factors affecting reaction rate

Experiment 1Factors Affecting Reaction

Rates

Chiu, Ina Cathrina R.Salindo, Elysse S.

Introduction

Chemical Kinetics Deals with rates of reactions

(how fast a reaction progresses)

Rate is a measure of quantity (formed/consumed) per unit of time

Introduction Collision Theory

For a reaction to proceed, successful collision must happen!

Collision frequency is directly proportional to reaction rate

Activation energy is inversely proportional to reaction rate

Transition State Theory

– For a reaction to occur, it must reach a transition state (with enough energy) before proceeding on to the products.

Introduction

Factors Affecting Reaction Rates Nature of Reactants Concentration of Reactants Temperature Surface Area Catalyst

RESULTS AND DISCUSSION

Part A. Nature of Reactants

Reactant Observation

MgBubbles (slower

reaction)

NaExplosion (faster

reaction)

Activation Energy (Ea)— the minimum amount of

energy required in order for a reaction to proceed

—Inversely proportional to the rate of reaction


↓Ea = ↑reactivity = ↑reaction rate

Na has a relatively

lower activation

energy than Mg.


Part B. Concentration of Reactants

Table 2: Effect of [Na2S

2O

3] on rate with constant [HCl]

[Na2S

2O

3][HCl

]ln

[Na2S

2O

3]

Time(s)

Rate (1/time

)

Ln Rate

0.125 0.5 -2.079 15 0.067 -2.703

0.100 0.5 -2.303 19 0.053 -2.937

0.075 0.5 -2.590 27 0.037 -3.297

0.050 0.5 -2.996 64 0.016 -4.135

0.025 0.5 -3.689 142 7.04 E-3

-4.956

Plot ln rate vs ln [Na2S

2O

3]


-5.5 -5 -4.5 -4 -3.5 -3 -2.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

ln rate

ln [Na

2S

2O

3]

Table 3: Effect of [HCl] on rate with constant [Na2S

2O

3]

[Na2S

2O

3][HCl] ln

[HCl]Time Rate

(1/time)

ln Rate

0.1 1.0 0.000 15 0.067 -2.703

0.1 0.8 -0.233 22 0.043 -3.147

0.1 0.6 -0.511 26 0.038 -3.270

0.1 0.4 -0.916 34 0.029 -3.540

0.1 0.2 -1.609 41 0.024 -3.730


Plot ln rate vs ln [HCl]


-1.8 -1.6 -1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

ln rate

ln [HCl]

Solving for the order with respect to Na

2S

2O

3:

ln (rate = k[Na2S

2O

3]x[HCl]y)

ln Rate = x ln [Na2S

2O

3] + y ln [HCl] + ln K

y = m x + b

r2 = 0.99

m = order with respect to [Na2S

2O

3] =

1.458

b = y ln [HCl] + ln K = 0.373


Solving for the order with respect to HCl:

ln (rate = k[Na2S

2O

3]x [HCl]y)

ln Rate = y ln[HCl] + x ln[Na2S

2O

3] + ln k

y = m x + b

r2 = 0.86

m = order with respect to [HCl] = 0.579

b = x ln[Na2S

2O

3] + ln k = -2.901


Actual vs. Theoretical Order

Actual Theoretical

Order wrt

Na2S

2O

3

1.458 2

Order wrt HCl

0.579 0

Overall order

2.037 2


Rate = k [Na2S

2O

3]1.458 [HCl]0.579

Solving for specific rate constant, k (differential method):

b = y ln [HCl] + ln K

0.373 = (0.579) ln (0.5) + ln k

k = 2.169 M-1 s-1

b = x ln[Na2S2O3] + ln k

-2.901 = (1.458) ln (0.1) + ln k

k = 1.578 M-1 s-1


As [Na2S

2O

3 ] increases, the rate of

reaction also increases As [HCl] increases, rate increases. BUT!

Theoretically, it shouldn’t. GENERALLY, ↑ concentration = ↑

collision frequency = ↑ reaction rate


Part C. Temperature

Temp (o C)

1/T (K)

Time (s)

Rate (1/tim

e)k ln k

173.45E-

0385 0.012 1.49 0.399

273.34E-

0354 0.019 2.36 0.859

373.23E-

0327 0.037 4.59 1.524

Arrhenius Plot

0.0032 0.00325 0.0033 0.00335 0.0034 0.00345 0.00350

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

In k

1/T (K)

Part C. Temperature

ln k = -EaR

1T+ ln A

y = mx + b

Ea = -R(slope)*find slope using linear regressionSlope = -5054.287569Ea = -8.314 J (-5054.287569)Ea = 42,021.35 J

Part C. Temperature

↑temperature = ↑rate of reaction

Increasing temperature increases the fraction of molecules that posses enough kinetic energy to overcome Ea, thus increasing reaction rate.

Fra

ctio

n of

mol

ecul

es

T2 > T1

Part C. Temperature

Part D. Surface Area

Reactants Visible Results

Strip of MgBubbles (slower

reaction)

Pieces of MgBubbles (faster

reaction)

↑surface area = ↑rate of reaction

—A greater surface area exposed increases the probability of effective collisions between reactant molecules and results to an increase in reaction rate.

Part D. Surface Area

Part E. Catalyst

Reactants Visible Results

H2O2 + Rochelle SaltSlow bubble formation

H2O2 + Rochelle Salt

+ CoCl2

Faster bubble formation; color change (pink → green → pink)

- a substance that speeds up reaction by providing an alternative pathway with lower activation energy for the reactant molecules, but is not consumed in the reaction

CoCl2 in aqueous solution is pink

because of [CoCl2 (H2O)4]●2H2O or

cobaltous chloride hexahydrate

formed an intermediate

activated complex of Co3+ (green) and

tartrate ions

converted back to its original form,

CoCl2

Color Change:

pink

pink

green

Conclusion

—5 main factors that can affect the rate of reaction – nature of reactants, concentration of reactants, temperature, surface area and presence of a catalyst.

—For a reaction to proceed, molecules must acquire enough energy to overcome the activation energy.

—Generally, increasing the frequency of collisions and decreasing the activation energy would hasten a reaction.

—Concentration of reactants, temperature and surface area are directly proportional to reaction rate. An increase in these factors would increase reaction rate.

—Adding a catalyst and using reactants with lower activation energy would also hasten the reaction rate.

Conclusion

Recommendations

—Use a stopwatch instead of a clock or watch because of its greater accuracy.

—It is also recommended that this experiment should not be performed spontaneously and without proper preparation (glassware, reagents, personal protective equipment, etc).

References:• Clark, J. (March 2011). Rates of Reaction Menu. Retrieved

April 30, 2011, from

• http://www.chemguide.co.uk/physical/basicratesmenu.html#top

• The Royal Society of Chemistry. Classic Chemistry Demonstration:A Visible Activated Complex. Retrieved May 1, 2011, from

• www.rsc.org/images/oscillating_tcm18-188828.pdf

• Purchon, N.D. (Novermber 10, 2006). Rates of Reaction. Retrieved April 30, 2011, from

• http://www.purchon.com/chemistry/rates.htm

• Engle, Harry and Luciana Ilao, Learning Modules in General Chemistry 2 (2007 Edition),

factors affecting reaction rate

Documents