Calculating Enthalpy

Question 1

Hess’ Law

Answer 1

• The enthalpy change for a reaction is INDEPENDENT of the route taken
• Provided that the INITIAL and FINAL STATES of the reactants and products are the SAME

Question 2

When is Hess’ Law used?

Answer 2

When the standard enthalpy change of reaction CANNOT be measured by EXPERIMENT because:

• Activation energy is too high
• Reaction rate is too low
• Multiple reactions taking place at the
  same time

Question 3

STEPS for calculating the standard enthalpy change of REACTION using the standard enthalpy change of FORMATION

Answer 3

• Write BALANCED equation for the reaction
• Write the ELEMENTS under the equation, include STATE symbols
• Draw ARROWS from the elements to the REACTANTS and PRODUCTS.
• SUBSTITUTE standard enthalpy change of formation value into this formula:

Standard enthalpy change of formation (PRODUCTS) - Standard enthalpy change of formation (REACTANTS)

Question 4

STEPS for calculating the standard enthalpy change of REACTION using the standard enthalpy change of COMBUSTION

Answer 4

• Write BALANCED equation for the reaction
• Write the COMBUSTION PRODUCTS under the equation, include STATE symbols
• Draw arrows from the REACTANTS and PRODUCTS to the combustion products
• BALANCE the combustion products and ADD OXYGEN to balance the Hess cycle
• SUBSTITUTE standard enthalpy change of combustion values into formula:

Standard enthalpy change of combustion (REACTANTS) - Standard enthalpy change of combustion (PRODUCTS)

Question 5

Breaking and making bonds in chemical reactions

Answer 5

• Energy is required to BREAK bonds in the reactants ~ ENDOTHERMIC PROCESS
• Energy is given out when bonds are FORMED in the products ~ EXOTHERMIC PROCESS

Question 6

Is the reaction exothermic or endothermic overall?

Answer 6

EXOTHERMIC ~ the energy required to breaks the bonds in the reactants is LESS than the energy released when the products are formed

ENDOTHERMIC ~ the energy required to break the bonds in the reactants is MORE than the energy releases when the products are formed.

Question 7

Bond enthalpy

Answer 7

The AVERAGE amount of energy required …

to break ONE MOLE of a particular covalent bond …

between two atoms in a GASEOUS molecules

Question 8

Bond enthalpy values

Answer 8

• AVERAGED over several different compounds

C-H bond enthalpy :
- Depends on the chemical environment
- Differs in an alkane , alkene, ester & alcohol

• Enthalpy changes calculated using bond enthalpies are an APPROXIMATION
• Calculations using average bond enthalpies requires all species to be in the GASEOUS STATE

-The enthalpy change calculated will NOT be STANDARD VALUES

• UNLESS the standard state of both reactants and products is gaseous

Question 9

STEPS for calculating standard enthalpy change of REACTION using BOND ENTHALPIES, E

Answer 9

1. Write the BALANCED EQUATION and the DISPALYED formula for each substance
2. Calculate the energy required to break all the bonds in the REACTANTS
3. Calculate the energy required to break all the bond in the PRODUCTS
4. Use equation:

Bond enthalpies in reactants - bond enthalpies in products