Chapter 20: Electrochemistry

Describe how to balance a redox reaction for an acidic vs a basic environment.

Answer explanation

To balance a redox reaction:

1. Split the reaction into two half-reactions – one for oxidation and one for reduction.

2. Balance all elements except hydrogen and oxygen in each half-reaction.

3. Balance oxygen atoms by adding H₂O.

4. Balance hydrogen atoms:

   • In acidic solution, add H⁺.

   • In basic solution, still add H⁺ for now; you’ll adjust this later.

5. Balance the charges by adding electrons (e⁻) to the more positive side of each half-reaction.

6. Make the number of electrons equal in both half-reactions by multiplying one or both reactions by a coefficient if needed.

7. Add the two half-reactions together and cancel out species that appear on both sides (e.g., electrons, H⁺, H₂O).

8. For acidic solution, the reaction is now balanced.

9. For basic solution, take the balanced acidic equation and:

   • Add the same number of OH⁻ ions as H⁺ ions to both sides of the equation.

   • Combine H⁺ and OH⁻ on the same side to form H₂O.

   • Cancel out any extra H₂O molecules that appear on both sides.

10. Double-check that:

• All atoms are balanced.

• The net charge on both sides is the same.

What is the primary purpose of a galvanic (voltaic) cell?

• To convert electrical energy into chemical energy

• To convert chemical energy into electrical energy

• To store electrical energy for later use

• To neutralize acids and bases

In a galvanic cell, where does oxidation occur?

• At the cathode

• In the salt bridge

• At the anode

• In the external circuit

Match each term about the voltaic cell to its definition.

• Salt Bridge

• Electrode where oxidation occurs; loses electrons.

• Anode

• Pathway through which electrons flow from anode to cathode.

• Cathode

• Electrode where reduction occurs; gains electrons.

• External Circuit

• Allows ion flow to maintain electrical neutrality between half-cells.

Answer explanation

In a voltaic cell, electrons move from ​ (a)   to ​ (b)   through the external circuit.

• anode
• cathode

​ (a)   is the gain of electrons, while ​ (b)   is the loss of electrons.

In the zinc and copper voltage cell example, the reduction occurs on the ​ (c)   side, while the oxidation occurs on the ​ (d)   side.

• Oxidation
• Reduction
• cathode
• anode

What role does the salt bridge play in a galvanic cell?

• It generates electrons for the circuit

• It prevents the flow of ions between half-cells

• It allows the flow of electrons between electrodes

• It maintains electrical neutrality by allowing ion flow

Which direction do electrons flow in a galvanic cell?

• From cathode to anode

• From anode to cathode

• From salt bridge to anode

• From electrolyte to salt bridge

What occurs at the cathode in a galvanic cell?

• Oxidation

• Reduction

• Electron loss

• Ionization

What is the function of the external circuit in a galvanic cell?

• To allow ions to move between half-cells

• To enable electron flow from anode to cathode

• To mix the solutions in both half-cells

• To maintain the temperature of the cell