Download Heat and Energy of Ractions

Heat and Energy of Ractions
Chapter 10
Energy
World of Chemistry
Zumdahl
Last revision Fall 2009
What is energy?

The ability to cause a change.
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
change in position or temperature
1st law of thermodynamics: Energy is neither
created nor destroyed, only transformed from
one type to another.
The unit is the
Energy transformations

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Joule (J) or
Kgm2/s2
Kinetic: motion
Potential: stored
Work: applied forces to make motion
Heat: really kinetic energy of molecules
Temperature and Heat

Temperature is a measure of the random
motions of the particles of a substance.

Hot water
(90. oC)
Cold water
(10. oC)
More motion
in particles
means higher
temperatures

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Absolute
Zero is the
temperature at
which all
motion of
particles stops.
Scientists have
yet to reach it.
State of Matter- Matter can be solid, liquid,
gas or plasma depending on how much kinetic
energy (or motion) they have.

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Solid: Particles vibrate in place. It has a definite
volume and a definite shape.
Liquid: Particles roll around each other. It has a
definite volume but no definite shape.
Gas: Particles move around colliding with one another.
It has no definite volume or shape.
Plasma: Particles are so hot and excited that the
electrons leave the nucleus and matter is ionized or
charged
Matter in Motion
The Kinetic Molecular Theory states
that atoms and molecules are always
in motion.
Remember, the temperature of a substance is
the measure of its kinetic energy.

That energy can only do one thing at a time:
1. Change the state of the substance (disrupt the
intermolecular forces that hold it in a phase).
2. Increase the temperature (motion) of a substance.
1. Changes in the states of Matter
Boiling
Heat of vaporization
Condensing
Freezing
Heat of fusion
Melting
Sublimating
Depositing
Heating Curve
This curve can also work in
reverse if energy is being taken
away.
condensing
vaporizing
Freezing
Melting
Energy in Reactions

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Exothermic Reactions – A reaction in
which energy is released. Feels hot.
Endothermic Reactions – A reaction which
energy must be provided for it to continue.
Absorbs energy and feels cold.
Dissolution Reactions – When ionic
compounds dissolve in water.
Will this be endothermic or exothermic?
(remember, breaking bonds requires energy)
Endothermic
What the heat?
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Heat is energy that transfers from one object to
another because of a temperature difference
between them.
Heat, itself, cannot be detected by the senses
or by instruments. Only the change can be
detected.
Heat always flows from a warmer object to a
cooler object.
Heat takes the perspective of the reaction so it
is positive if it is absorbed and negative if it is
released
CH 4 + 2O 2  CO 2 + 2H 2 O + Heat
The products have less
energy than the
reactants. It has to go
somewhere
Potential energy
Activation Energy
CH 4 + 2O 2
Heat
Energy of
Reactants
Released to the
surroundings (-)
Energy of Products
CO 2 + 2 H 2 O
Exothermic has heat as a product
N2 + O2 +heat  2NO
Potential energy
2NO
Absorbed from the
surroundings (+)
N2 + O2 Energy of Reactants
Heat
Heat must be
present for the
reaction to
happen
Energy of
Products
Endothermic has heat as a reactant
Energy, potential energy, and Enthalpy.
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Energy (q) is the ability to do work or produce
heat.
Kinetic energy is the motion of the molecules
and can be measured by taking it’s temperature.
Potential energy is the energy stored in a
substance because of its composition (like the
types of bonds it formed).
Enthalpy (H) is the heat released or absorbed in
a chemical reaction.
Heat Capacity and Specific Heat

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Heat Capacity (C) is the amount of heat energy a
substance can absorb before the substance will
increase its temperature by one degree Celsius.
Specific heat (s) of a substance is the amount of heat
required to raise the temperature of one gram of that
substance by one degree Celsius.
Calculating the Energy of a
Reaction
Any energy lost by the reaction must be gained by the
surroundings. Energy must always be conserved. The sign
on Q indicates if energy is leaving or entering the system.
Try it yourself:

What is the specific heat of a pure metal
that has a mass of 2.8g and requires 10.1J
of energy to raise the temperature from
21oC to 36 oC.
Try this (Remember heat is conserved):
-Qlost= Qgained

What is the specific heat of the rock below if
the rock begins at 100 oC and is placed in the
beaker of water at 20.0oC with a specific heat
capacity of 4.18J/g oC. They both end up at
23.5 oC.