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Bomb Calorimetry (Constant Volume Calorimetry). Reaction carried out under constant volume. Use a bomb calorimeter. Usually study combustion. Example - PowerPoint PPT Presentation
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TCq calrxn
• Reaction carried out under constant volume.
• Use a bomb calorimeter.• Usually study combustion.
Bomb Calorimetry (Constant Volume Calorimetry)
Example
A 0.5269 g sample of octane is placed in a bomb calorimeter with a heat capacity of 11.3 kJ °C-1. The original temperature of the water in the calorimeter is 21.93°C. After combustion, the temperature increases to 24.21°C. Determine the enthalpy of combustion per mole and per gram of octane.
Example
A 0.5269 g sample of octane is placed in a bomb calorimeter with a heat capacity of 11.3 kJ °C-1. The original temperature of the water in the calorimeter is 21.93°C. After combustion, the temperature increases to 24.21°C. Determine the enthalpy of combustion per mole and per gram of octane.
-48.9 kJ g-1
-5590 kJ mol-1
• Hess’s law: if a reaction is carried out in a number of steps, H for the overall reaction is the sum of H for each individual step.
• For example:
CH4(g) + 2O2(g) CO2(g) + 2H2O(g) H = -802 kJ
2H2O(g) 2H2O(l) H = -88 kJ
CH4(g) + 2O2(g) CO2(g) + 2H2O(l) H = -890 kJ
Note that: H1 = H2 + H3
Example
Given:
Fe2O3 (s) + 3 CO (g) 2 Fe (s) + 3 CO2 (g) ΔH = -23 kJ
3 Fe2O3 (s) + CO (g) 2 Fe3O4 (s) + CO2 (g) ΔH = -39 kJ
Fe3O4 (s) + CO (g) 3 FeO (s) + CO2 (g) ΔH =+18 kJ
What is ΔH for the following?
FeO (s) + CO (g) Fe (s) + CO2 (g)
Example
Given:
Fe2O3 (s) + 3 CO (g) 2 Fe (s) + 3 CO2 (g) ΔH = -23 kJ
3 Fe2O3 (s) + CO (g) 2 Fe3O4 (s) + CO2 (g) ΔH = -39 kJ
Fe3O4 (s) + CO (g) 3 FeO (s) + CO2 (g) ΔH =+18 kJ
What is ΔH for the following?
FeO (s) + CO (g) Fe (s) + CO2 (g)
-11 kJ
Example
The bombardier beetle uses an explosive discharge as a defensive measure. The chemical reaction involved is the oxidation of hydroquinone by hydrogen peroxide to produce quinone and water:
C6H4(OH)2 (aq) + H2O2 (aq) C6H4O2 (aq) + 2 H2O (l)
C6H4(OH)2 (aq) C6H4O2 (aq) + H2 (g) ΔH = +177.4 kJ
H2 (g) + O2 (g) H2O2 (aq) ΔH = -191.2 kJ
H2 (g) + ½ O2 (g) H2O (g) ΔH = -241.8 kJ
H2O (g) H2O (l) ΔH = -43.8 kJ
C6H4(OH)2 (aq) + H2O2 (aq) C6H4O2 (aq) + 2 H2O (l)
ΔH = ???
C6H4(OH)2 (aq) C6H4O2 (aq) + H2 (g) ΔH = +177.4 kJ
H2 (g) + O2 (g) H2O2 (aq) ΔH = -191.2 kJ
H2 (g) + ½ O2 (g) H2O (g) ΔH = -241.8 kJ
H2O (g) H2O (l) ΔH = -43.8 kJ
C6H4(OH)2 (aq) + H2O2 (aq) C6H4O2 (aq) + 2 H2O (l)
ΔH = ???
-202.6 kJ
• If 1 mol of compound is formed from its constituent elements, then the enthalpy change for the reaction is called the enthalpy of formation, Ho
f .
• Standard conditions (standard state): 1 atm and 25 oC (298 K).
• Standard enthalpy, Ho, is the enthalpy measured when everything is in its standard state.
• Standard enthalpy of formation: 1 mol of compound is formed from substances in their standard states.
• If there is more than one state for a substance under standard conditions, the more stable one is used.
• Standard enthalpy of formation of the most stable form of an element is zero.
Examples – write formation reactions for each:
1. H2SO4
2. C2H5OH
3. NH4NO3
Examples – write formation reactions for each:
1. H2SO4 H2 (g) + S (s) + 2O2 (g) → H2SO4
2. C2H5OH 2C (s) + 3H2 (g) + 1/2O2 (g) → C2H5OH
3. NH4NO3 N2 (g) + 2H2 (g) + 3/2O2 (g) → NH4NO3
Enthalpies of FormationEnthalpies of Formation
Using Enthalpies of Formation of Calculate Enthalpies of Reaction
• For a reaction
reactantsproductsrxn ff HmHnH
Examples
Use the tables in the back of your book to calculate ΔH for the following reactions:
1. 4 CuO (s) 2 Cu2O (s) + O2 (g)
2. C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
3. NH3 (g) + HCl (g) NH4Cl (s)
Examples
Use the tables in the back of your book to calculate ΔH for the following reactions:
1. 4 CuO (s) 2 Cu2O (s) + O2 (g) 283 kJ
2. C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l) -2220 kJ
1. NH3 (g) + HCl (g) NH4Cl (s) -175.9 kJ
1. Vaporize the metal (enthalpy of vaporization)• Na (s) Na (g)
2. Break diatomic nonmetal molecules (if applicable) (bond enthalpy)
• ½ Cl2 (g) Cl-
3. Remove electron(s) from metal (ionization energy)• Na (g) Na+ (g) + e-
4. Add electron(s) to nonmetal (electron affinity)• Cl (g) + e- Cl- (g)
5. Put ions together to form compound (lattice energy)• Na+ + Cl- NaCl (s)
Born-Haber CycleBorn-Haber CycleApplication of Hess’s LawApplication of Hess’s Law
• Overall Reaction:
– Na (s) + ½ Cl2 (g) NaCl (s)
• This is useful because all quantities are directly measurable except lattice energy. The Born-Haber cycle can be used to calculate lattice energy from the other values.
Step Energetics
1
2
3
4
5
Step Energetics
1 endothermic
2 endothermic
3 endothermic
4 Exothermic
5 EXOTHERMIC (highly)