ENERGETICS
ENERGETICS
CAPE Chemistry
Module 1
Objectives
Objectives
1.
State that chemical reactions take place through energy changes (usually in the form of heat)
associated with the breaking and making of bonds
2.
State that energy changes occur in chemical reactions associated with the making and
breaking of bonds
Explain the differences between exothermic and endothermic reactions using energy profile
diagrams
Explain the term ‘bond energy’
Explain how bond energy data may be used to show the relationship between strength of
covalent bonds and reactivity of covalent molecules
3.
4.
5.
6.
7.
8.
9.
Apply concepts associated with enthalpy changes
Explain the effect of ionic charge and radius on the magnitude of lattice energy
Calculate enthalpy changes from appropriate experimental data
State Hess’ law of constant heat summation
Energetics&
&Thermodynamics
Thermodynamics
Energetics
Energetics is the branch of physical
science that deals primarily
with energy and its transformations
Thermodynamics is the particular
branch of energetics which deals with
heat energy and its transformations
Enthalpy is the total energy associated
with any system
http://www.sophia.org/define-enthalpy/define-enthalpy-tutorial
Energy&
&Chemical
ChemicalReactions
Reactions
Energy
Chemical reactions take place
through energy changes (usually
in the form of heat)
−
Enthalpy (H) is the energy
possessed by a system
−
Enthalpy change or ΔH is
the energy change in a
reaction
http://www.sophia.org/define-enthalpy/define-enthalpy-tutorial
EnthalpyChange
Changein
inaaSystem
System
Enthalpy
Enthalpy Change (ΔH) = Eproducts – Ereactants

If Eproducts > Ereactants then ΔH is positive

reaction is endothermic
 reaction absorbs heat from the surroundings
 reaction vessel gets colder

If Eproducts< Ereactants then ΔH is negative

The reaction is exothermic
 reaction releases heat to the surroundings
 reaction vessel gets hotter
EnthalpyChange
Changein
inaaSystem
System
Enthalpy
ChemicalReactions
Reactions
Chemical
Two steps necessary in a chemical
reaction:
•FIRST
- bonds are broken
- energy is needed or absorbed during
bond breaking (endothermic)
•AFTER
- bonds are formed
- energy is released during bond
formation (exothermic)
BOTH processes ALWAYS occur in a
chemical reaction
http://employees.csbsju.edu/cschaller/Reactivity/thermo/TDbondmakebreak.png
ChemicalReactions
Reactions
Chemical
Chemical reactions may be classified as
endothermic or exothermic
•
Exothermic - more energy is released than is
absorbed in a chemical reaction (ΔH is negative)
•
Endothermic - less energy is released than is
absorbed in a chemical reaction (ΔH is positive)
http://www.gcsescience.com/rc24-energy-level-diagram.htm
EnergyProfile
ProfileDiagrams:
Diagrams:Exothermic
ExothermicReactions
Reactions
Energy
• Most reactions are exothermic eg.
acid-base reactions, combustion
reactions and respiration
• Temperature of the system increases
• ΔH is negative (Energy of reactants
is higher than energy of products)
EnergyProfile
ProfileDiagrams:
Diagrams:Endothermic
EndothermicReactions
Reactions
Energy
• Fewer
reactions are endothermic
eg. Photosynthesis, thermal
decomposition, dissolving some
salts in water
• Temperature
of the system
decreases
• ΔH
is positive (Energy of
reactants is lower than energy of
products)
http://www.gcsescience.com/rc25-exothermic-endothermic-reaction.htm
BondEnergy
Energy
Bond
Bond energy is :

the amount of energy required to dissociate a molecule into its respective
atoms

directly related to the strength of the covalent bond and is indirectly
related to its reactivity
Factorsaffecting
affectingBond
BondEnergy
Energy
Factors
1.
Strength of covalent bond
2.
Size of the atoms in the molecule
3.
Degree of orbital overlap in the covalent bond
4.
Bond Order (single, double or triple)
BondEnergy
Energyand
andReactivity
Reactivity
Bond
1. If bonds can be broken easily, the reaction would occur quickly
2. If the bonds cannot be broken easily, the reaction would occur
slowly
Which is more reactive?
N2 B.E = 945 kJ mol-1 ; bond length = 110 pm
OR
O2 B.E = 498 kJ mol-1 ; bond length = 121 pm
BondEnergy
Energy&
&Chemical
ChemicalStability
Stability
Bond
• The energy required to break:
O-H in H2O(g) is 463 kJ mol-1
O-O bond in H2O2 (g) is 139 kJ mol-1
• H2O2 is less stable than water because it contains the O-O
bond which is easier to break than the O-H bond in H2O
• Hydrogen peroxide will be more chemically reactive than
water
Homework
Homewor
State whether the following processes are
exothermic or endothermic:
• Reaction between sodium hydroxide and
hydrochloric acid
N2 + 3H2 → 2NH3 ΔHrxn= - 92 kJ mol-1
• The burning of gasoline
• The dissolving of potassium nitrate in
water
•
Standard Enthalpy Changes and definitions
StandardEnthalpy
EnthalpyChanges
Changes
Standard
•
StandardEnthalpy
EnthalpyChange
ChangeofofReaction
Reaction()()
Standard
The standard enthalpy change when the mole
quantities in the balanced equation react under
standard conditions (1 atm and 298 K)
At 1 atm and 25°C (298 K) reactants and products are in their
usual or standard states
StandardEnthalpy
EnthalpyChange
ChangeofofFormation
Formation()()
Standard
•
StandardEnthalpy
EnthalpyChange
ChangeofofFormation
Formation()()
Standard
•
Standard
Enthalpy
Change
of
combustion
()
Standard Enthalpy Change of combustion ()
StandardEnthalpy
EnthalpyChange
ChangeofofNeutralization
Neutralization()()
Standard
StandardEnthalpy
EnthalpyChange
ChangeofofAtomization
Atomization()()
Standard
•
StandardEnthalpy
EnthalpyChange
Changeofofhydration
hydration()()
Standard
StandardEnthalpy
EnthalpyChange
Changeofofsolution
solution()()
Standard
StandardEnthalpy
EnthalpyChanges
Changes
Standard
•
Factorsaffecting
affecting
Factors
1. Size of the ions
2. Charge on the ions
http://chemistry.tutorvista.com/inorganic-chemistry/lattice-energy.html
http://www.chemhume.co.uk/A2CHEM/Unit%202b/9%20Lattice%20enthalpy/Ch9Lattice.htm
Determining
Enthalpy
Determining Enthalpy
Changes
Changes
Determiningenthalpies
enthalpiesofofreaction
reaction) )
Determining
•
Measuring Experimentally
Experimentally
1.1.Measuring
−
−
−
−
−
Chemical reaction occurs in an insulated
container called a calorimeter
Styrofoam coffee cup with a lid serves as
calorimeter
Thermometer measures heat change of the
reaction
The change in temperature is proportional to the
energy released by the reaction
Since the pressure is constant, the heat evolved or
absorbed is equal to ΔHr
https://www.learner.org/courses/chemistry/text/text.html?dis=U&num=Ym5WdElUQS9PQ289&sec=YzJWaklUQS9OeW89
Assumptionsin
inmeasuring
measuring experimentally
experimentally
Assumptions
−
calorimeter perfectly prevents
the gain or loss of heat from
the solution to its surroundings
−
heat gained by the solution is
produced by the chemical
reaction or heat lost by the
solution is absorbed by the
chemical reaction
Measuringthe
the for
foracid-base
acid-basereactions
reactions
Measuring
Measuring ofofaasalt
salt
Measuring
Question
5g of ammonium nitrate (NH4NO3) was dissolved in 50 cm3 of
water in a Styrofoam cup. The temperature fell from 22 oC to
14 oC. Calculate the molar enthalpy change of solution for the
above reaction.
Measuringthe
the of
offuels
fuels
Measuring
 heat
from combustion of the fuel is used to heat up
the water
 assume
energy released by combustion of fuel is
absorbed by the water in the calorimeter
 heat
capacity of the water and the temperature rise
are needed to calculate how much heat was
released by a specific mass of fuel
 very
inaccurate method because of huge losses of
heat e.g. radiation from the flame
Measuringthe
the of
offuels
fuels
Measuring
Question
100 cm3 of water was measured into a calorimeter. A spirit
burner containing ethanol fuel, C2H5OH, weighed 18.62 g at the
start. The initial temperature of the water was measured. After
burning some time, the flame was extinguished and the water
stirred gently. The final temperature of the water was measured.
The burner and fuel are then reweighed to see how much fuel
had been burned. After burning, 17.14g of ethanol remained
and the temperature of the water rose from 18 to 89 oC.
Calculate the enthalpy of combustion for ethanol.
http://www.docbrown.info/page03/3_51energyD.htm#Calculations
Calculating from
fromBond
BondEnergies
Energies
2.2.Calculating
Calculating
from
Bond
Energies
Calculating from Bond Energies
Calculating from
fromBond
BondEnergies
Energies
Calculating
Use Bond enthalpies (found in data booklet) to
estimate the enthalpy change in the following
reaction:
CalculatingΔH
ΔHr rfrom
fromΔH
ΔHf f
3.3.Calculating
ΔHr = ΣΔHf, products– ΣΔHf, reactants
ΔHr is the enthalpy change of reaction
ΔHf, products is the enthalpy change of formation of products
ΔHf, reactants is the enthalpy change of formation of reactants
CalculatingΔH
ΔHr rfrom
fromΔH
ΔHf f values
values
Calculating
Question
Calculate the enthalpy of the following reaction:
C2H5OH(l) + 3 O2(g) → 2 CO2 (g) + 3 H2O (l)
Standard enthalpies of formation are:
C2H5OH = -228 kJ/mol
CO2 = -394 kJ/mol
H O = -286 kJ/mol
CalculatingΔH
ΔHrfrom
fromΔH
ΔHf values
Calculating
r
f
values
Answer:
C2H5OH + 3 O2 → 2 CO2 + 3 H2O
ΔHr= ΣΔHf, products– ΣΔHf, reactants
ΔHr = [(3 x -286) + (2 x -394)] – [(-228) + (3 x 0)]
= -1418 kJ mol-1
CalculatingΔH
ΔHrusing
usingHess'
Hess'Law
Law
4.4.Calculating
r
Hess’ Law states that the
enthalpy in converting
reactants into products is
the same regardless of the
route taken.
http://commons.wikimedia.org/wiki/File:Hess_Law.png
According to Hess' Law:
ΔH = ΔH1 + ΔH2 + ΔH3
Hess’ Law
Hess’Law
Hess’ Law
may be used to calculate
enthalpy changes which cannot be
determined experimentally e.g. lattice
enthalpies
sum of
enthalpies via route 1 =
sum of enthalpies via route 2
Sum of
clockwise enthalpies =
sum of anticlockwise enthalpies
http://www.chemhume.co.uk/ASCHEM/Unit%203/13%20Enthalpy/13%20Enthalpyc.htm
Hess'Law:
Law:Applications
Applications
Hess'
•
Hess'Law
LawCalculations:
Calculations:Energy
EnergyCycle
CycleDiagrams
Diagrams
Hess'
You are given the following:
ΔH°c (kJ mol-1)
C6H6(l)
-3267
C(s)
-394
H2(g)
-286
Hess’sLaw
LawCalculations:
Calculations:Chemical
ChemicalEquations
Equations
Hess’s
• Hess’s law can be applied when chemical equations are given
instead of a diagram
• The chemical equations will be treated like algebraic equations
Hess'Law
LawCalculations:
Calculations:Chemical
ChemicalEquations
Equations
Hess'
•
The reaction for enthalpy of combustion for ethane is:
C2H6(g) + O2(g) 2CO2(g) + 3H2O(l) ∆HC = ?
Arrange the three equations in such a way, that when they are added together
their sum will be the overall equation above.
C2H6(g) 2C(s) + 3H2(g)
2 x [C(s) + O2(g) CO2 (g)]
3 x [H2(g) + O2(g) H2O(l)]
∆H1 = +84.68 kJ/mol
∆H2 = -394 kJ/mol x2
∆H3 = -286 kJ/mol x3
C2H6(g) + O2(g) + 2C(s) + 3H2(g) 2CO2(g) +3H2O(l) + 2C(s) + 3H2(g)
therefore: ∆HC = -∆H1 + 2∆H2 + 3∆H3
∆HC = -1561.32 kJ/mol
http://intro.chem.okstate.edu/1314F00/Lecture/Chapter6/Lec102300.html
LatticeEnthalpy
Enthalpy))
Lattice
 Lattice enthalpy is a measure of
the strength of the forces
between the ions in an ionic
solid
 The greater the lattice enthalpy,
the stronger the forces
 Remember that energy is given
out when bonds are made, and
is needed to break bonds
LatticeEnthalpy
Enthalpy))
Lattice
Lattice Enthalpy (∆Hlatt)

The change when one mole of ionic
solid is formed from its gaseous ions
(downward arrow)
 ΔH0latt is negative
Reverse Lattice Enthalpy (-∆Hlatt)


The change when one mole of ionic
solid is broken up to form its scattered
gaseous ions (upward arrow)
Reverse lattice enthalpy is positive
value
eg. NaCl, the solid is more stable than
the gaseous ions by 787 kJ mol-1
0
0
0
Relationship
between
ΔH
ΔH
and
ΔH
0
0
0
,
,
Relationship between ΔH soln
ΔH hyd
sol,nΔH
lat,tand
hyd
latt
0
0 , and ΔH0
Relationship
between
ΔH
ΔH
,
soln
hyd
0
0
Relationship between ΔH soln, ΔH lattlatt, and ΔH0 hyd
Determine the enthalpy of solution of potassium fluoride, KF. Given the following
information:
ΔH0 of KF = -819 kJ mol-1, ΔH0 = -821 kJ mol-1
hyd
latt
Enthalpy of solution of KF,ΔHsolution = -ΔHlattice + ΔHhyd
= 821 + (-819) kJ mol -1
= +2 kJmol -1
Born-HaberCycle
Cycle
Born-Haber
 Born-Haber cycle relates the lattice energy of an
ionic compound to other enthalpies that can be
measured
 special application of Hess’Law
 indirectly measures the lattice enthalpies (ΔHlatt)
 lattice enthalpies are difficult to determine
experimentally
What steps are
involved in the
formation of
NaCl?
Born-HaberCycle
Cyclefor
forCaCl
CaCl2
Born-Haber
2
http://chemistry.tutorvista.com/inorganic-chemistry/born-haber-cycle.html