7.2 – RECOGNIZING REDOX REACTIONS

UNIT 7 – REDOX REACTIONS & ELECTROCHEMISTRY

EXAMPLE 1:

• Oxidation numbers are a convenient way of identifying redox reactions and also indicating which element is oxidized and which is reduced. Here's an example - the reaction between sodium metal and chlorine gas:

2 Na + Cl2 → 2 NaCl

• It is often useful to write the oxidation number for every element, in every compound, above the element in the equation. Thus for our reaction we have:

0 0 +1 -1

2Na + Cl2 → 2 NaCl

• Be sure to note that the balancing coefficients in the equation (the "2" in front of Na and in front of NaCl) do not affect the value of the oxidation numbers. We'll return to these coefficients soon.

A CHART IS A USEFUL WAY FOR US TO SUMMARIZE THE CHANGES IN OXIDATION NUMBER FOR EACH ELEMENT:

element initialox no final

ox nochange inelectrons

(e-)oxidized or

reduced

Na 0 → +1 lost 1 e- oxidized

Cl 0 → -1 gain 1 e- reduced

WE SEE SEVERAL IMPORTANT THINGS IN OUR TABLE:

• Since oxidation numbers did change, this was a redox reaction

• Na's oxidation number increased - from 0 on the reactant side to +1 on the product side. An element becomes more positive by losing electrons.

• Loss of electrons is Oxidation (LEO)

• Cl's oxidation number decreased, from 0 to -1, as chlorine gained electrons.

• Gain of electrons is Reduction (GER)

TO SUMMARIZE:

• An increase in oxidation number indicates oxidation

• A decrease in oxidation number indicates reduction

EXAMPLE 2:

• Consider the reaction

2 Mg + O2 → 2 MgO

• Determine oxidation numbers for all elements in every compound:

0 0 +2 -2

2 Mg + O2 → 2 MgO

• Summarize the changes, determine the number of electrons transferred per atom, and identify what is oxidized and what is reduced:

EXAMPLE 2:

element initialox no final

ox nochange inelectrons

(e-)oxidized or

reduced

Mg 0 → +2 lost 2e- oxidized

O 0 → -2 gain 2e- reduced

OXIDATION & REDUCTION AGENTS

• Two new terms before we continue. You will recall that we mentioned in the first section of this unit that oxidation cannot occur without reduction, and vice versa.

• Reducing agent

• the substance that is oxidized.It allows another element to be reduced.

• Oxidizing agent

• the substance that is reduced.It allows another element to be oxidized.

• In our last example

2 Mg + O2 → 2 MgO

• Magnesium was oxidized, therefore it was the reducing agent.

• Oxygen was reduced, therefore it was the oxidizing agent.

OXIDATION & REDUCTION AGENTS – EXAMPLE 3

• By convention we often refer to the oxidizing agent and reducing agents as the entire compound the element is in, not just individual element. Consider the following reaction. Oxidation numbers are shown only for substances whose oxidation numbers undergo a change:

0 +5 (N) +2 (Zn) +4 (N)

Zn + HNO3 → Zn(NO3)2 + NO2 + H2O

SUMMARIZE:

• We see that HNO3 is referred to as the oxidizing agent, not just N.

• One final thing to note in this example - notice that the nitrogen in Zn(NO3)2 did not undergo a change in oxidation number.

element initialox no final

ox no e-oxidized

orreduced

Agent

Zn 0 → +2 lost 2e- oxidized reducing agent: Zn

N +5 → +4 gain 1e- reducedoxidizing

agent: HNO3

EXAMPLE 4:

• Consider the reaction

N2 + 2H2 → 2 NH3

• Determine oxidation numbers for all elements in every compound:

0 0 -3+1

N2 + 2H2 → 2 NH3

EXAMPLE 4:

• Summarize the changes, determine the number of electrons transferred per atom, and identify what is oxidized and what is reduced, and identify the oxidizing agent and reducing agent:

element initialox no final

ox no e- oxidized orreduced Agent

N 0 → -3 gain 3e- reduced oxidizing agent: N2

H 0 → +1 lose 1e- oxidized reducing agent: H2