Transcript thermochemistry

INTRO TO THERMOCHEMISTRY
Chemical reactions involve changes in energy
Breaking bonds requires energy
Forming bonds releases energy
These energy changes can be in the form of heat
Heat is the flow of chemical energy
The study of the changes in energy in chemical reactions is called
thermochemistry.
The energy involved in chemistry is real and generally a measurable
value.
WHAT IS HEAT?
Hot & cold, are automatically associated with the words heat and
temperature
Heat & temp are NOT synonyms
The temperature of a substance is directly related to the energy of
its particles, specifically its Kinetic Energy
Kinetic Energy
 The Kinetic Energy defines the temperature
– Particles vibrating fast = hot
– Particles vibrating slow = cold
Vibrational energy is transferred from one particle to the next: One particle collides
with the next particle and so on; and so on – down the line
Thermal energy is a form of kinetic energy that the particles have that make up a
substance
Kinetic energy from vibrational energy (motion) in solids, and liquids and gases
it is vibrational, rotational, and translational energy that contribute to the KE
POTENTIAL ENERGY
Potential energy from molecular attraction (within or between
the particles)
PE is the energy stored in the bonds between the atoms and in
the nuclear forces that hold the nucleus together.
The PE of a molecule results from the interactions between
electrons and nuclei both between and within atoms. This
interaction is a chemical bond
The energy changes that occur during a chemical reaction are
mainly due to the PE changes that occur during the breaking of
chemical bonds in the reactants and the formation of new bonds
in the products..

Example: 2H2(g) + O2(g) → 2H2O(g) + heat


The bonds between the hydrogen atoms in the H2 and the oxygen atoms in the
O2 must be broken in order to make the H-O bonds in H2O. This breaking of
bonds requires energy and is therefore endothermic. However, in this example,
more energy is released in the making of the H-O bonds than is required to break
the H-H and the O=O bonds.
Therefore the overall reaction is exothermic. This means that the reverse
reaction would be endothermic
2H2O(g) + heat → 2H2(g) + O2(g

Thermal energy is dependent upon the amount or mass of material present



(KE =½mv2)
Thermal energy is also related to the type of material



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Different type of materials
– May have the same temp, same mass, but different conductivity
– Affected by the potential energy or the intermolecular forces
So it is possible to be at same temp (same KE) but have very different
thermal energies
The different abilities to hold onto or release energy is
referred to as the substance’s heat capacity
Thermal energy can be transferred from object to object through direct
object through direct contact
– Molecules collide, transferring energy from molecule to molecule
molecule to molecule
DEFINITION
THE FLOW OF THERMAL ENERGY FROM SOMETHING
WITH A HIGHER TEMP TO SOMETHING WITH A LOWER
TEMP
UNITS
MEASURED IN JOULES OR CALORIES
THROUGH WATER OR AIR = CONVECTION
TYPES
THROUGH SOLIDS = CONDUCTION
TRANSFERRED ENERGY BY COLLISION WITH PHOTON
= RADIANT ENERGY
HEAT CAPACITY
The measure of how well a material absorbs or releases
heat energy is its heat capacity
It can be thought of as a reservoir to hold heat, how
much it holds before it overflows is its capacity
Heat capacity is a physical property unique to a particular
material
Water takes 1 calorie of energy to raise the temp 1 °C
Steel takes only 0.1 calorie of energy to raise temp 1 °C
SPECIFIC HEAT CAPACITY(Cp)
The amount of energy it takes to raise the temp of a standard
amount (1 g) of an object 1°C
Specific heats can be listed on data tables
Smaller the specific heat  the less energy it takes the
substance to feel hot and the less time it takes the substance
to cool off
Larger the specific heat  the more energy it takes to heat a
substance up (bigger the heat reservoir) the longer time it
takes the substance to cool off
http://www.engineeringtoolbox.com/specific-heat-capacity-food-d_295.html
SUBSTANCE
SPECIFIC HEAT CAPACITY, CP
WATER, H2O
4.18J/g°C OR 1cal/g°C
ALUMINUM, Al
.992J/g°C OR .237cal/g°C
TABLE SALT, NaCl
.865 J/g°C OR .207cal/g°C
SILVER, Ag
.235 J/g°C OR .056cal/g°C
MERCURY, Hg
.139 J/g°C OR .033cal/g°C
CHEMICAL RXNS
There are 2 types of chemical rxns
– Exothermic rxns  rxns in which heat energy
is a product
 Exothermic rxns typically feel warm as the rxn
proceeds
– You might hear the word exergonic- these are
reactions that release energy, but not necessarily
HEAT!!
Ex: the hydration of any strong acid or base


Exothermic rxn
CH4 + 2O2 
CO2
+
2H2O + 2043kJ change
– To a cold camper, the important product
here is the heat energy
The other type of reaction is
–
Endothermic rxns  rxns in which heat energy is a reactant
(absorbs heat energy)
Endothermic rxns typically feel cooler the longer the rxn
proceeds
 You might hear the word endergonic these are
reactions that absorb energy, but not
necessarily HEAT!!


Ex: Citric acid and baking soda

Endothermic rxn
NH4NO3+H2O+ 752kJ NH4OH+HNO3
– Similar system as what is found in cold packs
H2O (s) + 752kJ  H2O (l)
CHANGE IN HEAT ENERGY (ENTHALPY)
The energy used or produced in a chem rxn
is called the enthalpy of the rxn
– Burning a 15 gram piece of paper
produces a particular amount of heat
energy or a particular amount of enthalpy
 Enthalpy is a value that also contains a
component of direction (energy in or energy
out)

HEAT
HEAT
HEAT
HEAT
CHANGE IN ENTHALPY
Most common version of enthalpy is
when we have a change in enthalpy
(H)
 The enthalpy absorbed or gained
(changed) in a rxn is dependent on the
amount of material reacting

Amount is usually in the form of
moles
– We can use the coefficient ratios of
the balanced chemical reactions to
energy ratios to calculate how much
energy a reaction used or produced
–
Endothermic Versus Exothermic Reactions
To further understand the difference between the two types of
reactions (exothermic and endothermic), we need to explore a
couple of other concepts. In addition to kinetic energy
(vibrational, rotational and translational motion), molecules also
have potential energy. Potential energy is energy due to position
and composition. It is stored in molecular bonds that exist within
molecules (intramolecular); between different molecules
(intermolecular), between different atoms of an element and
finally within atoms.
In endothermic reactions the
reactants have less potential energy
than the products do. Energy must
be added to the system from the
surroundings in order to raise the
particles up to the higher energy
level.
Energy + A + B --> AB
In exothermic reactions the
reactants have more potential
energy than the products have.
The extra energy is released to
the surroundings.
A + B --> AB + Energy
USING H IN CALCULATIONS

Chemical reaction equations are very powerful
tools.
– Given a rxn equation with an energy value,
We can calculate the amount of energy
produced or used for any given amount of
reactants.
EXAMPLE 1:
How much heat will be released if
1.0g of H2O2 decomposes in a
bombardier beetle to produce a
defensive spray of steam
2H2O2 2H2O + O2 Hº =-190kJ
2H2O2 2H2O + O2 Hº = -190kJ
THINK Moles and ratios! From the balanced
chemical equation, for every 2 mols of
H2O2 that decomposes, 190kJ of heat
is produced. Now, calculate how much
energy is produced when1.0 g of H2O2
decomposes.
Convert 1.0 g of H2O2 to moles of H2O2
2H2O2 2H2O + O2 Hº = -190kJ
Again, with 2 moles of H2O2, 190 kJ of energy is produced but
since there is only 0.02941 mols of H2O2 calculate
how much energy the bug produces?
Example #2
How much heat will be released when 4.77 g of ethanol (C2H5OH)
react with excess O2 according to the following equation:
C2H5OH + 3O2 2CO2 + 3H2O Hº=-1366.7kJ
 We
can also track energy changes due to
temp changes, using H=mCT:
H =
SPECIFIC
MASS
HEAT
FINAL TEMP –
INITIAL TEMP
Example #3:
If you drink 4 cups of ice water at 0°C, how much heat energy is
transferred as this water is brought to body temp? (each glass
contains 250 mL of water & body temp is 37°C). Density of
water is 1g/mL.
Enthalpy is dependent on the conditions of the rxn
– It’s important to have a standard set of conditions
– This allow us to compare the affect of temps, pressures, etc. On
different substances
 Chemist’s have defined a standard set of conditions
– Stand. Temp = 298K or 25°C
– Stand. Press = 1atm or 760mmHg
 Enthalpy produced in a rxn under standard conditions is the standard
enthalpy (H°)

Standard enthalpies can be found on tables of data measured as
standard enthalpies of formations (pg 799-800)
 Standard enthalpies of formations are measured values for the
energy to form chemical compounds (Hf°)
– H2 gas & O2 gas can be ignited to produce H2O and a bunch of
energy
– The amount of energy produced by the rxn is 285kJ for every mol
of water produced

H2(g) + ½02(g)  H2O(g)
Hf°=-285.8kJ/mol
STANDARD ENTHALPIES OF FORMATION
SYMBOL
FORMULAS
Hf°kJ/mol
AlCl3(s)
Al + 3/2Cl2  AlCl3
-705.6
Al2O3(s)
2Al + 3/2O2  Al2O3
-1676.0
CO2(g)
C + O2  CO2
-393.5
H2O(g)
H2 + 1/2O2  H2O
-241.8
C3H8(g)
3C + 4H2  C3H8
-104.7
CALORIMETRY

Calorimetry is the process of measuring heat energy
– Measured using a device called a calorimeter
–

Uses the heat absorbed by H2O to measure the
heat given off by a rxn or an object
The amount of heat soaked up by the water is equal to
the amount of heat released by the rxn
Hsys is the reaction that is taking
HSYS=-HSUR place in the main chamber (rxn
etc.) And Hsur is the
HSYS=±│q│ surroundings which is generally
water.
CALORIMETRY
 You calculate the amount of heat
absorbed by the water (using q= mCT)
 Which leads to the amount of heat given
off by the rxn HSYS=±│q│
– you know the mass of the water (by
weighing it)
– you know the specific heat for water
(found on a table)
– and you can measure the change in the
temp of water (using a thermometer)
A chunk of Al that weighs 72.0g is heated to 100.0°C is dropped in a
calorimeter containing 120ml of water at 16.6°C.
the H2O’s temp rises to 27.0°C.
- mass of Al = 72.0g
- Tinitial of Al = 100.0°C
- Tfinal of Al = 27.0°C
- CAl = .992J/g°C (from table)
q=
H
72g
=
.992J/g°C
-5214J
HSYS
27°C-100°C
 We can do the same calculation with the water info
– Mass of H2O= 120g
– Tinitial of H2O= 16.6°C
SUR
– Tfinal of H2O = 27°C
– CH2O= 4.18J/g°C (from table)
H
H =
120g
4.18J/g°C
H =
27°C-16.6°C
5216J
Equal but opposite, means that since the Al decreased in temp, it
released heat causing the H2O to increase in temp.