Thermochemistry - Madison Public Schools

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Transcript Thermochemistry - Madison Public Schools

Reaction Energy
and Reaction
1. Define heat and temperature.
2. Perform specific heat calculations.
Temperature and Heat
Temperature – measure of the average kinetic energy of
the particles of a substance.
What are the units?
oC, K
Heat – the flow of energy from one substance to
What are the units?
Joules, calories, Calories, kJ, kcal
1 cal = 4.184 J
Specific Heat
Specific Heat (C): the amount
of energy needed to raise the
temperature of 1 gram of a
substance by 1 degree celcius
For water:
C = 4.184 J/goC
C = 1 cal/g0C
Specific Heat
q  m  C  T
q = heat (joule or cal)
m= mass (grams)
C = specific heat (J/g*C) or (cal/g*C)
ΔT= change in temperature (oC)
Specific Heat Problems
1. Determine the specific heat of a material if a 35 g
sample absorbed 48 J as it was heated from 293K to
2. If 980 kJ of energy are added to 6.2 L of water at
18oC, what will the final temperature be?
Heat of Reaction
Enthalpy (H) – heat content of a system
Enthalpy change (ΔH) – amount of energy absorbed or
lost by a system during a process at constant pressure.
ΔH = Hproducts- Hreactants
Heat of Reaction
Heat of Reaction
Combustion of propane
C3H8 (g) + 5 O2(g)  3 CO2 (g) + 4 H2O(g) + 2043 kJ
ΔH = -2043 kJ (exothermic)
Formation of synthesis gas
C(s) + H2O(g) +113kJ  CO(g) + H2(g)
ΔH = +113 kJ (endothermic)
Sample Problems
1. How much heat will be released when 6.44 g of sulfur
reacts with excess O2 according to the following
2 S + 3 O2  2 SO3
ΔH = -791.4 kJ
2. How much heat is transferred when 9.22 g of glucose
(C6H12O6) in your body reacts with O2 according to the
following equation?
C6H12O6 (s) + 6 O2(g)  6 CO2 (g) + 6 H2O(l) ΔH= -2803 kJ
Heat of Formation
Molar Heat of Formation (∆Hf0 ) – energy change
that occurs when 1 mole of a compound forms from
the combination of its elements.
H2 (g) + 1/2 O2 (g)  H2O(l)
C(s) + O2 (g)  CO2 (g)
∆Hf0 = -285.8 kJ/mol
∆Hf0 = -393.5 kJ/mol
Stability and Heat of Formation
Al2O3 (s)
-1676.0 kJ/mol
-1206.92 kJ/mol
90.29 kJ/mol
142.7 kJ/mol
• Stability goes with a high negative heat of formation.
Heat of Combustion
Heat of Combustion (∆Hc0) –energy released with the
combustion of one mole of a substance.
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)
∆Hc0 = -2219.2 kJ/mol