Thermodyanmics

Question 1

Thermodynamics

Answer 1

transformation of energy from one form to another

Question 2

what is the zeroth law of thermodynamics?

Answer 2

if two systems are in thermal equilibrium with a third system, then the two initial systems are in thermal equilibrium with each other.

Question 3

heat flows from the body with the higher temp to the

Answer 3

body with the lower temp

Question 4

what is the first law of thermodynamics?

Answer 4

the total energy of the universe is constant

energy can be transformed from one form to another but it cannot be created or destroyed.

Question 5

for chemistry and physics, we define everything in terms of what is happening to the ?

Answer 5

system!
so energy flow into the system +
energy flow out of the system is -

Question 6

when energy flows into a system from surroundings, the energy of the system increases and the energy of the surroundings

Answer 6

decreases

Question 7

what is enthalpy?

Answer 7

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.

Question 8

when a bond is formed , the energy is released

Answer 8

H <0

Question 9

energy has to be put into a system to break a bond

Answer 9

H >0

Question 10

the enthalpy of a reaction is given by the difference between the enthalpy of the products and the enthalpy of the reactants

Answer 10

H products - H reactants = change in enthalpy or the heat of the reaction

Question 11

in an exothermic reaction energy is released and the products are ?

Answer 11

in a lower energy state

and H is negative

Question 12

in an endothermic reaction, energy is absorbed and the products are in a

Answer 12

higher energy state and the H is +

Question 13

catalyst will Lower the activation energy but it will not change ?

Answer 13

it will not change the equilibrium constant, enthalpy , entropy or free energy in any way

Question 14

the heat of a reaction can be calculated in a number of ways

Answer 14

The standard heat of formation
hess’s law
bond dissociation energies

Question 15

standard conditions

Answer 15

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.

Question 16

DO NOT confuse standard conditions with standard temp and pressure (STP)

Answer 16

STP is 0 degrees Celsius , while standard conditions means 25 degrees celcius

Question 17

The Standard heat of formation

Answer 17

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

Question 18

The standard heat of formation for diatomic elements is

Answer 18

zero! for ex O2, H2, CL2 all have standard heat of formation =zero!

Question 19

The standard heat of reaction can be calculated

Answer 19

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.

Question 20

Hess’s law of heat summation

Answer 20

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

Question 21

what are the two rules for Hess’s law?

Answer 21

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.

Question 22

summation of the average bond enthalpies

Answer 22

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)

Question 23

how are bond energies reported?

Answer 23

as the average of many examples of that type of bond.

Question 24

The second Law of thermodynamics

Answer 24

disorder of the universe increases in a spontaneous process

Question 25

we measure disorder or randomness as ?

Answer 25

entropy!

the greater the disorder of a system, the greater the entropy (S)

Question 26

the change in entropy is determined by?

Answer 26

S products - S reactants

Question 27

what is the third law of thermodynamics?

Answer 27

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.

Question 28

what is Gibbs free energy?

Answer 28

energy that is available (free) to do work from a chemical reaction.

Question 29

change in Gibbs free energy?

Answer 29

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

Question 30

what happens when both H and S are the same ?

Answer 30

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

Question 31

what happens when TS in the equation are very small

Answer 31

then G=H 
so G (diff between the products-reactants) = heat of the reaction or the H

Question 32

DO NOT confuse thermodynamics with Kinetics !

Answer 32

just bc a reaction is spontaneous (thermodynamically favourable) does not mean it will take place rapidly.

Question 33

when you reverse the reaction what changes and what stays the same ?

Answer 33

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