1. Experiment 12 Kinetic Investigation of Oxidative Reaction of Formic Acid with Bromine by Electrodynamic Potential
HCOOH + Br2 → 2H+ + 2Br- + CO2

2. Experimental objectives
Determine the reaction order, reaction constant and activation energy of oxidation of formic acid by bromine via Electrodynamic potential measurement.

3. Reaction rate constant
HCOOH + Br2 → 2H+ + 2Br- + CO2 
Reaction rate =k [HCOOH]m [Br2]n
K is the reaction rate
How to determine k, m , n?

4. Reaction rate =k [HCOOH]m [Br2]n
determine n
Reaction rate =k [HCOOH]m [Br2]n
During reaction, if HCOOH is present in far excess, and therefore [HCOOH] almost keeps constant.
Hence, Reaction rate =k’ [Br2]n; Where k’ = k [HCOOH]m
By measuring the function of [Br2] versus reaction time and then reaction order of Br2 and reaction constant k’ can be derived experimentally.

5. determine k, m
If we conduct two similar experiments, namely with two high HCOOH concentrations, then we obtain two k’ values.
k’1 = k [HCOOH]1m
k’2 = k [HCOOH]2m
Then, reaction order of HCOOH (m) and reaction constant k can be computed.

6. Measurement of [Br2] - Electrodynamic potential
We design a cell as
Hg，HgCl2|KCl(sateurated)| | Br－，Br2，Pt
The potential of this cell can be determined
Since [Br-] is in great excess, and hence we can treat it as a constant.

7. Or ln[Br]2 =Constant –k’t
If we assume bromine in the oxidation of formic acid is the first reaction order, then
Or ln[Br]2 =Constant –k’t As
Hence
plot E (cell potential) as a function of reaction time. If it presents a straight line, the reaction order of Br2 is thus confirmed to be 1(first order reaction). k’ can be derived from the slope of this line.

8. Determination of Ea -Activation Energy
Conduct the reaction at temperature T1 and T2, than we will get reaction constants KT1 and KT2 respectively.

9. Apparatus

10. Experimental procedure
1. Set the temperature of thermostatic bath to 25.0oC（another team to 35.0oC）, and connect the warm water to the reactor. Switch on the water pump.
2. Preparation of solutions.
Pipette 10 mL and 20 mL of 1.00 mol L-1 HCOOH solution to volumetric flasks, respectively. Dilute with distilled water to the marked level. Place them into the thermostatic bath allowing them to be warmed for at least 15 min.
With another pipette, pipette 10 mL of (1.0 mol L-1 KBr+0.01 mol L-1 Br mol L-1 HCl) aqueous solution to a 50 mL volumetric flask and dilute with distilled water to the marked level. Heat the solution in the water bath. Pipette another 10 mL of (1.0 mol L-1 KBr+0.01 mol L-1 Br mol L-1HCl) aqueous solution to a 50 mL volumetric flask and dilute with distilled water to the marked level. Transfer this solution to the reactor via the funnel on the reactor, switch on the stirrer and heat for 15 min.

11. 3. Take the Pt electrode and purge with distilled water (Don’t touch Pt piece). Then put the Pt electrode into the reactor. Meanwhile, put the calomel electrode into a small beaker containing saturated KCl solution. Connect the reactor to the KCl solution with a salt bridge as shown in Figure 1.
4. Connect following Figure 2. (Pay attention to the negative and positive electrode), and switch on the paperless recorder.
Wire-wound
potentiometer (1kΩ)

12. 5. Reaction
Pour the warmed HCOOH solution into the reactor rapidly via the funnel. Adjust the wire-wound potentiometer (1kΩ) as shown in Figure 2 to just reach the full scale on paperless recorder. Reset the paperless recorder and start to record E-t data set. When the value on paperless recorder is less than 3 mV， stop recording and save your data in the computer.
Pour out the solution in the reactor; wash the reactor with distilled water for three times (Pay Attention to the magnet in the reactor. Don’t pour your magnet to the sink.) Set up the apparatus again, and then introduce 20 mL heated HCOOH solution and 10 mL (1.0 mol L-1 KBr+0.01 mol L-1 Br mol L-1 HCl) aqueous solution into the reactor. Record the E-t dataset again.
Copy the data who conducted the reaction at 35.0 o C （or 25oC）.
Clean up your bench.

13. Data analysis
Plot two E-t curves with your data collected with two HCOOH concentrations. Derive the values of k’. According Equations, calculate reaction order m and reaction constant k.
According to the two reaction constants derived from 25oC and 35 oC, calculate the reaction activation energy (see Equation 10).

14. Discussion
Why we can determine the reaction order of one reactant while another reactant is present with significant excess?
During the experiment, most of the reaction potential was compensated by wire-wound potentiometer. Does it affect the results of our experiment? Why?
Why it is necessary to adjust the wire-wound potentiometer to just reach the full scale rather than zero on paperless recorder?