CHEM 3310

Experiment 1Factors Governing the Speed Factors Governing the Speed

of Chemical Reactions

Experiment 1: Factors Affecting p gReaction Rates

Part A

Effect of Concentration on Reaction Rate

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Iodine clock Reaction

S O 2 ( ) 2 ( ) 2 SO 2 ( ) ( )

Iodine clock Reaction

S2O82‐ (aq) + 2 I‐ (aq)    2 SO4

2‐ (aq) + I2 (aq) (              clear solution                )                (        pale yellow solution         )

Ammonium persulfate (NH ) S O

Reactants:

Ammonium persulfate (NH4)2S2O8

Potassium iodide KI

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Iodine clock Reaction

How do we do this?

S2O82‐ (aq) + 2 I‐ (aq)    2 SO4

2‐ (aq) + I2 (aq) ( clear solution ) ( pale yellow solution )(             clear solution                 )                  (       pale yellow solution        )

Observe a COLOUR CHANGE!Problem: The pale yellow solution is visually very difficult to judge.

Observe a COLOUR CHANGE!

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Iodine clock Reaction

S O 2 ( ) 2 ( ) 2 SO 2 ( ) ( )

Iodine clock Reaction

S2O82‐ (aq) + 2 I‐ (aq)    2 SO4

2‐ (aq) + I2 (aq) (              clear solution                )                (        pale yellow solution         )

Introduce two substances to help us observe the colour changemore accurately:

1. Starch indicator2. Sodium thiosulfate, Na2S2O3, of a fixed quantity, 2 2 3, q y

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Iodine clock ReactionS2O8

2‐ (aq) + 2 I‐ (aq)    2 SO42‐ (aq) + I2 (aq)      (1)

Iodine clock Reaction

( clear solution )

I2 (aq) + 2 S2O32‐(aq)    starch indicator     S4O6

2‐ (aq) + 2 I‐ (aq)             (2)thiosulfate ion ( coloured solution )

As the I2 is formed (1), it reacts with the fixed amount of sodium thiosulfate that hasbeen 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 colourcomplex, and turns the clear solution into a coloured solution. 

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Iodine clock Reaction

S2O82‐ (aq) + 2 I‐ (aq)    2 SO4

2‐ (aq) + I2 (aq) 

Make solutions of different

Prepare 14 solutions in 14 Erlenmeyer flasks withdifferent concentrations of S2O8

2‐ and I‐ solutions!different

concentrations.Label them:A1, A2, A3, A4, A5, A6, A7.B1 B2 B3 B4 B5 B6 B7B1, B2, B3, B4, B5, B6, B7.

SEPARATE the reactants until we are ready to mix them!

A solutions contains:potassium iodide (I-) sodium thiosulfate (S2O3

2‐ )

B solutions contains:ammonium persulfate (S2O8

2-)

starch indicator

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S O 2 ( ) 2 I ( ) 2 SO 2 ( ) I ( )Iodine clock Reaction

S2O82‐ (aq) + 2 I‐ (aq)    2 SO4

2‐ (aq) + I2 (aq) 

Total volume of ‘A’ solutions =

30.00 mL. CHEM 3310 8

S O 2‐ (aq) + 2 I‐ (aq) 2 SO 2‐ (aq) + I (aq)Iodine clock Reaction

S2O82‐ (aq) + 2 I‐ (aq)    2 SO4

2‐ (aq) + I2 (aq) 

Total volume of ‘B’ solutions =

20.00 mL.

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Iodine clock Reaction

S2O82‐ (aq) + 2 I‐ (aq)    2 SO4

2‐ (aq) + I2 (aq) 

dMIX and TIME: A1 + B1  A2 + B2 A3 + B3 A4 + B4 A5 + B5 A6 + B6 A7 + B7

Total volume of Total volume of the combined

solution =

50.00 mL50.00 mL.

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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

0 00050 l (NH ) S O 0 010 M (NH ) S O

( ) ( )(NH4)2S2O8

0.00050 moles (NH4)2S2O8 = 0.010 M  (NH4)2S2O80.05000 L

Total volume of the in the combined A1 + B1 solution

combined solution =

50.00 mL.CHEM 3310 11

Iodine clock Reaction

S2O82‐ (aq) + 2 I‐ (aq)    2 SO4

2‐ (aq) + I2 (aq) 

What is the reaction’s rate law?

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Iodine clock Reaction

Wh t i th ti ’ t l ?

S2O82‐ (aq) + 2 I‐ (aq)    2 SO4

2‐ (aq) + I2 (aq) 

The rate law for a chemical reaction is an experimentally

What is the reaction’s rate law?

determined mathematical equation that describes the progressof the reaction.

For the Iodine clock reaction, the reaction rate of formation of I2is proportional to the product of the concentrations each reactant each raised to some power m and nreactant each raised to some power, m and n.

Rate [I-]m [S2O82‐]n

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Iodine clock Reaction

Wh t i th ti ’ t l ?

S2O82‐ (aq) + 2 I‐ (aq)    2 SO4

2‐ (aq) + I2 (aq) 

What is the reaction’s rate law?

Rate [I-]m [S O 2‐]n

We can remove the proportional symbol and introduce a

Rate [I ]m [S2O82 ]n

Rate k [I-]m [S2O82‐]n

proportionality constant, k.

f

The proportionality constant, k, is known as the rate constant.

We will experimentally determine the values of m and n graphically.

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Rate k [I-]m [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 S2O8

2‐ .concentration of S2O8 .

Rate k’ [S2O82‐]n

where k’ = k [I-]m[ ]

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This will enable us to find n graphically.

Rate k’[S O 2 ]nRate k [S2O82‐]n

Take log of both sides,Take log of both sides,

Log (Rate) = n log [S2O82‐] + log k’

Since Rate time-1,

Log (time-1) = n log [S2O82‐] + log k’

Y = slope X + bGraph of “Log (time-1) Versus log [S2O8

2‐]” should yield a straight line with a slope of n.

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Rate k [I-]m [S2O82‐]n

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-the concentration of I .

Rate k’’ [I-]mwhere k’’ = k [S2O8

2‐]nwhere k k [S2O8 ]

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Similarly, this will enable us to find m graphically.

Rate k’’ [I ]mRate k [I-]m

Take log of both sides,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 + bGraph of “Log (time-1) Versus log [I-]” should yield a straight line with a slope of m.

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Iodine clock Reaction

S2O82‐ (aq) + 2 I‐ (aq)    2 SO4

2‐ (aq) + I2 (aq) 

R k [I ] [ 2 ]Rate k [I-]m [S2O82‐]n

Values of m and n should be very close to an integer If notValues 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 S2O8

2‐

The overall reaction order is m+n.

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Experiment 1: Factors Affecting p gReaction Rates

Part B

Effect of Temperature on Reaction Rate

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Iodine clock Reaction

How do we do this?

S2O82‐ (aq) + 2 I‐ (aq)    2 SO4

2‐ (aq) + I2 (aq) 

Keep the reactant concentrations constant, and let the

How do we do this?

p ,reaction react at different temperatures:

0oC 20oC 30oC 40oC0oC, 20oC, 30oC, 40oC(Part A)

Use A4 B4 solutions for all the temperatures!!Use A4, B4 solutions for all the temperatures!!

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Verify Arrhenius’ Equation

RTactE

Ak RTAek 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)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 is the activation energy the energy required by the reactingEact is the activation energy, the energy required by the reacting

species for their collisions to be effective (ie - those that leadto the formation of products)

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Rewrite Arrhenius’ Equation

RTactE

Aek ATR

Ek act ln1ln

TR y = slope x + b

1 1"versuslnPlot

T k " 1

or "versuslogPlot

T k " 1

should yield a straight line!!

Recall to convert between ln and logRecall, to convert between ln and log,

ln y = 2.303 log yy g y

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Recall, how to use the Linest function in Excel to

http://nobel.scas.bcit.ca/wiki/index.php/Linest

determine the statistical error in a set of data.

http://nobel.scas.bcit.ca/wiki/index.php/Linest

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