# Thermochemistry (Text Chapter 5)

Mechanics

Oct 27, 2013 (4 years and 7 months ago)

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Thermochemistry
(
Text Chapter

5
)

1 calorie = 4.184 Joules = 0.001 kcal (or Calorie)

I.
Reactions (5.4)

Exothermic

Endothermic

heat is released (
product
)

heat is absorbed (
reactant
)

H is

H is

example: NaOH
(s)

NaOH
(aq)

example: NaOH
(aq)

NaOH
(s)

H
final

H
initial

H
final

H
initial

Graph:

Graph:

Basic rule:

energy is

when bonds are broken
and

when bonds are formed

Another basic rule:

Substances like to be in their lowest
energy state because it’s the most stable. So, exothermic
processes are generally more likely to occur spontaneously.

Writing Thermo Equations (4

ways)

1.

NaOH
(aq)

NaOH
(s)

2.

NaOH
(aq)

NaOH
(s)

3.

NaOH
(aq)

NaOH
(s)

Q:

Calcium oxide and water are mixed to make cement. The
reaction produces 65.2kJ of heat.

<<note enthalpies of combustion
-

table 13 in data booklet>>

II.
Calculating

H (5.5
-
5.7)

1.

H forward =
-

H backward

(1) CH
4

+ 2O
2

CO
2

+ 2H
2
O

H =
-
890kJ

(2) CO
2

+ 2H
2
O

CH
4

+ 2O
2

H =

2.

Stoichiometry won’t go away!

H

am
ount of reactant used.

(1) CH
4

+ 2O
2

CO
2

+ 2H
2
O

H =
-
890kJ

(2) 2CH
4

+ 4O
2

2CO
2

+ 4H
2
O

H =

Q:

So… what would

H be for 4.50g of CH
4
?

Q:

If this room was filled with methane? (1
x10
4

L at 1atm,
298K)?

3.

Calorimetry

A

method of calculating

H of a reaction by measuring

T and
using the formula:

q = mC

T

(q = heat, C = specific heat, m = mass)

(Specific)
Heat Capacity
=

(high:
_________, low: __________)

Q:

A 200mL (=200g) puddle of water absorbs heat from the sun
and its temperature increases by 15
o
C. How much heat did it
absorb (C= 4.18J/g
o
C)
?

Demo: How many calories are in a Cheeto?

4.

Hess’s Law
:

If a reaction is carried out in a series of steps, the overall

H will
be the sum of the

H for each of the steps.

Q:

Calculate the

H for N
2

+ O
2

2NO using the reactions
below:

4NH
3

+ 3O
2

2N
2

+ 6H
2
O

H=
-
1530 kJ

4NH
3

+ 5O
2

4NO + 6H
2
O

H=
-
1170 kJ

Q:

Calculate

H for the reaction: 2C
(s)

+ H
2(g)

C
2
H
2(g)

using the
reactions
below:

C
2
H
2
+ 5/2 O
2

2CO
2

+ H
2
O

H=
-
1299.6 kJ

C + O
2

CO
2

H=
-
393.5 kJ

H
2

+ ½ O
2

H
2
O

H=
-
285.8 kJ

5
.

Bond Energies

(table 10 of data booklet)

Bond energy is the energy required to break a bond. When a
bond is formed, energy is released and when it is broken, energy
is absorbed. So…

H
o
reaction

=

bond energies of bonds broken

bond energies of bonds formed

Q:

What is the

H
o
rx
n

for the
reaction

of
H
2

and O
2

to make
water
?

Q:

What is the

H
o
rx
n

for the
reaction of CH
4

and O
2

to make
CO
2

and H
2
O?

Q:

What is the

H
o
rx
n

for the
reaction

of
ethane and
Cl
2

to make
1,2
-
dichloro ethane
?