Midterm #1 - Thermochemistry

thermodynamics

the general study of energy and its interconversions

energy (E)

the capacity to do work

* 1 J = 1 kgm^2/s^2
* 1 cal = 4.184 J
* 1000 cal = 1 kcal = 1 Cal

work (w)

a force acting over a distance

w = force (distance)

heat (q)

the flow of energy caused by a difference in temperature

* exchange of therma energy

kinetic energy

the energy of motion

thermal energy

a form of kinetic energy

* energy associated with temperature
* temperature is a measure of the thermal energy
* temperature is proportional to the heat transferred

potential energy

energy that is stored in an object or energy associated with the composition and position of the object

chemical energy

a form of potential energy

* energy stored in the structure of a compound

system

defined area of investigation (the subject)

surroundings

everything that the system can exchane energy with (everything else other than the system)

state function

depends only on the current state and not the path taken

* ex - temp, pressure, concentration, physical state, enthalpy

path function

depends on route taken to get between two states; “journey”

* ex - work and heat

extensive property

based on the amount of a substance

* ex - mass, volume, and length

intensive property

not based on the amount of a substance (inherent)

* ex - temperature, color, melting point, molecular weight

internal energy

kinetic energy + potential energy

* state function

first law of thermodynamics - law of conservation of energy

the total energy of the universe is constant

* energy can neither be created nor destroyed, only transferred

change in energy equation

delta E = q + w

\-q

system loses thermal energy

\+q

system gains thermal energy

\-w

work is done by the system

* using work and requires more energy

\+w

work is done on the system

* gaining energy

how much work is done during the combustion?

delta E = q + w

* use wording to access if it is - or +
* find w in kJ

heat exchange

occurs when there is a difference in temp

* flows from high to low temp
* continues until thermal equilibrium is reached

heat capacity

the amount of thermal energy a system must absorb through a heat transfer in order to undergo a specified change in temp

* like threshold
* q = C (delta T)
* q = J/ celcius

specific heat capacity

heat capacity per gram

* q = mass C (delta T)
* mass = g
* q = J/ g celcius

molar heat capacity

heat capacity per mol

* q = mol C (delta T)
* q = J/(mol Celcius)

heat capacity and thermal energy relationship

higher heat capacity = more thermal enregy needed to raise the temp

thermal energy transfer

(mC delta T) of system = - (mC delta T) of surroundings

pressure-volume work

when force is changing the volume against an exerted pressure

w = -pressure (delta V)

* p = L atm
* energy = J

pressure equation

pressure = force/area

pressure to energy converstion

1 L(atm) = 101.3 J

bomb calorimetery

constant volume

q rxn = -q cal = -C delta T

* C = kj/mol

solving bomb calorimetery

1. find heat capacity (c) of bomb calorimeter
2. use heat capacity to find E rxn

enthalpy

a measurement of energy in a thermodynamic system

* delta H = delta E + P(delta V)

enthalpy at a constant pressure

delta H = q

* usually for atmospheric pressure (coffee cup)

constant pressure calorimetry (coffee cup)

ΔHrxn = qrxn = -q of solution/ water

ΔH vs ΔE

they are almost the same

* will be similar if small change in volume

bond energy/ enthalpy

energy needed to break one mole of a bond in a compound

using bond enthalpy to find ΔHrxn

1. ΔHrxn = bonds broken - bonds formed
2. ΔHrxn = energy of all bonds in the products vs. all bonds in the reactants

solving for bond energy

ΔHrxn = broken (+ value) + formed (- value)

ΔHrxn (kJ/mol) = ΔH of product - ΔH of reactant

standard enthalpy change

the change in enthalpy for a process when all reactants and products are in their standard states (ΔH°rxn)

* ΔH°rxn = ΔH°f of product - ΔH°f of reactant
* catabolic = -
* anabolic = +

hess’s law

the change in enthalpy for a stepqise process is the sum of the enthalpy changes of the steps

multiplying equation for hess’s law

multiply everything including ΔH

reversing the equation in hess’s law

make the ΔH the opposite sign

lattice energy

energy of formation of a crystalline solid from gaseous ions; the two ions attract

* always - since energy is being released

ionization energy

remove an electron

bond energy

breaking bond

lattice energy trends

ion charge increases = magniture increases

ion size increases = magnitude decreases

ionic radii trend

down and to the left it increases