Unit 6: Electrochemistry Introduction to Redox

Chemistry 12

Chapter 18

What is electrochemistry?

• Electrochemistry it is branch of chemistry that studies : – How chemical reaction can produce electricity

– How electricity can cause chemical reaction

– Spontaneity of reactions

• Electricity or electric current are caused by a flow of electrons thus all reactions involved are characterized by transfer of one or more electrons

Redox Reactions

Some examples are…

• combustion of gas in a car

• rusting of metals

• bleaching hair

• reactions in batteries

• Cut apples turning brown

What do you think Redox stands for?

Historically…

• Oxidation was used to describe the combination of an element with oxygen

– Ex: 4Fe + 3O2 → 2Fe2O3

• Reduction was used to describe the loss of oxygen from a compound

– Ex: 2Fe2O3 + 3C → 4Fe + 3CO2

Redox = oxidation and reduction reactions

• Today it refers to the transfer of electrons

• Oxidation: loss of electrons

• Reduction: gain of electrons

Hint:

Leo the lion says Ger

Example of redox reaction

Na + Cl Na + Cl

Oxidation number

• The oxidation number, or state, of an atom is the charge which that atom would have if all covalent bonds were broken so that the more electronegative element kept all the electrons in the bond

• Use:

– Naming compounds

– Balancing chemical equations

– Determine if redox reaction took place

Oxidation number vs. charge +8

X

X-

• Oxidation numbers – +/- 1 – 8

– +/- I – VIII

Written directly above the symbol for an element.

• Charge – 1 – 8 +/-

Indicated by superscript to the right of the symbol of an element.

Review

• Remember all atoms want to achieve a stable octet by losing or gaining electrons

• Group 1 +1

• Group 2 +2

• Group 3 +3

• Group 4 +/- 4

• Group 5 - 3 (in ionic compounds)

• Group 6 - 2 (in ionic compounds)

• Group 7 - 1 (in ionic compounds)

Rules to Assigned Oxidation Numbers

• 1) The total oxidation number of the atoms in a species is equal to the electrical charges it carries.

2) Some elements almost always have the same ox.# in their compounds, with few exceptions.

Sample problems

a) PF3 b) Na3P c) NO3- d) N2

e) NH4+ f) MnSO4 g) Na2Cr2O7 h) Ba2+

i) CH4 j) NaAl(OH)2CO3 k) CH3OH e) CH2O

List the following oxides of nitrogen in order of decreasing oxidation number of nitrogen:

NO2 N2O5 NO N2O3 N2O N2O4

Predicting Redox Reaction

A redox reaction occurs when there is a change in oxidation numbers for a reaction

Sample problem 1

• Is the following reaction a redox reaction?

KOH(aq) + HBr(aq) → KBr(aq) + H2O (l)

1. Assign ox. # to each element in the rxn.

2. Check if any ox. # has changed.

- no change – it is not redox rxn

- change – it is redox rxn

+1 -2 +1 +1 -1 +1 -1 +1 -2

KOH(aq) + HBr(aq) → KBr(aq) + H2O (l)

There is no change in oxidation number for any element. Therefore, this is not a redox rxn.

Sample problem 2

Is the following reaction a redox reaction?

0 +1 -1 +2 -1 0

Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (l)

Assign ox. # to each element in the rxn.

0 +1 -1 +2 -1 0

Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (l)

Check if any ox. # has changed.

Both Mg and H have changed in their oxidation numbers. Therefore, this is a redox rxn.

• Is the following a redox reaction?

HClClCHClCH 324