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Unit 10:
Energy in Chemical
Reactions
Cypress Creek High School
Chemistry 1L
Chapter 10
Energy
• In this unit, we will study
chemical systems & their
energy
– Energy is the ability to do
work
• All of these systems contain
kinetic and potential energy
– Kinetic energy – all particles
are in motion
– Potential energy – stored in
the bonds of compounds
Chemical Reactions
• When a substance
undergoes a chemical
change, it takes part in a
chemical reaction.
– After it reacts, it no longer
has the same chemical
identity.
• What are the evidences
that a chemical change has
occurred?
Evidence for Chemical Changes
• Color Change
• Gas produced without
heating
• Precipitate formed
• New odor develops
• Large amount of heat
or light produced
Collision Theory
• The following three statements summarize the collision theory.
1) Particles must collide in order to react.
2) The particles must collide with the correct orientation.
3) The particles must collide with enough energy to form an
activated complex, which is an intermediate particle made
up of the joined reactants.
• An effective collision is one that results in a reaction; new
products are formed.
Activated Complex
• The activated complex is an
intermediate particle formed
when reactants collide and stick
together.
– Old bonds break while new
bonds form.
• The minimum amount of energy
colliding particles must have to
form an activated complex is
called the activation energy.
– Particles that collide with less
than the activation energy
cannot form an activated
complex.
Exothermic Reactions
• In exothermic reactions energy is
released into the surroundings
– Potential energy, which is
stored in the bonds, converts
to kinetic energy when it is
released.
• Exothermic reactions feel warm
because thermal energy is
entering the surroundings (i.e.
your hands)
• The exothermic reaction releases
heat; energy is a product.
Endothermic Reactions
• In endothermic reactions energy is
absorbed from the surroundings
– Kinetic energy converts to
potential energy when it is
absorbed and is stored in bonds
• Endothermic reactions feel cold
because thermal energy is leaving
the surroundings (i.e. your hands)
• The endothermic reaction absorbs
heat; energy is a reactant.
Enthalpy
• Enthalpy is a measure of heat energy of a system at
constant pressure
– It can’t be measured directly, we measure change in enthalpy.
• Change in enthalpy is called heat of reaction or ΔHrxn
ΔHrxn = ΣH(products) - ΣH(reactants)
– This information will come from a table
– “Δ” is the letter delta – it means “change”
– “Σ” is the letter sigma – it means “sum”
– “ o ” refers to standard temperature and pressure
(STP)
– Units: kJ/mol
Change in Enthalpy
• Negative ΔH indicates an
energy loss (exothermic)
• Positive ΔH indicates an
energy gain
(endothermic)
Chemical Equations Energy
• Photosynthesis: plants consume
carbon dioxide, water, and light
energy (from the sun) and convert
it into sugar (glucose), oxygen, and
water.
6 CO2(g) + 12 H2O(l) + energy →
C6H12O6(aq) + 6 O2(g) + 6 H2O(l)
• These chemical equations
•
are important principles
of biology.
• Which is endothermic
and which is exothermic?
Cellular Respiration: animals
consume sugar (glucose) and
oxygen to produce carbon dioxide,
water, and chemical energy (ATP)
C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6H2O(l) +
energy
Change in Enthalpy
• All elements in their standard
states have an enthalpy of zero
– Because there is no change
involved in their formation
Endothermic
• When N2 and O2 combine to
form NO2, the ΔH = +33.2
kJ/mol
Exothermic
• When S and O2 combine to
form SO3, the ΔH = -396 kJ/mol.
Check for Understanding!
• Do elements in their
standard states possess
zero energy?
• Why are elements in
their standard states
assigned enthalpies of
zero?
• What does the +33.2 on
the graph tell you?
• What does the -396 on
the graph tell you?