Question

MCAT Tutorial: Introduction to Thermochemistry

Answer 1

\({\bf{Basic~Terminology:}}\)
-state function: a property/function of a substance that only depends on the current conditions, enthalpy, entropy, temperature, pressure, volume, etc.
-path function: a property/function of a substance that depends on the pathway used to get to its current state, ex. heat and work
- system: the region of the universe being studied
- surroundings: everything not part of the system
- exothermic: process that releases heat
- endothermic: process that absorbs heat
- open system: allows energy and matter to exchange between sys. and surroundings
- closed system: only allows energy to be exchanged
- isolated system: allows neither to be exchanged

Answer 2

\({\bf{Enthalpy:}}\) total heat of a system at a given, constant pressure, symb. by H
- enthalpy equation: ΔHrxn = ΔH products - ΔH reactants = ΔH bonds broken - ΔH bonds formed, must also multiply by the molar quantity indicated by the balanced chemical equation
- if the equation is reversed simply multiply the heat of reaction by -1
- ΔH°f: standard heat of formation, 0 for a pure element, determined experimentally
- bond energy: average amount of energy required to break 1 mole of one bond of a gas

- Hess's Law: if an equation can be decomposed as a sum of multiple equations then the total enthalpy is the sum of the enthalpies of the component reactions
- tips for Hess's Law calculations:
1. start with the reactants of the desired equation, and find an equation that will allow you to write the reactants on the left side, and keep going for the entire equation
2. be careful with bond enthalpies, they have to be multiplied/divided, etc. according to the way the component equations are multiplied/divided
3. if you have the same compound in the product of one equation and the reactant of another equation they will cancel out when the equations are added
A+B -> C
C - > D
when the equations are added this yields

A+B -> D

Answer 3

\({\bf{Calorimetry:}}\) used to measure the temperature change associated with a particular rxn
- calorimeter: thermally isolated device used to conduct a reaction and measure the temperature change, based on the principle that (Δq) of all the chemical processes are 0 for an isolated system
- specific heat: amount of heat required to raise the temperature of 1 gram by 1 degree Celcius
- heat change: q = mCΔT where C = specific heat, T is temp, m is mass, q is heat; be careful with units
[will add calorimetry problems upon request]

\({\bf{Entropy:}}\) measure of disorder in a system, -ΔH/T, integral of dq/T
- entropy equation: ΔS[products] - ΔS[reactants]
- max entropy occurs at equilibrium
- explains why heat flows spontaneously from high T to low T

- Gibb's free energy: state function that allows us to predict the maimum amount of work a system that can do, or whether reaction is spontaneous or not
- note: spontaneity does not mean fast, it refers to the ability of a reaction to occur w/o energy input, ex. a ball will roll down a hill w/o energy input

ΔG = ΔH - TΔS
ΔG < 0 exergonic, spontaneous
ΔG = equilibrium
ΔG > 0 endergonic, non-spontaneous

therefore low values of ΔH and high values of ΔS are considered favorable

Answer 4

\({\bf{Laws~of~Thermodynamics:}}\)
- zeroth law: two objects in equilibrium have same temperature
- first law: energy is conserved, ΔE = q + w
sign convention for q and w: heat absorbed = positive q, heat released = negative q
work done by system: negative w, work done on system: positive w
- second law: entropy of universe is always increasing [for a system, work can be expended to decrease the entropy but the overall entropy of the universe still increases]
- third law: perfect crystal at 0K has 0 entropy (so everything else has positive entropy)

\({\bf{Heat~Transfer:}}\)
- conduction: contact between molecules
- convection: motion of fluids, ex. ocean and winds
- radiation: electromagnetic waves

\({\bf{Heat~of~Fusion~and~Vaporization:}}\)
- ΔHfus and ΔHvap, respectively, are the enthalpy change associated with the melting and vaporization of a substance
- deltaT during a phase change since all the heat involved is used to change the state not the temperature

Answer 5

\({\bf{PV~Diagrams:}}\) used to plot the change of a substance's pressure and volume over time

straight lines represent changes in one variable while the other stays constant

Answer 6

\({\bf{Carnot~Cycle:}}\) cyclical system used to do work via expansion/compression of gas and a hot/cold reservoir

Answer 7

efficiency: amount of work done compared to the potential to do work [some potential lost due to heat/entropy]

efficiency = 1 - Tc/Th
where Tc is the temp. of the cold reservoir and Th is temp. of hot reservoir
can never be 100% since Tc never reaches 0K

Answer 8

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT