ENTHALPY
Chemistry 11 - Energetics
Source: Brown and LeMay’s Chemistry the Central Science
Objectives




Define the terms exothermic reaction,
endothermic reaction, and enthalpy change of
reaction (ΔH)
State that combustion is an exothermic reaction.
Apply the relationship between temperature
change, enthalpy change, and the classification
of a reaction as endothermic and exothermic.
Deduce, from an enthalpy level diagram, the sign
of the enthalpy change of the reaction.
Energy




What is it?
How / why is it important?
Are there different kinds of energy?
How do we measure it?
Chemical reactions

Distinguish between a physical and chemical
reaction.
Chemical reactions




A physical reaction involves a change of state or
phase but the chemical composition of the
substance stays the same.
A chemical reaction involves a change in the
composition of the substance.
What are some chemical reactions / processes that
you can think of?
Apply what we have learned in bonding to explain
what a chemical reaction is.
Energy and chemical reactions

How do you think energy and chemical reactions
might be related?
The first law of thermodynamics



There are many types or forms of energy.
Name some types of energy that we have
encountered before.
Consider a rock sitting on top of a cliff that is
made to fall to ground level. Describe the
changes that have occurred in terms of energy.
The first law of thermodynamics



Energy can be converted from one form to the
other.
It can neither be created nor destroyed, it can
only transform from one type to another.
Is the amount of energy in the universe
changing? decreasing? increasing?
The first law of thermodynamics


The first law of thermodynamics states that the
amount of energy in the universe is constant.
So why are we always being told to conserve
energy if it’s not going anywhere?
The first law of thermodynamics

In this unit, we will often refer to systems, of
which there are many different types:
 isolated
 closed
 open
The first law of thermodynamics
A comparison of types of systems
Type of
system
isolated
Can MASS leave or
enter?
NO
Can ENERGY leave or
enter?
NO
closed
open
NO
YES
YES
YES
We will often work with isolated systems, because it
is the ideal situation. Do we actually work with
isolated systems in the “real world”?
The first law of thermodynamics


Define the internal energy of a system.
How would we represent a change in internal
energy?
The first law of thermodynamics
The change in internal energy of a system may be
expressed as:
ΔE = Efinal – Einitial
What would a positive ΔE imply?
A negative ΔE?
The first law of thermodynamics



Remember that the sign of a quantity only
signifies direction.
- ΔE: Energy is lost .
+ ΔE: Energy is gained.
The first law of thermodynamics
Internal energy, E
Changes in internal energy
initial state
final state
final state
initial state
In which process is energy lost? Energy gained?
Which process has + ΔE? – ΔE?
The first law of thermodynamics
Energy diagram for the interconversion of H2(g), O2(g),
and H2O(l)
Internal energy, E
H2(g), O2(g)
Which process has a +ΔE?
-ΔE?
H2O(l)
The first law of thermodynamics
Energy diagram for the interconversion of H2(g), O2(g),
and H2O(l)
Internal energy, E
H2(g), O2(g)
ΔE > 0
ΔE < 0
H2O(l)
What does it mean if there
are arrows going both
ways?
The first law of thermodynamics

Recall two terms that we have discussed before:
 heat,
Q
 work, W



What is heat?
What is work?
How do they relate to energy?
The first law of thermodynamics


If heat is added to a system, what happens to
internal energy? What if heat is taken away?
If work is done on a system, what happens to
internal energy? What if work is done by the
system?
The first law of thermodynamics
The effect of heat and work on internal energy
Quantity
heat added to sys
heat taken away from sys
work done on sys
work done by sys
Symbol Effect on internal energy,
E
The first law of thermodynamics
The effect of heat and work on internal energy
Quantity
heat added to sys
Symbol Effect on internal energy,
E
+Q
increase
heat taken away from sys
-Q
decrease
work done on sys
+W
increase
work done by sys
-W
decrease
Derive an expression that relates the change in
internal energy ΔE to heat Q and work W.
The first law of thermodynamics
ΔE = Q + W
This is the quantitative definition of the first law of
thermodynamics.
The first law of thermodynamics
Two gases, A(g) and B(g), are confined in a cylinder with
a moving piston. Substances A and B react to form a
solid product:
A(g) + B(g)  C(s)
As the reaction occurs, the system loses 1150J of heat
to the surroundings. The piston moves downward
as the gases react to form a solid. This action
requires 480 J of work to be done on the system.
What is the change in internal energy of the
system?
The first law of thermodynamics
Review of chemical equations:
A(g) + B(g)  C(s)
The first law of thermodynamics
Review of chemical equations:
2H2O(l)  2H2(g) + O2(g)
The first law of thermodynamics
ΔE = Q + W
ΔE = -1150 + 480
ΔE = -670 J
The first law of thermodynamics
Calculate the change in internal energy of the
system for a process in which the system
absorbs 140 J of heat from the surroundings and
does 85 J of work on the surroundings.
The first law of thermodynamics
ΔE =+55 J
Exothermic vs endothermic


Chemical reactions may be classified as
exothermic or endothermic.
One type means it releases heat and the other
means it absorbs heat. Which is which?
Exothermic vs endothermic


When a process absorbs heat, it is said to be
endothermic.
When a process releases heat, it is said to be
exothermic.
 endo- means “within”
 exo- means “outside”
 -therm means “heat”
Can you think of some processes (physical or
chemical) that are endothermic or exothermic?
State functions



Consider 50 g of water at 25C in a pot.
That system carries with it a certain amount of
internal energy.
What are processes that may have occurred to
reach this endpoint?
State functions

The pot of 50 g of water at 25C may have been
obtained either by:
cooling 50 g of water from 100C to 25C, or
 melting 50 g of ice and raising the temperature from
0C to 25C


Does the process by which that system reached
the endpoint affect its internal energy?
State functions



No, it does not.
The process by which we arrived at that result doesn’t
matter. The internal energy of 50 g of water at 25C is
the same no matter how it was done.
This is an example of a state function, a quantity for
which the magnitude only depends on the present
state of the system, not the path the system took to
reach that state.
State functions
You are travelling from Manila to Baguio. Manila is
at sea level, 0 m, and Baguio is much higher,
1500 m above sea level.
Is the altitude change a state function?
Is the distance traveled a state function?
State functions
There are several routes or paths to get to Baguio from
Manila:
via EDSA or C-5 and Katipunan
 via NLEX or National Highway
 via SCTEX or National Highway
 via Kennon Road or Marcos Highway

Does the path you take affect the altitude change from
Manila to Baguio?
Does it affect the distance traveled from Manila to
Baguio?
State functions
Quite obviously, the path taken does not affect the
altitude change. Baguio is still 1500 m higher
than Manila. Therefore, altitude change is a state
function.
Again, quite obviously, the path taken will affect
the distance you travel. Therefore, distance
traveled is not a state function.
Can you think of any other state functions?
State functions
Quite obviously, the path taken does not affect the
altitude change. Baguio is still 1000 m higher
than Manila. Therefore, altitude change is a state
function.
Again, quite obviously, the path taken will affect
the distance you travel. Therefore, distance
traveled is not a state function.
Can you think of any other state functions?
Enthalpy
Let’s go back to the First law of thermodynamics:
ΔE = Q + W
How would you measure heat?
How would you measure work?
Enthalpy

Heat can be measured using calorimeters, which
we discussed in physics.
 What

are the main parts of a calorimeter?
How is work defined in physics?
Enthalpy


Consider the following reaction:
Zn(s) + 2H+(aq)  Zn2+(aq) + H2(g)
Work is done by the system in this reaction. How
will we use W = Fd to measure work?
Enthalpy
Zn(s) + 2H+(aq)  Zn2+(aq) + H2(g)


The work is done by the expanding hydrogen
gas.
Work can be measured if that expanding gas can
be harnessed to exert a force on something.
Enthalpy
W = Fd
for gases, force is related to pressure:
P=F/A
F = PA
therefore:
W = PAd
W = P ΔV
where P is the pressure in the system and ΔV is the change
in volume.
Enthalpy
W = P ΔV
What does + ΔV imply? – ΔV?
Enthalpy
Relationship between change in volume and work
ΔV
+ΔV
-ΔV
Expansion or
compression?
Work done by or on the
system?
W
Enthalpy
Relationship between change in volume and work
+ΔV
Expansion or
compression?
expansion
Work done by or on the
system?
by the sys
-W
-ΔV
compression
on the sys
+W
ΔV
W
Therefore the more accurate relationship would be:
W = - P ΔV
Enthalpy

A thermodynamic function called enthalpy, H,
(enthalpein means “to warm”) accounts for heat
flow in processes at constant pressure, such that:
H = E + PV
where H is enthalpy, E is the internal energy of the
system, P is the pressure of the system and V is the
volume of the system.
Why is it important to specify “at constant pressure”?
Enthalpy
H = E + PV
Is internal energy a state function? pressure?
volume?
Enthalpy
H = E + PV
Internal energy E, pressure P, and volume V are all
state functions, therefore enthalpy H is also a
state function.
Enthalpy
H = E + PV



However, in the context of chemical reactions,
changes in these quantities are far more
relevant.
Instead of H, we are interested in ΔH.
Derive an expression for change in enthalpy ΔH.
Enthalpy
H = E + PV
ΔH = Δ(E + PV)
ΔH = ΔE + PΔV
Which of these quantities can be measured?
Enthalpy
ΔE cannot be measured so where do we go from
here?
ΔH = ΔE + PΔV
Is ΔE related to any quantities that can be
measured?
Enthalpy
ΔH = ΔE + PΔV
ΔH = (Q + W) + PΔV
ΔH = Q – PΔV + PΔV
(at constant pressure)
ΔH = QP
therefore the change in enthalpy ΔH is the heat that
flows into or out of a system at constant pressure.
What does +ΔH imply? –ΔH?
Enthalpy
Classifying reactions as endothermic or
exothermic based on change in enthalpy
ΔH
+ΔH
-ΔH
Heat flow
Exo- or endothermic?
Enthalpy
Classifying reactions as endothermic or
exothermic based on change in enthalpy
ΔH
+ΔH
Heat flow
into system
Exo- or endothermic?
endothermic
-ΔH
out of system
exothermic
Enthalpy


We can describe the energy changes in a system
during a chemical reaction using either ΔE or
ΔH.
What is the advantage of using ΔH to describe
energy changes?
Enthalpy
Indicate the sign of the enthalpy change ΔH in each
of the following processes carried out at
constant pressure:
(a)
(b)
an ice cube melts
1 g of butane is combusted completely to form
carbon dioxide and water
Enthalpy
(a)
(b)
heat flows into the system, +ΔH, endothermic
explosion: heat flows out of the system, -ΔH,
exothermic
Enthalpy
Suppose we confine 1 g of butane and sufficient
oxygen in a cylinder with a movable piston. The
cylinder is perfectly insulated so no heat can
escape to the surroundings. A spark initiates
the combustion of the butane which forms
carbon dioxide and water vapor. If we used this
to measure enthalpy, would the piston rise, fall,
or stay the same?
2C4H10(g) + 13O2(g)  8CO2(g) + 10H2O(g)
Enthalpy
Combustion reaction
2C4H10(g) + 13O2(g)  8CO2(g) + 10H2O(g)
If it is used to measure enthalpy, pressure must
remain constant.
Notice how many moles of gas are on each side of
the reaction.
Enthalpy
Combustion reaction
2C4H10(g) + 13O2(g)  8CO2(g) + 10H2O(g)
The piston would rise to make room for the
additional molecules of gas.
Ideal gas simulation
Enthalpies of reaction
Combustion reaction
2H2 (g) + O2(g)  2H2O(g)
Predict the sign of the change in enthalpy ΔH of
this reaction.
Enthalpies of reaction
Combustion reaction
2H2 (g) + O2(g)  2H2O(g)
This is a combustion reaction so we would expect
heat to be released from the system.
Is it an exothermic or endothermic reaction?
Enthalpies of reaction
Combustion reaction
2H2 (g) + O2(g)  2H2O(g)
This is an exothermic reaction.
ΔH = - 486.3 kJ/mol
Enthalpies of reaction
Combustion reaction
2H2 (g) + O2(g)  2H2O(g)
ΔH = - 486.3 kJ/mol




ΔH in this case would be referred to as the
enthalpy of reaction or the heat of reaction.
The symbol is ΔHrxn
The equation above is called a thermochemical
equation. What information does it provide?
Is that a lot of energy?
Enthalpies of reaction
Enthalpy
Combustion reaction
2H2 (g) + O2(g)  2H2O(g)
ΔH = - 486.3 kJ/mol
Enthalpies of reaction
Combustion reaction
2H2 (g) + O2(g)  2H2O(g)
ΔH = - 486.3 kJ/mol
2H2 (g) + O2(g)
Enthalpy
ΔH < 0
2H2O(g)
Enthalpies of reaction
Enthalpy
Combustion reaction
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(l)
ΔH = - 890 kJ/mol
Enthalpies of reaction
Combustion reaction
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(l)
ΔH = - 890 kJ/mol
CH4 (g) + 2O2(g)
Enthalpy
ΔH < 0
CO2(g) + 2H2O(l)
Enthalpies of reaction
Combustion reaction
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(l) ΔH = - 890 kJ/mol
How much energy would be released by combusting 2
mol of methane?
Enthalpies of reaction
Combustion reaction
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(l)
ΔH = - 890 kJ/mol
Twice the amount of methane combusted would mean
twice the amount of energy is released (in sufficient
oxygen).
ΔH = - 1780 kJ
The enthalpy of reaction is directly proportional to
the amount of reactants consumed in the
process.
Enthalpies of reaction
Combustion reaction
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(l)
ΔH = - 890 kJ/mol
Predict the value of ΔH for the following reaction:
CO2(g) + 2H2O(l)  CH4 (g) + 2O2(g)
Enthalpies of reaction
Reverse of a combustion reaction
CO2(g) + 2H2O(l)  CH4 (g) + 2O2(g)
CH4 (g) + 2O2(g)
CH4 (g) + 2O2(g)
CO2(g) + 2H2O(l)
ΔH = 890 kJ
Enthalpy
Enthalpy
ΔH = -890 kJ
CO2(g) + 2H2O(l)
Enthalpies of reaction
Combustion reaction
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(l)
ΔH = - 890 kJ/mol
CH4 (g) + 2O2(g)
Enthalpy
ΔH = -890 kJ
ΔH = +890 kJ
CO2(g) + 2H2O(l)
The enthalpy change
of a reaction is equal
in magnitude but
opposite in sign to its
reverse reaction.
Enthalpies of reaction
Combustion reaction
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(l)
ΔH = - 890 kJ/mol
Compare the enthalpy of the above reaction with the
reaction below:
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(g)
Which would have a larger ΔH?
Enthalpies of reaction
Consider the following process:
2H2O(l)  2H2O(g)
Is heat absorbed or released?
Is this an exothermic or endothermic process?
Enthalpies of reaction
Consider the following process:
2H2O(l)  2H2O(g)
The evaporation of water requires energy.
Therefore this is an endothermic process.
ΔH is positive.
Enthalpies of reaction
SO
Process
ΔH (kJ/mol)
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(l) - 890
2H2O(l)  2H2O(g) +88
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(g) ???
Enthalpies of reaction
SO
Process
ΔH (kJ/mol)
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(l) - 890
2H2O(l)  2H2O(g) +88
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(g) -802
The enthalpy change of reaction depends on
the state of the reactants and products.
Enthalpies of reaction
1.
2.
3.
The enthalpy of reaction is directly proportional
to the amount of reactants consumed in the
process.
The enthalpy change of a reaction is equal in
magnitude but opposite in sign to its reverse
reaction.
The enthalpy change of reaction depends on the
state of the reactants and products.
Enthalpies of reaction
How much energy is released when 4.50 g of methane
gas (CH4) is burned in a constant pressure system?
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(l)
ΔH = - 890 kJ/mol
Enthalpies of reaction
CH4 (g) + 2O2(g)  CO2(g) + 2H2O(l)
ΔH = - 890 kJ/mol
4.50 g of methane would release 250 kJ of heat.
Enthalpies of reaction
Hydrogen peroxide can decompose to water and
oxygen by the following reaction:
2H2O2(l)  2H2O(l) + O2(g)
ΔH = -196 kJ/mol
Calculate the value of ΔH if 5.00 g of hydrogen
peroxide decomposes at constant pressure.
Enthalpies of reaction
Hydrogen peroxide can decompose to water and
oxygen by the following reaction:
2H2O2(l)  2H2O(l) + O2(g)
ΔH = -196 kJ
ΔH = -14.4 kJ
Objectives




Define the terms exothermic reaction,
endothermic reaction, and enthalpy change of
reaction (ΔH)
State that combustion is an exothermic reaction.
Apply the relationship between temperature
change, enthalpy change, and the classification
of a reaction as endothermic and exothermic.
Deduce, from an enthalpy level diagram, the sign
of the enthalpy change of the reaction.
Energetics

So what is the relationship between energy and
chemical reactions?
Other learning activities


Enthalpy worksheet
Applications of chemistry
 You
will be assigned an article about a particular
application of Energetics.
 I will ask random people to share with the class on
Tuesday. Be prepared.

Enthalpy QUIZ on Tuesday, March 10.