Topic 5A- Thermochemistry Flashcards
energy
ability to do work or transfer heat
-energy used to move object= work
-energy used to increase temp= heat
kinetic energy
energy an object gains by motion
-eg bike going down a hill
potential energy
energy an object gains from its position or chem composition
-E el
-smaller separation, greater repulsion (more work) , higher E el
-greater separation, less repulsion (less work) , lower E el
units of energy
-Joules
-also calorie
-1 cal= 4.184 J
system and surroundings
-system: includes molecules, the reference point
surroundings- everything else
work equation
W= f x d
force and distance
heat flows from
warmer to cooler objects
conversions of energy
-energy can be converted from one type to another
-eg- the cyclist has potential energy at top of hill then turns into kinetic when going down
first law
- total energy is constant
-if system loses energy, it must be gained by surroundings
-if surroundings lose energy, it will be gained by systemi
if energy is lost to surroundings what will the ^E be? Gained from surroundings?
loss- negative
gain- positive
internal energy
-sum of kinetic energy and potential energies of all components of the system (E or U)
change of internal energy
-change in internal energy is the final energy of system minus the initial energy of the system
-^E= Efinal - Einital
-if Efinal > Einitial, the system absorbed energy from the surroundings, endergonic, pos ^E
-if Efinal < Einitial, system released energy to surroundings, exergonic, neg ^E
how does energy being exchanged between system and surroundings work?
whats its formula
-its exchanged as heat or work
-^E= q + w
if ^sys is pos whats the surr? vis versa
^sys= -^surr
-^sys= ^surr
what do + and - mean in terms of q, w, ^E (system is the reference)
q= + gain heat, - lose heat
w= + work done on sys, - work done by sys
^E= + energy gain, - energy loss
when heat is absorbed by system from surroundings (heat enters to something cold) what is it
endothermic
when heat is released by system to surroundings (heat exiting to surroundings)
-utilized more
-exothermic
why do we not know internal energy of a system
eg- if something is hot it will cool to room temp, and if we have ice it will melt to room temp which ends up being the same so we can tell
what is the internal energy of a system dependent on
-depends on present state of the system, not by the path
-state function
-so only depends on initial and final
what isnt a state function
-q and w
-the battery will always be the same from start to finish whether
enthalpy and its formula
internal energy plus the product of pressure and volume
^H= q (heat)
-at constant pressure, the change in enthalpy is the heat gained or lost: products- reactants
three truths about enthalpy
- enthalpy depends on amount
- ^H forward= ^reverse but opposite in size
- ^H depends on state of prod and reactants- not the path
calorimetry
- we dont know exact enthalpy of reactants, so we measure ^H through calorimetry which is the measurement of heat flow
-calorimeter measures the q surr so we can then measure the sys
heat capacity
-amount of energy required to raise the temp of a substance by 1 K (1 C degree)
J/g-K or J/m-k
-al heat capacity is low (could grab it out of oven)
-Fe is much higher (burn out of oven)
^H formula
q= m x Cs x ^T
-determine if endo or exo and then apply sign at end
qsys= ^Hrxn
bomb calorimetry
-constant volume
-carried out in a sealed bomb
-measures the ^E not ^H
-difference is very small
hess law
-if a reaction is carried out in a series of steps, ^H overall will be equal to the sum of all indiv steps
^Hrxn=^H1 + ^H2 + ^H3…..
-depends only on initial state and final state of products
enthalpies of formation
^Hf is the enthalpy change where a compound is made from its elements in there elemental forms
-in bar
tips for hess law
- “calculate the ^H for the formation”
-just break apart what its asking for
-make the equation you want
-watch stoich mols
-may have to switch around equation to cross out what you need - might need to switch signs if switching
-need to multiply by mols
-^Hf= + + + +
standard heat of combustion formula
sum if products- sum of reactants
-formula + O2 and ends in CO2 and H2O
-multiply mols by the ^Hf on data sheet
if heat capacity is given in J/mol or J/g what does that determine
your m in the formula will be mass or moles depending on this
