How are they related?
CHEMISTRY AND ENERGY
Energy Encountered Daily
Thermochemistry
 The study of heat used or released in a
chemical reaction.
 Let’s investigate heat as it compares to
temperature using the Heat vs. Temperature
Handout
CHEMICAL ENERGETICS 1:
All chemical reactions involve an energy change.
The transfer of energy, usually heat, into or out of
the reaction mixture’.
For example:
When petrol burns heat is given out.
When ammonium nitrate dissolves in water heat
is taken in.
CHEMICAL ENERGETICS 2:
As chemists are lazy people instead of writing ‘energy
change’ all the time they use the symbols:
H
Pronounced ‘delta aitch’.
=greek letter ‘delta’ meaning change.
H=heat.
So, H means ‘heat change’.
Temperature Changes 1
Some reactions cause the temperature of
the reaction mixture to increase.
This type of reaction is called exothermic.
Heat energy is given out by the reaction
mixture hence the surroundings increase in
temperature.
Temperature Changes 2
Some reactions cause the temperature of
the reaction mixture to decrease.
This type of reaction is called endothermic.
Heat energy is taken in by the reaction
mixture hence the surroundings decrease in
temperature.
Exothermic & Endothermic
Exothermic reactions
1. An exothermic reaction is one which
releases heat energy to the
surroundings
2. The temperature of the surroundings
increases
Exothermic Reactions
Exothermic reactions increase in temperature.

Examples include:



Burning reactions including the combustion
of fuels.
Detonation of explosives.
Reaction of acids with metals.
Magnesium
reacting with
acid
Thermit reaction
Activity
Say whether these processes are exothermic.
yes
1. Charcoal burning
yes
2. A candle burning.
3. A kettle boiling
no
4. Ice melting
no
5. A firework exploding
yes
You have to put heat in for boiling and melting.
You get heat out from all the other processes
Exothermic Reactions

Magnesium + Hydrochloric acid
25o C
45o C
magnesium
Gets hot
Hydrochloric
acid
Heat
energy
given
out
Exothermic Reactions

Almost immediately the hot reaction products start to
lose heat to the surroundings and eventually they
return to room temperature.
25o C
45
Chemical energy becomes heat
energy.
The reaction mixture gets hotter.
Eventually this heat is lost to the
surroundings.
It follows that reaction products have
less chemical energy than the
reactants had to start with.
Energy Level Diagram for an
Exothermic Reaction
Energy / kJ)
reactants
Reactants have more
chemical energy.
Some of this is lost as
heat which spreads out
into the room.
products
Progress of reaction (time)
Products now have
less chemical energy
than reactants.
Energy Level Diagram for an
Exothermic
Exothermic
Reaction
Reaction
2.
H is how
much energy
is given out
Energy / kJ
reactants
H=negative
products
Progress of reaction
H is negative
because the
products have
less energy than
the reactants.
Exothermic Reaction - Definition
Energy / kJ)
Exothermic reactions
give out energy. There
is a temperature rise
and H is negative.
reactants
H is negative
products
Progress of reaction
Activity
Endothermic reactions
1. An endothermic reaction is on which
takes in heat energy from the
surroundings
2. The temperature of the surroundings
decreases
Endothermic
Reactions
Endothermic reactions cause a decrease in temperature.



Endothermic chemical reactions are
relatively rare.
A few reactions that give off gases are highly
endothermic - get very cold.
Dissolving salts in water is another process
that is often endothermic.
Endothermic Reactions
Endothermic reactions cause a decrease in temperature.
Ammonium
nitrate
Cools
Water
Starts 25°C
Cools to 5°C
Heat
energy
taken
in as
the
mixture
returns
back to
room
temp.
Returns to 25°C
Endothermic Reactions

The cold reaction products start to gain heat from
the surroundings and eventually return to room
temperature.
o oCC
25
5
The reactants gain energy.
This comes from the substances used in the
reaction and the reaction gets cold.
Eventually heat is absorbed from the
surroundings and the mixture returns to
room temperature.
Overall the chemicals have gained energy.
Energy Level Diagram for an
Endothermic Process
Energy / kJ)
products
H=+
reactants
Progress of reaction
This is how
much energy
is taken in
This is positive
because the
products have
more energy
than the
reactants.
Endothermic Reaction Definition
Endothermic reactions
take in energy. There is
a temperature drop and
H is positive.
Energy / kJ
products
H=+
reactants
Progress of reaction
Activity
Are these endothermic or exothermic?
1. A red glow spread throughout the mixture
and the temperature rose.
2. The mixture bubbled vigorously but the
temperature dropped 150C.
3. Hydrazine and hydrogen peroxide react so
explosively and powerfully that they are
used to power rockets into space.
4. The decaying grass in the compost maker
was considerably above the outside
temperature.
exo
endo
exo
exo
Endothermic and exothermic
reactions
Step 1: Energy must be
SUPPLIED to break bonds:
Step 2: Energy is RELEASED
when new bonds are made:
A reaction is EXOTHERMIC if more energy is RELEASED
then SUPPLIED. If more energy is SUPPLIED then is
RELEASED then the reaction is ENDOTHERMIC
Representing Energy Changes:
The energy changes in a chemical reaction can be
conveniently represented using
energy level diagrams
Energy level diagrams make it easier to decide
whether a reaction is exothermic (gives out heat
and gets hotter) or endothermic (takes in heat and
gets cooler).
Energy Level Diagrams
This energy level diagram shows a
reaction where H is negative (an
exothermic reaction).
This energy level diagram shows a
reaction where H is positive (an
endothermic reaction).
Reactants
Products
energy
energy
H negative,
exothermic.
Products
H positive,
endothermic.
Reactants
Calorimetry
 Method used to determine the heat involved
in a physical or chemical change.
 Relies on the law of conservation of energy
Calorimeter
Simple Calorimeter
Specific Heat Calculations
 q = mCΔT
q = heat (J or cal or Cal)
4.184 cal = 1 Joule
1000 cal = 1 Cal (dietary calorie)
m = mass (g)
C = specific heat (J/g oC or cal/g oC)
ΔT = change in temperature (o C or K)
= T f - Ti
Specific Heat
 Specific heat of water = 1 cal /g o C or
= 4.184 J / goC
 Specific heat of most metals = < 1 J / goC
Practice Problem
 How much energy is required to heat 120.0 g
of water from 2.0 oC to 24.0oC?
q = mCΔT
m= 120.0 g
C = 4.184 J/goC
ΔT= (24.0 – 2.0)oC = 22.0oC
q = (120.0g)(4.184 J/goC)(22.0oC) =
Practice Problem
 How much heat (in kJ) is given off when 85.0
g of lead cools from 200.0oC to 10.0 oC?
(Specific heat of lead = 0.129 J/g oC)
q = mCΔT
m = 85.0 g
C = 0.129 J/g oC
ΔT = (10.0 – 200.0)oC = - 190.0oC
q = (85.0 g)(0.129 J/g oC)(- 190.0oC) = -
Practice Problem
Thermochemical Equations
 An equation that includes the heat change
 Example: write the thermochemical equation
for this reaction
 CaO(s) + H2O(l) Ca(OH)2(s)
H = -65.2 kJ
CaO(s) + H2O(l) Ca(OH)2(s) + 65.2 kJ
Calorimetry Math
 Heat gained by the water = q
 Heat lost by the system = -q
mCT = q
T = Tf –Ti , m = mass, C = specific heat
q gained by water = q lost by system
 q water = - q system
 mCT = -mCT
(mass H2O)(spec. heat H2O)(T H2O) = - (mass
sys)(spec. heat sys)(T sys)
Standard Heat of Reaction
Standard Heat of Reaction
 Heat change for the equation as it is written
H = Hf(products) - Hf(reactants)
Standard Heats of Formation (Hf)
 Change in enthalpy when 1 mole of the
compound is formed from its elements in
their standard states at 25oC and 101.3 kPa
Hess’s Law
 A way to calculate the heat of a reaction
that may be too slow or too fast to collect
data from.
 Add together several reactions that will
result in the desired reaction. Add the ΔH
for these reactions in the same way.
 Htotal = Hproducts - Hreactants
Making and Breaking Bonds:
Making chemical bonds is always exothermic.
Breaking chemical bonds is always endothermic.
All chemical reactions involve bond ‘rearrangements’.
Bonds are broken and new ones are formed.
Chemical reactions are in two stages:
1. Breaking bonds, an endothermic process.
2. Making new bonds, an exothermic process.
Activity
Copy the summary using the words from the box to
fill in the gaps:
endothermic
exothermic
lose
common
positive
1. Exothermic reactions are _____.
common
endothermic
2. Reactions that get cold are called _____.
3. Bond forming is an _______
exothermic process.
positive H.
4. Endothermic reactions have a _____
5. In exothermic reactions the chemicals ___
lose
chemical energy.
12.7 Activation Energy
Chemical Reactions must go over an energy
hill like a mountain (Swiss Alps).
Catalyst
 A substance that speeds up a reaction without
being consumed or makes a tunnel through the
Activation Energy Hill.
Chlorofluoro
Carbons (CFC’s) are
acting as catalysts
to decompose the
ozone (O3) layer.
The ozone layer is
formed from cosmic
radiation and
protects us from UV
light.
77
Energy level diagrams
Energy
level
Activation
energy
Using a catalyst
might lower the
activation energy
Energy given
out by
reaction
Reaction progress
Exothermic vs endothermic:
EXOTHERMIC – more
energy is given out than is
taken in (e.g. burning,
respiration)
ENDOTHERMIC –
energy is taken in but
not necessarily given out
(e.g. photosynthesis)
Burning Methane
CH4 + 2O2
To burn methane
you have to break
all of these bonds:
And then you
have to make
these ones:
2H2O + CO2
Burning Methane
CH4 + 2O2
2H2O + CO2
Methane
Oxygen
Carbon dioxide
Water
Bond energies
C-H = 435 Kj
O=O = 497 Kj
Total for breaking bonds = 4x435 + 2x497 = 2734 KJ/mol
C=O = 803 Kj
H-O = 464 Kj
Total for making bonds = 2x803 + 4x464 = 3462 KJ/mol
Total energy change = 2734-3462 = -728 KJ/mol
Drawing this on an energy
diagram:
2734 Kj
3462 Kj
-728 Kj
More energy is given out (3462) than is given in (2734) –
the reaction is EXOTHERMIC. The total (“nett”) energy
change is –728 Kj. An endothermic reaction would have a
positive energy change.
Activity
Copy the summary using the words from the box to fill in the
gaps:
more
endothermic
activation
1. The energy needed to start off a reaction is
called the activation
______ energy
2. In endothermic reactions bond breaking
more energy than is produced by
requires ___
bond forming.
endothermic process.
3. Bond breaking is an _________
Which of the following is an endothermic process?
A.
B.
C.
D.
Burning wood
Reacting an acid and alkali
Reacting magnesium with acid
Dissolving ammonium nitrate in water
Which of the following is true for an exothermic
process?
products
Energy / kJ
A. The reactants lose
energy to the
surroundings
B. The reaction gets cold
C. The energy diagram will
be as shown
D. The reaction will have a
H that is positive (+).
reactants
Progress of reaction
Which of the following is true when chemical
bonds are broken?
A. The process is exothermic
B. Energy is given out
C. A physical change is occurring
D. The reaction will have a H that is
positive(+).
Which of the following is true for an exothermic
reaction?
A. Bond breaking involves a bigger energy
change than bond making
B. Bond making involves a bigger energy
change than bond breaking
C. Bond making involves the same energy
change as bond breaking
Which of the following is the activation energy:
X
Energy / kJ)
A. X
B. Y
C. Z
Z
Y
Progress of reaction
© Boardworks Ltd 2001