IGCSE CHEMISTRY

ENERGETICS
Exothermic and endothermic reactions
Exothermic and endothermic reactions

In a chemical reaction there is usually a

temperature change.
Exothermic and endothermic reactions

In a chemical reaction there is usually a

temperature change.

In an EXOTHERMIC
reaction, the
temperature goes
UP. Heat energy is
given OUT.
Exothermic and endothermic reactions

In a chemical reaction there is usually a

temperature change.

In an EXOTHERMIC In an ENDOTHERMIC
reaction, the reaction, the
temperature goes temperature goes
UP. Heat energy is DOWN. Heat energy
given OUT. is taken IN.
Exothermic and endothermic reactions

Examples of EXOTHERMIC reactions:

Combustion is a common example of an exothermic

reaction.

Methane + oxygen  Carbon dioxide + water +

Heat

energy

Carbon + oxygen  Carbon dioxide + heat energy

(coal)
Exothermic and endothermic reactions

Examples of EXOTHERMIC reactions:

Neutralisation is another example of an exothermic

reaction.

Acid + Alkali  Salt + Water + Heat energy

Exothermic and endothermic reactions

Examples of EXOTHERMIC reactions:

Neutralisation is another example of an exothermic

reaction.

Acid + Alkali  Salt + Water + Heat energy

The symbol to show a change in the amount of

heat energy is ∆H (pronounced delta H)
Exothermic and endothermic reactions

reactants

For an EXOTHERMIC

Loss of energy
Increasing energy
reaction, ∆H is negative.
This means that heat has
been lost from the
reaction.

products

Time
Exothermic and endothermic reactions

Examples of ENDOTHERMIC reactions:

Endothermic reactions tend to be less common.

Dissolving ammonium nitrate crystals in water is an

endothermic reaction.

Ammonium + Water  ammonium nitrate – Heat

nitrate solution energy

Temperature of reactants = 20oC

Temperature of products = 13oC
Exothermic and endothermic reactions
products

For an ENDOTHERMIC

Gain in energy
Increasing energy
reaction, ∆H is positive.
This means that heat has
been gained in the
reaction.

reactants

Time
Exothermic and endothermic reactions

Let’s just
recap
Exothermic and endothermic reactions

Let’s just
recap

EXOTHERMIC
Exothermic and endothermic reactions

Let’s just
recap

EXOTHERMIC

Heat is given out

Exothermic and endothermic reactions

Let’s just
recap

EXOTHERMIC

Heat is given out

∆H is negative
Exothermic and endothermic reactions

Let’s just
recap

ENDOTHERMIC
Exothermic and endothermic reactions

Let’s just
recap

ENDOTHERMIC

Heat is taken in
Exothermic and endothermic reactions

Let’s just
recap

ENDOTHERMIC

Heat is taken in

∆H is positive
Exothermic and endothermic reactions

Another word
for the heat
energy content
of a compound is
ENTHALPY
Exothermic and endothermic reactions

You need to know that the unit

of energy is the JOULE (J).
Exothermic and endothermic reactions

You need to know that the unit

of energy is the JOULE (J).

1000 joules = 1 kilojoule (kJ)

Calculating enthalpy changes
Calculating enthalpy changes
The enthalpy change of a reaction is the
enthalpy of the products minus the
enthalpy of the reactants.
Calculating enthalpy changes
The enthalpy change of a reaction is the
enthalpy of the products minus the
enthalpy of the reactants.

Eg. Enthalpy of combustion of a fuel

Calculating enthalpy changes
Eg. Enthalpy of combustion of a fuel

The amount of energy released or

absorbed in a reaction can be measured
using this formula:
Q = mc∆t

heat energy = mass of x rise in x specific heat

produced water temperature capacity of water

(joules) (grams) ( oC) J/kg/oC

 Calculating enthalpy changes
thermometer

Measurements: Draught shield

Temperature of Insulating card

water before and
after heating
water
Mass of water
Copper calorimeter
used

Mass of fuel used

Spirit burner
Calculating enthalpy changes

EXAMPLE: the temperature

of 100g of water was
increased by 32.6oC by heating
with 0.46 grams of ethanol.

Assuming no heat losses from

the apparatus, what was the
enthalpy of combustion of
ethanol, C2H5OH?
Calculating enthalpy changes
heat energy = mass of x rise in x specific heat
produced water temperature capacity of water

(joules) (grams) (oC) J/kg/oC

Q = 100 x 32.6 x 4.2

Q= 13692 J (for 0.46g of ethanol)

Calculating enthalpy changes
heat energy = mass of x rise in
ed
x specific heat
produced water
uc
temperature capacity of water

rod J/kg/ C
(joules) (grams) ( C)
o

y p o

rg l?
n e 32.6
n o x 4.2
heat energy = 100
t ex
a
produced
a
e f et h
(joules)
h
t h e le o
is = mo13692 (for 0.46g of ethanol)
heat energy
a t er
h (joules)
produced
w p
S o
Calculating enthalpy changes
Rememberheat thatenergy = mass of x rise in
e d
x specific heat
Another wordproduced water
uc
temperature capacity of water
a mole of a
for the heat rod J/kg/ C
substance is the
(joules) (grams) ( C)
o

y p o

energy content r g l?
relative
of a compound is n e 32.6n o x 4.2
heat energy
molecular mass
= 100
t e x
a
ENTHALPY
produced
e a t h
in grams
(joules)
h f e
h e e o
s t o13692
l
i m
heat energy =
t
(for 0.46g of ethanol)
a r
wh pe
produced
(joules)

So
Calculating enthalpy changes
heat energy = 13692 (for 0.46g of ethanol)
produced
(joules)

Ethanol C2H5OH = 24 + 5 + 16 + 1 = 46
Calculating enthalpy changes
heat energy = 13692 (for 0.46g of ethanol)
produced
(joules)

Ethanol C2H5OH = 24 + 5 + 16 + 1 = 46

So the mass of one mole of ethanol is 46g

Calculating enthalpy changes
heat energy = 13692 (for 0.46g of ethanol)
produced
(joules)

Ethanol C2H5OH = 24 + 5 + 16 + 1 = 46

So the mass of one mole of ethanol is 46g

n of C2H5OH = 0.46/46 = 0.01 mol
The energy released from the combustion of one
mole of ethanol

= 13692/0.01 = 1369200 J

= 1369.2 kJ
Calculating enthalpy changes

Burning ethanol is an EXOTHERMIC

reaction, so heat energy is lost
(∆H is –ve)

The molar enthalpy of combustion of

ethanol is therefore:

= -1369.2 kJ/mol
Calculating enthalpy changes

Use a simple calorimeter to

measure the enthalpy change
in reactions such as
dissolving, neutralisation and
displacement.
Calculating enthalpy changes

thermometer

Insulating lid

Chemical mixture (eg. acid + alkali)

Polystyrene cup (insulated)

Calculating enthalpy changes

Enthalpy of neutralisation = the

heat produced when an acid and
alkali react to produce one mole
of water.
Calculating enthalpy changes
eg. A volume 50cm3 of 2M
hydrochloric acid were added to
50cm3 of 2M sodium hydroxide in
a polystyrene cup. The
temperature rose by 13.7oC.

If there were no heat losses,

and a final volume of 100cm3,
what was the enthalpy of
neutralisation?
‘M’ means mol/dm3 which is a unit of concentration C.
Number of moles=Concentration x Volume
n=CxV
Calculating enthalpy changes
Calculating enthalpy changes
HCl + NaOH  NaCl + H2O
Calculating enthalpy changes
HCl + NaOH  NaCl + H2O
1 1 1 1
Calculating enthalpy changes
HCl + NaOH  NaCl + H2O
1 1 1 1

Number of moles in 50cm3 of 2M HCl

= C x V = 50 x 2 = 0.1 mol
1000
Calculating enthalpy changes
HCl + NaOH  NaCl + H2O
1 1 1 1
Number of moles in 50cm3 of 2M
HCl = 50 x 2 = 0.1
1000

From the equation, 0.1 moles of HCl

will produce 0.1 moles of H2O
 Calculating enthalpy changes
HCl + NaOH  NaCl + H2O
1 1 1 1
Number of moles in 50cm3 of 2M
HCl = 50 x 2 = 0.1
1000

From the equation, 0.1 moles of HCl

will produce 0.1 moles of H2O

Q = Heat released in
producing 0.1 moles = 100 x 13.7 x 4.2 = 5754J
of water
Calculating enthalpy changes
Enthalpy of neutralisation ∆H = Q/n

Enthalpy of = 5754 J / 0.1 mol

neutralisation
= - 57540 J/mol

= - 57.54 kJ/mol

(remember: because this is an

exothermic reaction, ∆H is negative)