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Unformatted text preview: 1 Chemical Kinetics What factors affect the rate of a reaction? Reaction Rate Factors Reaction Rate Factors Reaction Rate Factors Reaction Rate Factors-Concentration of reactants-Temperature (Increasing temperature increases the reaction rates. In what direction is a different question.)-Surface Area-Catalysis (Substance that participates but is not changed by the reaction. It is not included in the balanced equation. Inhibitors slow down reactions. Plotting the concentration of a chemical in a reaction as concentration of a chemical in a reaction as concentration of a chemical in a reaction as concentration of a chemical in a reaction as a function of time a function of time a function of time a function of time is called a Kinetic Curve or Concentration vs. Time Curve Calculate the slope at a given time (Instantaneous rate) by drawing a tangent at that point and calculating the slope (derivative) of the line. Always use a positive value for rate Rate = C products / t Rate = - C reactants / t Where C is the concentration Once the rate of one component is determined, others can be determined (i.e. the rate of appearance and disappearance) using stoichiometric ratios. (More on this in a moment) Units: mol/Ls or M/s 2N 2 O 5 4NO 2 + O 2 Secant vs tangent lines 0.882M 0.697M Question: This picture shows the decomposition of hydrogen peroxide by the reaction H 2 O 2 H 2 O + O 2 a. Use the diagram to determine the initial rate of the reaction by-Using the first two concentration measurement points-Estimating the slope of the line at t = 0 (the blue line) b. Once you have done this, calculate [H 2 O 2 ] at t = 30sec using the value from above you feel is best for this measurement. (remember brackets means concentration (molarity)) 2 Results: a. Using the first two data points (Secant):- [H 2 O 2 ]/ t = (0.697M 0.882M)/(60s 0s) = 3.08x10 3.08x10 3.08x10 3.08x10-3 M/s M/s M/s M/s Using the estimated slope of the tangent (best for initial rate): Slope = (0 -0.882)M/(275-0)s = -3.21x10-3 M/s The initial rate is the negative of the slope of the tangent line = 3.21x10 3.21x10 3.21x10 3.21x10-3 M/s M/s M/s M/s b. Initial Rate = - [H 2 O 2 ]/ t 3.08x10-3 = - [H 2 O 2 ]/30s (The average is better) [H 2 O 2 ] = -0.092M [H 2 O 2 ] 30s = 0.882M - 0.092M = 0.790M 0.790M 0.790M 0.790M The general rate general rate general rate general rate of reaction is obtained by dividing the rate of disappearance of a reactant or the rate of formation of a product by the stoichiometric coefficient of that reactant or product in the balanced chemical equation. For equation aA + bB cC + dD The general rate of reaction general rate of reaction general rate of reaction general rate of reaction is: Rate = -(1/a) [A]/ t = -(1/b) [B]/ t = (1/c) [C]/ t = (1/d) [D]/ t This represents the average rate of a reaction over a time interval, as the rate is generally greater when the concentration of reactants are greater.are greater....
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