Thermochemistry

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Thermochemistry

• The study of energy changes in chemical reactions
  and physical changes

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Energy

• The ability to do work
• Energy changes in chemical reactions and physical
  changes are measured in the form of heat
• There are two types of energy involved in these
  changes
• Potential
• Kinetic

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Potential Energy

• The energy due to composition or position of an
  object

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Kinetic Energy

• Energy of motion

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Heat (q)

• The total kinetic energy of the random motion of
  particles in a substance
• Will always flow from a warmer object to a cooler
  one
• Measured in Joules (J) or calories (cal)
• 1 cal 4.184 J
• Heat is different from temperature

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Brownian motion is the random motion of a
molecule, or other very light object. It is
observed when the object is big enough to see, as
a speck of dust in a light beam, either in air or
in water. The speck of dust in air is being
struck at random by molecules of air, and keeps
changing direction because of that.
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Temperature

• A measure of the average kinetic energy of random
  motion of particles in a sample of matter
• The more particles, the greater the amount of
  heat must be transferred to raise the average
  kinetic energy of all the particles

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Specific Heat

• Relates temperature changes to heat changes
• Defined as the amount of heat energy required to
  increase the temperature of one gram of a
  substance by one degree Celsius
• Specific heat -Varies if pressure and temperature
  are not kept constant
• A physical property
• Varies depending on the substance
• Symbol is Cp, units are J/gC

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Specific Heat

• Must be measured!
• Substances with low specific heats require less
  energy to feel hot than those with high specific
  heats
• Specific heat can be used to calculate changes in
  heat

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Substance Specific Heat J/gC
Water (liquid) 4.184
Water (solid) 2.03
Water (steam) 2.01
Ethanol (liquid) 2.44
Aluminum (solid) 0.897
Granite (solid) 0.803
Iron (solid) 0.449
Lead (solid) 0.129
Silver (solid) 0.235
Gold (solid) 0.129
Copper (solid) 0.385
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Heat Calculations

• Change in heat of a substance can be calculated
  using the following equation
• q m?TCp
• q change in heat
• m mass of the substance
• ?T change in temperature of the substance
• Cp specific heat of the substance

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Total Energy Changes

• The amount of heat (q) involved in a reaction is
  positive () if the sample warms up. The sample
  is gaining heat.
• The amount of heat (q) involved in a reaction is
  negative (-) if the sample cools off. The sample
  is releasing, or losing, heat.

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Specific Heat Problems

• Use the sheet which has the chart on it to find
  the specific heat of the element or substance in
  the problem
• Solve using algebra and the equation
• q m?TCp

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Sample Problems
If the temperature of 34.4 g of ethanol increases
from 25.0 ?C to 78.8 ?C, how much heat has been
absorbed by the ethanol? The specific heat of
ethanol is 2.44 J/(g??C)
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Sample Problems
A 4.50 g nugget of pure gold absorbed 276 J of
heat. What was the final temperature of the gold
if the initial temperature was 25 ?C ? The
specific heat of gold is 0.129 J/(g??C).
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Sample Problems
A 155-g sample of an unknown substance was heated
from 25.0?C to 40.0 ?C. In the process, the
substance absorbed 5696 J of energy. What is the
specific heat of the substance?
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Calorimeter Questions

• Transfer of heat is measured by measuring the
  difference in temperature transferred to water
  from an object
• Specific heat of water (4.184 J/gC) and its mass
  is used to solve the problem.

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Sample Problem

• A piece of metal is placed in a calorimeter, and
  causes the 335 g of water to increase in
  temperature from 21.0C to 50.1C. What is the
  amount of energy released by the piece of metal?

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Enthalpy

• Defined as the total absolute amount of energy in
  a system.
• This cannot be measured or calculated directly
• Changes in energy, however, CAN be measured

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Enthalpy Change

• Enthalpy change is represented as ?H
• Defined as the heat energy released (-) or
  absorbed () by a system during a physical or
  chemical change
• System must be at a constant pressure throughout
  the change

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Enthalpy Change

• ?H SHproducts SHreactants
• Exothermic
• Heat is released
• ?H is NEGATIVE
• Reaction feels warm
• Endothermic
• Heat is absorbed during a reaction
• ?H is POSITIVE
• Reaction feels cold

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Activation Energy

• Defined as the minimum amount of energy that must
  be supplied to a system to start a chemical
  change.
• Endothermic reactions must have a source from
  which to draw their energy (usually their
  surroundings)

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Heat in Physical Changes

• Changes in state are reversible processes that
  can be reversed by adjusting the temperature
• Each change in state requires an energy transfer

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Examples

• Assume both samples are heated at a constant
  temperature
• Sample 1
• 1 ice cube melts quickly needs less heat to
  boil
• Sample 2
• 8 ice cubes melts slowly needs more heat to
  boil

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Heat of Fusion

• Symbol ?Hfus
• Defined as heat necessary to convert a given
  amount of a solid to a liquid
• ?H mol substance x ?Hfus
• Total heat change the number of moles of a
  substance x its heat of fusion
• Heat of fusion is measured in kJ/mol

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Heat of Vaporization

• Symbol ?Hvap
• Defined as heat necessary to vaporize a given
  amount of a liquid
• ?H mol substance x ?Hvap
• Total heat change the number of moles of a
  substance x its heat of vaporization
• Heat of vaporization is measured in kJ/mol

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Four More Heats!!

• Heat of Solution heat changes involved in
  dissolving a solute in a solvent
• Heat of Reaction the amount of energy absorbed
  or released during a chemical change
• Heat of Formation heat changes involved in the
  synthesis of a mole of compound from its elements
• Heat of Combustion heat energy released when a
  substance reacts with oxygen to form CO2 and H2O

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Heat in a Chemical Reaction

• Energy can be converted into other forms
• Measures of changes in heat energy can be made in
  a calorimeter
• Changes in heat energy can be used to calculate
  specific heat
• These heats refer to the total flow of energy
  during a chemical change

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How can I use the mole ratio in these problems?

• Because the heats of chemical reactions are
  expressed in kJ/mol, these amounts can be used in
  stoichiometric problems as if it were a mole
  ratio!

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Sample Problems

• Calculate the heat required to melt 25.7 g of
  solid methanol at its melting point. The ?Hfus
  of methanol is 3.22 kJ/mol.

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Sample Problem

• What mass of methane must be burned in order to
  liberate 12.880 kJ of heat? The ?Hcomb of
  methane is -891 kJ/mol.

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Sample Problem

• Calculate the ?H of the following reaction
• 2SO2 (g) O2 (g) ? 2SO3 (g)

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Hesss Law

• States that the total enthalpy change for a
  chemical or physical change is the same whether
  it takes one step or several steps.

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Rules for Manipulating Reactions

• If the coefficients of an equation are multiplied
  by a factor, the enthalpy change is multiplied by
  the same factor
• If an equation is reversed, the sign of ?H is
  reversed also

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Sample Problem

• Use the thermochemical equations a and b to
  determine ?H for the decomposition of hydrogen
  peroxide.
• 2H2O2(l) ? 2H2O(l) O2(g)
• 2H2(g) O2(g) ? 2H2O (l) ?H -572 kJ
• H2(g) O2(g) ? H2O2(l) ?H -188kJ

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Sample Problem

• Use reactions a and b to determine ?H for the
  following reaction
• 2CO(g) 2NO(g) ? 2CO2(g) N2(g)
• a. 2CO(g) O2(g) ? 2CO2(g) ?H -566.0 kJ
• b. N2(g) O2(g) ? 2NO(g) ?H 180.6 kJ

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Sample Problem

• Use reactions a, b and c to determine ?H for the
  following reaction.
• H2S(g) 4F2(g) ? 2HF(g) SF6(g)
• a. 1/2 H2(g) 1/2 F2(g) ? HF(g) ?H -273 kJ
• b. S(s) 3F2 (g)? SF6(g) ?H -1220 kJ
• c. H2(g) S(s) ? H2S(g) ?H -21 kJ

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Spontaneity

• Spontaneous change a change that proceeds on
  its own, without any outside intervention
• Occurs primarily in one direction
• Does not mean the reaction will occur quickly

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Spontaneity

• Some can occur when a small amount of energy is
  added to the system
• Some can be reversed if conditions change
• The more energy released, the lower enthalpy is
  and therefore the more likely the reaction will
  be spontaneous

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Entropy and Stability

• Entropy (S) a measure of the randomness or
  disorder of the system
• The tendency of nature is toward more disorder
• Its effects increase with temperature
• Measured in units of J/K

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Entropy

• ?S Sproducts Sreactants
• If entropy of a system increases during a
  reaction or process, Sproducts ? Sreactants and
  ?S is positive
• If entropy of a system decreases,
  Sproducts lt Sreactants and ?S is negative

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Entropy

• When a solid turns to a liquid and a liquid to a
  gas, entropy increases (?S ? 0)
• When a gas dissolves in a liquid, entropy
  decreases (?S ? 0)

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Sample Problem

• What is the entropy change for the single
  replacement reaction between sodium chloride and
  fluorine? SNaF 51.5 J/mol K

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Sample Problem

• What is the change in entropy for the
  decomposition of potassium chlorate? SKClO3(s)
  143.7 J/mol K