Thermochemistry
The relationship between chemical reactions and
energy changes is known as thermochemistry
The relationship between chemical reactions and energy changes is known as
thermochemistry
Energy is the capacity to do work.
Kinetic energy
Potential energy
The energy of a moving object is the energy due to condition,
is called kinetic energy.
position or composition
(is stored energy)
ek =
1
2
mv2
Kg * (meters/second)2 = 1 joule
Energy can be created or destroyed it can only be converted
from one form into another
The relationship between chemical reactions and energy changes is known as
thermochemistry
A System
A System is a part of universe that we are interested.
Surroundings
are the rest of the universe which system interacts.
System
An open system
exchange the matter
and energy with
surroundings
A closed system
exchange energy but not matter with the
surroundings.
Thus, the system will contain the same
mass after an experiment, but the
system can lose or gain energy (in the
form of heat, work, or both).
An isolated system
exchange neither
matter nor
energy.
System &
Surroundings
any energy entering the system
carry positive (+) sign.
Any energy leaving system,
carry negative (-) sign.
Heat
is an energy that transfers as a result of a
temperature differences.
one calorie The quantity of heat required to change the
temperature of one gram of water by one degree
(cal).
Celsius has been defined as one calorie (cal).
1 cal = 4.184 J.
heat
capacity
The quantity of heat required to change a system by
one degree is called heat capacity
Heat flows from hotter to colder
heat capacity.
The quantity of heat required to change a system
by one degree
molar heat capacity
The heat capacity of 1 mol of a substance is called its
molar heat capacity
specific heat capacity
The heat capacity of 1 gram of a substance is called its
specific heat capacity
The specific heat of a substance can be determined experimentally by measuring the
temperature change (T) that a known mass (m) of the substance undergoes when it gains
or loses a specific quantity of heat (q):
q = mcT
question: How much heat is required to raise the temperature of 250g of
water from 22 °C to 98 °C? (specific heat of water is 4.18 J g-1 K-1).
q = (4.18 J g-1 K-1)*(250g)*(371-295 K)
q = 79420 J
determining the specific heat
qlead = -qwater
Question: a 150.0 g sample of lead is heated to 100 °C and added to
50.0g of water at 22.0 °C in thermally isolated beaker. the
temperature of the system after lead addition changed to 28.8 °C,
calculate the specific heat of lead.
Question: a 150.0 g sample of lead is heated to 100 °C and added to
50.0g of water at 22.0 °C in thermally isolated beaker. the
temperature of the system after lead addition changed to 28.8 °C,
calculate the specific heat of lead.
qwater = mcT = (50.0 g)(4.184 J/g °C)(28.8 - 22.0)°C
qlead = -1.4x103 J = mcT = (150.0 g)(c)(28.8 - 100.0)°C
clead = 0.13 Jg-1°C-1
The measurement of heat flow is called calorimetry
An apparatus that measures heat flow is called a calorimeter
Bomb calorimeter
Coffee Cup Calorimeter
qrxn = -qcal
qrxn = -qcal
qcal = qbomb + qwater
Define the heat capacity of the
calorimeter:
qcal = miciT = CT
qrxn = -qcal
Ccal= mwater x C water
Question
H2 + Cl2 → 2HCl
Reaction studied in a bomb clorimeter when 1.00g sample of H2 reacts
completely, the temperatures rises from 20.0 to 29.82. the heat capacity of
calorimeter is 9.33kJ/°C, calculate the amount of heat evolved in the
reaction
Question: when 1.00g amonium nitrate dissolves in 50.0g of water in a coffee
cup calorimeter. The temperature drops from 25.00 to 23.32 °C calculate q
for the reaction system.
• heat effects chemical
reactions may also do
work.
(Pressure- volume work.)
Work is an energy transfer between a system and its surroundings.
(Pressure- volume work.)
Work = Force x distance
F = P.A , w= P. A. h ,
( V = (Vf-Vi) )
w = - P V negative sign shows that system loses energy.
Question: how much work in joules is involved when 0.225 mol N2 at
constant temperature of 23°C is allowed to expand 1.50L in volume
againts an external pressure of 0.750atm
w = - P V
• Internal energy (U) of a system is the total energy
contained within the system.
• Internal energy is a function of state depends on amount
of matter, structure of matter, temperature and pressure.
A system contains only internal energy.
A system does not contain heat or work. These only occur during a
change in the system.
First law of thermodynamic
in a chemical or physical change, energy can be
exchanged between a system and its
surroundings, but no energy can be created or
destroyed.
U = q + w or
U = q - P V
A system does not contain heat or work These only occur during
a change in the system.
• In a constant volume
•
U = qv
qv is the quantity of heat at the constant volume
any energy entering the system and work done on
the system carry positive (+) sign.
Any energy leaving system, and work done by
system carry negative (-) sign.
question: a gas expanding, absorbs 225 J of heat and
does 243 J of work. What is the E for the gas?
Enthalpy
heat of reaction.
Energy, released or absorbed during a
chemical reaction, is called energy of reaction
or heat of reaction.
enthalpy H.
Under conditions of constant pressure the heat
absorbed or released is termed enthalpy H.
H=qp
U = qp - P V
qp is the quantity of
pressure.
H = U + P V
or
H = E+ P V
heat at the constant
for a reaction,
H =Hfinal – H initial
or
H = H(products) - H(reactants)
When 1 mole butane is burned at 1 atm pressure, it makes 3kJ
work and 2658kJ energy is produced. Calculate the enthalpy and
E for the reaction of 1 mole butane.
• a reaction that gives off heat is an exothermic
reaction and in this case H is negative
CH4 + O2
 CO2 + 2H2O
H = -880.3 kJ
• A reaction that absorb heat is an endothermic
reaction. In this case H is positive.
N2(g) + O2(g)

NO(g)
H = + 180.5 kJ
Properties of enthalpy:
Enthalpy is an extensive property. The magnitude of DH is dependent
upon the amounts of reactants consumed. Doubling the reactants,
doubles the amount of enthalpy.
N2(g) + O2(g) → 2 NO(g)
½N2(g) + ½O2(g) → NO(g)
H = +180.50 kJ
H = +90.25 kJ
Reversing a chemical reaction results in the same magnitude of enthalpy
but of the opposite sign.
NO(g) → ½N2(g) + ½O2(g)
H = -90.25 kJ
the enthalpy change for a reaction depends upon the state of the
reactants and products. The states (i.e. g, l, s or aq) must be
specified.
• Calculate the internal energy changes of the following
system at 25°C and 1atm
• 2 CO + O2 → 2 CO2
∆H= -566 kj
Calculatin Enthalpy changes in reactions
Hess’s law,
Enthalpies of
formation
Bond Energy
Hess’s law,
• if a reaction is carried out in a series of steps, ∆H
for the reaction will be equal to the sum of the
enthalpy changes for the individual steps.
• the overall enthalpy change for the process is
independent of the number of steps or the
particular nature of the path by which the reaction
is carried out.
İf you reverse reaction equation, change the sign of its ∆ H,
if you multiple a reaction equation, multiple ∆ H value by same factor.
question:
½ N2(g) + ½ O2(g)
NO(g) + 1/2O2(g)


NO(g)
NO2(g)
• What is the ∆ H of the reaction of
½ N2(g) + O2(g)

NO2(g)
∆H1= + 90.25 kJ
∆ H2= -57.07 kJ
Standart enthalpies of formation
The standard enthalpy of formation of a substance is the
enthalpy change that occurs in the formation of 1mol of the
substance from its elements.
∆ H0=  ∆ Hf (products) -  ∆ Hf(reactants)
Substance
H0f, kJ/mol
Substance
H0f,
kJ/mol
CO(g)
-110,5
H2O(g)
-241,8
CO2(g)
-393,5
H2O(l)
-285,8
CH4(g)
-74,81
N2O(g)
82,05
HCl(g)
-92,31
NO2(g)
33,18
NH3(g)
-46,11
SO2(g)
-296,8
NO(g)
90,25
SO3(g)
-395,7
C2H4(g)
H+(aq)
52,26
0
CH3CH2OH(l)
OH-(aq)
-277,7
-230
The standard enthalpy of formation of a
pure element in its reference from is 0
• question: use the standard formation enthalpies from the
table to calculate the standard enthalpy change for the
reactions ?
3CH4 (g) + 2H2O (s) + CO2 (g)  4 CO (g) + 8 H2 (g)
C2H4 (g) + H2O (l)

CH3CH2OH (g) ∆H0 = ?
H+(suda) + OH-(suda) 
H2O (l)
∆H0 = ?
• question:
• C2H4 (g) + H2O (l)

CH3CH2OH (g)
• question:
• H+(aq) + OH-(aq) 
H2O (l)
∆H0 = ?
∆H0 = ?
question: Calcium carbide (CaC2) reacts with water to form
acetylene (C2H2) a gas used as a fuel in welding.
CaC2 (s)
+ H2O (l)  C2H2(g)
+ Ca(OH)2 (s)
∆H=
-128,0kJ
how many kilojoules of heat are evolved in the reaction of 3.5g
CaC2 reacts with 125ml H2O?
question: determine the enthalpy change for the oxidation of
ammonia
4NH3(g) + 5O2(g)  4NO(g) + 6 H2O(l) from the following data
• N2(g) + 3H2(g)  2 NH3(g)
• N2(g) + O2(g) 
2NO(g)
• 2H2(g) + O2(g) 
2H2O (l)
∆H = -92,22kJ
∆H = +180,5kJ
∆H = -571,6kJ
Dimethylhidrazine (N2H2(CH3)2) is used as a rocket fuel. When it reacts
with oxygen, the thermochemical equation for the reaction is
½ N2H2(CH3)2(l) + 2O2(g) → CO2(g) + 2H2O(g) + ½ N2(g)
∆H = -898,1kJ
a- the heat formation of CO2(g) and H2O(g) are -393,5 kJ/mol and –
241,8 kJ/mol respectively. calculate the heat of formation of N2H2(CH3)2(l)
(H:1,0 g/mol N: 14,0 g/mol O: 16,0 g/mol C: 12,0 g/mol)
(R= 8,314J/mol K
R=0,082 L atm /mol K
R= 1,987 cal/mol K)
Dimethylhidrazine (N2H2(CH3)2) is used as a rocket fuel. When it reacts
with oxygen, the thermochemical equation for the reaction is
½ N2H2(CH3)2(l) + 2O2(g) → CO2(g) + 2H2O(g) + ½ N2(g)
∆H = -898,1kJ
1,00g N2H2(CH3)2(l) is
burned in an open container?
b- how much heat is evolved when
(H:1,0 g/mol N: 14,0 g/mol O: 16,0 g/mol C: 12,0 g/mol)
(R= 8,314J/mol K
R=0,082 L atm /mol K
R= 1,987 cal/mol K)
Dimethylhidrazine (N2H2(CH3)2) is used as a rocket fuel. When it
reacts with oxygen, the thermochemical equation for the reaction is
½ N2H2(CH3)2(l) + 2O2(g) → CO2(g) + 2H2O(g) + ½ N2(g)
∆H = -898,1kJ
for the reaction ,∆H = qp= -898,1kJ calculatre ∆U at 25°C
(H:1,0 g/mol N: 14,0 g/mol O: 16,0 g/mol C: 12,0 g/mol )
(R= 8,314J/mol K
R=0,082 L atm /mol K
R= 1,987 cal/mol K)
0,212 g sample of pure Na2C2O4 (Mw=134g/mol) was titrated with 43,31ml of KMnO4 what is the molarity of the KMnO4?
Write the balanced eauation for this reaction?
MnO4- + C2O42- + H+ → Mn2+ + CO2
Calculate the root mean square speed of H2 molecules at 20°C
The spectral lines of hydrogen in infrared region represent electron transitions to the n=5 from higher levels. What is the
electron transition that corresponds to the 3740nm spectral line? Rh=2.179x10 -18 J h= 6,626x10-34Js c=3,0x108m/s
Find the emprical and molecular formula of A molecule that has mass of 182 amu and a mass percent composition of 39,56
%C, 7,74%H and 52,70% O.
In the Combustion of glucose, if the reaction is carried out in an expandable container at 35°C and 780 torr, what volume of
CO2 is produced from 18,0g glucose and excess O2?
C6H12O6 + 6O2 → 6CO2 + 6H2O
An 8,07g Ag2O (88,3% by mass) decomposes into solid silver and O2(g). The O2(g) is collected over water at 25°C and
749,2mmHg pressure. The vapor pressure of water at 25°C is 23,8mmHg. What is the volume of gas collected?
Rank the ions in each of the following sets in order of increasing size, and explain your ranking.
a) Li+, K+, Na+
b) Se2-, Rb+, Br c) O2-, F -, N 3How much energy, in joule, must be absorbed to convert to K + all atoms present in 10 mg of gaseous K? The first ionization
energy of K is 418.8 kJ/mol.