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-Types of Redox Reactions- Balancing Redox Reactions

Mr. Shields Regents Chemistry U14 L02

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Types of Redox ReactionsTypes of Redox Reactions

We’ve talked about 5 different types of chemical reactionsBefore:

1)Synthesis2)Decomposition3)Combustion4)Single Replacement5)Double Repalcement

The first 4 of these reaction types are examples of RedoxReactions. Why isn’t double replacement an example of aRedox Reaction?

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In double replacement two ions simply replace each otherIn the compounds undergoing reaction. There is no REDOXOccuring.

For example: BaCl2 + AgNO3 AgCl + Ba(NO3)2

What’s the oxidation nos. of the reactants and ProductsIn the equation above?

Ba+2 Ba+2Cl-1 Cl-1Ag+1 Ag+1N+5 N+5O-2 O-2

Has any oxidation or reduction taken place?

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SynthesisSynthesis

In Synthesis, elements or compounds combine to form One product.

Example: N2 + 3H2 2NH3

This is a Redox Reaction. What is being Oxidized and What’s being Reduced?

N0 and H0 change to N-3 and H+1. Nitrogen has been reducedAnd Hydrogen has been oxidized

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DecompositionDecomposition

In Decomposition a compound is broken down into simplerCompounds and/or elements.

Example: 2HgO 2Hg + O2

This too is a Redox reaction. What is being Oxidized & What’s Reduced?

Hg+2 and O-2 become Hg0 and O0. Hg+2 has been reducedAnd O-2 has been oxidized.

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CombustionCombustion

In Combustion a compound reacts with oxygen to formseveral compounds, if combustion is complete the productsAre always water and carbon dioxide.

Example: CH4 + 2O2 2H20 + CO2 + Heat

Again, this is a Redox reaction. What’s being Oxidized &What is being Reduced?

C-4 and O0 become C+4 and O-2. Carbon-4 has been oxidizedAnd O0 has been reduced.

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Single ReplacementSingle Replacement

In Single Repalcement rxns a free element replaces anotherElement that is combined in a compound.

Example: Zn + Cu(NO3)2 Zn(NO3)2 + Cu

Since this is a Redox rxn what is being Oxidized & Reduced?

Zn0 and Cu+2 become Zn+2 and Cu0 . Zinc (0) has been Oxidized and Cu (+2) has been reduced.

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Remember when we’re discussing single replacement rxnsWe need to consider what their position is relative to oneAnother on TABLE J

Metals higher on theList will replace metalsLower on the list.

Metals higher on theList are more reactive

And lastly, metalsHigher on the list areMore easily oxidized.

Notice that halogens accept electrons so higher on the listmeans more easily reduced!

Most easily oxidized

Less easily oxidized

Most easily reduced

Leasteasily reduced

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Problem: in the following reaction

Sr + FeCL2 SrCl2 + Fe

What gets oxidized?

What gets reduced?

What is the oxidizing agent?

What is the reducing agent?

Write the Oxidation & Reduction half cell rxns

Sro

Iron +2

Iron +2

Sro

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Balancing Redox EqsBalancing Redox Eqs

We’re now going to return to a topic we’ve discussedBefore… BALANCING CEHEMICAL EQUATIONS

For example: Fe + O2 Fe2O3

1) 1st we count atoms on the reactant side and then on the Product side

#R atom #P1 Fe 22 O 3

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Balancing Redox EqsBalancing Redox Eqs2) We then pick one atom to balance, let’s say oxygen

#R atom #P1 Fe 2

3x 2 O 3 x2

Then see what we’ve changed: Fe + 3O2 2Fe2O3

3) After we’ve adjusted our table we then balance the next atom, Fe

#R atom #P4x 1 Fe 4

6 O 64Fe + 3O2 2Fe2O3

balanced

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Balancing Redox RxnsBalancing Redox RxnsMany reactions can be balanced “visually” orusing the RAP Method we just reviewed.

However, many redox reactions are difficult to balanceUsing these methods.

Consider the difficulty of balancing this REDOX equation:

S + HNO3 SO2 + NO + H2O

We could balance it with RAP tables but it would be difficult

To more easily balance this equation we need to applyA method that will utilize oxidation numbers.

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Balancing Redox RxnsBalancing Redox RxnsThere are some rules you need to follow to balance Redoxreactions. By balancing reactions this way we will not onlyBalance the equation but we will BALANCE THE HALF CELLS

1)The number of e- lost in an oxidation process must equal the number of electrons gained in the reduction process

2) Assign oxidation numbers to all atoms in the chem. Eq.

3) Identify which reactants are oxidized & which are reduced

0 +1 +5 -2 +4 -2 +2 -2 +1 -2 S + HNO3 SO2 + NO + H2O

oxidized reduced

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Balancing Redox RxnsBalancing Redox Rxns

4) Visually balance any atoms that change from ions to atoms or vice versa

EX: Cr + Cl2 CrCl3

becomes Cr + 3Cl2 2CrCl3

And

NH3 + O2 N2 +H20

becomes 2NH3 + O2 N2 + 2H2O

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Balancing Redox RxnsBalancing Redox Rxns

5) Connect the atoms that change oxidation number by a line

6) Write the change in electrons on the line

0 +1 +5 -2 +4 -2 +2 -2 +1 -2 S + HNO3 SO2 + NO + H2O

Ox. -4 (e-)

Red +3 (e-)

Note that these brackets define the 2 half cell reactions

S0 S+4 + 4e- and N+5 + 3e- N+2

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Balancing Redox RxnsBalancing Redox Rxns7) Multiply the change in electrons by a number that makesthe # of ox. electrons equal to the # of red. electrons

Ox. -4 (e-) x 3

Red +3 (e-) x4

0 +1 +5 -2 +4 -2 +2 -2 +1 -2 S + HNO3 SO2 + NO + H2O

8) These nos. become the coefficients in the eq.

3S + 4HNO3 3SO2 + 4NO + H2O

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Balancing Redox RxnsBalancing Redox Rxns8) Balance the rest of the equation by visual inspection or RAP tables

3S + 4HNO3 3SO2 + 4NO + H2O

Need 4 H on product side

Is oxygen also balanced?

Yes. There are 12 O on each side of the Eq.This equation is now balanced.

Increase H to 4

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Problem: Balance the following equation!

Ag + HNO3 AgNO3 + NO + H20

Solution:

0 +1 +5 -2 +1 +5 -2 +2 -2 +1 -2Ag + HNO3 AgNO3 + NO + H20

Ox -1 e- x3

Red. +3 e- x1

3Ag + HNO3 3AgNO3 + NO + H20

1)

2)

3) 3Ag + 4HNO3 3AgNO3 + NO + 2H20

Visually or using RAP tables Balance the Rest of the Eq.

1) I Need 4 N 2) SO… I need 4 H Is “O” OK?

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0 +1 +5 -2 +1 +5 -2 +2 -2 +1 -2Ag + HNO3 AgNO3 + NO + H20

Ox -1 e- x3

Red. +3 e- x1

3Ag + 4HNO3 3AgNO3 + NO + 2H20

Notice that the half cell reactions are now alsobalanced

Balanced half cell reactions:3Ag0 3Ag+1 + 3e-

1 N+5 + 3e- 1 N+2

Unbalanced half cell reactions:Ag0 Ag+1 + 1e-

N+5 + 3e- N+2

# of e- lostAnd gained Are equal

=

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Conservation of Mass & ChargeConservation of Mass & Charge

Remember !!Remember !!

In ALL chemical reactions there is not only In ALL chemical reactions there is not only conservation of conservation of massmass

- - Equal number of atoms on both sidesEqual number of atoms on both sides

There must also be There must also be conservation of conservation of chargecharge

- - Number of e- lost = Number of e- gainedNumber of e- lost = Number of e- gained