Ch 6 - Thermochemistry Flashcards
thermochemistry
the study of the relationships between chemistry and energy
Exothermic Reaction
a reaction that gives off energy to the surrondings
Energy
the capacity to do work
Work
the result of a force acting through a distance
heat
the flow of energy caused by temperature differences
energy VS heat/work
Energy - something an object possesses
Heat/work - ways objects exchange energy
kinetic energy
the energy associated with the motion of an object
thermal energy
the energy associated with the temperature of an object.
A type of kinetic energy based on the motion of atoms or molecules within a substance.
potential energy
the energy associated with the position or composition of an object.
chemical energy
the energy associated with the relative positions of electrons and nuclei in atoms and molecules.
law of conservation of energy
energy can neither be created nor destroyed.
It can be:
- Transferred from one object to another
- assume different forms(potential to kinetic to thermal etc)
System
the specific setup being examined.
Example: reactants in a flask or a hand warmer in winter.
Surroundings
The systems surroundings are everything with which the system can exchange energy.
Example: Chemicals in a beaker can react with the solution, the beaker, air, the table, etc
Kintetic Energy equation
KE = 1/2mv^2
KE = kinetic energy m = mass v = velocity(m/s)
Mass
kg
Velocity
m/s
Joule
Named for James Joule.
SI unit of energy.
1 J = (1kg)(m^2/s^2)
Kilojoule
often used instead of the joule.
1 kJ = 1000J
“c” calorie(cal)
1 cal = 4.184 J(exact)
A larger unit than the Joule.
“C” Calorie(Cal) or kilocalorie
This is a kilocalorie.
1 Cal = 1000 cal
or
1 kcal = 1000 cal
or
1 kcal = 4184 J
kilowatt-hour(kWh)
1kWh = (3.60)*(10^6) J
First law of thermodynamics
the total energy of the universe is constant.
The is no free lunch. Everything costs something.
internal energy(E)
the sum of the kinetic and potential energies of all of the particles that compose a system
State function
the value of internal energy depends only on the state of the system not on how the system arrived at that state.
Flying or climbing to 10,000 feet are different avenues but either way you are at 10,000 feet(the state)
the change in state function only accounts for the initial and final states and does not care how it got there.
State function formula
change of E(the internal energy) = E(final) - E(initial)
Chemical system state function
change of E(the internal energy) = E(products) - E(reactants)
Energy Flow between system and surroundings
change E(sys) = - change(surr)
Whatever one loses the other gains the exact opposite of.
q(heat)
+ means system gained thermal energy
- means system lost thermal energy
w(work)
+ means work done on the system
- means work done by the system
change E(internal energy)
+ energy flows into the system
- energy flows out of the system
Energy, work and heat
Change E = q(heat) + w(work)
\+ = energy into the system - = energy out of the system
Temperature
a measure of the thermal energy within a sample of matter
heat
a transfer of thermal energy
thermal energy
always flows from matter with high temperature to low
thermal equilibrium
the point in which heat transfer stops because the two systems are the same temperature
heat capacity (C)
An extensive property(dependent on the amount of matter)
the quantity of heat in a system required to change its temperature by 1 degree C.
heat capacity formula
C = q/change in temperature = J/degree C
heat formula
q = (C)(change in temperature)
q = heat C = heat capactiy
Specific Heat
intrinsic property.
the amount of heat required to raise the temperature of 1 gram of a substance by 1 degree C.
Specific Heat formula
Cs = (J/g)(degree C)
Molar Heat capacity
the amount of heat required to raise the temperature of 1 mole of a substance by 1 degree C.
Heat/temperature/amount relationship
q = (m)(Cs)(change in temperature)
q = heat
m = mass
Cs = specific heat capacity(J/g*Celsius)
Change in temperature(Tfinal - Tinitial)
Thermal Energy Transfer
Since Qmetal = -Qwater:
Mmetal)(Cs,metal)(change temperature,metal) = -(Mwater)(Cs,water)(change temperature,water
Pressure-Volume-Work
when a force is caused by a volume change against an external pressure.
Work Formula
w= F * D
OR(based on force definition)
w = (P*A)(D)
w = work F = force D = distance
Pressure formula
P = F/A
P = pressure F = Force A = area
Force formula
variation on pressure formula.
F = PA
F = force P = pressure A = area
Work and Volume
w = PAChange in H
change in V = A*(change in H)
So:
w = -(P)(change in V)
J to (L)(atm) conversion
101.3J = (1L)(atm)
Calorimetry
the measure of the thermal energy exchanged between the reaction(defined by the system) and the surroundings by observing the change in temperature of the surroundings.
Bomb Calorimeter
a piece of equipment designed to measure the change of E for combustion reactions.
ensures the reaction occurs at a constant volume
Water Specific Heat
4.18J/(g*celcius)
Enthalpy(H)
the sum of its internal energy and the product of its pressure and volume.
H = E + PV
deltaH = deltaE+P*deltaV
Endothermic Reaction
a chemical reaction with a positive deltaH absorbs heat from its surroundings
A cold pack absorbs heat from your skin.
Exothermic Reaction
a chemical reaction with a negative deltaH releases heat to its surroundings.
A chemical hand warmer gives off heat to your skin.
Enthalpy of reaction
or
Heat of Reaction
extensive property.
delta Hrxn.
The amount of heat generated or absorbed when a chemical reaction occurs depends on the amounts of the reactants that actually react.
Coffee-cup calorimeter
measures enthalpy changes for chemical reactions in solutions.
Qsoln = (Msoln)(Cs,soln)(delta T)
Qrxn = -(Qsoln)
Calorimetry Summary
Bomb Calorimetry occurs at a constand volume and measures the deltaE for a reaction.
Coffee-cup calorimetry occurs at a constant pressure and measures deltaH for a reaction.
