Big Idea 5 : Thermochemistry

Thermochemisty is the thermodynamics of chemistry , and thermodynamics is a branch of physics that studies temperature ,heat, work and energy. It also studies how heat and energy transfer between objects (and in thermochemistry ,between molecules in reactions). First of all , lets explain KE=1/2mv^2 ,which is the formula for kinetic energy or the energy of movement. KE is Kinetic energy, m is mass and v is velocity. You don't use this formula that much in AP Chemistry, so lets move on. The universe can be divided into the system and the surroundings , where the system is what you are studying and the surroundings is everything else. The first law of Thermodynamics states that energy is conserved. The temperature of a substance is ( the measure of ) the average kinetic energy of the molecules in that substance, So at higher temperatures the molecules are moving faster. There are 3 temperature scales : Kelvin , Celsius ( centigrade ) and Fahrenheit. The most commonly used in AP Chemistry is K or Kelvin ( Note that Kelvin's symbol does not have a ° symbol , so you do not say degrees Kelvin , but just Kelvin ) , but °C ( Celsius ) is used sometimes too. The conversion between K and °C is: K=273.15+°C and °C=K-273.15 . Heat is the  kinetic energy transferred between objects, when they have different temperatures and collide. In this collision heat goes from the one with a higher average kinetic energy to the one with a lower average kinetic energy until they are the same temperature.  Heat Is the total kinetic energy in an object while temperature is the average kinetic energy. The specific heat capacity of a substance is the energy required to raise one gram of that substance by one degree Celsius. The Specific heat capacity of a substance = amount of heat added / (the mass of the substance )( the change in temperature ) , in units of J/g*K . Metals have low specific heat capacities and water has a high one. So water changes temperature slower ( or with more energy ) than metal. You can measure the heat transferred using q=mcΔT , where q= heat transferred , m = mass, c= specific heat capacity and ΔT = change in temperature. 

heating curve

Heating curves are a graph with temperature on the y-axis and Energy added on the x-axis, that starts with a solid and ends in a gas. During the phase changes ( Melting and boiling ) the temperature stays the same because all the energy added is used to change phase. During the time the temperature increases and there is no phase change you can use q=mcΔT, but during the phase changes you have to use another equation. This is q=(ΔHfusion/vaparazation )(moles), where ΔHfusion/vaparazation Is either the standard enthalpy of fusion ( which is the energy ( heat ) change when 1 mole of a substance is converted from solid to liquid )  , or vaporization ( which is the energy ( heat ) change when 1 mole of a substance is converted from liquid to gas ). An Endothermic change for the system is when energy is added to the system , and an Exothermic change for the system is when energy leaves the system. Cooling curves are like heating curves except they start with a gas and take away energy until you get a solid. ΔH° = Change in standard enthalpy , or change in heat (in regular conditions ) ( the standard conditions are shown by ° and are 1 bar and 1 mole and so on ) . The way to find ΔH° of a reaction is to use the equation 
ΔH° rxn = the sum of ( ΔH°f products ) - the sum of ( ΔH°f reactants ) , you can look up the ΔH°f ( the change in enthalpy of formation ) of compounds, then multiply them by the number of moles there is in the reaction ( the coefficient ) and add them together to get the ΔH°f reactants or ΔH°f products. Also if you want a ΔH rxn of a reaction but only have ΔH rxns  for other reactions that are similar to yours , you can us Hess's Law to find the ΔH rxn of your wanted reaction. The rules are this: 1 If you multiply a reaction by a number you must also multiply the ΔH rxn by that same number. 2 If you flip a reaction equation around then you change the sign of it's ΔH rxn . And 3 when everything cancels except your desired reactants and products , you can add up the ΔH rxns of all the similar reactions you were given to get your desired ΔH rxn. The second law of thermodynamics states that entropy ( disorder ) increases as time goes on. Entropy's symbol is S and is measured in J/mol*K. Since entropy is disorder it increases with changing states from solid to liquid to gas , and thus with temperature ( Also larger molecules have more entropy , than smaller ones ). ΔS is change in entropy , and can be calculated the same way as      ΔH°rx can with products - reactants:
ΔS° rxn = the sum of ( S°products ) - the sum of ( S° reactants ) where S° is the absolute entropy of something ( you can look these up too ). Gibbs Free energy is the free energy available to systems to do work and has the symbol ΔG. It can be calculated like ΔH and ΔS:
ΔG° rxn = the sum of ( G°f products ) - the sum of ( G°f reactants ), But it can also be calculated with            ΔG°rxn= ΔH°-TΔS°*  , Determining the sign of the change in Gibbs free energy for a reaction lets you determine the thermodynamic favorability of the reaction or if the reaction is thermodynamically favorable. If ΔG is negative the reaction is thermodynamically favorable, while if it is positive it is not thermodynamically favorable, or it is thermodynamically unfavorable.
If something is thermodynamically favorable it will happen without outside influence. But somethings that are thermodynamically favorable just happen to slow for us , like graphite turning into diamond, this means they are under kinetic control. Kinetics and Thermodyanmics are separate in AP Chemistry, Kinetics focusing on the rate or speed and Thermodyanmics on favorability. 
When something is thermodynamically unfavorable it needs outside energy to make it happen and the reactant formation is favored ( unless you add energy ) , while in thermodynamically favorable reactions the product is favored. The favorable signs for ΔH and ΔS are - and + respectively. So for a Reaction to be thermodynamically favorable, at least one of its  ΔH and ΔS has to be favorable, and if only one is then the favorability depends on temperature. for example if both are positive ( ΔH° is unfavorable and ΔS° is favorable ) then to have a negative ΔG° you would need a high temperature ( and the other way around if both are negative ). If both are favorable ( ΔH° negative and ΔS° positive ) then the favorability of the reaction does not depend on the temperature and ΔG° is always negative ( it is the other way around if both are unfavorable , then ΔG° is always positive , and you have to add energy to make the reaction go ). You can couple unfavorable reactions with favorable ones to make an overall favorable reaction, this is called coupling. 

FYI
its been some time since I studied for the test so my information may not be completely accurate and there may been some things not covered, my apologies. 

  


* when you do the math with this equation you have to make ΔH° and ΔS° have the same energy unit ( either kJ or J ) since ΔH° is usually in kJ/mol and ΔS° is usually in J/K*mol ( the Kelvin cancels out with the Kelvin in T ) 


Note: The sources for these blogs ( this , big idea 2, bonding , IMFs and molecular shapes, and AP Chemistry : Kinetics and Rate ) : Wikipedia, my AP class, and my review books ( AP Chemistry crash course, REA and Princeton review 2015 )