Chemistry: Endothermic and Exothermic Reactions
Standards:
7b. Students know chemical processes can either release
(exothermic) or absorb (endothermic) thermal energy.
7c. Students know energy is released when a material
condenses or freezes (exothermic) and is absorbed when a
material evaporates or melts (endothermic).
What is Energy?
---> The ability to do work
Energy
Kinetic
energy
(EK)
Energy due
to motion
Potential
energy
(EP)
Energy due to
postition
(stored energy)
Total Energy =
Kinetic Energy
E
=
EK
+
+
Potential Energy
EP
Kinetic energy & potential energy are interchangeable
Ball thrown upwards
slows & loses kinetic
energy but gains
potential energy
The reverse happens
as it falls back to
the ground
Law of Conservation of
Energy:
the total energy of the universe is
constant and can neither be
created nor destroyed; it can only
be transformed.
Systems & Surroundings
the world is divided into a system and its surroundings
A system is the part of the world we want to study (e.g. a
reaction mixture in a flask)
The surroundings consist of everything else outside the
system
SYSTEM
OPEN
ISOLATED
CLOSED
OPEN SYSTEM: can exchange both
matter and energy with the
surroundings (e.g. open reaction flask,
rocket engine)
CLOSED SYSTEM: can exchange
only energy with the surroundings
(matter remains fixed) e.g. a sealed
reaction flask
ISOLATED SYSTEM: can exchange
neither matter nor energy with its
surroundings (e.g. a thermos flask)
What is heat?
HEAT is the energy that transfers from one object to
another when the two things are at different
temperatures and in some kind of contact
e.g. kettle heats on a stove
cup of tea cools down (loses energy as heat)
Heat is energy in transit
UNITS OF ENERGY
S.I. unit of energy is the joules (J)
Heat and work ( energy in transit) also measured in joules
1 kJ (kilojoule) = 103 J
Calorie (cal): 1 cal is the energy needed to raise the
temperature of 1g of water by 1oC
1 cal = 4.184 J
ENTHALPY (H)
Heat content of a substance
H = Hproducts - Hreactants
H
difference of heat content of
products and reactants
First Law of Thermodynamics:
*the internal energy of an isolated system is
constant
*energy is always conserved
Signs (+/-) will tell you if energy is entering or
leaving a system
+ indicates energy entering a system
- indicates energy leaving a system
EXOTHERMIC & ENDOTHERMIC REACTIONS
Exothermic process: a change (e.g. a chemical reaction)
that releases heat.
Burning fossil
fuels is an
exothermic
reaction
Endothermic process: a change (a chemical reaction)
that requires (or absorbs) heat.
Photosynthesis is an
endothermic reaction
(requires energy input
from sun)
Forming Na+
and Cl- ions
from NaCl is an
endothermic
process
Measuring Heat
reaction
reaction
Exothermic reaction, heat
given off & temperature of
water rises
Endothermic reaction, heat
taken in & temperature of
water drops
All chemical reactions either
release or absorb heat
Exothermic reactions:
Reactants
products + heat as energy
e.g. burning fossil fuels
Endothermic reactions:
Reactants + heat as energy
e.g. boiling water
products
∆H (‘delta H’) is the
symbol for the ‘change
in energy’.
 In an exothermic
reaction the products
have less energy than
the reactants.
 ∆H is negative for an
exothermic reaction.

In an endothermic
reaction the products
have more energy than
the reactants.
 ∆H is positive for an
endothermic reaction.

Concept Check
Look at the chemical reaction below.
6 CO2 + 6 H2O + energy --> C6H12O6 + 6 O2
Which statement BEST describes the chemical reaction?
A. The reaction absorbs energy
B. The reaction releases energy
C. The reaction neither releases nor absorbs energy
D. The reactants have energy but the products are energy-free
Concept Check
Look at the chemical reaction below.
6 CO2 + 6 H2O + energy --> C6H12O6 + 6 O2
Which statement BEST describes the chemical reaction?
A. The reaction absorbs energy
B. The reaction releases energy
C. The reaction neither releases nor absorbs energy
D. The reactants have energy but the products are energy-free
Evaporation
Energy has to be supplied to a liquid to enable it to overcome
forces that hold molecules together
• endothermic process (positive)
Melting
Energy is supplied to a solid to enable it to vibrate more
vigorously until molecules can move past each other and flow
as a liquid
• endothermic process ( positive)
Freezing
Liquid releases energy and allows molecules to settle into a
lower energy state and form a solid
• exothermic process ( negative)
(we remove heat from water when making ice in freezer)
Bond Strengths
• bond breaking requires energy
• bond making releases energy
Lattice (ex. NaCl)
energy is required to break up solids
Concept Check
Which term BEST describes the energy process
involved in forming a chemical bond?
A. Endothermic
B. Exothermic
C. Kinetic
D. Potential
Concept Check
Which term BEST describes the energy process
involved in forming a chemical bond?
A. Endothermic
B. Exothermic
C. Kinetic
D. Potential
Concept Check
Which term BEST describes the energy process
involved in breaking a chemical bond?
A. Endothermic
B. Exothermic
C. Kinetic
D. Potential
Concept Check
Which term BEST describes the energy process
involved in breaking a chemical bond?
A. Endothermic
B. Exothermic
C. Kinetic
D. Potential
Concept Check
Why must heat be absorbed in order for water to
boil?
A. To make more molecules
B. To form the bonds for the gas
C. To break the bonds of the liquid
D. To reduce the motion of molecules
Concept Check
Why must heat be absorbed in order for water to
boil?
A. To make more molecules
B. To form the bonds for the gas
C. To break the bonds of the liquid
D. To reduce the motion of molecules
WHY DO THINGS DISSOLVE?
•
Substances dissolve because energy and matter tend
to disperse (spread out in disorder)
• If it dissolves and solution heats up : exothermic
•If it dissolves and solution cools down: endothermic
Second Law of Thermodynamics:
*the disorder (or entropy) of a system tends to
increase
*any physical or chemical change must result in an
increase in the entropy of the universe
ENTROPY (S)
•Entropy is a measure of disorder
• Low entropy (S) = low disorder
•High entropy (S) = greater disorder
Total entropy
change
=
entropy change
+
of system
entropy change
of surroundings
Dissolving
disorder of
solution
disorder of
surroundings
• must be an overall increase in disorder for dissolving
to occur
1. If we freeze water, disorder of the water
molecules decreases , entropy decreases
2. If we boil water, disorder of the water molecules
increases , entropy increases (vapor is a highly
disordered state)