CHEM 3310 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 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 CHEM 3310 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. CHEM 3310 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 2 S 2 O 3, of a fixed quantity CHEM 3310 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) S 4 O 6 (aq) + 2 I (aq) (2) thiosulfate ion ( clear solution ) I 2 (aq) + starch I 2 starch complex (3) ( 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 (3), and turns the clear solution into a coloured solution. CHEM 3310 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- ) CHEM 3310 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. CHEM 3310 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. CHEM 3310 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. CHEM 3310 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. 0.00050 moles (NH 4 ) 2 S 2 O 8 = 0.010 M (NH 4 ) 2 S 2 O 8 0.05000 L in the combined A1 + B1 solution CHEM 3310 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? 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 CHEM 3310 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? Rate [I - ] m [S 2 O 8 ] 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. CHEM 3310 13
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 CHEM 3310 14
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, so Rate = a(time -1 ) where a is a constant Log (time -1 ) = n log [S 2 O 8 ] + log (k /a) 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. Graph #1. CHEM 3310 15
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 2 O 8 ] 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
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. CHEM 3310 18
Experiment 1: Factors Affecting Reaction Rates Part B Effect of Temperature on Reaction Rate CHEM 3310 19
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!! CHEM 3310 20
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 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) CHEM 3310 21
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 22
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 23