Lesson 5/6 - Hess' Law #1/2

When Calorimetry Doesn’t work

1. Slow reactions

2. Reactions with multiple products

3. Combustions with bomb calorimeter not available

Two methods to Enthalpy Predictions

1. Heat summation - Hess’ Law

2. Enthalpies of Formation - Hess’ Law

Step 1 of Hess’ Law #1

Write net equation if not given

Step 2 of Hess’ Law #2

Flip equations to yield net equation

Step 3 of Hess’ Law #3

Add coefficients to match net equation

Step 4 of Hess’ Law #4

Determine molar enthalpy required

Yes

Yes or no: If a chemical equation is reversed will the △rH change?

Yes

Yes or No: If coefficients of a chemical equation is altered by multiplying/dividing will the △rH also be altered

Molar Enthalpy of Formation

1. Enthalpy change when one mole of a compound forms from its elements

2. This of an element assumed to be 0 kJ

3. Copy what you see in the data booklet and don’t change this

Meaning of Formula △cH = En△fPHm - EN△fRHm

1. Permanent resident

2. Change of enthalpy is total chemical potential NRG of (products - reactants)

No

Yes or No: When you are doing Hess’ Law #2 should you ever flip the equation?

Molar Enthalpy of Formation

1. Enthalpy change when one mole of a compound forms from its elements

2. Enthalpy of formation of an element assumed to be 0 kJ

3. When trying to find this copy what you see in the data booklet and DONT CHANGE ANYTHING

Meaning of Formula △cH = En△fPHm - EN△fRHm

1. Permanent resident

2. Change of enthalpy is total chemical potential NRG of (products - reactants)

3. NEVER FLIP THE EQUATION

4. Hess’ Law #2