Electrochemistry Pre-Lab Questions

Pre – Lab Questions
1. A decomposition reaction may be defined as any reaction in which one reactant, a compound, breaks down to give two or more products. Write the balanced chemical equation for the decomposition of water to its elements.
2H2O(l) 2H2(g) + O2(g)
2. (a) Assign oxidation states to the hydrogen and oxygen atoms in each substance in the above chemical equation.
The oxidation state of H2(g) and O2(g) is zero, because they are in their normal state.
(b) Based on the changes in oxidation states for each atom, identify the atom that is oxidized and the atom that is reduced in the decomposition of water.
Hydrogen atom is reduced because it gains electrons.
2H+ H2(g)
Oxygen atom is oxidized because it loses electrons.
O2- O2(g)
3. Balance the following oxidation and reduction half – reactions for the decomposition of water.
2H2O O2 + 4H+ + 4e-
2H2O + 2e- H2 + 2OH-
4. Explain how the oxidation and reduction half – reaction may be combined to give the balanced chemical equation for the decomposition of water. What happens to the electrons to the H+ and OH- ions?
2H2O O2 + 4H+ + 4e-
(2H2O + 2e- H2 + 2OH-) x 2
2H2O O2 + 4H+ + 4e-
4H2O + 4e- 2H2 + 4OH-
6H2O O2 + 2H2 + 4H+ + 4OH-
6H2O O2 + 2H2 + 4H2O
2H2O O2 + 2H2
When we multiply the second reaction by 2, we get same number of electrons in both reaction which can be canceled out. In the net equation, we get same moles of H+ and OH- ions which combines and to form H2O. Then we can subtract the moles of H2O on the product side by the moles of H2O on the reactant side, and we get the balanced chemical equation for the decomposition of water.
Data Table
Initial indicator color of the electrolysis solution
Dark Blue
Chnages occuring at the positive (+) electrode
Bright Yellow
Changes occuring the negative (-) electrode
Dark Blue
Final indicator color of the mixed electrolysis solution
Dark Brown

Post – Lab Questions
1. Suggest an explanation for the initial indicator color of the electrolysis solution.
Sodium sulfate is a base. Bromthymol blue is blue color when it is present in the basic solution. Therefore, the initial indicator color is based on solution’s basicity.

2. Describe at least three observations that indicate a chemical reaction had occurred during the electrolysis of water.
The color of the solution changed from dark blue to light yellow at the positive electrode.
The u – tube was cold, which indicated that temperature has affected the reaction.
Bubbles were formed on the surface of the surface of the u – tube which indicated the gas was being released.

3. What are the two functions of the pencil lead electrodes?
It transfers electrons from the alligator clip to the solution.
With the use of the pencil lead, we can avoid the contacting alligator clips to the solution.

4. (a) Compare the color changes observed at the positive (+) and negative (-) electrodes. What ions produced at each electrode?
On the positive electrode (+), the color of the solution changed from dark blue to light yellow, and on the negative electrode (-), the color of the solution did not change; it remained blue. On the positive electrode, H+ ions were produced and O2(g) was released. On the negative electrode, OH- ions were formed.
(b) Write out the oxidation and reduction half reactions for the decomposition of water and identify which reaction occurred at each electrode, based on the indicator color changes.
Positive electrode: Oxidation -- 2H2O O2 + 4H+ + 4e-
Negative electrode: Reduction -- 2H2O + 2e- H2 + 2OH-

5. Compare the rates of gas evolution at the positive (+) and negative (-) electrodes. What gas was produced at each electrode? Explain, based on the balanced chemical equation for the decomposition of water.
Rate of gas evolution on positive electrode was slower compared to the rate of gas evolution on negative electrode. On the positive electrode, the oxidation process occurred and oxygen gas was released. On the negative electrode, the reduction process occurred and hydrogen gas was released.

6. Suggest an explanation for the final indicator color of the mixed electrolysis solution.
The color of bromthymol blue changes from yellow to blue when the pH range is somewhat above 7.0. The brown color that causes the transition is near a pH of 7.5. The reason for this is because there is less weak acid than there is weak base. Hence, the pH would be above 7.5.

7. Think about the flow of electrons and current in the electrolysis of water. What do the positive and negative signs on a battery signify?
Electricity always flows from the anode to the cathode; oxidation occurs at the anode and reduction occurs at the cathode. The positive end of the battery is the cathode and the negative end is the anode.

Mahavir Patel
Electrolysis of Water
Period 8
Chemistry AP
February 11, 2015