enthalpy

1
Lecture 33

• enthalpy

2
bio

• Galileo Galilei

3
His parents wanted him to be a doctor but he
wanted to be an engineer and architect

• Invented
• 1) anatomical clock
• 2) theories on motion
• 3) a thermometer
• 4) advanced the telescope
• 5)Device to raise water levels
• 6) theories on planetary motion

4
Galileo said in the 1600s that heat was not a
material but the movement of atoms and that our
perception of heat was our ability to sense that
motion
5
Tycho Brahe said that all planets except the
earth and moon revolve around the sun

• Galileo because of his studies of the phases of
  the moons of Venus advocated Copernican system,
  in which everything goes round the Sun.

6
Galileo had a little problem with attitude

• He seems to have made a lot of enemies by making
  his opponents look fools.

he was a fierce controversialist, who, not
content with refuting adversaries, was bent upon
confounding them. Moreover, he wielded an
exceedingly able pen, and unsparingly ridiculed
and exasperated his opponents
7

• "The boldness, may we not say the recklessness,
  with which Galileo insisted on making proselytes
  of his enemies, served but to alienate them from
  the truth."

8
Galileo GalileeHe enemies of those that disagreed

• Benjamin Franklin

Glass, China, and Reputation, are easily crack'd,
and never well mended.
9
Go over homework

• Block 8 turn in lab and go to slide 27

10
Homeworkhandout 33

• Block 8 do quiz in addition to homework

11
Quiz 32

• Block 8 take quiz 31
• Do quiz 32 for homework

12
Last time we spoke of the equation qsh(mass) ?T
13
30 g sample of a unknown metal is heated from 22
C to 59.2 during the process 1000 joules of
heat is absorbed by the metal. What is the
specific heat of the metal
14
A 50 Co aluminum cube (density 2.7 g/ml) slides
up against a beaker of 100ml of water. The water
increases in temperature from 30 to 45 degrees
Co. The aluminum cube decreases in temperature
to 45 degrees. The specific heat of aluminum is
.89 How many joules of heat entered the water.
15

• Using our nifty equation no matter what the
  substance just insert the number of joules of
  heat that enters a system plug in the mass of the
  substance and its specific gravity whippo sappo
  we find the new and altered temperature.

16
Tanks hidden in the sand during the Iraq war
17
Heat of reaction

• Heat of Reaction is the general term denoting the
  amount of energy that is gained or lost during a
  chemical process. It is expressed by the symbol
  ?H rxn.
• If the sign of the term is negative , then the
  process is exothermic. If the sign of the term is
  positive, then the process is endothermic.

18

These are called a bomb calorimeter because they
have been known to explode
calorimeter
19
You record the change in water temp ?T you know
the mass of water in the calorimeter and you know
the specific heat of water. Now you can
calculate the q that was either sucked in or
expelled from the chemical reaction
q s (m) ?T
20
Lavoisier wrote how he, in a special chamber,
ignited Hydrogen and oxygen and 259 pounds of ice
melted from the heat.

• One pound of Charcoal melts 96pounds 8oz of
  ice
• One pound of phosphorus melts 100 pounds of ice

21
A typical reaction that is exothermic and puts
out heat is C(s) O2 (g) ?CO2 (g) ? H 394 kJ

Here we see that the product has lost 394 kJ in
the formation of CO2. It is said that the heat
of formation of CO2 is 394 kJ or ? Hf 394 kJ
22
exothermic
23
endothermic
24
What about the heat of formation of water where
the equation is

• 2H2 O2 ? 2H2O ? Hf -483.6 kJ

This gives us the heat of formation of How many
H2O?
2
25
Heat of Reaction is for one mole of product

• 2H2(g) O2(g) ? 2 H2O(l) H? - 572 kJ
  (Exothermic)
• H2(g) ½ O2(g) ? H2O(l) H? - 286 kJ
  (Exothermic

Always write a thermochemical reaction in
regards to 1 mole
26
The Heat of Reaction is broken down into two
different classes of reactions

• Heat of Formation ?Hf going from most basic
  into 1 mole of a compound
• 2. Heat of Combustion ? Hc going from most basic
  plus oxygen gas into one mole of compound

Hf
?Hf
?Hc
27
Example 1When 1 mole of CH4 is burned,
(enthalpy, H) 890 kJ of energy is released as
heat. Calculate ?H for a process in which a 5.8
g sample of CH4 is burned.
28
Example 2
C O2 ? CO2 ?H -393.5 kJ

• The above equation indicates how many kJ of
  enthalpy (heat) is released for every mole of
  carbon dioxide produced from Carbon and oxygen
  gas. If we need to heat a fire alarm in order to
  warn the world that Lex Luthor (arch enemy of
  Superman) is on the loose, how many grams of
  carbon do we need to burn to activate the 1000 kJ
  alarm.

29
There are Tables that have the heats of formation
on them. They are

• Compound Heat of Formation, kJ/mole  
• LiF -612
• LiCl -409
• LiBr -350
• LiI -271
• NaF -569
• NaCl -411
• NaBr -360
• NaI -288

2Li F2 ? 2LiF Li ½ F2 ?LiF ?H -612
30
Reversing a chemical reaction results in the same
magnitude of enthalpy but of the opposite sign.
C O2 ? CO2 ?H -393.5 kJ
CO2 ? C O2 ?H 393.5 kJ
31
It is important to be able to calculate the
amount of heat that is available in a chemical
reaction so you can predict how much energy will
be released during the experiment you can also
get an idea of how much energy is being locked in
an compound ie food
32
Books and catalogues have heats of formation for
many standard chemical equationsbut not all

• C ½ O2 ? CO ? H -100.5 kJ
• Elements ? compounds
• S O2 ? SO2 ? H -297kJ

33
These books and charts in addition to giving
heats of formation also give heats of combustion.
These are written ?Hcomb

• The ?Hcomb is the heat of combustion of one mole
  of a reactant burned with oxygen

34
What if you wanted to calculate the heat of
formation for a specific equation that was not in
a chart or table?

• How could you calculate a enthalpy from known
  values????????/

35

• All you know is that
• The price of a coke and fries is 1.25
• The price of a hot dog and fries is 1.50
• The price of a coke and hot dog is 1.75
• Can you determine the price of fries????

36

• C F 1.25
• H F 1.50
• C F H F 2.75
• We know that C H 1.75 so if we take this out
  of the equation then and 2F 1.00

So we see that by combining known facts we can
by adding and subtracting factors determine a
unknown value.
37
Can we really add chemical equations the same way
you would find the cost of French fries from the
known value of coke and fries and the value of
hot dog and fries

• Hesss Law

38
This is the essence and importance of the state
function

• It doesn't matter what turns and shifts you take
  its where you end up.

I was up then down now I am down 10 dollars
39
We know that when buying a hot dog the math works
but what about adding chemical equations does the
math hold true?

• Victor Franz Hess (1883-1964), Austrian-American
  Nobel laureate, studied this field

Hess determined this was also the case for
chemical reactions. It is Hesss law.
40
Hesss law states that the overall heat of a
reaction is equal to the sum of the overall steps
in the reaction.
41
The idea that you can add or subtract chemical
equations to calculate unknown values is called
Hesss Law

• When calculating the unknown value you must
  account for each element or compound

42
To find the value of the unknown heat of
formation you calculate the heat of the known
reactions either formation or combustion
In order to do this problem we need to build the
unknown equation using the known!!!!
CH4?C 2H2
C O2 ?CO2 H? -491.9KJH2 1/2O2 ?H2O
H? -357.2KJCO2 2H2O ?CH4 2O2 H? 1113.1KJ
43
Methane Oxygen gas yields how much energy

• CH4 O2 ? ? Hf ?

Balance the equation
44
CH4 2O2 ? CO2 2H2O

• You must consider each item
• CH4 formation
• 2O2 no change
• CO2 formation
• 2H2O formation

45
according to chart
The equation we are trying to build CH4 2O2 ?
CO2 2H2O

• C 2H2 ?CH4 ? Hf -74.8 kJ
• Flip the equation change the charge
• CH4 ? C 2H2 ? H 74.8

2O2 ? 2O2 ? Hf 0 you cant burn O2
46
C O2 ? CO2 ? Hf -393.5kJ
The equation we are trying to build CH4 2O2 ?
CO2 2H2O

• We need 2 H2O
• H2 ½ O2? H2O ? Hf -285.8kJ
• 2H2 O2?2H2O? Hf 2(-285.8) -571.6 kJ

47
So to react CH4 and 2O2to create CO2 and 2H2O we

• CH4 ? C 2H2 ? H 74.8
• 2O2 ? 2O2 ? Hf 0
• C O2 ? CO2 ? Hf -393.5kJ
• 2H2 O2?2H2O ? Hf -571.6 kJ

CH4 2O2 C O2 2H2 O2 ? C 2H2 2O2 CO2
2H2O
CH4 2O2? CO2 2 H2O ? Hf -890.36 kJ
48
C 2H2 ? CH4 ?Hfo ?

• We know from the book that
• C O2 ? CO2 ?Hcomb 393.5 kJ
• H2 ½O2 ? H2O ?Hcomb -285.8 kJ
• CH4 2O2?CO2 2H2O ?Hcomb 890.8

49
We need the CH4 on the other side so just switch
it and change the charge on the ?Hcomb kJ

• CH42O2?CO22H2O ?Hcomb 890.8

Switched sides switched kJ CO22H2O ? CH4 2O2
?Hcomb 890.8
50
C 2H2 ? CH4

• C O2 ? CO2 ? Hcomb -393.5kJ
• 2H2 O2 ? 2H2O ?Hcomb-571.6 kJ
• CO2 2H2O ? CH4 2O2 ?Hcomb 890.8
• CO2 2H2 O2 CO2 2H2O?CO2 2H2O CH42O2

C 2H2? CH4 ?Hf -74.3
51
Calculate the enthalpy change for

• 2H2O2(l) ? O2 (g) H2O (l)

2CO (g) O2(g) ? 2CO2(g)