Thermochemistry
Sayed Hussaini and Jack sun
•Energy
Thermochemistry: Part of thermodynamics that
involves the relationship between chemical
reactions and energy changes involving heat.
• Energy: The capacity to do work or transfer heat.
• Work: Energy used to move an object with mass
against a force. W = F x d
• Heat: The energy transferred from a hotter
object to a colder one.
• The unit of energy is a Joule. 1J=1kg-m2/s2
•
•Types Of Energy
•
Kinetic Energy: The energy an object has just by being in
motion. KE=1/2 mv2
Potential Energy: The energy an object has because of its
position and the forces acting on it. Such as the force of Gravity
or the electrostatic force that arises between two charged
particles.
•
PEg=mgh
•
PEe= kQ1Q2/d
•Internal Energy
The sum of all the kinetic and potential
energies of a system.
• Since we know Energy is always conserved
(first law of thermodynamic) we can say that
ΔE= E final – E initial
• ΔE= q + w
•
•Enthalpy
• The
work done by the expansion or
compression of a gases at constant
pressure is given by: W=-PΔV
• Enthalpy: The heat flow in processes
occurring at constant pressure.
• ΔH=ΔE+PΔV
ΔH=(qp+W)-W
•Enthalpies of reaction
• Enthalpy
of a reaction is the heat of
reaction can be exothermic or
endothermic.
• For example:
• 2 H2 + O2
2 H2O
ΔH=-483.6KJ
• The magnitude of H is directly
proportional to the amount of reactant.
• If you reverse the above equation,
ΔH=483.6KJ.
•Calorimetry
ΔH can be determined by measuring the
temperature change the heat flow
produces.
• Constant Pressure: ΔH=-Cs x m x ΔT=qp
•
•
Bomb Calorimetry (Constant-volume):ΔH= -Ccal x ΔT
•Hess’s Law
• If
a reaction consists of multiple steps,
the sum of the ΔH for each step will
equal the overall ΔH.
Example:
•Enthalpy of Formation
• The
enthalpy changes associated with
the formation of a substance from its
components(ΔHf).
•
•
Standard enthalpy of formation: Change in
enthalpy for a reaction that forms one mole
of the compound and is in standard state (
1atm;298K).
The standard enthalpy of formation of the
most stable form of an element is zero.
•Foods and fuels
Fuel value: Energy release when one gram of
a material is combusted; represented by
positive values.
• The average fuel value of carbohydrates and
proteins is 17kJ/g.
• The average fuel value of fats is 38kJ/g.
•
•Quiz!!
• Calculate
the change in internal
energy of the system for a process in
which the system absorbs 140J of heat
and does 85J of work.
• ΔE=q + w
•
=140+(-85)=55J
• 2H2o2
2H2O + O2 ΔH=-196KJ=qp
• Calculate the value of q when 5.00g of
H2O2 decomposes at constant pressure.
5gH2O2
1mol H2O2
34gH2O2
-196KJ
2molH2O2
=
-14.4KJ
• 1.
What is the specific heat of aluminum if
the temperature of a 28.4 g sample of
aluminum is increased by 8.10C when 207 J of
heat is added?
ΔT=8.10C
• Q=207J
•
•
q = Cs x m x ΔT
Cs=q/mΔT
Cs=0.90J/g-oC
•
•
•
•
•
Find the ΔH for the reaction below, given the following
reactions and subsequent ΔH values:
2CO2(g) + H2O(g) → C 2H2(g) + 5/2O2(g)
C2H2(g) + 2H2(g) → C2H6(g)
ΔH =-94.5 kJ
H2O(g) → H2(g) + 1/2O2 (g)
ΔH =71.2 kJ
C2H6(g) + 7/2O2(g) → 2CO2(g) + 3H2O(g) ΔH =-283 kJ
•
•
•
•
•
•
•
•
2 CO2(g) + H2O(g) → C 2H2(g) + 2½ O2(g)
C2H2(g) + 2 H2(g) → C2H6(g)
H2O(g) → H2(g) + ½ O2 (g)
C2H6(g) + 3½ O2(g) → 2 CO2(g) + 3 H2O(g)
C2H6(g) → C2H2(g) + 2 H2(g)
2 H2(g) + O2 (g) → 2 H2O(g)
2 CO2(g) + 3 H2O(g) → C2H6(g) + 3½ O2(g)
2 CO2(g) + H2O(g) → C 2H2(g) + 2½ O2(g)
ΔH = - 94.5 kJ
ΔH = + 71.2 kJ
ΔH = - 283 kJ
flip
flip x 2
flip
ΔH = + 94.5 kJ
ΔH = - 142.4 kJ
ΔH = + 283 kJ
235.1 kJ