AP Thermodynamics/Thermochemistry Concepts

Question 1

Ways to find enthalp (delta H)

Answer 1

• find q (specific heat) and divide by moles
• Hess’ Law
• Standard entalpies (products - reactants)
• bond energies (reactants - products)

Question 2

What are molar heat, heat of reaction, heat of dissolution, heat of formation

Answer 2

all names for enthalpy (delta H)

Question 3

Breaking bonds ___ energy

Answer 3

absorbs

Question 4

Creating bonds ___ energy

Answer 4

releases

Question 5

What is potential energy?

Answer 5

stored (chemical) energy (think: batteries)

Question 6

What are the Three Laws of Thermodynamics?

Answer 6

1: Energy can only be transferred and transformed (enthalpy)
2: disorder always increases (entropy)
3: a crystal a 0 Kelvin has 0 entropy

Question 7

What is the enthalpy of formation of elements (in their standard state)?

Answer 7

0 kJ/mol

Question 8

If reaction is exothermic enthalpy is

Answer 8

negative

Question 9

If reaction is endothermic enthalpy is

Answer 9

positive

Question 10

If reaction is exothermic heat is

Answer 10

released as a product

Question 11

If reaction is endothermic heat is

Answer 11

absorbed as a reactant

Question 12

How to read a heating curve

Answer 12

x-axis: heat is added
y-axis: temperature increases
slanted lines (temp increases same phase)
flat line (phase change)

Question 13

Why are meting and boiling points the same and condensation and evaporation points the same?

Answer 13

because EITHER the temperature changes OR a phase changes (not both at the same time)

Question 14

What is calorimetry?

Answer 14

measuring heat exchange

Question 15

What are the units for q = mcdelta T

Answer 15

q- heat in joules
m- mass grams
c- specific heat J/g*degrees C
delta T- Tf - Ti in degrees C

Question 16

If two substances are in an isolated container and do not change phases…

Answer 16

they will equilibrate to the same temperature (calorimetry)

Question 17

When calculating energy needed to heat/cool something that will undergo phase changes

Answer 17

1) use q = mcdeltaT to find energy for first phase change temp
2) use q = mdeltaH (fus, vap) –> constant for substance
3) use q = mcdeltaT to find energy for next phase change temp
4) use q = mdeltaH (fus, vap)
5) add up all the energies

Question 18

What is the specific heat of water?

Answer 18

4.18 J/g*degrees C

Question 19

How to find heat of reaction of a specific substance?

Answer 19

1) use q = mcdeltaT to find energy in joules and convert to kJ
2) make sure the substance is converted into moles
3) divide joules by moles

Question 20

If asked to find how much energy is released and you have equation and enthalpy

Answer 20

1) convert substance into moles
2) make mole ratio of enthalpy in numerator and substance’s mol (coefficient) from equation
3) multiply to find energy

Question 21

How to find the enthalpy of the equation

Answer 21

products’ enthalpies - reactants’ enthalpies

Question 22

What is heat of formation?

Answer 22

the energy (enthalpy) that takes one mole of the product to form

Question 23

Hess’ Law Rules

Answer 23

reversing the equation –> reverses the sign of enthalpy
doubling the equation –> doubles the enthalpy
keeping the equation the same –> keeps the enthalpy the same
*Make changes to partial equations until you get final equation and add enthalpies

Question 24

How to find enthalpy of reaction when given equation and standard enthalpies?

Answer 24

1) find enthalpies of products by multiplying their standard enthalpies (in kJ/mol) by their mol ration (coefficient in equation) and add them
2) find the enthalpies of reactants using the method above and add them
3) enthalpy of products - enthalpy of reactants

Question 25

What is bond energy?

Answer 25

the amount of energy it takes to break a bond

Question 26

How to find enthalpy of reaction with bond energies and equation?

Answer 26

1) write Lewis Dot Structure for each molecule 2) count how many of what kind of bond is in each molecule and multiply it by the bond energy (given in kJ/mol) *don't forget to multiply this value by the mole ratio (coefficient) from the equation 3) add the reactants up, add the products up 4) reactants - products

Question 27

True/False: the more bonds you have the more energy it takes to break them

Answer 27

True

Question 28

If there is more ordered, entropy (delta S) is

Answer 28

negative

Question 29

If there is more disordered, entropy (delta S) is

Answer 29

positive

Question 30

What is entropy?

Answer 30

disorder in a system

Question 31

True/False: change in entropy of the universe must always decrease

Answer 31

False --> entropy of the universe always increases

Question 32

What are some examples of entropy?

Answer 32

- a pure substance becoming impure - changing from a more ordered state to a less ordered state (ex: liquid to gas) - going from less moles to more moles

Question 33

What does spontaneous mean?

Answer 33

the reaction occurs naturally

Question 34

If reaction is not spontaneous, delta G is

Answer 34

positive

Question 35

If reaction is spontaneous, delta G is

Answer 35

negative

Question 36

Products are favored when

Answer 36

delta G is more negative

Question 37

Reactants are favored when

Answer 37

delta G is more positive

Question 38

What are the units of delta G, enthalpy, and entropy?

Answer 38

delta G: kJ/mol enthalpy: kJ/mol entropy: J/mol*K

Question 39

If enthalpy is negative and entropy is positive, delta G is

Answer 39

negative and spontaneous

Question 40

If enthalpy is negative and entropy is negative, delta G is

Answer 40

spontaneous at low temperatures

Question 41

If enthalpy is positive and entropy is positive, delta G is

Answer 41

spontaneous at high temperatures

Question 42

If enthalpy is positive and entropy is negative, delta G is

Answer 42

not spontaneous