Thermochemistry

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Thermochemistry Heat and Chemical Change

• Thermochemistry - study of
• heat transfer in chemical and physical processes.

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Energy Transformations

• Energy is the capacity to do work or transfer
  heat.

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ENERGY
Conservation of Energy gtThe energy of a system
is constant. gtEnergy cannot be created or
destroyed. gtEnergy can change from one form
to another.
Common Units of Energy Joules (J) , calories
(cal)
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Potential Energy

• Is stored energy
• Examples
• Water behind a dam
• Compressed spring
• Chemical bonds in gasoline or coal
• Food

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Kinetic Energy

• Is energy of motion
• Examples
• Hammering a nail
• Water flowing over a dam
• Working out
• Boxing
• Burning gasoline

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The object must have velocity to have kinectic
energy
Who has kinetic energy?
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A. The Nature of Energy

• Potential energy (PE) Kinetic energy (KE)

the energy of position
the energy of motion
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Total energy is the sumof both the kinetic
energy and the potential energy.
No friction!
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Some Forms of Energy

• Mechanical
• Electrical
• Thermal (heat)
• Chemical
• Radiant (light)

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• Temperature vs. Heat

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HEAT TRANSFER
Temperature and heat are different.
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• Hot cold, are automatically associated with the
  words heat and temperature
• Heat temp are NOT the same
• The temperature of a substance is directly
  related to the energy of its particles,
  specifically its

• The Kinetic Energy (motion) defines the
  temperature
• Particles vibrating fast hot
• Particles vibrating slow cold

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Temperature

• Temperature is a measure of the average kinetic
  energy in matter.

Hot water (90oC) Cold water (10oC)
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Learning Check

• Suppose you place water in a freezer.
• A. The water particles move
• 1) faster 2) slower 3) the same
• B. The water will get
• 1) hotter 2) colder 3) stay the same
• C. The temperature of the water will be
• 1) higher 2) lower 3) the same

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Solution

• Suppose you place water in a freezer.
• A. The water particles move
• 2) slower
• B. The water will get
• 2) colder
• C. The temperature of the water will be
• 2) lower

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Heat

• the energy that transfers between two objects due
  to a temperature difference between them.
• Heat is transferred from a hot object to a colder
  object (never in the other direction).

Hot water (90oC) Cold water (10oC)

• Water (50oC) Water (50oC)

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Learning Check

• A. When you touch ice, heat is transferred
• from
• 1) your hand to the ice
• 2) the ice to your hand
• B. When you drink a hot cup of coffee, heat
• is transferred from
• 1) your mouth to the coffee
• 2) the coffee to your mouth

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Solution

• A. When you touch ice, heat is transferred
• from
• 1) your hand to the ice
• B. When you drink a hot cup of coffee, heat
• is transferred from
• 2) the coffee to your mouth

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Learning Check

• When you heat 200 g of water for 1 minute,
  the water temperature rises from 10C to 18C.
• If you heat 400 g of water at 10C in the same
  pan with the same amount of heat for 1 minute,
  what would you expect the final temperature to
  be?
• 1) 10 C 2) 14C 3) 18C

400 g
200 g
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Solution

• 2)14C
• Heating twice the mass of water using the
  same amount of heat will raise the temperature
  only half as much.

400 g
200 g
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Some Equalities for Heat

• Heat is measured in calories or joules
• 1 kcal 1000 cal
• 1 calorie 4.18J
• 1 kJ 1000 J

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Question

• How many joules are in 375 calories?
• Answer
• 375 cal x 4.18 J/cal 1567.5 joules

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In food, a Calorie is the same as a kilocalorie.

• If an apple provides 120 Calories, that is the
  same as
• 120,000 calories,
• or 501,600 joules of energy.
• (120 Cal)(1000 cal/1 Cal)(4.18 J/cal) 501,600 J

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Specific heat capacity is
specific heat

• The amount of energy needed to change the
  temperature of 1.0 gram of a substance by 1.0
  degree Celsius (or 1.0 kelvin).
• Units cal/gºC or J/gºC

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Table of Specific Heats

• Smaller the specific heat ? the less energy it
  takes the substance to feel hot
• Larger the specific heat ? the more energy it
  takes to heat a substance up (bigger the heat
  reservoir)

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The specific heat of water

• The specific heat of water is 4.18 J/g . K
• The amount of heat required to raise the
  temperature of one gram of water 1C 1 Calorie
• The specific heat of aluminum is 0.902 J/g . K.
• It takes over 4 times as much heat to raise the
  temperature of a gram of water by a certain
  amount than a gram of aluminum.

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Learning Check

• A. A substance with a large specific heat
• 1) heats up quickly 2) heats up slowly
• B. Oceans change temperature slowly. Oceans have
  a
• 1) high specific heat 2) low specific heat
• C. Sand in the desert is hot in the day, and
  cool
• at night. Sand must have a
• 1) high specific heat 2) low specific heat

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Solution

• A. A substance with a large specific heat
• 2) heats up slowly
• B. Oceans have a
• 2) high specific heat
• C. Sand in the desert is hot in the day, and
  cool
• at night. Sand must have a
• 2) low specific heat

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CHEMICAL RXNS

• There are 2 types of chemical rxns
• Exothermic energy flows out of the system
  (surroundings get warmer)
• Endothermic energy flows into the system
  (surroundings get cooler)

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SURROUNDINGS
HEAT
HEAT
HEAT
HEAT
SYSTEM
SYSTEM
EXOTHERMIC
ENDOTHERMIC
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Exothermic Reactions

• Feel warm as the rxn proceeds
• Gives off heat energy
• The energy stored in the chemical bonds of the
  reactants is greater than the energy stored in
  the bonds of the products
• ?H
• Example Combustion
• AB CD Heat

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Endothermic Reactions

• Endothermic rxns typically feel cooler the longer
  the rxn proceeds
• Absorbs heat energy from surroundings, sometimes
  enough to get very cold
• Cause temperature to fall
• More energy is stored in the bonds of the
  products than in the bonds of the reactants
• - ?H
• AB Heat CD

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EXOTHERMIC
ENDOTHERMIC
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Specific Heat
Lets define some variables Q heat lost or
gained m mass c specific heat ti tf
initial final temperature
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Calculating Heat Transfer

• To calculate the energy required for a reaction

Heat transferred specific heat X mass X change
in temp
q C X m X ?T
?T (Tfinal - Tinitial).
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Problem

• How many joules must be transferred from a cup of
  coffee to your body if the temperature of the
  coffee drops from 60.0oC to 37.0oC (normal body
  temp)? Assume the cup holds 250. mL (with a
  density of 1.0 g/mL, the coffee has a mass of
  250. g), and that the specific heat of coffee is
  the same as that of water.

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Answer

• q Cm?T
• 2) q (4.18 J/g . K) (250. g) (37.0oC 60.0oC)
• 3) -24100 J -24.1 kJ
• Note the negative value denotes the direction of
  heat transfer it shows that energy is
  transferred from the coffee as the temperature
  declines.

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Problem

• In the previous problem, the cup of coffee lost
  24.1 kJ when cooled from 60.0oC to 37.0oC. If
  this same amount of heat is used to warm a piece
  of aluminum weighing 250. g. what would be the
  final temperature of the aluminum if its initial
  temperature is 37.0oC? (The specific heat of
  aluminum is 0.902 J/g . K.)

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Answer

1. q Cm?T
2. 24100 J (0.902 J/g . K) (250. g) (Tfinal -
   37.0oC)
3. Tfinal 144oC

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Calorimetry

• Calorimetry - process of measuring heat energy
• Measured using a calorimeter
• Uses the heat absorbed by H2O to measure the heat
  given off by a rxn or an object
• The amount of heat soaked up by the water is
  equal to the amount of heat released by the rxn

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Problem

• Suppose you heat a 55.0 g piece of iron in the
  flame of a Bunsen burner to 425oC and then you
  plunge it into a beaker of water. The beaker
  holds 600. mL water (density 1.00 g/mL), and
  its temperature before you drop in the hot iron
  is 25.0oC. What is the final temperature of the
  water and the piece of iron?

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Answer (set up)

• Heat lost by metal - (Heat gained by water)
• SO
• qmetal - qwater
• WHICH MEANS
• Cm?Tmetal - Cm?Twater
• (55.0 g) (0.451 J/g . K) (Tfinal - 425oC)
• - (600. g) (4.18 J/g . K) (Tfinal
  - 25oC)

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Problem solved

• 1) (55.0 g) (0.451 J/g . K) (Tfinal - 425oC)
  -(600. g) (4.18 J/g . K) (Tfinal - 25oC)
• 2) 24.805 (Tfinal - 425) -2508 (Tfinal - 25oC)
• 24.805 Tfinal 10542 -2508 Tfinal 62700
• 2532.8 Tfinal 73242.
• Tfinal 29oC

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Remember

• always subtract the initial temp from the final
  temp this results in a calculation that
  indicates an increase () or a decrease () in
  the heat transferred.
• If q is , heat is transferred into the object
  (endothermic),
• if q is , heat is transferred out of the object
  (exothermic).

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Problem

• You want to cool down a cup of coffee, and you do
  so by dropping in a cold block of aluminum. The
  aluminum block has a mass of 250. g and you want
  to cool 250. g of coffee (with a specific heat of
  4.18 J/g . K) from 60.oC to 45oC. To accomplish
  this, what must the initial temperature of the
  aluminum block be?

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Answer

1. Heat transferred from coffee (250. g) (4.18 J/g
   . K) (45oC 60.oC) -15675 J
2. Heat transferred to aluminum 15675 J (250. g)
   (0.902 J/g . K) (45oC Tinitial)
3. Tinitial -25oC

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Enthalpy

• For systems at constant pressure, the heat
  content is also called the enthalpy (H) of the
  system.
• All of the heat changes we have discussed occur
  at constant pressure, so for these processes, q
  ?H.

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Thermochemical equations

• CaO H2O ? Ca(OH)2 65.2 kJ
• You can treat heat change in a chemical reaction
  like any other reactant or product in a chemical
  equation.
• An equation that includes the heat change is
  called a thermochemical equation. This equation
  includes a heat of reaction .

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In thermochemical equations

• If heat is a reactant, energy is absorbed and the
  reaction is endothermic.
• If heat is a product, energy is released and the
  reaction is exothermic.

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Heat of Fusion

• The heat absorbed by one mole of a substance in
  melting is the molar heat of fusion (DHfus).
• The heat lost when one mole of a substance
  solidifies is the molar heat of solidification
  (DHsolid).

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DHfus - DHsolid

• The amount of heat absorbed by melting a solid is
  exactly the same as the amount of heat lost when
  the liquid solidifies (but opposite in direction
  of heat flow).
• DHfus - DHsolid

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Heat of vaporization

• The amount of heat needed to vaporize one mole
  of a substance is called its molar heat of
  vaporization (DHvap).
• The amount of heat released when one mole of
  vapor condenses is called the molar heat of
  condensasion (DHcond).

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DH vap - DHcond

• The amount of heat absorbed by melting a solid is
  exactly the same as the amount of heat lost when
  the liquid solidifies (but opposite in direction
  of heat flow).
• DHvap - DHcond

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During a phase change

• As a substance changes phase, no temperature
  change occurs so the heat involved is described
  as the latent heat of the phase change.
• Heat is added, but no temperature change is
  observed as a substance boils, melts, etc.

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• To solve for the amount of heat involved in a
  change of phase
• q m (DH)

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Heating curve
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Practice

• A. When water freezes, the temperature of the
  water ______(gets hotter, colder or stays the
  same), the surroundings get _______ (hotter or
  colder)
• B. When water boils, the temperature of the water
  ______(gets hotter, colder or stays the same),
  the surroundings get _____ (hotter or colder)

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Problem

• How much heat is absorbed when 75.0 g H2O(l) at
  100.oC is converted to steam at 100.oC?
• (The DHvap for water is 40.7 kJ/mol.)

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Answer

• Convert grams to moles
• (75 g H2O)(1 mol/18.02 g) 4.16 mol
• q m (DH)
• q (4.16 mol) (40.7 kJ/mol) 169 kJ