Experiment 1 Factors Governing the Speed of Chemical Reactions
Experiment 1: Factors Affecting Reaction Rates Part A Effect of Concentration on Reaction Rate 2
Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) ( clear solution ) ( pale yellow solution ) Reactants: Ammonium persulfate (NH 4 ) 2 S 2 O 8 Potassium iodide KI 3
How do we do this? Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) ( clear solution ) ( pale yellow solution ) Observe a COLOUR CHANGE! Problem: The pale yellow solution is visually very difficult to judge. 4
Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) ( clear solution ) ( pale yellow solution ) Introduce two substances to help us observe the colour change more accurately: 1. Starch indicator 2. Sodium thiosulfate, Na 2S 2O 3 3, of a fixed quantity 5
Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) (1) ( clear solution ) I 2 (aq) + 2 S 2 O 3 (aq) starch indicator S 4 O 6 (aq) + 2 I (aq) (2) thiosulfate ion ( coloured solution ) As the I 2 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 I 2 that is formed reacts with the starch indicator to form a colour complex, and turns the clear solution into a coloured solution. 6
Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) Prepare 14 solutions in 14 Erlenmeyer flasks with different concentrations of S 2 O 8 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 (S 2 O 3 ) starch indicator B solutions contains: ammonium persulfate (S 2 O 8 2- ) 7
Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) Total volume of A solutions = 30.00 ml. 8
Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) Total volume of B solutions = 20.00 ml. 9
Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (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. 10
When solutions A1 and B1 are combined, what is the concentration of (NH 4 ) 2 S 2 O 8 in moles/l? (0.0050 L) x (0.100 moles/l) = 0.00050 moles (NH 4 ) 2 S 2 O 8 Total volume of the combined solution = 50.00 ml. 0.0005000050 moles (NH 4 ) 2 S 2 O 8 = 0.010010 M (NH 4 ) 2 S 2 O 8 0.05000 L in the combined A1 + B1 solution 11
Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) What is the reaction s rate law? 12
Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (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 I 2 is proportional to the product of the concentrations each reactant each raised to some power, m and n. Rate [I - ] m [S 2 O 8 ] n 13
Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) What is the reaction s rate law? Rate [I - ] m [S 2 O 82 ] n We can remove the proportional symbol and introduce a proportionality constant, k. Rate k [I - ] m [S 2 O 8 ] n The proportionality constant, k, is known as the rate constant. We will experimentally determine the values of m and n graphically. 14
Rate k [I - ] m [S 2 O 8 ] 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 S 2 O 8. where k = k [I - ] m Rate k [S 2 O 8 ] n 15
This will enable us to find n graphically. Rate k [S 2 O 8 ] n Take log of both sides, Log (Rate) = n log [S 2 O 8 ] + log k Since Rate time -1, Log (time -1 ) = n log [S 2 O 8 ] + log k Y = slope X + b Graph of Log (time -1 ) Versus log [S 2 O 8 ] should yield a straight line with a slope of n. 16
Rate k [I - ] m [S 2 O 8 ] n Since the concentration of S 2 O 8 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 [S n 2 O 8 ] 17
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, Log (time -1 ) = m log [I - ] + log k Y = slope X + b Graph of Log (time -1 ) Versus log [I - ] should yield a straight line with a slope of m. 18
Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) Rate k [I - ] m [S 2 O 8 ] 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 S 2 O 8 The overall reaction order is m+n. 19
Experiment 1: Factors Affecting Reaction Rates Part B Effect of Temperature on Reaction Rate 20
Iodine clock Reaction S 2 O 8 (aq) + 2 I (aq) 2 SO 4 (aq) + I 2 (aq) How do we do this? Keep the reactant concentrations constant, and let the reaction react at different temperatures: 0 o C, 20 o C, 30 o C, 40 o C (Part A) Use A4, B4 solutions for all the temperatures!! 21
Verify Arrhenius Equation k E act RT Ae RT 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 E act 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) 22
Rewrite Arrhenius Equation k Ae Eact RT ln y = slope x + b1k E act R 1 T ln A Plot" ln k versus or Plot log k versus 1" " T T " 1should yield a straight line!! Recall, to convert between ln and log, ln y = 2.303 log gy 23
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 24