Thermodynamics And Thermochemistry

Question 1

Heat of a reaction in a bomb calorimeter equation

Answer 1

q_rxn = -C x ΔT

Question 2

In an exothermic reaction at equilibrium, increasing the temperature will:

Answer 2

According to Le Chatlier’s principle, increasing the temperature of an exothermic reaction will:
Increase the heat (a product), shifting the equilibrium towards the reactants
As the concentration of the reactants increase, the K_c of the reaction will decrease
The rate of the forward and reverse reactions will increase causing the reaction to proceed at a faster rate
The higher the temperature, the more energy the molecules have to hit each other with enough force to cause a reaction

Increasing temperature causes reaction to proceed at a faster rate while its K_c decreases

Question 3

Formula for calculating the enthalpy change from bond enthalpy values:

Answer 3

ΔH = ΣΔH_{bonds broken} - ΣΔH_{bonds formed}
Bonds broken = reactants and bonds formed = products —> breaking bonds require energy (+) and forming bonds release energy (-)

Question 4

Equation that governs enthalpy for a homogenous system:

Answer 4

(All gas or all liquid) H = U + PV
U = internal energy
P = pressure
V = volume
enthalpy of a system of a gas (or liquid) is dependent on pressure, volume and internal energy

Question 5

what is enthalpy?

Answer 5

enthalpy (H) is the amount of heat energy contained within a system: ΔH = H_products - H_reactants

Question 6

endothermic

Answer 6

a process in which heat is transferred from surroundings to the system (surrounding will feel cold): ΔH > 0

Question 7

exothermic

Answer 7

a process in which heat is transferred from the system to the surroundings (surrounding will feel hot): ΔH < 0

Question 8

what phase changes are endothermic?

Answer 8

fusion, vaporization, sublimation

Question 9

what phase changes are exothermic?

Answer 9

condensation, recrystalization and deposition

Question 10

conduction

Answer 10

the transfer of heat via direct contact

Question 11

convection

Answer 11

the transfer of heat due to motion of a liquid of gas

Question 12

radiation

Answer 12

the transfer of heat via electromagnetic radiation

Question 13

what is the equation used to determine whether expansion or compression when referring to gaseous systems?

Answer 13

ΔE = q + w
w = -PΔV
ΔE = change in internal energy
q = heat
w = work
P = pressure (always a positive value)
ΔV = change in volume

Question 14

when work is a positive value, the means ΔV is a ________ value and the volume has been ________

Answer 14

negative, decreased –> the surrounding is doing work on the system to compress it

decreasing volume compresses it

Question 15

when work is a negative value, that means ΔV is a ____ value, and volume has been ________

Answer 15

positive, increased –> system is doing work on the surrounding causing it to expand

increasing volume expands it

Question 16

what is specific heat (C)?

Answer 16

• the amount of energy that is required to raise the temperature of 1.0 gram of a substance by 1°C
• units are J/g °C
• specific heat varies from one substance to another and between phases of a substance

Question 17

equation for calculating specifc heat of a substance and q (heat) it absorbs or releases and its mass:

Answer 17

q = mcΔT
* q = heat absorbed or released by substance
* m = mass of the substance (grams)
* C = specific heat of a substance
* ΔT = change in temperature of a substance

only used when calculating the q of a substance not undergoing a phase change (ex: increasing the temperature of water from 20°C to 90°C, water is a liquid at both temperatures and therefore it is not undergoing a phase change

Question 18

equation for calculating the heat requirement while a phase change is occuring:

Answer 18

q = (m)ΔH
* ΔH = kJ/mol
* mass = mole
* if phase change involves conversion of solid to liquid then ΔH_fusion will be used etc.

mass must be converted to moles!

Question 19

second law of thermodynamics:

Answer 19

the entropy of the universe is always increasing (the universe is becoming more and more disordered)

Question 20

ΔG: (+)
ΔH: (+)
ΔS: (-)
-TΔS: (+)

Answer 20

nonspontaneous

Question 21

ΔG: (-)
ΔH: (-)
ΔS: (+)
-TΔS: (-)

Answer 21

spontaneous

Question 22

ΔG: (+/-)
ΔH: (-)
ΔS: (-)
-TΔS: (+)

Answer 22

high temp: nonspontaneous
low temp:spontaneous

Question 23

ΔG: (+/-)
ΔH: (+)
ΔS: (+)
-TΔS: (-)

Answer 23

high temp: spontaneous
low temp: nonspontaneous

Question 24

-ΔG° means K is:

Answer 24

K > 1 products are favored at equilibrium

Question 25

+ΔG° means K is:

Answer 25

K < 1 reactants are favored at equilibrium