C10 Thermodynamics Flashcards
what is thermodynamics? what does it allow us to predict?
- a branch of physical chemistry concerned with the transfer of energy
- allows predictions as to whether chemical reactions / processes will ‘go’
- allows for prediction of physical stabilities
what is energy?
- the capacity to do work (motion against an opposing force)
what are 2 forms that energy can take?
kinetic:
- the energy something possesses due to its motion
- any mass that is moving has kinetic energy
potential:
- the energy something possesses due to orientations / positions of its components
- eg. gravitational
what are the 4 laws of thermodynamics?
- zeroth law: thermodynamic equilibrium
- first law: conservation of energy
- second law: principles of entropy
- third law: absolute zero temperature
what is meant by system and surroundings?
system:
the bit we are interested in
eg. chemical reaction
surroundings:
everything else and where we make our observations
eg. in air / in a water bath
describe an open system
- both matter and energy can be transferred between the system and the surroundings
- mass transfer and heat transfer
- eg. test tube containing hot water
describe a closed system
- only energy can be transferred between the system and the surroundings
- only heat transfer
- eg. a stoppered test tube containing hot water
describe an isolated system
- no matter or energy can be transferred
- no heat or mass transfer
- eg. stoppered and insulated test tube containing hot water
what is internal energy (U)?
- the sum of all the kinetic energy and potential energy contributions of all the components in the system
- total potential and kinetic energy in a system
- impossible to calculate
- can only calculate changes in U
how can ΔU be calculated?
U final - U initial = ΔU
what 2 things can the internal energy of a system by changed by?
heat
work
what formula links work (w), ΔU and heat (q)?
ΔU = w + q
describe heat and work in relation to systems
- work (w) and heat (q) are done on or done by the system
- work and heat are what transfer energy into and out of our system
describe what it means if q is positive or negative
- q is positive if heat is absorbed by the system
- q is negative if heat is given off by the system
describe what it means if w is positive or negative
- w is positive if work is done on the system
- w is negative is work is done by the system
what does enthalpy take into account?
- enthalpy (H) accounts for pressure and volume
- heat content of an open system at constant pressure
what is the enthalpy change of a reaction?
- energy changes during chemical reactions
- enthalpy change between products and reactants
what does Hess’ law state?
- the standard reaction enthalpy is the sum of all the standard reaction enthalpies into which the overall reaction is divided
- can take any path for a reaction (any arrows) and will always get the same answer
what is enthalpy of formation of a substance?
- standard enthalpy (per mol) for its formation from its elements in their reference state
- they can be used to work out overall reaction enthalpy changes
is bond breaking endothermic or exothermic?
endothermic (energy is taken in)
is bond making endothermic or exothermic?
exothermic (energy is release)
what is entropy (S)?
- the disorder of a system
how do spontaneity and work relate to each other?
- spontaneous processes are driven by entropy and don’t require work to be done
- non-spontaneous processes require work to be done
describe entropy changes in isolated, open and closed systems
- isolated: entropy increase is spontaneous
- open/closed: you have to take into account the entropy of the system and its surroundings
sodium thiosulphate spontaneously transitions from liquid to solid at room temperature.
this doesn’t make sense because it is a spontaneous DECREASE in entropy. explain this
- chemical process gives out large amounts of heat (exothermic) - transferred to surroundings
- surroundings become hotter and thermal motion is greater = increases their entropy
- dispersion of energy is greater so entropy of surroundings increases
- total entropy of surroundings and system is increasing meaning it can be spontaneous
explain the third law / absolute zero
- at 0 Kelvin, all thermal motion is eliminated
- there is no disorder, S = 0
- this is Absolute Zero which we cannot go below
what is the equation for Gibbs free energy?
ΔG = ΔH - TΔS
what must ΔG be for a reaction to be spontaneous?
less than 0
if ΔS is positive and ΔH is negative, what does T have to be (if anything) for the reaction to be spontaneous?
spontaneous at all T
if ΔS is positive and ΔH is positive, what does T have to be (if anything) for the reaction to be spontaneous?
only spontaneous at high T
if ΔS is negative and ΔH is negative, what does T have to be (if anything) for the reaction to be spontaneous?
only spontaneous at low T
if ΔS is negative and ΔH is positive, what does T have to be (if anything) for the reaction to be spontaneous?
not spontaneous for any values of T
in biology, what drives non-spontaneous reactions?
ATP
what is the formula for equilibrium constant, Keq?
[products] / [reactants]
what does the numerical value of Keq tell us about the proportions in a reaction?
- if Keq > 1, products dominate
- if Keq < 1, reactants dominate
- if Keq = 0, reaction is at equilibrium
a reaction is spontaneous if ΔG < 0. when else is a reaction spontaneous?
if Keq is much greater than 1
(reaction favours products)
what does the Van’t Hoff equation allow us to see?
temperature can have an effect on the Keq
state the equation linking Keq and ΔG
ΔG = -RTln(Keq)
