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With the Law of Conservation of Energy, it doesn’t matter if the reaction is one step or several. The overall change in energy will be equal.

Note to Self: AP will not give you the steps in order or in the correct direction…

So we’ll need to manipulate reaction steps (reverse, multiply, etc) in order to get the steps of the reaction to match the overall reaction.

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Here is an example!

How to approach these problems:

1) Match with the overall equation

2) Manipulate as needed (multiply, reverse, etc.)

3) Rewrite

4) Simplify

5) Double-Check

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Here is an example!

How to approach these problems:

1) Match with the overall equation

2) Manipulate as needed (multiply, reverse, etc.)

3) Rewrite

4) Simplify

5) Double-Check

Manipulate Reaction 2: Reverse the Reaction, Multiply Everything by 2, Reverse the sign of ΔH

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4 NO_2(g) → 4 NO_(g) + 2 O_2(g)

ΔH = -2 (-114.2 kJ)

Rxn # 2

Time to Rewrite the Equations

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Here is an example!

How to approach these problems:

1) Match with the overall equation

2) Manipulate as needed (multiply, reverse, etc.)

3) Rewrite

4) Simplify

5) Double-Check

Hmmm… Oxygen’s in two spots - Save for last!

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Here is an example!

How to approach these problems:

1) Match with the overall equation

2) Manipulate as needed (multiply, reverse, etc.)

3) Rewrite

4) Simplify

5) Double-Check

Manipulate Reaction 1: Multiply Everything by 2

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4 NO_2(g) → 4 NO_(g) + 2 O_2(g)

ΔH = -2 (-114.2 kJ)

Rxn # 2

Time to Rewrite the Equations

4 NO_(g) + 3 O_2(g) → 2 N₂O_5(s)

ΔH = 2 (-223.7 kJ)

Rxn # 1

4 NO_2(g) + O_2(g) → 2 N₂O_5(s)

ΔH = -219.0 kJ

Final

Here is an example!

How to approach these problems:

1) Match with the overall equation

2) Manipulate as needed (multiply, reverse, etc.)

3) Rewrite

4) Simplify

5) Double-Check

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Here is an example!

How to approach these problems:

1) Match with the overall equation

2) Manipulate as needed (multiply, reverse, etc.)

3) Rewrite

4) Simplify

5) Double-Check

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Here is an example!

How to approach these problems:

1) Match with the overall equation

2) Manipulate as needed (multiply, reverse, etc.)

3) Rewrite

4) Simplify

5) Double-Check

Manipulate Reaction 3: Multiply Everything by 3

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3 C_(s) + 3 O_2(g) → 3 CO_2(g)

ΔH = 3(-393.5 kJ)

Rxn #3

Time to Rewrite the Equations

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Here is an example!

How to approach these problems:

1) Match with the overall equation

2) Manipulate as needed (multiply, reverse, etc.)

3) Rewrite

4) Simplify

5) Double-Check

Manipulate Reaction 1: Multiply Everything by 2

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3 C_(s) + 3 O_2(g) → 3 CO_2(g)

ΔH = 3(-393.5 kJ)

Rxn #3

Time to Rewrite the Equations

4 H_2(g) + 2 O_2(g) → 4 H₂O_(l)

ΔH = 2(-571.7 kJ)

Rxn #1

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Here is an example!

How to approach these problems:

1) Match with the overall equation

2) Manipulate as needed (multiply, reverse, etc.)

3) Rewrite

4) Simplify

5) Double-Check

Manipulate Reaction 2: Reverse and Change Sign of ΔH

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3 C_(s) + 3 O_2(g) → 3 CO_2(g)

ΔH = 3(-393.5 kJ)

Rxn #3

Time to Rewrite the Equations

4 H_2(g) + 2 O_2(g) → 4 H₂O_(l)

ΔH = 2(-571.7 kJ)

Rxn #1

3CO_2(g) + 4H₂O_(l) → C₃H_8(g) + 5O_2(g)

ΔH = -1(-2220.1 kJ)

Rxn #2

3 C_(s) + 4 H_2(g) → C₃H_8(g)

ΔH = -103.8 kJ

Final

Here is an example!

How to approach these problems:

1) Match with the overall equation

2) Manipulate as needed (multiply, reverse, etc.)

3) Rewrite

4) Simplify

5) Double-Check

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Hold On!

What do we do if we aren’t given an overall reaction?

We look for clues and get a rough idea of what the overall equation is...

C_(s) + O_2(g) → CO_2(g)

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Hold On!

What do we do if we aren’t given an overall reaction?

We look for clues and get a rough idea of what the overall equation is…

Basic rearrangement shows us that oxygen will cancel out because it is a reactant and product in the two reactions.

C_(s) + O_2(g) → CO_2(g)

Fe₂O_3(s) → 2Fe_(s) + 3/2O_2(g)

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Hold On!

What do we do if we aren’t given an overall reaction?

We look for clues and get a rough idea of what the overall equation is…

After we get the rough draft of the overall equation, we can balance it.

C_(s) + O_2(g) → CO_2(g)

Fe₂O_3(s) → 2Fe_(s) + 3/2O_2(g)

C_(s)+ Fe₂O_3(s) → 2 Fe_(s) + CO_2(g)

3C_(s)+ 2Fe₂O_3(s) → 4 Fe_(s) + 3CO_2(g)

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How to approach these problems:

1) Match with the overall equation

2) Manipulate as needed (multiply, reverse, etc.)

3) Rewrite

4) Simplify

5) Double-Check

Reaction #2 - Multiply everything by three

3C_(s)+ 2Fe₂O_3(s) → 4 Fe_(s) + 3CO_2(g)

3 C_(s) + 3 O_2(g) → 3 CO_2(g)

ΔH = 3(-394 kJ)

Rxn #2

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How to approach these problems:

1) Match with the overall equation

2) Manipulate as needed (multiply, reverse, etc.)

3) Rewrite

4) Simplify

5) Double-Check

Reaction #2 - Multiply everything by three

Reaction #1 - Reverse, multiply everything by 2 and change the sign of ΔH

3C_(s)+ 2Fe₂O_3(s) → 4 Fe_(s) + 3CO_2(g)

3 C_(s) + 3 O_2(g) → 3 CO_2(g)

ΔH = 3(-394 kJ)

Rxn #2

2 Fe₂O_3(s) → 4 Fe_(s) + 3 O_2(g)

ΔH = -2(-822 kJ)

Rxn #1

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How to approach these problems:

1) Match with the overall equation

2) Manipulate as needed (multiply, reverse, etc.)

3) Rewrite

4) Simplify

5) Double-Check

Reaction #2 - Multiply everything by three

Reaction #1 - Reverse, multiply everything by 2 and change the sign of ΔH

3C_(s)+ 2Fe₂O_3(s) → 4 Fe_(s) + 3CO_2(g)

3 C_(s) + 3 O_2(g) → 3 CO_2(g)

ΔH = 3(-394 kJ)

Rxn #2

2 Fe₂O_3(s) → 4 Fe_(s) + 3 O_2(g)

ΔH = -2(-822 kJ)

Rxn #1

3C_(s)+ 2Fe₂O_3(s)→ 4Fe_(s)+3CO_2(g)

ΔH = +462 kJ

Final

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Learning Goals

6.9

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Homework

1 - You can also divide reactions to get the numbers to work out