Lab 3: Reaction Order and Rate Laws

Purpose / IntroductionThis lab will provide a physically-observable demonstration of how concentrations will

or will not affect reaction rates. The reaction rate of a reactant is the time elapsed for the reaction to occur. Changes of certain factors can alter the speed of a reaction rate: surface area, temperature, catalysts, and concentrations. In this lab the concentrations are altered, and the chemical reaction is observable by a change in color and precipitate formation. By timing the chemical reaction, the rate constants and orders –orders are usually positive integers, but can also be fractions or negative integers– can be calculated to express the "Rate Law" equation

Rate=k [ A ]x [ B ]y

In this lab, the reactants will be HCl and Na2 S2 O3. In one part of the lab, HCl concentration will be varied while Na2 S2 O3 concentration is held constant. In the second part of the lab, Na2 S2 O3 concentration is varied while HCl concentration is held constant.

I expect the reactions between the reactants will produce a precipitate that will cloud the mixtures' visibilities. I also expect the time for the reaction rate will increase as the solution's dilution increases.

Prelab QuestionsN/A

ProcedureRefer to CHE 112-BK01 lab manual

Lab 3: Reaction Order and Rate Laws (page 104)

Data / ResultsPart II: Varying the Concentration of 1.0 M HCl

Varying the Concentration of 1.0 M HCl Concentrations

Well #

# dropsHCl

# dropsWater

# dropsNa2 S2 O3

InitialHCl

InitialNa2 S2 O3

FinalHCl

FinalNa2 S2 O3

Reaction Time (sec) ReactionRate (sec−1)Trial 1 Trial 2 Average

1 12 0 8 1.0M 0.30M 0.60M 0.12M 30.34 34.37 32.36 0.03090

2 6 6 8 0.50M 0.30M 0.30M 0.12M 43.01 46.17 44.59 0.02243

3 4 8 8 0.33M 0.30M 0.20M 0.12M 50.09 50.69 50.39 0.01985

Colorless solution yields a milky white precipitate that eventually becomes a milky lemon yellow color.

Part III: Varying the Concentration of 0.30 M N a2 S2 O3

Varying the Concentration of 0.30 M HCl Concentrations

Well #

# dropsHCl

# dropsWater

# dropsNa2 S2 O3

InitialHCl

InitialNa2 S2 O3

FinalHCl

FinalNa2 S2 O3

Reaction Time (sec) ReactionRate (sec−1)Trial 1 Trial 2 Average

1 8 0 12 1.0M 0.30M 0.40M 0.18M 28.46 27.95 28.21 0.03545

2 8 6 6 1.0M 0.15M 0.40M 0.09M 69.42 64.54 66.98 0.01493

3 8 8 4 1.0M 0.10M 0.40M 0.06M 153.52 153.89 153.71 0.006506

Colorless solution yields a milky white precipitate that eventually becomes a milky lemon yellow color.

Part IV: CalculationsVaried 1.0 M HCl Varied 0.30 M Na2 S2O3

Initial Final Initial Final(1.0 M ) (12 drops )

12 drops=1.0 M

(1.0 M ) (12 drops )20 drops

=0.60 M (0.30 M ) (12 drops )

12 drops=0.30 M

(0.30 M ) (12 drops )20drops

=0.18 M

(1.0 M ) (6drops )12 drops

=0.50 M (0.50 M ) (12drops )

20 drops=0.30 M

(0.30 M ) (6drops )12drops

=0.15 M (0.15 M ) (12 drops )

20drops=0.09 M

(1.0 M ) (4 drops )12 drops

=0.33 M (0.33 M ) (12drops )

20drops=0.20 M

(0.30 M ) (4 drops )12 drops

=0.10 M (0.10 M ) (12 drops )

20drops=0.06 M

Constant 0.30 M Na2 S2O3 Constant 1.0 M HCl(0.30 M ) (8 drops )

8 drops=0.30 M

(0.30 M ) (8 drops )20 drops

=0.12 M (1.0 M ) (8 drops )

8 drops=1.0 M

(1.0M ) (8 drops )20 drops

=0.40 M

12 Concentrations

13 Concentrations

[ HCl ] (M ) [ Na2 S2 O3 ](M ) Rate [ HCl ] (M ) [ Na2 S2 O3 ](M ) Rate1.0 0.30 3.3 ×10−2 1.0 0.30 3.3 ×10−2

0.50 0.30 2.2 ×10−2 0.33 0.30 2.0 ×10−2

1.0 0.15 1.5 .× 10−2 1.0 0.10 0.65 ×10−2

12 Concentrations

13 Concentrations

2.2× 10−2

3.3× 10−2 =[ 0.50 ]x

[1.0 ]x

23=( 1

2 )x

x=0 [ HCl ]0

2.0× 10−2

3.3× 10−2 =[ 0.33 ]x

[1.0 ]x

2033

=( 13 )

x

x=0 [ HCl ]0

1.5.×10−2

3.3× 10−2 =[ 0.15 ]y

[ 0.30 ]y

[ Na2 S2 O3 ]1 0.65× 10−2

3.3× 10−2 =[ 0.10 ] y

[ 0.30 ] y [ Na2 S2 O3 ]1

511

=( 12 )

y

511

=( 510 )

y

y=1

1366

=(13 )

y

1366

=( 1339 )

y

y=1

Rate Law:Rate=k [ HCl ]0 [ Na2 S2O3 ]1 Rate=k [Na2 S2O3]

Rate Law:Rate=k [ HCl ]0 [ Na2 S2O3 ]1 Rate=k [Na2 S2O3]

Postlab QuestionsC.

1. For the varied HCl portion of the lab, the reaction order was calculated to be 0; thus the reaction order is of the zero order: concentration does not affect the rate. 2. The varied Na2 S2 O3 concentration's calculated reaction order is 1: first order reaction; the rate is directly proportional to the concentration, thus a doubled concentration results in a doubled rate.

D. The rate law for the reaction of HCl and Na2 S2 O3 is Rate=k [ Na2 S2 O3 ]1.E. Using the equation Rate=k [ Na2 S2 O3 ]1 the constant k can be calculated.

3.3×10−2 Ms

=k (0.30 )1 M

k=3.3 × 10−2 M

s0.30 M

=(3.3 ×10−2 Ms )( 1

0.30 M )k=1.1× 10−1 s−1

F. Potential errors in the experiment can be located in several areas. First, if one's calculations are not correct, errors can be found from the concentrations of HCl and Na2 S2 O3 to the reaction orders, rate law, and even the constant k . Another potential error could be misconstruing the change of concentrations between the three wells for each part of the lab; between the first well and the second well the concentrations are halved, and between the first well and the third well the concentrations differ by a third; however, one may misconstrue the differences and assume each well possess half the concentration of the well before it. Additionally, calculations for this lab compared to the book's prescribed directions vary, and may confuse individuals during calculations.

Another potential error that my group found was that by observing the X sign for a prolonged period of time almost created a "burned" image of the X sign, thus making it difficult to distinguish when the sulfur precipitate completely obscured the X sign. As a result, the times recorded are possibly inaccurate. Also, if the well plate was not cleaned in a timely manner, precipitate formed to the well walls, and could become a nuisance to clean to prevent future contamination for other labs.

ConclusionPrior to the lab I predicted two outcomes: produced precipitation and increase of reaction

rate as the reactant became more dilute. The lab produced results that support my predictions: solid sulfur formed the precipitate, and the elapsed time of chemical reaction increased as the reactant concentration become more dilute. Other observations were made during the lab. After

the lab was completed and calculations were made other determinations were made. Different concentrations of HCl were hardly "dented" the reaction rates and reaction time: change of reaction time between well 1 and well 3 was 18.03 seconds. On the other hand, different concentrations of Na2 S2 O3 proved excellent success in altering the reaction rates and reaction time: change of reaction time between well 1 and well 3 was 125.50 seconds. As a result, the order of reaction for HCl is of the zero order and does not affect rate constant k ; whereas the order of reaction for Na2 S2 O3 is the first order and directly affects rate constant k .