Hess

1
Hesss LawandEnthalpies of Formation

• Sections 5.6-5.7

2
Objectives

• Establish standard values for enthalpy changes in
  chemical reactions
• Use these standards to calculate enthalpy changes
  for reactions

3
PA Academic Standards

• 3.4.12
• Apply and analyze energy sources and conversions
  and their relationship to heat and
  temperature.  Determine the heat involved in
  illustrative chemical reactions.  Evaluate
  mathematical formulas that calculate the
  efficiency of specific chemical and mechanical
  systems.

4
Key Terms

• Enthalpy of formation
• Standard enthalpy change
• Standard enthalpy of formation

• Enthalpy
• State Function
• Hesss Law

5
Hesss Law

• ?Hs have been measured and recorded for many
  reactions
• We can estimate ?H for a given reaction from the
  published ?H of other reactions
• Forgo calorimetric measurements

6
Hesss Law

• Hesss law If a reaction is carried out in a
  series of steps, ?H for the overall reaction will
  be equal to the sum of the enthalpy changes for
  the individual steps.

7
Hesss Law

• Because ?H is a state function, the total
  enthalpy change depends only on the initial state
  of the reactants and the final state of the
  products.
• The in between does not matter. Thus the steps
  are additive.

8
Hesss Law
9
Example

• C O2 ? CO2
• Publications tell us that this can occur as 2
  steps
• C ½O2 ? CO ?H? 110.5 kJ
• CO ½O2 ? CO2 ?H? 283.0 kJ

C CO O2 ? CO CO2 ?H? 393.5 kJ

• C O2 ? CO2 ?H? 393.5 kJ
• Hesss law allows us to add equations.
• We add all reactants, products, ?H? values

10
Tabulated Enthalpies

• Experimental data is grouped according to the
  type of process
• Examples
• Enthalpies of vaporization
• Enthalpies of fusion
• Enthalpies of combustion
• Enthalpies of formation

11
Enthalpies of Formation ?Hf

• Enthalpy change for the reaction in which a
  compound is made from its constituent elements in
  their elemental forms.
• Subscript f indicates substance has been formed

12
Magnitude of Enthalpy

• Temperature
• Pressure
• State (gas, liquid, solid) of reactants and
  products
• In order to compare enthalpies, a set of
  conditions for tabulation must be defined
  standard state

13
Standard State
?

• Standard enthalpies of formation, ?Hf, are
  measured under standard conditions (25C and 1.00
  atm pressure).

14
Standard Enthalpy of Change

• Enthalpy change when all reactants and products
  are in their standard states
• ?H

15
Standard Enthalpy of Formation

• Change in enthalpy for the reaction that forms
  one mole of the compound from its elements with
  all substances in their standard states
• Usually reported at 298 K
• ?Hf

16
Standard Enthalpy of Formation

• Standard enthalpy of formation of the most stable
  form of any element is zero because there is no
  formation reaction needed when element is in its
  standard state

17
(No Transcript)
18
PRACTICE EXERCISE Write the equation
corresponding to the standard enthalpy of
formation of liquid carbon tetrachloride (CCl4).
19
Calculation of ?H
C3H8 (g) 5 O2 (g) ?? 3 CO2 (g) 4 H2O (l)

• Imagine this as occurring
• in 3 steps

C3H8 (g) ?? 3 C 4 H2 (g)
3 C 3 O2 (g) ?? 3 CO2 (g) 4 H2 (g) 2 O2 (g)
?? 4 H2O (l)
20
Calculation of ?H
C3H8 (g) 5 O2 (g) ?? 3 CO2 (g) 4 H2O (l)

• The sum of these equations is

C3H8 (g) ?? 3 C(graphite) 4 H2 (g) 3
C(graphite) 3 O2 (g) ?? 3 CO2 (g) 4 H2 (g) 2
O2 (g) ?? 4 H2O (l)
C3H8 (g) 5 O2 (g) ?? 3 CO2 (g) 4 H2O (l)
21
Calculation of ?H

• We can use Hesss law in this way
• ?H ??n??Hf(products) -??m??Hf(reactants)
• where n and m are the stoichiometric
  coefficients.

?
?
22
Calculation of ?H
C3H8 (g) 5 O2 (g) ?? 3 CO2 (g) 4 H2O (l)

• ??????H 3(-393.5 kJ) 4(-285.8 kJ) -
  1(-103.85 kJ) 5(0 kJ)
• (-1180.5 kJ) (-1143.2 kJ) - (-103.85
  kJ) (0 kJ)
• (-2323.7 kJ) - (-103.85 kJ)
• -2219.9 kJ