Half Reaction A half reaction is either the oxidation or reduction reaction component of a redox...
-
Upload
austen-owens -
Category
Documents
-
view
225 -
download
0
Transcript of Half Reaction A half reaction is either the oxidation or reduction reaction component of a redox...
![Page 1: Half Reaction A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the.](https://reader036.fdocuments.us/reader036/viewer/2022082404/5697bfb61a28abf838c9df7e/html5/thumbnails/1.jpg)
Half Reaction
A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction.
Step 1: Write the skeletons of the oxidation and reduction half-reactions.
Step 2: Balance all elements other than H and O. Step 3: Balance the oxygen atoms by adding H2O molecules
where needed. Step 4: Balance the hydrogen atoms by adding H+ ions
where needed.
![Page 2: Half Reaction A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the.](https://reader036.fdocuments.us/reader036/viewer/2022082404/5697bfb61a28abf838c9df7e/html5/thumbnails/2.jpg)
Half Reaction - continued
Step 5: Balance the charge by adding electrons, e-. Step 6: If the number of electrons lost in the oxidation half-
reaction is not equal to the number of electrons gained in the reduction half-reaction, multiply one or both of the half- reactions by a number that will make the number of electrons gained equal to the number of electrons lost.
Step 7: Add the 2 half-reactions as if they were mathematical equations. The electrons will always cancel. If the same formulas are found on opposite sides of the half-reactions, you can cancel them. If the same formulas are found on the same side of both half-reactions, combine them.
Step 8: Check to make sure that the atoms and the charges balance.
![Page 3: Half Reaction A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the.](https://reader036.fdocuments.us/reader036/viewer/2022082404/5697bfb61a28abf838c9df7e/html5/thumbnails/3.jpg)
Half Reaction - continued
Electron donor: CHCl3 (Chloroform) Electron acceptor: O2 (1/4O2 + H+ + e- → 1/2H2O)Step 1: C → CO2; N → NO3; Clx → Cl-
CHCl3 → CO2 + Cl- Step 2: CHCl3 → 1CO2 + 3Cl- Step 3: CHCl3 + 2H2O → CO2 + 3Cl-
Step 4: CHCl3 + 2H2O → CO2 + 3Cl- + 5H+ Step 5: CHCl3 + 2H2O → CO2 + 3Cl- + 5H+ + 2e-
Step 6: DoneStep 7: CHCl3 + 2H2O → CO2 + 3Cl- + 5H+ + 2e- + 1/4O2 + H+ + e- → 1/2H2O × 2Step 8: CHCl3 + 1/2O2 + H2O → CO2 + 3Cl- + 3H+
![Page 4: Half Reaction A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the.](https://reader036.fdocuments.us/reader036/viewer/2022082404/5697bfb61a28abf838c9df7e/html5/thumbnails/4.jpg)
Half Reactions
CH2Cl2 (Dichloromethane or methylene chloride)
CH2Cl2 + 2H2O → CO2 + 2Cl- + 6H+ + 4 e-
1/2O2 + 2H+ + 2e- → H2O 2
CH2Cl2 + O2 → CO2 + 2H+ + 2Cl-
COD for 100 mg/L of CH2Cl2
2 mol 16 g/mol 2/(112+22+235.45)g 100 mg/L = 73.6 mg/L
4
![Page 5: Half Reaction A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the.](https://reader036.fdocuments.us/reader036/viewer/2022082404/5697bfb61a28abf838c9df7e/html5/thumbnails/5.jpg)
Theoretical Oxygen Demand
Determine the ThOD for glycine (CH2(NH2)COOH) using the following assumptions
1. In the first step, the organic carbon and nitrogen are converted to carbon dioxide (CO2) and ammonia (NH3), respectively.
2. In the second and third steps, the ammonia is oxidized sequentially to nitrite and nitrate.
3. The ThOD is the sum of the oxygen required for all three steps.
5
![Page 6: Half Reaction A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the.](https://reader036.fdocuments.us/reader036/viewer/2022082404/5697bfb61a28abf838c9df7e/html5/thumbnails/6.jpg)
Solution
1. Write balanced reaction for the carbonaceous oxygen demand.
CH2(NH2)COOH + 1.5O2 NH3 + 2CO2 + H2 0 2. Write balanced reactions for the nitrogenous
oxygen demand. NH3 + 1.502 HNO2 + H20 HNO2 + 0.5O2 HNO3
NH3 + 2O2 HNO3 + H2O3. Determine the ThOD.
ThOD = (1.5 + 2) mol O2/mol glycine = 3.5 mol O2/mol glycine 32 g/mol O2
43 g/mol glycine= 2.6 g O2/g glycine
6
![Page 7: Half Reaction A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the.](https://reader036.fdocuments.us/reader036/viewer/2022082404/5697bfb61a28abf838c9df7e/html5/thumbnails/7.jpg)
Example
A groundwater contains the following: 150 mg/L ethylene glycol; 100 mg/L phenol; 40 mg/L sulfide (S2-); 125 mg/L ethylene diamine hydrate (ethylene diamine is essentially nonbiodegradable).
(a) Compute the COD and TOC. (b) Compute the BOD5 if the k10 is 0.2/day
(BODult = 0.92 COD). (c) After treatment, the BOD5 is 25 mg/L.
Estimate the COD (k10 = 0.1/day)
7
![Page 8: Half Reaction A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the.](https://reader036.fdocuments.us/reader036/viewer/2022082404/5697bfb61a28abf838c9df7e/html5/thumbnails/8.jpg)
Solution
(a) COD: Ethylene glycol: C2H6O2 + 2.5 O2 2 CO2 + 3 H2O
COD = 2.532/62 150 mg/L=194 mg/L Phenol: C6H6O + 7 O2 6 CO2 + 3 H2O
COD = 732/94 100 mg/L = 238 mg/LEthylene diamine hydrate: C2H10N2O + 2.5 O2 2 CO2 + 2 H2O + 2 NH3
COD = 2.532/78 125 mg/L = 128 mg/LSulfide: S2- + 2 O2 SO4
2-
COD = 232/32 40 mg/L = 80 mg/LTotal COD: 194 + 238 + 128 + 80 = 640 mg/L
8
![Page 9: Half Reaction A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the.](https://reader036.fdocuments.us/reader036/viewer/2022082404/5697bfb61a28abf838c9df7e/html5/thumbnails/9.jpg)
Solution - continued
(a) TOC:Ethylene glycol:
Phenol:
Ethylene diamine hydrate:
Total TOC: 58 + 77 + 39 = 174 mg/L
mg/L 58mg/L 15062
122TOC
mg/L 77mg/L 10094
126TOC
mg/L 93mg/L 12578
122TOC
9
![Page 10: Half Reaction A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the.](https://reader036.fdocuments.us/reader036/viewer/2022082404/5697bfb61a28abf838c9df7e/html5/thumbnails/10.jpg)
(b) Ultimate BOD: BODult = 0.92 COD = 0.92 (194 + 238 + 80) mg/L = 471
mg/L
BOD5 = 471 mg/L 0.9 = 424 mg/L
(c) BODult in the effluent:
CODeff = 128 mg/L + 40 mg/L + residual byproducts
9.0101BOD
BOD 2.05
ult
5
L/mg 5.36101
L/mg 251.05
mg/L 40mg/L 39.70.92
mg/L 36.5
0.92
BODCOD ult
10
Solution - continued