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ap chemistry > AP Thermodynamics/Thermochemistry Concepts > Flashcards

AP Thermodynamics/Thermochemistry Concepts Flashcards

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1
Q

Ways to find enthalp (delta H)

A
  • find q (specific heat) and divide by moles
  • Hess’ Law
  • Standard entalpies (products - reactants)
  • bond energies (reactants - products)
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2
Q

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

A

all names for enthalpy (delta H)

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3
Q

Breaking bonds ___ energy

A

absorbs

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4
Q

Creating bonds ___ energy

A

releases

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5
Q

What is potential energy?

A

stored (chemical) energy (think: batteries)

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6
Q

What are the Three Laws of Thermodynamics?

A

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

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7
Q

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

A

0 kJ/mol

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8
Q

If reaction is exothermic enthalpy is

A

negative

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9
Q

If reaction is endothermic enthalpy is

A

positive

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10
Q

If reaction is exothermic heat is

A

released as a product

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11
Q

If reaction is endothermic heat is

A

absorbed as a reactant

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12
Q

How to read a heating curve

A

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

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13
Q

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

A

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

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14
Q

What is calorimetry?

A

measuring heat exchange

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15
Q

What are the units for q = mcdelta T

A

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

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16
Q

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

A

they will equilibrate to the same temperature (calorimetry)

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17
Q

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

A

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

18
Q

What is the specific heat of water?

A

4.18 J/g*degrees C

19
Q

How to find heat of reaction of a specific substance?

A

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

20
Q

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

A

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

21
Q

How to find the enthalpy of the equation

A

products’ enthalpies - reactants’ enthalpies

22
Q

What is heat of formation?

A

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

23
Q

Hess’ Law Rules

A

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

24
Q

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

A

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

25
Q

What is bond energy?

A

the amount of energy it takes to break a bond

26
Q

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

A

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

27
Q

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

A

True

28
Q

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

A

negative

29
Q

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

A

positive

30
Q

What is entropy?

A

disorder in a system

31
Q

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

A

False –> entropy of the universe always increases

32
Q

What are some examples of entropy?

A
  • 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
33
Q

What does spontaneous mean?

A

the reaction occurs naturally

34
Q

If reaction is not spontaneous, delta G is

A

positive

35
Q

If reaction is spontaneous, delta G is

A

negative

36
Q

Products are favored when

A

delta G is more negative

37
Q

Reactants are favored when

A

delta G is more positive

38
Q

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

A

delta G: kJ/mol

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

39
Q

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

A

negative and spontaneous

40
Q

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

A

spontaneous at low temperatures

41
Q

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

A

spontaneous at high temperatures

42
Q

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

A

not spontaneous

ap chemistry (10 decks)

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