Chapter 5 - Thermochemistry

thermal energy

results from motion of molecules

heat

amount of energy transferred between substances

exothermic

energy releasing -

endothermic

energy requiring +

temperature

average kinetic energy of the particles in a sample of matter

opne system

matter and energy can change

closed system

mattter can’t change but energy can

isolated systen

neither matter nor energy can change

bomb vs cup calorimeter

bomb measures combustion and has water not directly in contact with the system

molar enthalpy

enthalpy chnage per mole of a reaction (h=nh^o)

what are thr 3 calorimeter assumptions

no heat transferred to environment

any heat absorbed or released by the cal is negligable

a dilute solution has heat capacity and density of water

thermochemical equation

represent enthalpy of reaction within the equation

method 2 of representing H: beside equation

negative for exo positive for endo, and in kJ/mol

standard molar enthalpy of reaction

Standard molar enthalpies allow chemists to create tables to compare enthalpy values. ° "nought" represents enthalpy change of a chemical reaction

that occurs at SATP (25°C and 100 kPa)

enthalpy

shows E(kJ) over the reaction progress

state function

path independant; change in altitude relys only on the difference between start and end

hess’s law

H can be written in steps and is equal to the sum of the enthalpy of each individual step

using the 3rd means of showing enthalpy

use a balanced equation of formation for exact;y 1 mol of the compound of interest. then enthaply = ehtalpy products - enthalpy reactants

• must add standard states to equation as well

enthalpy of elements in standard state

in standard state, the enthalpy of a substance is just 0. eg o2, Na, Cl2

exmaples of potential energies

bond energy, nuclear pe of protons and neutrons

exmaples of kinetic energy

motion of elections, vibrations, rotations and translation of atoms and molecules

enthalpt change: physical change

10^0-10² kj/mol overcoming intermolecular force; particles remain unchanges

enthalpy change, chemical change

10² - 10^4 kJ/mol. overcoming electronic structure + bonds; new substance with nre bonds eg combustion

enthalpy change: nuclear change

10^10 - 10^12 kj/mol. overcome forces between protons and nucleus; new atoms form eg nuclear decay

spontaneous reaction

given the necessary activation energy, proceeds without continuous assitance on an open system

non- spontaneous

contiuous supply of energy is needed to sustain the reaction

what is entropy

disorder or randomness, entropy increases when disorder increases; S = Sproducts - Sreactants

predicting entropy

s is positive when there is increasing disorder (volume increases, temp increase, state change from less movement to more, more moles of products than reactants, mixing/dissolving,complex molecule being broken down)

first law of thermodynamics

total e in the universe is constant, e is not created nor destroyed, but can be transfered or transformed

second law of thermodyanmics

total entropy of the universe is constantly increases. suniverse = ssystem+ssurrounding

if Suniverse is positive

a process is spantaneous in the direction written

if SUniverse is negative

a process is spantanrous in the oppsoite direction than written

if SUniverse = 0

no tendancy to occur and system at equilibrium

entropy of a reaction:

sum of products - sum of reactants

gibbs free energy

energy available to do useful work. DECREASES when a reaction is spantaneous (negative for spontaneous)

G>0

spantenous for reverse reaction

G<0

spantaneous as written

G=0

no reaction

equation for gibs free energy (h-TS) uses… for temp

kelvins (273k+C)