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## Determining Rate Laws and Rate Constants

❶We ran the reactions in a cuvette in a spectrometer, which measured the rate of change in absorbance of the solution as the concentration of I2 which is yellow in colour increased. From there, our next purpose was to determine the activation energy for the reaction, as well as the Arrhenius prefactor explained next Important Equations Data Part I Part I After the first round of experiments varying concentration , we found that the rate orders were all 1, and the rate constant for the reaction was

## Who can edit:   The sample calculation comes from Part II. Part II Data Cont. Solving for the Activation Energy To do this, we use the graph of ln k vs.

Using algebra, m was solved for, which was multiplied by R the ideal gas constant to get activation energy for the reaction Data Cont. Solving for the Arrhenius Prefactor For this calculation, we again refer to the linear regression from the graph in the Data section.

The completed Arrhenius equation is shown below. Sources of Error Activation Energy?? Think of the progression of the car as the "reaction," and the gate as the activation energy barrier. Only when the car is going with enough speed energy will the car crash through the energy "gate," allowing the "car" reaction to move forward!

Though we were careful, there are several places in which errors occurred that could have hampered our data. Inexact measuring of mL of each reactant solution. Lack of agitation of reactants in cuvette before placing in spectrometer. In both parts, the slight heating up of materials from the hands.

Varying amounts of time spent in reaction beaker before being pipetted into cuvette and placed in the spectrometer. This doesn't affect Part II since the concentrations were held constant, and are therefore irrelevant. The completed Arrhenius equation for the reaction is shown below. Conclusion Our purposes for the experiments were to 1. Discover the rate of reaction equation for the given reaction, and 2. Determining the Arrhenius prefactor and the activation energy for the reaction to be able to determine the rate constant for the reaction at a different temperature.

In Part I of the experiments, we were able to determine the rate orders and the rate constant for the above reaction of ions in water by varying the concentration of one reactant keeping the other two constant , and running the reaction again. We ran the reactions in a cuvette in a spectrometer, which measured the rate of change in absorbance of the solution as the concentration of I2 which is yellow in colour increased. By creating plots of the ln of the ion vs.

We then used the equation for the rate of reaction with the only remaining variable being k to solve for the rate constant for each trial, averaging them to get an k value of In Part II of the experiments, we were able to determine the Arrhenius prefactor and the activation energy of the reaction by varying the temperature at which the reaction occurred, recording the rate, and again solving for the rate constant for each trial.

Then, using a manipulation of the Arrhenius equation, we were able to plot the ln k vs. Once this was accomplished, algebra was used to solve for the activation energy 2. The significance of this experiment is that we were able to determine the rate of reaction equation for the given reaction, and we were also able to witness how temperature affects how quickly the reaction takes place.

Furthermore, we're now able to predict the behaviours of this reaction, regardless of the concentrations or temperature. Popular presentations See more popular or the latest prezis. Blog 31 August Prezi at Dreamforce The proof of concept Latest posts. Creating downloadable prezi, be patient. Delete comment or cancel. Cancel Reply 0 characters used from the allowed.

Send link to edit together this prezi using Prezi Meeting learn more: Reset share links Resets both viewing and editing links coeditors shown below are not affected. Send this link to let others join your presentation: One of the earliest methods used to measure concentrations at specified times is to quench the reaction either by flash freezing it or by adding a substance that severely inhibits the reaction.

Both of these techniques are problematic because one can't be sure that the reaction has completely stopped. The reaction may still be going on during the analysis. Additionally, the reaction mixture is destroyed for the purposes of kinetic experiments, so the chemist must make multiple trial runs and waste a large amount of reagents to observe the concentrations at multiple points in time.

A more modern technique to measure concentration is absorbance spectroscopy. This experiment may be used when a product or reactant has an absorbance frequency unique to those of other components of the reaction mixture. By measuring the absorbance of a particular product or reactant at a variety of known concentrations, you can construct a plot of absorbance versus concentration called a Beer's Law plot.

This calibration chart allows you to calculate the unknown concentration given the reaction solution's absorbance. The advantage of this method is that a large number of data points with well known times can be quickly collected using only one reaction mixture.

When looking at the expression for the , you should notice that the variables in the equation are the concentration terms and the powers p and q:. ## Main Topics

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Determining the Rate Law Kinetics Experiments The goal of a kinetics experiment is to measure the concentration of a species at a particular time during a reaction so that a rate law can be determined.

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Rate Laws from Rate Versus Concentration Data (Differential Rate Laws) A differential rate law is an equation of the form In order to determine a rate law we need to find the values of the exponents n, m, and p, and the value of the rate constant, k.

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If the rate law for a reaction is known to be of the form rate = k [A] n where n is either zero, one or two, and the reaction depends (or can be made to depend) on one species and if the reaction is well behaved, the order of the recation can be determined graphically. Determination of a Rate Law. Objectives. 1. To determine the rate law for a chemical reaction. 2. To use graphical techniques in the analysis of experimental data.

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Determining Rate Laws and Rate Constants This is an exercise in the analysis of basic kinetic data. When you press "New Problem", a set of kinetic data for the reaction of three species A,B and C will appear in a table to the right of the scoring table.