Thermochemistry and Thermodynamics

first law of thermodynamics

energy cannot be created or destroyed (law of conservation of energy)

system

the chemical reaction

surrounding

the container in which the reaction happens and everything beyond it

enthalpy

(H): amount of heat energy contained within a system

endothermic

heat goes into the reaction : H>0

exothermic

heat is released from the reation : H<0

endothermic phase changes

• sublimation

• melting

• evaporation

exothermic phase changes

• freezing

• deposition

• condensation

3 types of heat transfers

• conduction

• convection

• radiation

conduction

transfer of heat via direct contact

convection

transfer of heat due to motion of a liquid or gas

radiation

transfer of heat via electromagnetic radiation

formula for change in internal energy

E = q + w

formula for work

w = -PdeltaV

If delta V is negative…

work is done onto the system = positive work

If delta V is positive…

work is done by the system, which is negative work

compression is…

positive work

expansion is…

negative work

For enthalpy +q =

endothermic

for enthalpy -q =

exothermic

for heat, + w =

compression

for heat - w =

expansion

specific heat capacity

the amount of energy that is required to raise the temperature of 1.0 gram of a substance by 1 degree celsius

How to calculate H^o of reaction

sum H^o bonds broken - sum H^o bonds formed

specific heat capacity of phase changes, the specific heat capacity for the same molecule….

differs depending on the molecular state

How to calculate heat absorbd/released

q = mCAT

Heat of fusion and vaporization

q = mAH

Bomb calorimetry

used to measure the heat emitted by a sample burned under an oxygen atmosphere in a closed vessel surrounded by water

bomb calorimetry equation

qcal = CAT, qrxn = -qcal

standard enthalpy change

refers to the enthalpy change that occurs in a chemical reaction under standard conditions, with all reactants and products in their standard states

3 ways to calculate standard enthalpy change

• bond energies/ bond enthalpies

• enthalpies of formation

• hess’s law

forming bonds is equal to

release energy

breaking bonds is equal to

requiring energy

The standard enthalpy of formation, Delta H^o_f, is the ….

change in enthalpy that happens when one mole of the substance is made in its standard state

Standard enthalpy of formation for an element…

in its standard state is always zero

Hess’s Law

regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes

To find the overall enthalpy change for a reaction…

take the sum of the enthalpy changes for its intermediate reactions

If there is a coefficient, you multiply Delta H by…

the same coefficient

Entropy

disorder or randomness a system contains

2nd law of thermodynamics

the entropy of the universe is always increasing

3rd law of thermodynamics

the entropy of a pure crystalline substance at absolute zero temperature is 0

Delta S calculation

Sum S(products)n - Sum(reactants)n

entropically favorable reaction

S > 0; products are more disordered than reactants

Entropically unfavored reaction

S < 0: reactants are more disordered than products

Dissolution/Aqueous Solution trend (entropy)

Solute particles become separated from one another when a solution is formed: increasing dissolution = increasing entropy

As total number of product molecules is greater than the total number of …

reactant molecules, then entropy increases

Spontaneous Reactions

reactions that happen on their own, without a force or something acting upon them

Delta G =

Delta H - T(Delta S)

Exergonic reaction

DG < 0, energy is released to surrounding

Endergonic reaction

DG > 0, energy is absorbed from the universe

If DG < 0…

spontaneous

If DG > 0…

Non-spontaneous

If DG = 0…

the reaction is in equilibrium

If a reaction is spontaneous at all temperatures…

DH < 0, DS > 0, DG < 0

If a reaction is spontaneous at low temp, and non-spontaneous at hight temp…

DH < 0, DS < 0, DG + or -

If a reaction is Non-spontaneous at low temp, and Spontaneous at high temp…

DH > 0, DS > 0, DG is + or -

If a reaction is Non-spontaneous at all temperatures…

DH > 0, DS < 0, DG > 0

If DG is negative, than K_eq

is greater than one, products are favored at equilibrium

If DG is positive, then K_eq

is less than one, reactants are favored at equilibrium

If DG is zero, ten K_eq

is equal to one, and products and reactants are equally favored at equilibrium