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Transcript
Chemistry
Heat Energy
Energy
• Work: The result of a force acting on a distance
• Energy: The Capacity to do Work (or the ability to cause
a change)
• All samples of matter have energy! Matter is driven in
part by energy.
Law of Conservation of Energy
Energy is neither created nor destroyed.
•
Energy can be changed from one form to another
•
Energy can be transferred from one object to another
•
Energy can NOT be created out of nothing and does not
vanish into nothing
Potential Energy
• Potential energy is stored energy. No motion involved.
• Dependent on its position (location) and its condition.
• It is used to “run” many things.
Potential Energy
• Potential energy is stored energy. No motion involved.
• Dependent on its position (location) and its condition.
• It is used to “run” many things.
Forms of Potential Energy
Forms of Potential Energy
•
Chemical (food, gasoline)
• Gravitational (position/place)
• Stored Mechanical (strecthed rubber band)
• Nuclear (fusion, fission)
Kinetic Energy
• Energy in the form of motion
• Dependent on mass and velocity of
the object
• The greater the mass and velocity, the
more kinetic energy the object has
Kinetic Energy
Things have energy depending on how fast they move
Which one has more kinetic energy?
Forms of Kinetic Energy
Sound
Radiant
Electrical
Heat
Law of Conservation of Energy
• Kinetic energy can turn into potential energy,
which can turn into kinetic energy and so on.
Units of Energy
• SI Unit for Energy: Joule (J)
• Secondary unit: calorie (cal)
• Note: calorie is different from a nutritional Calorie (Cal)
• 1000 cal = 1 Cal
• 1 cal = 4.184 Joules
Heat Energy
• The kinetic energy of molecular motion.
• A material feels hot because the atoms of molecules of
which it is made are moving. (i.e., the faster they move,
the hotter the material feels).
• Temperature is used to help quantify this phenomena.
Temperature
• Temperature measures the average kinetic
energy of molecules in a material.
• It does not measure the total amount of
heat energy a material contains.
Heat Capacity
• Different materials have different
capacities for storing energy as heat.
Heat Capacity
• The quantity of heat (usually in J) needed to change the
temp. of a given amount of the substance by 1 ºC
• Some factors that attribute to varying heat capacities of
substances
1. Vibrational modes of molecules
2. Strength of the chemical bonds involved.
Heat Capacity
• Specific Heat Capacity – a more precise term to describe heat
capacity when expressed in grams. ( cal / g ºC ) or ( J / g ºC )
Heat Capacity
• To quantify the amount of heat mathematically:
Heat = Mass x Specific Heat Capacity x Temperature Change
q = m x C x DT
• Equation describes amount of heat either gained or lost by material
as temperature changes.
• Question: If a substance loses heat when the temp. goes down, will
the value of q be negative or positive? Value of DT?
Heat Capacity
• The total heat in a closed system must remain constant, which is
represented by the equation.
qsystem + qsurroundings = 0
Exothermic v. Endothermic Process
• If q is positive:
A system gains heat and is referred to as Endothermic
• If q is negative:
A system loses heat and is referred to as Exothermic.
• “System” refers to the particular sample of material whose
temperature and/or phase is changing.
Exothermic v. Endothermic Process
• Exothermic and Endothermic can be
applied to chemical reactions as well.
Specific Latent Heat
• Two types:
1. Heat of vaporization – quantifies the transfer of energy when a
substance changes from a liquid to gas or gas to liquid.
2. Heat of Fusion - quantifies the transfer of energy when a
substance changes from a solid to liquid or liquid to solid
• No temperature changes are considered.
q = m x L
Heat Capacity
How much energy (in kcal) is needed to heat exactly one
liter of water from room temperature to its boiling point?
Heat Capacity
How much energy (in kcal) is needed to heat exactly one
liter of water from room temperature (22 ºC) to its boiling
point?
• You need 3 things:
1) The heat capacity of the substance being heated
2) The temperature change
3) The mass of the sample
Heat Capacity
How much energy (in kcal) is needed to heat exactly one liter
of water from room temperature (22 ºC) to its boiling point?
Heat Capacity
How much energy (in kcal) is needed to heat exactly one liter
of water from room temperature (22 ºC) to its boiling point?
Heat Capacity
How much energy (in kcal) is needed to heat exactly one liter
of water from room temperature (22 ºC) to its boiling point?
Heat Capacity
If 250.0 g of water at 83 ºC loses 20.0 kcal of heat, what
will its final temperature be?
Heat Capacity
If 250.0 g of water at 83 ºC loses 20.0 kcal of heat, what
will its final temperature be?
Heat Capacity
If 250.0 g of water at 83 ºC loses 20.0 kcal of heat, what
will its final temperature be?
Heat Capacity
If 250.0 g of water at 83 ºC loses 20.0 kcal of heat, what
will its final temperature be?
Problem
If you hold Gallium in your hand, it melts from your body
heat. How much heat must 2.5 g of gallium absorb from your
hand to raise the temperature of the gallium from 25 ºC to
29.9 ºC? The specific heat of gallium is 0.372 J / (g ºC).