Download 5.1 Energy Changes in Chemical and Nuclear Reactions

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5.1 Energy Changes in Chemical and Nuclear Reactions

Thermochemistry is the study of energy changes during a physical or chemical
process.
o Energy is the ability to do work or transfer heat; work is energy
transferred to an object to make it move (both energy and work are in SI
units of joules). Heat is energy transferred to an object to make it warm
up or move its internal particles faster
 Energy is classified as either potential energy or kinetic energy:
o Potential energy is energy due to position or composition of matter;
o Kinetic energy is the energy of motion of matter
 Energy associated with chemical bonds is potential energy.
 In a chemical process, the amount of energy released or absorbed equals the
potential energy difference between the bonds in the reactants and the bonds in
the products.
Show example of combustion engine  releases energy because bonds of products
have less potential energy than those of reactants  is transferred to products as
kinetic energy of gases which are then used to turn into mechanical energy.
Thermal Energy vs. Temperature
 The total quantity of potential energy and kinetic energy contained by a
substance is called thermal energy  depends on how fast its component atoms
are moving.
 Substance absorbs thermal energy from its surroundings, its component particles
move faster and it warms up; substance releases thermal energy, its components
slow down and it becomes cooler.
 Heat is the transfer of thermal energy from one warm object to a cool object, i.e.
heating a beaker of water over a Bunsen flame transfers thermal energy from the
burner to the water molecules (and to the beaker molecules, the air molecules,
etc).
 Temperature is a measure of the average kinetic energy of a particle in a
substance.
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 So, comparing a cup of tea with a swimming pool, tea = higher temperature
because of higher average kinetic energy within the substance; pool = higher
thermal energy because of far more particles of water – more total energy.
Law of Conservation of Energy
 Energy can neither be created nor destroyed, but only changed from one form to
another.
 A chemical system represents all of the reactants and products of a chemical
reaction; can be open or closed systems.
o Open system – the reaction exchanges energy and matter with its
surroundings. E.g. propane barbecue
o Closed system – the reaction exchanges energy, but not matter, with its
surroundings. E.g. ideal lab and industry reactions
Exothermic vs. Endothermic Rxns
 During a chemical process, bonds are broken and reformed as reactants become
products.
o If the product bonds have less potential energy than the reactant bonds,
thermal energy is released  exothermic reactions (more energy is
released by the formation of new product bonds than is required to
break the old reactant bonds
o If the product bonds have more potential energy than the reactant
bonds, energy is absorbed from the surroundings  endothermic rxn.
Show potential energy differences in model rxns. (like p 288 fig 4)
Nuclear Reactions
 We have previously looked at different types of nuclear reactions, mainly
fission and fusion reaction.
 All nuclear reactions are very exothermic, the breaking of the nuclear bonds
gives off far more energy than it requires.
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 All types of physical, chemical, and nuclear processes involve energy
exchanges.
 Physical processes usually involve the lowest amounts of energy change
(involve intermolecular forces); e.g. condensation of steam to liquid water.
 Chemical reactions can be 10 to 300 times more energetic (due to breaking and
making chemical bonds – ionic, covalent, metallic); e.g. combustion rxn
 Nuclear reactions can be a further million times more energetic (breaking and
making nuclear bonds); e.g. solar fusion rxn.
Practice Questions: Pg. 291 # 1, 2, 4, 5, 8, 9; Worksheet 5.1 # 6 – 18.