Cheatography
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Thermochemistry for first year of engineering
This is a draft cheat sheet. It is a work in progress and is not finished yet.
Systems
A system can be open (freely exchanges energy and matter with the surroundings), closed (exchanges energy but not matter) or insulated (does not interact with the surroundings) |
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Sign conventions
If the system gains heat, it is an endothermic process. If the system releases heat, it is an exothermic process.
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Heat
Q=m•Hs•ΔT |
not a state function |
Work
pressure-volume work |
W=Pext•V (if P=constant) |
if carried out in a reversible way |
W=-nRT•ln(V2/V1) |
Ideal gas law
P•V=nRT |
R=0,082 atm•L/(K•mol) |
Calorimeter bomb
Q+W=0=ΔE |
W=0 |
Q=0 |
Qreleased+Qabsorbed=0 |
Qreaction+Qwater+Qcalorimeter=0 |
Qreaction+Mwater•(Tf-Ti)+Kcalorimeter•(Tf-Ti)=0 |
Qreaction=Qv=ΔE |
Laws of thermochemistry
Lavoisier and Laplace laws |
The energy change accompanying any transformation is equal and opposite to energy change accompanying the reverse process. |
Hess Law |
The energy change accompanying any transformation is the same whether the process occurs in one step or many. The combination of chemical equations allows to determine unknown heats of reaction. |
Standard States. Standard enthalpy of reaction
enthalpy change of a reaction in which all reactants and products are in their standard states |
the standard enthalpy of formation of a pure element in its standard state is 0 |
ΔHº=Σvp•ΔHºf(products)-Σvp•ΔHºf(reactants) |
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Units
1cal=4,184J |
Specific heat |
quantity of heat required to change the temperature of one gram of the substance by one degree celsius |
Molar heat |
quantity of heat required to change the temperature of one mol of the substance by one degree celsius |
Heat capacity |
quantity of heat required to change the temperature of a system by one degree celsius |
First law of thermodynamics
Internal energy is the total energy in a system |
E=ΣEi |
Principle of conservation of energy |
ΔE=Q+W |
For isolated system |
Q+W=0=ΔE |
Heat and work are means by which a system exchanges energy with the surroundings
Enthalpy (H)
It is the change in the internal energy when there is only pressure-volume work, and the pressure is constant |
ΔE=Q+W=Qp-P•ΔV |
Qp=ΔE+P•ΔV |
H=E+P•V |
ΔH=ΔE+VΔP+PΔV +ΔPΔV |
if P=constant |
ΔH=Qp=m•Hs•ΔT=n•Hs•ΔT |
Relation of Qv and Qp (gases) |
ΔH=ΔE+PΔV |
if P and T are constant |
ΔH=ΔE+ΔnRT |
Relation of Qv and Qp (solids and liquids) |
if P=constant-->change in volume really small |
Qv≈Qp-->ΔE≈ΔH |
Enthalpy and bond energy
ΔHr=ΣHbroken bonds-ΣHformed bonds |
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