Chapter 6: Thermochemistry Flashcards
System + Surroundings =
Universe
E(univ) = E(syst) + E(surr) = 0
The first law of Thermodynamics:
The total energy of the universe remains constant.
Energy is conserved, and is neither created nor destroyed.
Energy is transferred in the form of heat and/or work.
A change in the energy of the system must be
accompanied by an equal and opposite change in the energy of the surroundings.
the sum of the potential and kinetic energies of all the particles present
E = Internal energy
q + w =
(+) (+)
(+) (-)
(-) (+)
(-) (-)
q+ = system gains heat / q- = system loses heat
w+ = work done by system / w- = work done on it
delta E (+) depends on size of q and w depends on size of q and w (-)
[deltaE = w + q] (value of work + value of heat)
change in the internal energy of a system
Q (p) = n C (p) delta T
[ C(p) in J/mol*K ]
molar heat capacity
rearrange to find moles:
n = q (p) / (C (p) delta T)
q = m * c * delta T
[ C(p) in J/g*C ]
to calculate how much heat energy
q =Heat Energy m = mass c = specific heat delta T = change in temperature delta H = the heat change when a reaction occurs
the amount of heat absorbed by the solution =
the amount of heat released by the reaction
*heat ‘released’ is (-)
C (p) =
Specific heat
The specific heat capacity (c) of a substance is the
quantity of heat required to change the temperature of
1 gram of the substance by 1 K.
1 J = 1 kg∙m^2/s^2
The SI unit of energy is the joule (J).
1 cal = 4.184 J
Enthalpy (H) is defined as E + PV so
deltaH = deltaE + deltaPV
*If a system remains at constant pressure and its volume does not change much, then
deltaH ≈ deltaE
-for reactions that do not involve gases
-for reactions in which the total amount (mol) of gas
does not change
-for reactions in which q is much larger than PdeltaV, even if the total mol of gas does change.
Enthalpy:
Chemical Change at Constant Pressure
*deltaE = q + w ; To determine deltaE, both heat and work must be measured.
*The most common chemical work is PV work:
the work done when the volume of a system changes in the presence of an external pressure.
deltaH is the change in heat for a system at constant pressure.
q(p) = deltaE + PdeltaV = deltaH
*If pressure remains constant, the change in enthalpy equals the change in heat for a system
deltaH overall =
deltaH1+ deltaH2+ ………. + deltaHn
*deltaH for an overall reaction can be calculated if the deltaH values for the individual steps are known.
Hess’s law states that the enthalpy change of an overall
process is the sum of the enthalpy changes of its
individual steps.
- Identify the target equation, the step whose deltaH is unknown.
* Note the amount of each reactant and product. - Manipulate each equation with known deltaH values so that the target amount of each substance is on the correct side of the equation.
*Change the sign of deltaH when you reverse an equation.
*Multiply amount (mol) and deltaH by the same factor. - Add the manipulated equations and their resulting deltaH
values to get the target equation and its deltaH.
* All substances except those in the target equation must cancel.
To calculate the change in enthalpy for an overall process
mcDT = mcDT
heat lost = heat gained
