1. Module 3

1.6. Page 4

Lesson 1

Module 3—Electrochemical Reactions

Read

 

Electrochemical change involves a transfer of electrons. In the virtual investigation, “Observing the Reactivity of Zinc,” you saw that silver ions must have gained electrons to become solid silver. How is such a change represented?

 

Read pages 561–564 of the textbook to learn how to represent electrochemical changes using a balanced chemical equation. Carefully work through “Sample problem 13.1” and “Communication example 1” on page 563 to practise using chemical equations to explain electrochemical changes.

 

Remember to use this reading (particularly “Sample problem 13.1”) to check your answers to the analysis questions for the virtual investigation, “Observing the Reactivity of Zinc.” If you identify significant differences between your answers and the textbook reading, you may wish to contact your teacher for help.

 

Half-Reactions

 

In the textbook reading you will have noticed that half-reactions are used to describe the changes observed in chemical systems. Half-reactions are one type of balanced chemical equation. You will recall from previous science courses that balanced chemical equations demonstrate two conservation laws:

  • law of conservation of mass—in any physical or chemical change, the total initial mass of reactant(s) is equal to the total final mass of product(s)
  • law of conservation of charges—in any isolated system, the sum of all of the charges remains constant

Conservation of mass is quite easy to identify in an equation because the type and numbers of each type of atom are the same on both sides of the equation.

For example,

Ag+(aq) + 1 e– → Ag(s)

Reactants

Products

Ag = 1

Ag = 1

 

Conservation of charge involves adding up the charges on each side of the equation. If the net charge on each side is equal, the equation is balanced for charge.

For example,

Cu(s) → Cu2+(aq) + 2 e–

Reactants

Products

Cu = 1

Cu = 1

charge = 0

charge = +2 + (–2) = 0

 

Self-Check

 

SC 1. Complete “Practice” questions 7–11 on page 564 of the textbook.

 

SC 2. Use the table “Colours of Common Aqueous Ions” on page 11 of the Chemistry 30 Data Booklet or a similar table to determine the colour of the resulting solution if nitric acid were reacted with each of the following metals:

  1. nickel
  2. copper
  3. chromium

SC 3. Write balanced half-reactions for the oxidation and reduction that occurred in the virtual investigation “Observing the Reactivity of Zinc.”

 

Check your work.