Thermochemistry

1
Thermochemistry
The study of energy during physical changes and
chemical changes.
2
Objectives

• Be able to define and correctly use
  energy-related terminology.
• Identify and understand endothermic and
  exothermic processes.

3
Energy

• energy the ability to do work (push things)
• chemical bonds store chemical potential energy.
• Some substances store much energy (example
  fuels, explosives) but others contain little
  energy (H2O, CO2).

4
Heat Terms

• thermal energy total KE of particles in a
  substance
• (depends on g and oC)
• heat thermal energy that flows from warmer to
  cooler areas.
• Heat transfers by
• (1) conductioncolliding particles
• (2) radiationelectromagnetic waves
• (3) convectioncurrents
• enthalpy a measure of total energy (KE PE)
  stored in a system at constant pressure

5
Energy Transfer
exothermic process system releases energy and
the surroundings typically get warmer
Example coffee cools down, air gets warmer.
surroundings (includes thermometer)
energy
system
enthalpy decreases -DH
6
Energy Transfer
endothermic process system gains energy from
the surroundings (the surroundings may get colder)
Example the surroundings get cooler as ice melts.
surroundings
energy
system
enthalpy increases DH
7
Objectives

• Be able to define and correctly use the common
  units of thermal energy.
• Be able to define and understand the concept of
  specific heat.
• Be able to make calculations related to thermal
  energy and temperature changes.

8
Calorimetry

• calorimetry calculating or measuring heat
  transfer
• common energy units calories (cal) or joules (J)
• It takes about 4000 J to heat 1 kg of water by
  1oC. (4 J for 1 g)

9
Specific Heat

• specific heat the amount of energy required to
  raise 1.00 g of a substance by 1.00oC.
• molecules higher Cp (change T slowly)
• metals lower Cp (change T quickly)

Substance Cp (J/goC)
water 4.184
rubber 2.0
plastic 1.7
concrete 0.9
steel 0.5
mercury 0.14
10
Energy and Temperature Changes

• temperature changes involve changes in kinetic
  energy
• Q m DT Cp
• Q is enthalpy ( J)

Q enthalpy increase endothermic -Q
enthalpy decrease exothermic
11
Calorimetry Problems

• Example What is the change in enthalpy when a
  cup of water (227 g) cools from boiling to room
  temperature (97oC to 22oC)?
• Example A wedding ring absorbs 16.4 J of energy
  when it is placed on a finger (the temperature
  rises from 21oC to 38oC). If the mass of the
  ring is 4.80 g, what is the specific heat of
  the metal?

12
Objectives

• Understand the concept of latent heat and how it
  corresponds to potential energy.
• Be able to make latent heat calculations.

13
Latent Heat

• Changes in state involve changes in potential
  energy.
• This stored energy is often called latent heat.
• Temperature (KE) is constant during a phase
  change.

Endothermic Processes
Exothermic Processes
VAPOR
-DH Latent Heat of Condensation
Latent Heat of Vaporization DH
WARM LIQUID
COOL LIQUID
-DH Latent Heat of Solidification
Latent Heat of Fusion DH
SOLID
14
Latent Heat Values
Substance DH (kJ/mol) ( melt, - solid) DH (kJ/mol) vapor, - cond
ammonia, NH3 5.65 23.4
ethanol, CH3CH2OH 4.60 43.5
hydrogen, H2 0.12 0.90
methanol, CH3OH 3.16 35.3
oxygen, O2 0.44 6.82
water, H2O 6.01 40.7
Size of value depends on the strength of
intermolecular bonds!
15
Latent Heat Calculations

• Temperature remains constant during a phase
  change, so dont use Q m DT Cp
• use Q (m/M) DH
• Example How much energy is needed to boil 19.75
  g of ethanol (CH3CH2OH)?
• Example How much water (at 0oC) is freezing if
  2.5 kJ of energy is released?

16
Latent Heat and Flathead Cherries!

• Sometimes water is sprayed on cherries to protect
  them from freezing. Why?
• As the water freezes, it releases latent heat
  which warms/protects the fruit.

17
Objectives

• Be able to draw a heating curve or cooling curve
  for a substance.
• Be able to correctly label the regions where DKE
  and DPE are occurring on a heating or cooling
  curve.

18
Heating and Cooling Curves

• Imagine heating an ice cube. What energy changes
  take place as it is continually heated?

changes of state Q m/M DH
D PE molecules pulled apart when boiling
D KE molecules speed up
D PE molecules pulled apart when melting
TEMPERATURE
D KE molecules speed up
heating or cooling Q m DT Cp
D KE molecules speed up
TIME
19
Objectives

• Understand the concept of a standard heat of
  formation.
• Be able to calculate the heat of reaction using
  Hesss Law and determine if a reaction is
  endothermic or exothermic.

20
Standard Heat of Formation

• standard heat of formation (DHf0) change in
  enthalpy
• that accompanies the formation of one mole of
  a
• compound from its elements at 25oC and
  101.3kPa.

DHf0 for any uncombined element in its normal
state 0 kJ/mol
21
Heat of Reaction
heat of reaction (Q) the change in enthalpy
(energy lost or gained) in a chemical
reaction Use Hesss Law to calculate the heat of
reaction Q S(nDHf0)products -
S(nDHf0)reactants Calculate Q, state if
endo/exothermic, and write the thermochemical
equation (A) CH4(g) 2O2(g) ? CO2(g)
2H2O(g) (B) 2CO2(g) ? 2CO(g) O2(g)
22
Objective

• Be able to draw a potential energy diagram for
  a chemical reaction using a thermochemical
  equation.
• Be able to correctly determine the temperature
  changes that occur in the surroundings for both
  endothermic and exothermic reactions.

23
Kerr Dam
Cu2 2e- energy ? Cu

• Energy is needed to make copper from copper ore.
• Kerr Dam was built to produce electricity for
  copper production in Anaconda. Plus, the copper
  could be used to make wiring for homes in the
  region.

24
Thermochemical Equations and PE Diagrams

• endothermic reaction (DH0)
• 2CO2(g) energy ? 2CO(g) O2(g)
• 2CO2(g) 566 kJ ? 2CO(g) O2(g)

PE diagram
2CO(g) O2(g)
566 kJ DH (enthalpy increases)
2CO2(g)
endothermicproducts have more energy, so they
are less stable
25
PE Diagrams

• exothermic reaction
• CH4(g) 2O2(g) ? CO2(g) 2H2O(g) 890 kJ

CH4(g) 2O2(g)

• 890 kJ
• - DH (enthalpy decreases)

CO2(g) 2H2O(g)
exothermicproducts have less energy stored in
their bonds and are more stable