Download Enthalpy of combustion File

Learning objective:
01/05/2017
• To measure enthalpy of combustion of alcohols
• To explain the trend in terms of molecules
• To identify and evaluate sources of error
• Enthalpy = heat energy (H)
• The enthalpy of a substance can’t be measured…
…but the change can be.
system
reactants
and
products
surroundings
the rest of the
universe!
Enthalpy change (ΔH)
Enthalpy of reactants > enthalpy of products
reactants
Enthalpy
(H)
Enthalpy lost to surroundings
Surroundings heat up
EXOTHERMIC
ΔH -ve
products
Enthalpy change (ΔH)
products
Enthalpy of reactants < enthalpy of products
Enthalpy
(H)
Enthalpy taken from surroundings
Surroundings cool down
reactants
ENDOTHERMIC
ΔH +ve
Standard enthalpy of combustion
ΔHcӨ
• The standard enthalpy of combustion is
the enthalpy change when one mole of a
substance completely reacts with oxygen
under standard thermodynamic conditions
• 1 atm and 298K
SAFETY
GOGGLES must be worn; hair and loose clothing
must be secured; remove excess papers; alcohols
are flammable and TOXIC
Always have calorimeter over the base of the stand
Calculation
• Enthalpy change (ΔH) of system (J)
= - (mass water (g) x specific heat capacity x ΔT)
• Enthalpy per gram
• Enthalpy per mole:
calculate amount of alcohol burned
• Comment on the trends in your results
Things to think about
• What is the relationship between number of
chemical bonds and energy released per mole?
• Why do you need to light the fuel if the reactions
are so exothermic?
• How are chemical bonds breaking and forming
related to the energy change?
methanol
ethanol
propanol
butanol
+ O=O + O=O + O=O  O=C=O + O=C=O + H-O-H + H-O-H + H-O-H
•Why won’t the burner light until the lit splint arrives?
•Is the splint supplying or taking energy from the alcohol and
oxygen?
+ O=O + O=O + O=O  O=C=O + O=C=O + H-O-H + H-O-H + H-O-H
•What must happen to the bonds between the atoms for the
reaction to start?
•Does breaking bonds require or release energy?
Bonds and energy changes
• Watch the MSS slideshow
• The changes can be represented in an energy level
diagram:
H+H+H+H+O+O
Heat
energy H-H + H-H + O=O
ENDOTHERMIC – energy
taken from surroundings
BONDS BREAKING
This is the ACTIVATION
ENERGY
Heat energy to the
surroundings – overall
the reaction is
exothermic
EXOTHERMIC –
energy released to
surroundings
BONDS FORMING
H-O-H + H-O-H
Energy required to break bonds: 2 x H-H + 1 x O=O = 2 x 436 + 1 x 496 = 1367 kJ/mol
Energy released by forming bonds = 4 x O-H = 4 x 463 = 1852 kJ/mol
Overall energy released = 1852 – 1367 = 485 kJ/mol
Practice calculation
• Try the bond energy method to calculate the
energy released on burning 1 mole of ethanol
• Include an energy level diagram in your calculation
+ O=O + O=O + O=O  O=C=O + O=C=O + H-O-H + H-O-H + H-O-H
Bond Energies (kJ/mol)
C-C C-H C-O O-H O=O C=O
346 436 358 463 496 743
1.
2.
3.
4.
Write down the number of each bond broken
Write down the number of each bond formed
Calculate the total energies of the broken and formed bonds
Subtract the energy of the broken bonds (ENDOTHERMIC) from the energy of
the formed bonds (EXOTHERMIC)
+ O=O + O=O + O=O  O=C=O + O=C=O + H-O-H + H-O-H + H-O-H
C-C
346
Bond Energies (kJ/mol)
C-H C-O O-H O=O C=O
436 358
463
496
743
1.
2.
3.
4.
Write down the number of each bond broken
Write down the number of each bond formed
Calculate the total energies of the broken and formed bonds
Subtract the energy of the broken bonds (ENDOTHERMIC) from the energy of
the formed bonds (EXOTHERMIC)
1.
2.
3.
4.
5.
1 x C-C + 5 x C-H + 1 x C-O + 1 x O-H + 3 x O=O
4 x C=O + 6 x O-H
1 x 346 + 5 x 436 + 1 x 358 + 1 x 463 + 3 x 496 = 4835 kJ/mol
4 x 743 + 6 x 463 = 5750
Overall reaction: 5750 - 4835 = 915 kJ/mol
HHHHHHOOOOOOOCC
4835 kJ/mol
BONDS BREAKING
+ O=O + O=O + O=O
This is the ACTIVATION
ENERGY
5750 kJ/mol
BONDS FORMING
Heat
energy
915 kJ/mol to
surroundings
O=C=O + O=C=O +
H-O-H + H-O-H + H-O-H
Sources of error
• Why is your experimental result so much lower than
your calculated value?
• Estimate the percentage error
• Is this error likely to be systematic (the same each
time)?
• Are you still confident in your conclusions about the
relationship between bonds and enthalpy?
• How could we reduce the experimental error?
Bomb calorimeter
Actual values
•
•
•
•
•
Ө
of ΔHc
Methanol = -726.3 kJmol-1
Ethanol = -1366.7 kJmol-1
Propanol = -2017.3 kJmol-1
Butanol = -2674.9 kJmol-1
Why is this different from the bond energy
calculation?
• Which one is right?