Experiment 1 Factors Governing the Speed of Chemical Reactions

Experiment 1: Factors Affecting Reaction Rates Part A Effect of Concentration on Reaction Rate CHEM 3310

Iodine clock Reaction S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) ( clear solution ) ( pale yellow solution ) Reactants: Ammonium persulfate (NH4)2S2O8 Potassium iodide KI CHEM 3310

Observe a COLOUR CHANGE! How do we do this? Iodine clock Reaction S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) ( clear solution ) ( pale yellow solution ) Observe a COLOUR CHANGE! Problem: The pale yellow solution is visually very difficult to judge. CHEM 3310

Iodine clock Reaction S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) ( clear solution ) ( pale yellow solution ) Introduce two substances to help us observe the colour change more accurately: Starch indicator Sodium thiosulfate, Na2S2O3, of a fixed quantity CHEM 3310

S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) (1) Iodine clock Reaction S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) (1) ( clear solution ) I2 (aq) + 2 S2O32-(aq) S4O62- (aq) + 2 I- (aq) (2) ( clear solution ) thiosulfate ion I2 (aq) + starch I2 – starch complex (3) ( coloured solution ) As the I2 is formed (1), it reacts with the fixed amount of sodium thiosulfate that has been added to the reaction mixture (2). When the sodium thiosulfate is used up, the next quantity of I2 that is formed reacts with the starch indicator to form a colour complex (3), and turns the clear solution into a coloured solution. CHEM 3310

S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) Iodine clock Reaction S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) Prepare 14 solutions in 14 Erlenmeyer flasks with different concentrations of S2O82- and I- solutions! Label them: A1, A2, A3, A4, A5, A6, A7. B1, B2, B3, B4, B5, B6, B7. Make solutions of different concentrations. SEPARATE the reactants until we are ready to mix them! A solutions contains: potassium iodide (I-) sodium thiosulfate (S2O32- ) starch indicator B solutions contains: ammonium persulfate (S2O82-) CHEM 3310

Total volume of ‘A’ solutions = Iodine clock Reaction S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) Total volume of ‘A’ solutions = 30.00 mL. CHEM 3310

Total volume of ‘B’ solutions = Iodine clock Reaction S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) Total volume of ‘B’ solutions = 20.00 mL. CHEM 3310

Total volume of the combined solution = Iodine clock Reaction S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) MIX and TIME: A1 + B1 A2 + B2 A3 + B3 A4 + B4 A5 + B5 A6 + B6 A7 + B7 Total volume of the combined solution = 50.00 mL. CHEM 3310

When solutions A1 and B1 are combined, what is the concentration of (NH4)2S2O8 in moles/L? (0.0050 L) x (0.100 moles/L) = 0.00050 moles (NH4)2S2O8   0.00050 moles (NH4)2S2O8 = 0.010 M (NH4)2S2O8 0.05000 L in the combined A1 + B1 solution Total volume of the combined solution = 50.00 mL. CHEM 3310

What is the reaction’s rate law? Iodine clock Reaction S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) What is the reaction’s rate law? The rate law for a chemical reaction is an experimentally determined mathematical equation that describes the progress of the reaction. For the Iodine clock reaction, the reaction rate of formation of I2 is proportional to the product of the concentrations each reactant each raised to some power, m and n. Rate  [I-]m [S2O82-]n CHEM 3310

What is the reaction’s rate law? Iodine clock Reaction S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) What is the reaction’s rate law? Rate  [I-]m [S2O82-]n We can remove the proportional symbol and introduce a proportionality constant, k. Rate = k [I-]m [S2O82-]n The proportionality constant, k, is known as the rate constant. We will experimentally determine the values of m and n graphically. CHEM 3310

Rate = k [I-]m [S2O82-]n Rate = k’ [S2O82-]n Since the concentration of I- is kept constant for A1 to A4, the reaction rates for A1/B1, A2/B2, A3/B3, and A4/B4 will only depend on the concentration of S2O82- . Rate = k’ [S2O82-]n where k’ = k [I-]m CHEM 3310 14

Log (Rate) = n log [S2O82-] + log k’ This will enable us to find n graphically. Rate = k’[S2O82-]n Take log of both sides, Log (Rate) = n log [S2O82-] + log k’ Since Rate  time-1, so Rate = a(time-1) where a is a constant Log (time-1) = n log [S2O82-] + log (k’/a) Y = slope  X + b Graph of “Log (time-1) Versus log [S2O82-]” should yield a straight line with a slope of n. Graph #1. CHEM 3310 15

Rate = k [I-]m [S2O82-]n Rate = k’’ [I-]m Since the concentration of S2O82- is kept constant for B4 to B7, the reaction rates for A4/B4, A5/B5, A6/B6, and A7/B7 will only depend on the concentration of I- . Rate = k’’ [I-]m where k’’ = k [S2O82-]n CHEM 3310 16

Similarly, this will enable us to find m graphically. Rate = k’’ [I-]m Take log of both sides, Log (Rate) = m log [I-] + log k’’ Since Rate  time-1, and Rate = a(time-1) Log (time-1) = m log [I-] + log (k’’/a) Y = slope  X + b Graph of “Log (time-1) Versus log [I-]” should yield a straight line with a slope of m. Graph #2. CHEM 3310 17

S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) Iodine clock Reaction S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) Rate = k [I-]m [S2O82-]n Values of m and n should be very close to an integer. If not, round m and n to the nearest integer. m is the order with respect to I- n is the order with respect to S2O82- The overall reaction order is m+n. CHEM 3310 18

Experiment 1: Factors Affecting Reaction Rates Part B Effect of Temperature on Reaction Rate CHEM 3310

S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) Iodine clock Reaction S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) How do we do this? Keep the reactant concentrations constant, and let the reaction react at different temperatures: 0oC, 20oC, 30oC, 40oC (Part A) Use A4, B4 solutions for all the temperatures!! CHEM 3310

Verify Arrhenius’ Equation A is the pre-exponential or frequency factor, a constant related to the collision frequency R is the gas constant (8.314 J / K mole) T is the absolute temperature (K) k is the rate constant at temperature T Eact is the activation energy, the energy required by the reacting species for their collisions to be effective (ie - those that lead to the formation of products) CHEM 3310

Rewrite Arrhenius’ Equation y = slope  x + b or should yield a straight line!! Recall, to convert between ln and log, ln y = 2.303  log y CHEM 3310

Recall, how to use the Linest function in Excel to determine the statistical error in a set of data. http://nobel.scas.bcit.ca/wiki/index.php/Linest CHEM 3310