CHEM 2510 / Topic 9: Electrochemistry

How do you balance chemical equations?

• Assign variables to coefficients.

• Write down equations based on atom conservation.

• Assign one of the coefficients to 1 and solve the system.

• Adjust coefficient to make sure all of them are integers. Multiply all coefficients by some number to arrive at the balanced equation with integer coefficients.

What’s the procedure for the half-reaction method of balancing aqueous redox equations?

• Assign oxistates.

• Sep overall rxn into two half-reactions.

  • Bal all elements other than H and O.

  • Bal O by adding H₂O.

  • Bal H by adding H⁺.

  • (Basic) Neutralize H⁺ by adding enough OH^- to neutralize each hydrogen ion. Add the same number of hydroxide ions to each side of the equation.

• Balance each half-reaction with respect to its charge by adding electrons.

• Make the number of electrons in both half-reactions equal by multiplying one or both half reactions by a small whole number.

• Add the two half-reactions together.

• Verify if balanced.

What is electric charge (q)?

q=\left(n\right)\left(N\right)\left(F\right)

amount of charge transferred.

What is electric current (A)?

A=\frac{C}{\sec}=\frac{V}{\Omega}

amount of charge flowing through a circuit per unit time.

(A ≈ V ; A ≈ 1/R)

• What creates an electric potential?

• What is voltage?

• What does a high voltage indicate?

V=\frac{J}{C}

• electric potential: presence of electric charges that causes attraction/repelling.

• V: electric potential difference b/w two pts / work per unit charge needed to be done when one charge moves from one pt to another.

• high V: ↑ potential diff b/w two pts ↑ push strength on charged particles travelling b/w those pts

What is power?

P=\frac{work}{time}=E\times I

work done per unit time.

• What type of reaction is used to produce electricity in a voltaic cell?

• What energy conversion occurs in a voltaic cell due to differences in chemical potential?

• spont rxn.

• Diff in chemical potential b/w higher energy reactants and lower energy products → electrical energy.

• What type of reaction is driven by electricity in a electrolytic cell?

• What energy conversion occurs in a electrolytic cell due to differences in chemical potential?

• nonspont rxn.

• Electrical energy from power supply converts lower-energy reactants to higher-energy products.

What happens to electrons when half-reactions are physically separated and connected by an external circuit?

• e^- travel thru circuit.

• Movement forms an electrical current.

Where are the components of each half-reaction placed in a voltaic cell?

In separate containers called half-cells → each half-cell has an electrode in an electrolyte sol (solution containing mobile ions).

How are the two half-cells connected in a voltaic cell?

By an ext circuit → consists of a wire and a salt bridge.

What happens if there is no salt bridge?

• Charge builds up in each half-cell

• One side becomes too +, the other too −

• Electron flow stops

• Cell stops working

• By convention, how are the anode and cathode arranged in a voltaic cell diagram?

• What does the double vertical line || represent?

• What does a single vertical line | represent within a half-cell?

• How are species ordered in each half-cell in cell notation?

• Anode (oxidation) on the left and cathode (reduction) on the right.

• Sep of two half-cells, repping wire + salt bridge.

• A phase boundary.

• Written in the same order as the half-reaction. Electrons appear at the extreme ends.

• What is electrolysis?

• What’s Faraday’s Law of Electrolysis?

• Electrolysis: Process in which a chemical reaction is forced to occur at an electrode by an imposed voltage.

• Law: subst amt prod at each electrode ≈ electric charge amt flowing through cell.

How do you relate mass plated to current in an electrolysis problem?

• Mass → moles substance (÷ FW)

• Moles substance → moles e^- (from balanced ½-rxn)

• Moles e^- → charge (× F)

• Charge ÷ time → current

• I = q / t