# Thermochemistryx - Duluth High School

Mechanics

Oct 27, 2013 (4 years and 6 months ago)

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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
-
m
2
/s
2

Types Of Energy

Kinetic Energy: The energy an object has just by being in
motion. KE=1/2 mv
2

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.

PE
g
=
mgh

PE
e
= kQ
1
Q
2/
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
Δ
E= E final

E initial

Δ
E= q + w

Enthalpy

The work done by the expansion or
compression of a gases at constant
pressure
is given by: W
=
-
P
Δ
V

Enthalpy: The heat flow in processes
occurring at constant pressure.

Δ
H=
Δ
E+P
Δ
V
Δ
H=(qp+W)
-
W

Enthalpies of reaction

Enthalpy of a reaction is the heat of
reaction can be exothermic or
endothermic.

For example:

2 H
2
+ O
2

2 H
2
O
Δ
H=
-
483.6KJ

The magnitude of H is directly
proportional to the amount of reactant.

If you reverse the above equation,
Δ
H=483.6KJ.

Calorimetry

Δ
H can be determined by measuring the
temperature change the heat flow
produces.

Constant Pressure:
Δ
H=
-
C
s

x m x
Δ
T=
q
p

Bomb
Calorimetry

(Constant
-
volume):
Δ
H=
-
C
cal

x
Δ
T

Hess’s Law

If a reaction consists of multiple steps,
the sum of the
Δ
H for each step will
equal the overall
Δ
H.

Example:

Enthalpy of Formation

The enthalpy changes associated with
the formation of a substance from its
components(
Δ
H
f
)
.

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.

Δ
E=q + w

=140+(
-
85)=55J

2H
2
o
2
2H
2
O + O
2

Δ
H=
-
196KJ=
q
p

Calculate the value of q when 5.00g of
H
2
O
2
decomposes at constant pressure.

5gH
2
O
2

-
196KJ

=

-
14.4KJ

2molH
2
O
2

34gH
2
O
2

1mol

H
2
O
2

1.

What is the specific heat of aluminum if
the temperature of a 28.4 g sample of
aluminum is increased by
8.1
0
C
when 207 J of
heat is

Δ
T=8.1
0
C q = C
s

x m x
Δ
T

Q=207J C
s
=q/m
Δ
T

C
s
=0.90J/g
-
o
C

Find the
Δ
H for the reaction below, given the following
reactions and subsequent
Δ
H values:

2CO2(g) + H2O(g) → C 2H2(g) + 5/2O2(g)

C2H2(g
) + 2H2(g) → C2H6(g)
Δ
H =
-
94.5 kJ

H2O(g) → H2(g) + 1/2O2 (g)
Δ
H =71.2 kJ

C2H6(g) + 7/2O2(g) → 2CO2(g) + 3H2O(g)
Δ
H =
-
283 kJ

2 CO2(g) + H2O(g) → C 2H2(g) + 2½ O2(g)

C2H2(g
) + 2 H2(g) → C2H6(g)
Δ
H =
-

94.5 kJ flip

H2O(g) → H2(g) + ½ O2 (g)
Δ
H = + 71.2 kJ flip x 2

C2H6(g) + 3½ O2(g) → 2 CO2(g) + 3 H2O(g)
Δ
H =
-

283 kJ flip

C2H6(g) → C2H2(g) + 2 H2(g)
Δ
H = + 94.5 kJ

2 H2(g) + O2 (g) → 2 H2O(g)
Δ
H =
-

142.4 kJ

2 CO2(g) + 3 H2O(g) → C2H6(g) + 3½ O2(g)
Δ
H = + 283 kJ

2 CO2(g) + H2O(g) → C 2H2(g) + 2½ O2(g
) 235.1 kJ