Chapter 6 - Thermochemistry

what qualifies as energy

the capacity to do work (ability to move things) or to produce heat (a currency of energy)

thermal en contains BOTH

kin en and pot en (the energy an obj possesses due to its position, condition or composition)

SI unit for en

J, kJ

system; surroundings

part of the universe that is being examined

universe =

system + surroundings

• so you can rearrange into universe - system = surroundings blah blah blah kill me lol

extensive property

a property that depends on the amount of substance in the system

• physically divisible, like V, n, mass

intensive property

property that does NOT depend on the amount of substance in the system

• physically indivisible, like P and T

work

movement against a force

= force (vector, can be pos or neg) x dist (vector, can be pos or neg)

total energy of a system =

internal en, denoted as U

• internal en can be increased by doing work ON the system or decreased by the system doing work BY the system

• in other words, work is pos if work is done on system, and neg if work was done by the system

• en is stored in molecular pot and kin en

you can’t calculate the total internal energy of a system, but you CAN calculate the

CHANGE in total internal en of the system

• ∆U = q + w

• q = heat transferred

• w = work transferred

heat (q)

form of energy that flows between two samples because of their difference in temperature.

• During heat flow, Temperature may change or Phase may change (an isothermal process).

if only the heat transfer contributed to the change in internal energy

∆U = q

• q = pos if heat enters sys (increases temp)

• q = neg if heat leaves sys (decreases temp)

• heat transfer changes molecular pot and kin en

heat capacity

quantity of heat required to change the temperature of a system by one degree is called the heat capacity of the system

• units → C (J/g°C), w C denoting the heat capacity

transfer of heat formula

q = mC∆T = mC(T2 - T1) → the sign (pos or neg) of the change in temp dictates the sign of q, and therefore tells us whether the system gained or lost heat

chemical reactions and how they relate to heat

when a reaction occurs in the system, heat can be released or absorbed by the system.

heat of rxn: q(rxn)

The quantity of heat exchanged between a system and its surroundings when a chemical reaction occurs within the system, at constant temperature

• the system = the reaction

• the surroundings = water or calorimeter

• if q(rxn) is neg → the rxn released/lost heat

• if q(rxn) is pos → the rxn gained/absorbed heat

chemical reactions and how they relate to work

• in addition to heat effects, chemical reactions may also do work

• work is done on the system from the surroundings when the system is compressed in volume – “PV” work

• the atmosphere (P → pressure) presses on the system and changes the volume

W(sys) (work done on system)

wsystem = -P∆V = -P(V2 - V1)

→ rmbr that the unit for work = J

total internal en theeeee fuckin specifics

• a bunch of movement of the dumbass molecules happens y’know which amounts to both kin and pot en

a system contains ONLY

• internal en (shit like kin and pot en) and does NOT “contain” work or heat, which are quantities that are important during an energy CHANGE in the system

key points of the conservation of energy (god how many fucking times have i learned about this at this point)

• The energy of an ISOLATED system is constant

• Energy can be converted from one form to another but can neither be created nor destroyed.

molar heat capacity (still C)

amount of heat needed to raise the temperature of 1 mol (instead of 1 gram) of substance by 1° C.

∆U universe =

∆U system + ∆U surroundings= 0

enthalpy

denoted by ∆H and represents the heat gained or lost by a chemical reaction

working under constant volume conditions

you would not have the “work” term in any eqn since the change in V = 0

standard enthalpy values

• measured enthalpy changes under STANDARD CONDITIONS (SATP)

• For a gas, pressure is exactly 1 bar

• For a solution, concentration is exactly 1 molar

• Pure substance (liquid or solid), it is the pure liquid or solid.

standard molar enthalpy of formation

• denoted as ∆Hof

• the enthalpy change when 1 mol of compound is formed from the reference form of the elements under standard conditions (and its NATURAL STATE)

• basically how much heat it absorbs or releases during the chemical formation of this compound

• The standard enthalpy of formation of a pure element in its reference state is 0 (bc it exists in its natural state and didn’t “form” anything new)

W =

-∆nRT

where ∆n = diff b/w coeffs of the gases in the chem eqn (prod gases - reactant gases)

• only use this relationship when the change in volume is ONLY due to the change in the amount of gas (bc more gas means less space)

enthalpies of formation of ions in aq solns

• select H+ as the ref point (new “anchor” pt) and have every other enthalpy be relative to H+

• the reason is bc we can’t acc det an eqn of FORMATION for ions since they aren’t made from elements combining together but from a change in charge)

change in temp is inversely proportional to

the amount of en (the more shit there is, the more the heat needs to spread)

in a system with CONSTANT VOLUME, neglect

work in calculations for the change in internal en (∆U)

∆rH

= MOLAR enthalpy → heat change PER MOLE

∆H = ∆U + P∆V =

∆H = ∆U + ∆nRT

OR

∆rH = ∆rU + ∆nRT

phase changes and ∆H

• solid → lq = endothermic (since it needs heat to melt)

watts unit breakdown

J/s

1 atm =

101.325 J

caloric content of food =

enthalpy of combustion (ΔHcomb) per gram

the mass of “solvent” (not really) in a bomb calorimeter is

negligible

• so you ignore mass in q = mcΔT

at constant volume, ΔrU and ΔU =

q (the heat transfer) bc w would be rendered = 0

ΔrH =

ΔrU + ΔnRT (where ΔrU is the molar enthalpy of only heat and Δn is the difference between the coeffs of prod gases - reactant gases)

heat capacity of water

4.182 J / g*C

a higher heat capacity means

more heat is required to raise the temperature of that substance

difference between U and H

U = total energy

H = total heat

to calc the amount of water that was boiled away

take the difference between the heat that was lost by the extremely hot substance and the heat that was actually absorbed by the water (since the boiling temp of water is 100 degrees celcius) → the remaining heat is the heat available for turning water to steam