3.1.4.3 Applications Of Hess's Law

Question 1

What is Hess’s Law?

Answer 1

The enthalpy change for a given reaction is independent of the route by which the reaction is achieved

Question 2

What is the symbol for enthalpy change?

Answer 2

🔼Hr = enthalpy change 
🔼Hr = 🔼H1 + 🔼H2

Question 3

What are the symbols for endothermic and exothermic reactions?

Answer 3

-Ve = exothermic
\+Ve = endothermic

Question 4

What are the units for enthalpy change?

Answer 4

Kilojoules per mole

Question 5

What is the enthalpy change of formation?

Answer 5

Arrows point upwards
Reactants to products on top row
Elements in their standard state on bottom row
🔼Hr = 🔼Hf (products) -🔼Hf (reactants)

Question 6

What is the enthalpy change of combustion?

Answer 6

Arrows point down
Reactants to products on top row
Combustion products on bottom row
🔼Hr = +🔼Hc (reactants) - 🔼Hc (products)
Multiply 🔼Hf / 🔼Hc by the number of moles in the equation

Question 7

How do you use bond enthalpies to determine enthalpy change?

Answer 7

🔼Hr = positive bond enthalpies (reactants) + negative bond enthalpies (products)

Question 8

How do you work out the enthalpy change in a reaction when given bond enthalpies?

Answer 8

1. Draw the structure of the reaction
2. Total the number of bonds
3. Input the values of bonds
4. (Bond enthalpies of reactants) - (Bond enthalpies of products)