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EXPERIMENT 4CHEMICAL KINETICSIntroductionChemical kinetics studies how fast reactants change into products in a chemical reaction It focuses on the reaction rate of an equation the change in the amount of reactants or products over time The rate varies between reactions and is determined from the nature of the reactants Reactions such as explosions and the ripening of fruit occur over a different range of time Since the reaction rate is given by the change in amount of reactants or products over time we can derive the equationRatechange in concentrationchange in timeIf the concentration of the reactant decreases as the reaction occurs then we put a negative sign around the rate equation to ensure it is positive For the rate of an entire reaction with multiple reactants the rate can be expressed asmnRatekABWhere ABProductThis is called the rate law and it expresses the rate as a function of the reactant or product concentrations K is a proportionality constant called the rate constant and it is specific for a given reaction at a given temperature The square brackets around A and B indicate that it is the concentration of the reactants we are multiplying The exponents m and n are the reaction orders and depending on what the value of the exponent is they determine how the rate is affected by the reactant concentration The reaction orders have to be found experimentally and are not related to the balancing coefficients in the chemical equationOne method to finding the reaction orders is through the emergence of a colored species over time For this we use a spectrophotometer to find the transmittance and absorbance of light in the solution that becomes colored over time Transmittance is the ratio of the intensity of light after it passes through a medium to the intensity of light before it passes through that mediumTIIOAbsorbance is the capacity of a substance to absorb light of a certain wavelength in this experiment it is 545 nm and it can be found through the following equationAlogTAbsorbance needs to be found because it is related to the concentration as shown in this equationAbcThe symbolis the molar absorptivity coefficient B is the path length and c is the concentration of the absorbing species in the solution In this experiment the path length is constant so the only changing variable is the concentration Graphing the concentration over time and graphing the absorbance over yields no difference and so we form a graph to show the change in CrIII over timeIn order to make the last graph the rate needs to be found The rate for the reaction in this experiment can be expressed asabRatedCrIIIdt dCrIIIEDTAdtkCrIIIHThe concentration of the EDTA is neglected in the equation because it is in excess and so its concentration did not change H is a reactant that we include because there is an excess of it because of the acidic pH of the EDTA but it is also constant because the solutions were buffered to prevent the concentration of H from changing The reaction then gets rewritten as
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