Using Hess' Law

Question 1

What do you calculate using hess’ law cycles

Answer 1

ΔHr from ΔHf

Question 2

The products can be directly formed from the elements = ……….
OR

The products can be indirectly formed from the elements = ………….

Answer 2

ΔH2

ΔH1 + ΔHr

Question 3

The enthalpy change from elements to products (direct route) is ……………….to the enthalpy change of elements forming reactants and then products (indirect route)

Answer 3

equal

Question 4

equation

Answer 4

ΔH2 = ΔH1 + ΔHr

Therefore,

ΔHr = ΔH2 – ΔH1

PRACTICE THIS

Question 5

STEPS

Answer 5

Step 1: Write the balanced equation at the top

Step 2: Draw the cycle with the elements at the bottom

Step 3: Draw in all arrows, making sure they go in the correct directions. Write the standard enthalpy of formations

Step 4: Apply Hess’s Law

Question 6

Calculating average bond energies using Hess’s cycles
Bond energies cannot be found directly so

Answer 6

enthalpy cycles are used to find the average bond energy

Question 7

Bond energies cannot be found directly so enthalpy cycles are used to find the average bond energy
This can be done using

Answer 7

enthalpy changes of atomisation and combustion or formation

Question 8

The enthalpy change of atomisation (ΔHatꝋ ) is

Answer 8

the enthalpy change when one mole of gaseous atoms is formed from its elements under standard conditions.
Eg. ΔHatꝋ [H2] relates to the equation:
½ H2(g) → H(g)

Question 9

enthalpy change of atomisation symbol

Answer 9

ΔHatꝋ

Question 10

PRACTICE QUESTIOND

Question 11

Take into account the number of moles of each reactant and product.

For example, there are two moles of NaHCO3(s) so the ΔHf value is multiplied by 2.