AP Thermodynamics/Thermochemistry Concepts Flashcards
Ways to find enthalp (delta H)
- find q (specific heat) and divide by moles
- Hess’ Law
- Standard entalpies (products - reactants)
- bond energies (reactants - products)
What are molar heat, heat of reaction, heat of dissolution, heat of formation
all names for enthalpy (delta H)
Breaking bonds ___ energy
absorbs
Creating bonds ___ energy
releases
What is potential energy?
stored (chemical) energy (think: batteries)
What are the Three Laws of Thermodynamics?
1: Energy can only be transferred and transformed (enthalpy)
2: disorder always increases (entropy)
3: a crystal a 0 Kelvin has 0 entropy
What is the enthalpy of formation of elements (in their standard state)?
0 kJ/mol
If reaction is exothermic enthalpy is
negative
If reaction is endothermic enthalpy is
positive
If reaction is exothermic heat is
released as a product
If reaction is endothermic heat is
absorbed as a reactant
How to read a heating curve
x-axis: heat is added
y-axis: temperature increases
slanted lines (temp increases same phase)
flat line (phase change)
Why are meting and boiling points the same and condensation and evaporation points the same?
because EITHER the temperature changes OR a phase changes (not both at the same time)
What is calorimetry?
measuring heat exchange
What are the units for q = mcdelta T
q- heat in joules
m- mass grams
c- specific heat J/g*degrees C
delta T- Tf - Ti in degrees C
If two substances are in an isolated container and do not change phases…
they will equilibrate to the same temperature (calorimetry)
When calculating energy needed to heat/cool something that will undergo phase changes
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
What is the specific heat of water?
4.18 J/g*degrees C
How to find heat of reaction of a specific substance?
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
If asked to find how much energy is released and you have equation and enthalpy
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
How to find the enthalpy of the equation
products’ enthalpies - reactants’ enthalpies
What is heat of formation?
the energy (enthalpy) that takes one mole of the product to form
Hess’ Law Rules
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
How to find enthalpy of reaction when given equation and standard enthalpies?
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
What is bond energy?
the amount of energy it takes to break a bond
How to find enthalpy of reaction with bond energies and equation?
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
True/False: the more bonds you have the more energy it takes to break them
True
If there is more ordered, entropy (delta S) is
negative
If there is more disordered, entropy (delta S) is
positive
What is entropy?
disorder in a system
True/False: change in entropy of the universe must always decrease
False –> entropy of the universe always increases
What are some examples of entropy?
- 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
What does spontaneous mean?
the reaction occurs naturally
If reaction is not spontaneous, delta G is
positive
If reaction is spontaneous, delta G is
negative
Products are favored when
delta G is more negative
Reactants are favored when
delta G is more positive
What are the units of delta G, enthalpy, and entropy?
delta G: kJ/mol
enthalpy: kJ/mol
entropy: J/mol*K
If enthalpy is negative and entropy is positive, delta G is
negative and spontaneous
If enthalpy is negative and entropy is negative, delta G is
spontaneous at low temperatures
If enthalpy is positive and entropy is positive, delta G is
spontaneous at high temperatures
If enthalpy is positive and entropy is negative, delta G is
not spontaneous
