THERMOCHEMISTRY
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THERMOCHEMISTRY. COLLISION THEORY, ENDOTHERMIC/EXOTHERMIC, ENTHALPY, STANDARD ENTHALPIES, CALORIMETERY . INTRO TO THERMOCHEMISTRY. The study of the changes in energy in chem rxns is called thermochemistry .

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THERMOCHEMISTRY

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Thermochemistry

THERMOCHEMISTRY

COLLISION THEORY,

ENDOTHERMIC/EXOTHERMIC, ENTHALPY,

STANDARD ENTHALPIES,

CALORIMETERY


Thermochemistry

INTRO TO THERMOCHEMISTRY

  • The study of the changes in energy in chemrxns is called thermochemistry.

  • The energy used or produced in a chemrxn is called the enthalpy of the rxn (ΔHrxn)

    • Burning a 15 g piece of paper produces a particular amount of thermal or heat energy (enthalpy)

  • Enthalpy is a value that also contains a component of direction (energy in or energy out)


Thermochemistry

INTRO TO THERMOCHEMISTRY

  • Where does that energy come from?

    • When bonds are broken that requires energy.

    • When new bonds are formed that produces energy.

  • Let’s look at a simple reaction of NO + O3 NO2 + O2

    • Unpacking it…there is 1 O2-O bondbeing broken

    • And 1 new bond being formed, NO-O.


Thermochemistry

Energy change, ΔH

= Hproducts- Hreactants

Energy (kJ)

Activated

complex

Exothermic

Activation

energy

Hproducts > Hreactants

reactants

Energy

transferred to

break bond

Energy released as heat to surroundings

ΔH = negative

products

Time (s)


Thermochemistry

INTRO TO THERMOCHEMISTRY

  • Sometimes it takes more energy to break the old bonds than we get back when the new bonds are formed

    • These reactions absorb energy and if there isn’t enough energy they don’t happen naturally

  • If we get back more energy than needed to break the old bonds

    • These reactions produce energy and if there is enough it will be in the form of heat and light.


Collision theory

COLLISION THEORY

  • To visualize the bonds being broken and formed in a chemrxn chemists use a model called collision theory

  • According to collision theory, atoms, ions, and molecules form products when they collide

    • provided that the particles have enough kinetic energy

    • And that the molecules are oriented properly


Collision theory1

COLLISION THEORY


Collision theory2

COLLISION THEORY

  • The minimum amount of energy that the particles or reactants must have in order to react is called the rxn’sactivation energy.

    • In a sense the activation energy is a barrier that reactants must get over to be converted to products

    • The higher the barrier the larger the investment of energy in order to get the rxn to proceed


Thermochemistry

Activated complex


Collision theory3

COLLISION THEORY

  • During a rxn, a particle that is neither reactant nor product forms momentarily, called an activated complex

    • if there is sufficient energy and if the atoms are oriented properly

  • An activated complex is a kind of transition molecule which has similarities to reactants & products

    • An activated complex is the arrangement of atoms at the peak of the activation-energy barrier


Collision theory4

COLLISION THEORY

  • We can determine the change in enthalpy of a reaction (ΔH)

    • ΔHrxn = ∑Hproducts - ∑Hreactants

  • If Hproducts < Hreactants then -ΔHrxn

    • Indicates an exothermic reaction

    • Always get the initial activation energy back and then some

  • If Hproducts > Hreactants then +ΔHrxn

    • Indicates an endothermic reaction

    • Constantly need to put energy into the rxn


Thermochemistry

ENDOTHERMIC VS. EXOTHERMIC

  • Chemical rxns can be classified as either:

    • Exothermic  a rxn in which heat energy is generated (a product)

      • These rxns would feel warm to the touch

      • ΔHrxnis negative

    • Endothermic rxn in which heat energy is absorbed (a reactant)

      • These rxns would feel cool to the touch

      • ΔHrxnis positive


Thermochemistry

2043kJ

+

+

+

C3H8

5O2

3CO2

4H2O

ENDOTHERMIC VS. EXOTHERMIC

  • Exothermic rxn

  • To a cold camper, the important product here is the heat energy

C3H8 + O2


Thermochemistry

ENDOTHERMIC VS. EXOTHERMIC

  • Endothermic rxn

NH4NO3+H2O+ 752kJ  NH4OH + HNO3

  • Similar system as what is found in cold packs

NH4OH + HNO3

NH4NO3 + H2O


Thermochemistry

ENTHALPIES

  • There are literally as many enthal-pies as there are types of rxns and processes.

    • The ones published are standard enthalpies symbolized by a degree

      • Standardized means measured at 25 °C and 1 atm and 1 Molar

    • Enthalpy of formation (ΔH°f) is the amnt of energy involved in the formation of a compnd from its component elements.


Thermochemistry

ENTHALPIES

  • Enthalpy of combustion (ΔH°comb) is the amount of energy produced in a combustion rxn.

  • Enthalpy of solution (ΔH°sol) is the amount of energy involved in the dissolving of a compound

  • Enthalpy of fusion (ΔH°fus) is the amount of energy necessary to melt a substance.

  • Enthalpy of vaporization (ΔH°vap) is the amount of energy necessary to convert a substance from a liquid to a gas.


Thermochemistry

ENTHALPIES

  • Enthalpy of neutralization (ΔH°neut) is the amount of energy produced when an acid reacts with a base.

  • Enthalpy of sublimation (ΔH°sub) is the amount of energy involved in the subliming of a substance.


Thermochemistry

CALCULATING HEATS OF RXNS

  • There are four ways to calculate the energy of a reaction.

    • Stoichiometry (uses ΔHrxnpropor-tionally with a given amount of material)

    • ΔH=mCΔT (conservation of energy and a ΔT to calculate a ΔHrxn)

    • Enthalpy of Formation (takes data from a table to calculate the ΔHrxn)

    • Hess’s Law (energies of several reactions can be combined to find ΔHrxn for a more complex rxn)


Thermochemistry

CALCULATING HEATS OF RXNS

  • Chemrxn 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.

How much heat will be released when 4.77 g of ethanol (C2H5OH) react with O2according to the following eqn:

C2H5OH+3O22CO2+3H2O H= -1366.7kJ


Thermochemistry

CALCULATING HEATS OF RXNS

C2H5OH+3O22CO2+3H2O

H= -1366.7kJ

1mol C2H5OH

4.77g C2H5OH

46g C2H5OH

-1366.7kJ

1mol C2H5OH

= -142 kJ


Thermochemistry

CALCULATING HEATS OF RXNS

Ethyl alcohol burns according to the following balanced equation:

C2H5OH+3O22CO2+3H2O H= -1366.7kJ

How many molecules of water are pro-duced if 5000 KJ of energy are released?

6.02x1023

molecules

3mol H2O

5000 kJ

-1366.7 kJ

1mol H2O

= 6.61 x 1024 molecules


Thermochemistry

CLASSROOM PRACTICE 1

Ethanol, C2H5OH, is quite flammable and when 1 mole of it burns it has a reported ΔH of -1366.8 kJ. How much energy is given off in the combustion of enough ethanol to produce 12.0 L of CO2 @ 755 mmHg and 25.0°C?


Thermochemistry

MEASURING ENTHALPY

  • The only way to measure enthalpy is through a process called calorimetry (measuring heat).

    • 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 system/rxn

Hsuris generally H2O

HSYS=-HSUR


Thermochemistry

A COFFEE CUP

CALORIMETER

Used for a rxn

in water, or just

a transfer of heat.

A BOMB

CALORIMETER

used when trying

to find the amount

of heat produced by

burning something.


Thermochemistry

MEASURING ENTHALPY

  • With calorimetry we use the sign of what happens to the water

    • When the water loses heat into the system it obtains a negative change (-ΔHsurr)

      • Endothermic (+ΔHsys)

    • When the water gains heat from the system it obtains a positive change (+ΔHsurr)

      • Exothermic (-ΔHsys)


Thermochemistry

MEASURING ENTHALPY

  • You calculate the amount of heat absorbed by the H2O (using ΔH= mCΔT)

  • Which leads to the amount of heat given off by the rxn

    • 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)


Thermochemistry

MEASURING ENTHALPY

When a 4.25 g sample of solid NH4NO3 dissolves in 60.0 g of water in a calorimeter, the temp drops from 21.0°C to 16.9°C. Calculate the energy involved in the dissolving of the NH4NO3.

ΔHH2O= (mwater)(Cwater)(ΔTwater)

ΔHH2O=(60g)(4.18J/g°C)(16.9°C-21.0°C)

ΔHH2O= -1.03 x 103 J

- ΔHH2O= ΔHNH4NO3

ΔHNH4NO3= 1.03 x 103 J


Thermochemistry

MEASURING ENTHALPY

A chunk of Al that weighs 72.0g is heated to 100°C is dropped in a cal-orimeter containing 120ml of water at 16.6°C. The water ends up at 27°C. What is the specific heat of Al?

ΔHH2O= (mwater)(Cwater)(ΔTwater)

ΔHH2O=(72g)(4.18J/g°C)(27°C -16.6°C)

ΔHH2O= +5216 J

ΔHAl= -5216 J = (72.0g)(x)(27°C -100°C)

CAl= .992 J/g°C


Thermochemistry

CLASSROOM PRACTICE 2

A coffee-cup calorimeter is filled with 250g of H2O. The H2O temp was 24.2°C before 3.2 g of NaOH pellets was added to the H2O. After the NaOH pellets had dissolved the temp of the H2O registered 85.8°C. How much heat did the H2O absorb, & how much heat did the NaOH produce?

41.0g of glass at 95°C is placed in 175g of H2O at 19.5°C in a calorimeter. The temps are allowed to equalize. What is the final temp of the glass/water mixture? (H2O = 4.18J/g°C; Glass = 8.78 J/g°C)


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