Thermodyanmics Flashcards
Thermodynamics
transformation of energy from one form to another
what is the zeroth law of thermodynamics?
if two systems are in thermal equilibrium with a third system, then the two initial systems are in thermal equilibrium with each other.
heat flows from the body with the higher temp to the
body with the lower temp
what is the first law of thermodynamics?
the total energy of the universe is constant
energy can be transformed from one form to another but it cannot be created or destroyed.
for chemistry and physics, we define everything in terms of what is happening to the ?
system!
so energy flow into the system +
energy flow out of the system is -
when energy flows into a system from surroundings, the energy of the system increases and the energy of the surroundings
decreases
what is enthalpy?
measure of the heat energy that is released or absorbed when bonds are broken and formed during a reaction that’s run at constant pressure
symbol for enthalpy is H
- enthalpy is a measure of the energy stored in the bonds of molecules.
when a bond is formed , the energy is released
H <0
energy has to be put into a system to break a bond
H >0
the enthalpy of a reaction is given by the difference between the enthalpy of the products and the enthalpy of the reactants
H products - H reactants = change in enthalpy or the heat of the reaction
in an exothermic reaction energy is released and the products are ?
in a lower energy state
and H is negative
in an endothermic reaction, energy is absorbed and the products are in a
higher energy state and the H is +
catalyst will Lower the activation energy but it will not change ?
it will not change the equilibrium constant, enthalpy , entropy or free energy in any way
the heat of a reaction can be calculated in a number of ways
The standard heat of formation
hess’s law
bond dissociation energies
standard conditions
most constants, heat of formation , enthalpies and so on are determined for standard conditions.
under standard conditions, the temperature is 298 K (25 degrees Celsius) and the pressure is 1 atm
all solids and liquids are assumed to be pure, and solutions are considered to be @ 1 M .
values determined to be at standard conditions are given a 0 superscript.
DO NOT confuse standard conditions with standard temp and pressure (STP)
STP is 0 degrees Celsius , while standard conditions means 25 degrees celcius
The Standard heat of formation
amount of energy needed to make one mole of a compound from its constituent elements in their natural or standard state.
- which is how the compound exists under standard states
The standard heat of formation for diatomic elements is
zero! for ex O2, H2, CL2 all have standard heat of formation =zero!
The standard heat of reaction can be calculated
by:
(n) standard heat of formation of the products - standard (n) heat of formation of reactants
both of these you have to multiply by n where n= stoichiometric coefficient applied to each species.
Hess’s law of heat summation
Hess’s law states that if a reaction occurs in several steps then the sum of the energies absorbed or given off in all sets will be the same as that for the overall reaction.
this is because enthalpy is a state function, which means its independent of the pathway of the reaction ! so H is independent of the pathway
what are the two rules for Hess’s law?
if a reaction is reversed the sign of delta H is reversed too
if an equation is multiplied by a coefficient then delta H must also be multiplied by that same value.
summation of the average bond enthalpies
enthalpy can be viewed as the energy stored in the chemical bonds of a compound. bonds have characteristic enthalpies that denote how much energy is needed to break them. this is the third method of determine the heat of the reaction or the enthalpy of the reaction. if a question gives you a list of bond enthalpies , the heat of the reaction can be determined by the equation sum of ( BDE bonds broken)- sum of (BDE of bonds formed)
how are bond energies reported?
as the average of many examples of that type of bond.
The second Law of thermodynamics
disorder of the universe increases in a spontaneous process
we measure disorder or randomness as ?
entropy!
the greater the disorder of a system, the greater the entropy (S)
the change in entropy is determined by?
S products - S reactants
what is the third law of thermodynamics?
the third law defines absolute zero to be a state of zero entropy. at absolute zero, thermal energy is absent . therefore S=0 . kelvin =0 on the temp scale.
what is Gibbs free energy?
energy that is available (free) to do work from a chemical reaction.
change in Gibbs free energy?
delta G = delta H - T delta S ( this equation follows for a reaction under any set of conditions)
t= absolute temperature in kelvins
when delta G is + non spontaneous reaction in the forward direction
when delta G= 0 the reaction is at equilibrium
when delta G is - the reaction is spontaneous in the forward direction
what happens when both H and S are the same ?
G= H-TS
when H and S are positive , G is negative at high temp
but it will be + at low temp
when H and S are both negative , G is positive at high temp, but - at low temp
important note about this equation : make sure H and S have the same units.
- G will be spontaneous
+ G will be non spontaneous
what happens when TS in the equation are very small
then G=H so G (diff between the products-reactants) = heat of the reaction or the H
DO NOT confuse thermodynamics with Kinetics !
just bc a reaction is spontaneous (thermodynamically favourable) does not mean it will take place rapidly.
when you reverse the reaction what changes and what stays the same ?
the reverse reaction has the same magnitude for all thermodynamic values ( G, H,S) BUT OF OPPOSITE SIGN
one thing that changes is the Ea or the activation barrier which can be diff for the reverse or the forward reactions
