Download Lecture 4. - ChemWeb (UCC)

1
Changes of State
• Phase Transitions
H 2O( l )
Vapourisation
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→
←⎯
⎯
Condensation
H 2O( g )
• There is a vapour pressure over a liquid at all times
• Forward and reverse processes are said to be in
dynamic equilibrium
• Boiling point is reached when:
vapour pressure = atmospheric pressure
H 2O( s )
⎯Melting
⎯⎯
⎯→
←⎯
⎯
⎯⎯
Freezing
H 2O( l )
• Melting or freezing occurs when:
rate of melting = rate of freezing
Are melting points affected by pressure changes?
CO 2 ( s )
⎯⎯
⎯⎯
⎯⎯
⎯
⎯⎯
→
←
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Condensation
Sublimation
CO 2 ( g )
• Phase transitions are accompanied by heat changes
Thermochemistry
2
• The study of heat changes in chemical reactions
Chemical / Physical
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→ Energy
Heat ←⎯
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Change
Energy
• The capacity to do work (change matter)
Potential Energy
• Energy due to position in a field of force
Kinetic Energy
• Energy due to motion
Thermal Energy
• Energy associated with random motion of atoms
and molecules
Chemical Energy
• Energy stored in structural units of chemicals
Units = Joules
• 1 Joule (J) = 1 kg m2 /s2
• 1 Calorie (cal) = 4.184 J
Conservation of Energy
• Energy may be converted from one form to another
but the total energy remains constant
Heat
3
• Almost all chemical reactions absorb or produce
energy in the form of Heat
Heat (q)
• The transfer of thermal energy between two
bodies at different temperatures
• In chemical and physical changes the two bodies
are the SYSTEM and its SURROUNDINGS
• If the system and its surroundings are in thermal
contact then heat will flow to establish thermal
equilibrium
What is the difference between Heat & Temperature?
Heat of Reaction
• The value of q required to return a system to thermal
equilibrium with its surroundings
• If heat is evolved, energy is lost by system
Exothermic process (q is -ve)
•If heat is absorbed, energy is gained by system
Endothermic process (q is +ve)
Enthalpy
4
• Heat of reaction depends on reaction conditions:
• Amount of reactants, their physical state,
• Pressure, Temperature
Enthalpy is an extensive property of a substance
used to obtain the heat of reaction
Enthalpy = H
• Enthalpy is a measure of the internal energy of a
given amount of substance
Internal Energy = Kinetic + potential
energy
•Kinetic energy is due to motions of atoms and
molecules
•Potential energy is due to chemical bonds and
intermolecular forces
Enthalpy of Reaction
5
• The change in enthalpy for a reaction at a given
temperature and pressure
ΔH = H(Products) - H(Reactants)
•Most reactions take place at atmospheric pressure
Heat of reaction = qp
ΔH =qp
• Reactions can also give rise to volume changes
H = U + PV
• U = internal energy
ΔU = ΔH-PΔV
• Internal energy can change in 2 ways:
Energy leaves or enters the system as heat ΔH
The system increases or decreases in volume
against constant pressure -PΔV
Thermochemical Equations
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Rules
• Multiply both sides of an equation by a factor
then ΔH must also change by the same factor
• Reverse the equation, change the sign of ΔH
• H depends on Temperature, so does ΔH
•Always specify physical states of species
2H2(g) + O2(g) → 2H2O(g)
ΔH = -483.7 kJ
2H2(g) + O2(g) → 2H2O(l)
ΔH = -571.7 kJ
2H2O(l) → 2H2O(g)
ΔH = 88.0 kJ
Natrium Explosion
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
ΔH = -367.5 kJ
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W9pQT8s&feature=related