpoint.
Determine the order of the reaction experimentally.* This involves varying the initial concentrations of the reactants and observing the effect on the reaction rate.
Data Analysis
1. Calculate the initial rate of reaction. The initial rate of reaction is the slope of the graph of [NaOH] versus time.
2. Determine the order of the reaction with respect to each reactant. Plot the log(initial rate) versus log([A0]), log([B0]), and log([HCl0]). The slope of each graph gives the order of the reaction with respect to that reactant.
3. Calculate the rate constant. The rate constant is the intercept of the graph of log(initial rate) versus log([A0][B0][HCl0]).
Significance
This experiment allows students to experimentally determine the rate law and rate constant for a chemical reaction. This knowledge is important for understanding the kinetics of chemical reactions and for predicting the rate of reaction under different conditions.