chem 101 thermochem

what is energy

the capacity to do work (w) or produce heat (q)

is absolute energy measureable

no, only the energy transferred can be measured (change in E) → measured indirectly by heat or work

system vs surroundings

system = part of universe we are studying (often represents reaction taking place)

surrounding = everything else

what is work

energy transfer due to an applied force

heat

energy transfer due to a difference in temperature

OR

energy transfer due to anything other than work

open, closed, isolated and adiabatic system

adiabatic (thermos)

state function

functions or properties that define the present condition of a system (not dependent on path taken)

T or F: heat and work are state functions

false, they are a transfer of E

first law of thermodynamics

the energy of an isolated system is constant

(equal to zero)

internal energy

just U or E (no change) all the motion inside molecule

consists of translational energy (motion)

energy stored in bonds (rotational, vibrational and electronic)

energy stored in the intermolecular forces

positive delta U, positive q, positive w

energy of system increases

system absorbs heat (endothermic)

work done ON system by surroundings (prespective of system)

negative delta U, negative q, negative w

energy of system decreases, system releases heat, work done BY system on surroundings (perspective of system)

extensive property + example

property that depends on the amount of substance

ex. heat, mass, volume

intensive properties + examples

property that does not depend on amount of substance

ex. temp, density, and melting point

since heat is an extensive property, what does heat depend on

1. how many degrees is the substance heated or cooled

2. amount (mass)

3. nature

heat capacity

amount of heat required to changed the temperature of a system by one degree

extensive property

specific heat capacity c

(J/gK)

amount of heat required to change to temperature of one gram of a system by one degree

intensive

molar heat capacity C

(J/ mol*K)

amount of heat required to change the temperature of a system by one degree per mol

intensive

calorimeter

an isolated system where no energy or matter is exchanged with surroundings

what is calorimeter used to measure

heat released or absorbed during a reaction

bomb calorimetry

exothermic reaction inside bomb will heat up water to measure energy change

determine caloric content of food

similarities and differences between regular calorimeter vs bomb

similar = both are isolated sytems

differences = bomb is constant volume, regular is constant pressure

when doing bomb calorimeter calculations, what is the most important thing to keep in mind

q(cal)= q(rxn)

what are the most common type of work encountered in chemical processes

work done by a gas the work is negative (expansion)

work done to a gas is positive (compression)

external vs internal pressures of gas expansion

Pext < Pint

Vf > Vi

external vs interal pressure of gas compression

Pext > Pint

Vi > Vf

enthalpy

change in systems internal energy + product and energy changes (work and internal energy at constant pressure)

what happens to pressure volume system if volume is constant

work=0, measuring delta U which is equal to enthalpy (delta H)

what happens when pressure volume system if pressure is constant

delta U= qp - PdeltaV (some work will still be done)

qp represents enthalpy

what are the 3 ways to calculate enthalpy

1. Hess’s Law

2. Standard enthalpies of formation

3. Bond Dissociation Enthalpies

Hess’s Law

change in enthalpy of a reaction is the sum of all steps in the reactions

OR

is the sum of enthalpy of any reactions that sum to give the overall reaction

ehthalpy is a ____ and _____ property

state function, extentsive property

standard enthalpies of formation

change in enthalpy that accompanies the formation of 1 mol of substance from its elements (from their standard states)

standard enthalpy of formation of a pure element in its reference form is

ZERO (O2, n2, C, H+)

why is enthalpy of bond dissociation mainly for gas phase

no intermolecular forces