Stoichiometry

The property of stoichiometry is important in determining the relationship that occurs between the reactants and the output product of a chemical reaction. Traditional analytical methods for assessing the concentration composition of an unknown substance are time consuming and labor intensive since they require the use of chemical samples, examination, and comparison of results with identified elements to determine its origin. To make work easier, a more user-friendly form of continuous concentration is evaluated in an experiment that uses light absorbance at a certain wavelength and path distance to calculate the number of moles of iron iii thiocyanate formed when excess amount of iron iii react with thiocyanate ion in dilute solution. Using the formation constant the formula of the reaction can be verified and the concentration of unknown reagent determined.

Introduction

Concentration of a substance determines its chemical property. Continuous concentration method is one method that can be used to determine the number of moles of a product of chemical reaction. The formation constant is of importance in finding the relationship between reactant and the product formed. Colorimetry is a technique that determines the concentration of a product that is colored i.e. the more the concentration the more the intensity of the color is observed. This phenomenon is due to the presence of molecules in the product that absorb or reflect photon in visible spectrum. Beer’s law governance the relationship between the absorbance, concentration, and path length.

A=ᶓbc, where A is absorbance, B the path length, c the concentration and ᶓ a constant.

It is possible to calculate the concentration of unknown if both the absorbance of two components is known and the concentration of one of the component. The path length and constant are same for both samples and the absorbance is measured at the same frequency.

Cunknown=Unknown/Aknown×Cknown

In this experiment, job method is employed in determining the concentration of the product formed using absorbance property of iron iii thiocyanate.

Material and methods

The materials used in this experiment are:

40 ml beaker

Pipette

Test tube

0.003M aqueous Fe(NO3)3

0.003M aqueous KSCN

The procedure provided in lab manual followed to the letter.

Results

wavelength

concentration

400

0.086

425

0.126

450

0.154

475

0.149

500

0.117

525

0.075

550

0.046

575

0.024

600

0.015

625

0.003

650

0.001

675

0.007

700

0.007

Table 1 shows the results of concentration of solution A with varying wavelength

Figure 1 shows a graph of absorbance with varying wavelength

The wavelength that results in maximum concentration is 450 nm and will be used in further experiment procedure.

solution

absorbance

1

0.025

2

0.105

3

0.154

4

0.059

Table 2 shows the result of absorbance of solutions at a maximum wavelength

Of 450 nm

Part B

Wavelength in nm

absorbance

400

0.146

425

0.223

450

0.277

475

0.271

500

0.214

525

0.147

550

0.090

575

0.050

600

0.029

625

0.016

650

0.010

675

0.007

700

0.005

Table 3 shows absorbance of solution B2 with varying wavelength

Figure 2 shows the graph of absorbance of solution 2 with varying wavelength

The max absorbance is realized at a wavelength of 450 nm and will be used for further analysis

solution

absorbance

B1

0.039

B2

0.277

B3

0.188

B4

0.056

Table 4 shows the results of absorbance of solutions at a wavelength of 450 nm

Calculations

Part A

The ionic equation of the reaction for the formation of the complex ion

Fe3+ + SCN- ↔ Fe (SCN) 2+

Formation constant is givenby:

Kf= [Fe (SCN) 2+] /[Fe3+] [SCN-]

We used a concentration of 0.001M of Fe(NO3)3 and 10ml of 0.001M of KSCN

An assumption that all thiocyanate ions react completely make the moles of product equal to thiocyanate ion

Fe (SCN) 2+=[SCN-]

For B3 set as standard

Moles of SCN- = 1 x 10^-3 x 10^-3 = 1.0 × 10^-6

[Fe (SCN) 2+]std= (1x 10^-6)/0.010 L=1x 10^-4 M

Astd= 0.154

For solution B1

[Fe (SCN) 2+] eqm= (0.025/0.154) × 0.0001

= 1.6233 × 10^-5 M

Initial concentration of iron iii ions

[Fe3+] initial=(0.5 × 0.2 x 10^-3)/0.01 =1 x 10^-2 M

[Fe3+] equil= 1 x 10^-2 – (1.6233 × 10^-5)= 9.984x 10^-3 M

[SCN-] eqm = 1x 10^-4 -1.6233 x 10^-5

=8.3767 x 10^-5M

Kf= (1.6233 x 10^-5)/ {8.3767x 10^-5 x 9.984 x 10^-3}

=19

Table 5 below shows summary of the equilibrium constant obtained for rest of the solutions

solution

absorbance

FeSCN2+

Fe3+

SCN

kf

B2

0.025

1.62338E-05

0.179984

8.37662E-05

1.076755

B3

0.11

7.14286E-05

0.009929

2.85714E-05

251.7986

B4

0.059

3.83117E-05

0.000562

6.16883E-05

1105.689

Part B

Use B2 as the standard

The moles of SCN= 1 x 10^-4

[Fe3+] initial = 1 x 10^-3 x 0.2 /0.01

= 0.02 M

Astd= 0.277

[FeSCN2+]eq=0.188/0.277 x10^-4

= 6.787 x 10^-5 M

[Fe3+]eq= 0.02-(6.787 x 10^-5)

=1.993 x 10^-2 M

[SCN-]=(1 x 10^-4) –(6.787 x 10^-5)

= 3.213 x 10^-5 M

Kf= (6.787 x 10^-5)/ (1.993 x 10^-2 x 3.213 x 10^-5)

= 106

solution

Abs

FeSCN2+

Fe3+

SCN-

Kf

B2

0.039

1.40794E-05

0.179986

8.59206E-05

0.910435

B3

0.188

6.787E-05

0.009932

3.213E-05

212.6794

B4

0.056

2.02166E-05

0.00058

7.97834E-05

437.0488

Table 6 shows the summary of equilibrium constant of solution B1, B3 and B4

solution

Average Kf

B2

1.076755

B3

251.7985612

B4

1105.689078

Table 7 shows the average equilibrium constant

Discussion

The value of equilibrium constant of solution B2 is near a unit meaning that the reaction reached a state of equilibrium at an intermediate mixture. The average of B3 and B4 is more than one, which implies that the product i.e. the iron(iii) thiocyanate complex ions are majority that supports the assumption that the reaction was to completion.

It would be hard to get reliable results if the absorbance is measured at 675 nm. A standard solution of FeSCN2+ absorbs light at λ=447 nm. At wavelength close to this value the absorbance measurement will correlate with the concentration of the complex ion. This value would give an accurate calibration curve. 650 nm is value too large and therefore it cannot give the correct calibration which provides the relationship between the absorbance at a given wavelength and the concentration.

The total number of moles or the substances reacting is kept constant for various measurements. Each measurement is done using different ratio of the number of moles of the reactants. The greatest change occurs when the ratio of the number of moles is close to the optimum ratio. For instance, measurable quantity can be heat absorption or evolution, color change, precipitate formation among other physical quantities. An endothermic reaction accompanied by heat absorption (de Berg, Maeder, & Clifford, 2016). One measures the initial temperature and prior the reaction and then starts to mix the two reactants appropriately maintaining the total number of moles constant. The optimal ratio is arrived at when the thermometer registers the lowest temperature.

For reaction between two gaseous reactants the property measured is he volume of the gas evolved. The ratio of the number of moles at which the largest amount of gas is produced gives the maximum amount of consumed reactants.

Conclusion

The experiment conducted on the formation of complex ion is of greater scientific importance since it shows that the stoichiometry properties of unknown product can be determined by use of continuous variation method. Although there were some sources of errors that lead to deviation of results from the actual, the experiment successfully showed that complex iron (iii) thiocyanate complex ion is completely complex. The recommendation on the future experiment is use of more accurate apparatus and instrument for more exact results.

Reference

de Berg, K., Maeder, M., & Clifford, S. (2016). A new approach to the equilibrium study of iron (III) thiocyanates which accounts for the kinetic instability of the complexes particularly observable under high thiocyanate concentrations. InorganicaChimicaActa, 445, 155-159.