Chapter 15 - Standard enthalpy change of a reaction

1
Chapter 15 - Standard enthalpy change of a
reaction
2
Standard Enthalpy Changes Hana Amir and Madeley
3
Definition

• Any reaction that depends on temperature,
  pressure and state
• The enthalpy change happens when all reactants
  and products are in their standard state.

4
Some enthalpies

• Enthalpy of reaction
• Enthalpy of formation
• Enthalpy of neutralization
• Enthalpy of hydration
• Enthalpy of combustion
• Enthalpy of solution
• Enthalpy of atomization

5
Standard Enthalpy Changes of Reaction

• Definition
• The heat change when molar quantities of
  reactants as specified by the chemical equation
  react to form products at standard conditions
• It depends in the physical state of reactants and
  the products and the conditions under which the
  reaction occurs.
• Standard conditions are 298K (25o C) and
  1.00105 Pa

6
Standard Enthalpy change of Formation

• Enthalpy change that occurs when one mole of
  substance is formed from its elements in the
  standard state under standard conditions.
• Standard conditions
• Temperature 298K (25o C)
• Pressure 1.00105 Pa
• 2C(graphite) 3H2(g) 1/2O2(g)-------gt C2H5OH
  (I)

7

• All elements in their standard states (oxygen
  gas, solid carbon in the form of graphite, etc.)
  have a standard enthalpy of formation of zero, as
  there is no change involved.
• Eg
• O(g) O(g) ----gtO2(g)

8
Energy Cycle
9

• From the diagram we get
• The chemical change elements to products
• can either occur directly or indirectly .
• The Total enthalpy change must be the same for
  both routes.
• S?H?f (Products) S?H?f (Reactants)
  ?H?reaction
• This gives the general expression of
• ?H? reaction S?H?f (Products) - S?H?f
  (Reactants)

10
Example

• Calculate the enthalpy change for the reaction
• C3H8(g) 5O2(g)-------- 3CO2(g) 4H2O(g)
• Standard change of formation ?H?f /kJ mol-1
• C3H8(g) -105
• CO2(g) -394
• H2O(l) -286

11

• Steps
• Write down the equation with the corresponding
  enthalpies of formation underneath
• C3H8(g) 5O2(g)-------- 3CO2(g) 4H2O(g)
• (-105) 0 3(-394)
  4(-286)
• Note The standard enthalpies of formation are
  given in per mole , hence, they should be
  multiplied by the numbers of moles in the balance
  equation .

12

• ?H?reaction S?H?f (Products) - S?H?f
  (Reactants)
• ?H?reaction 3(-394) 4(-286) -(-105)
• -2221 KJ mol-1

13
Thermochemical Equations Balanced chemical
equation for a reaction including the enthalpy of
the reaction shown immediately after the
equation. In a thermochemical equation, the
coefficients represent moles and can therefore be
fractional. The following is an example IB Data
booklet gt -227 kj mol -1 Ethanol (C2H5OH) is
made from the elements (C (Graphite)) and
hydrogen (H2(g)) and oxygen (O2(g)) __C
(graphite) __H2 (g) ___O2(g)-----------gt
C2H5OH(I) ?H227kj mol -1 Balance the C, H and
0 2C (graphite) 3H2 (g) 1/2
O2(g)-------------- C2H5OH(I) ?H227 kj mol -1
14
Questions!
1 .Use the table of standard enthalpies of
formation at 25C to calculate enthalpy change
for the reaction 4NH3(g) 5O2(g) ? 6H2O(g)
4NO(g)
2 .Write the thermodynamically equation for the
standard enthalpy of formation of propanone
enthalpy change CH3COCH3
15
Answers! 1. 1031.76 kJ mol1 2. 3C (Graphite)
3H2(g) 1/2O2(g)-----------CH3COCH3
16
Standard enthalpy change of Combustion
17
What is standard enthalpy change of combustion?

• The standard enthalpy of combustion is the
  enthalpy change that occurs when one mole of
  substance burns completely under the standard
  conditions of 25 ? and 1 atm.
• Eg C6H14(l) 9O2(g) 6CO2(g)
  7H2O(l)
• The standard enthalpy of combustion is always
  negative

18
Exercise!

• Write down the enthalpy of combustion equations
  for the following reactions!
• Methane
• CH4 (g) 2O2(g) CO2(g) 2H2O(l)
• Ethanol
• C2H5OH(l) 3O2(g) CO2(g)
  2H2O(l)
• Propane
• C3H8(g) 5O2(g) 3CO2(g)
  4H2O(l)

19
Calculating standard enthalpy change

• ?H?reaction

Products
Reactants

• ?H?c(products)

• ?H?c(reactants)

Combustion Products
S?H?c(reactants) S?H?(products) ?H ?H
S?H?c(reactants) - S?H?c(products)
20
Question!
May 2010 Paper 1 TZ 2B
21
Question!

• Give an equation for the formation of glucose.
• 6C(graphite) 6H2 (g) 3O2(g)
  C6H12O6(s)
• Calculate the enthalpy of formation of glucose
• C ?H? -394 Kjmol-1 H2?H? -286
  Kjmol-1
• C6H12O6 ?H? -2803 Kjmol-1
• ?H S?H?c(reactants) - ?H?c(products)
• ?H ( 6(-394) 6(-286) 3(0) ) - (-2803)
• -1277 Kjmol-1

22
Question!

• Calculate the enthalpy change for the following
  reaction!
• C(s, graphite) C(s, diamond)
• C(s, graphite) O2(g) CO2(g) ?H?
  -393 Kjmol-1
• C(s, diamond) O2(g) CO2(g) ?H?
  -395 Kjmol-1
• Solution

?H?
C(s, graphite) O2(g)
C(s, diamond) O2(g)
-395 Kjmol-1
-393 Kjmol-1
CO2(g)
-393 Kjmol-1 -395 Kjmol-1 ?H? ?H 2
Kjmol-1
23
Question
May 2008 Paper 1 TZ 1A
24
Question
Nov 2007 Paper 1 D
25
Comparison
Standard enthalpy of combustion
Standard enthalpy of formation

• ?H?reaction

Products
Reactants

• ?H?c(reactants)

• ?H?c(products)

Combustion Products
?H S?H?c(reactants) - ?H?c(products)
Think First Price
Its not CPR in chem! Its CRP!
26
Question!
May 2010 Paper 1 TZ 2A
27
Questions!
May 2010 Paper 1 TZ1A
May 2010 Paper 1 TZ1C
28
Question!
Nov 2009 Paper 1 TZ1C
Answers to all MCQ questions is the last letter
in the identification of the paper from which the
question was taken! )