Thermochemistry, Thermodynamics, Participation Reactions

Since the start of second trimester we have spent the last few weeks talking about thermochemistry and thermodynamics. Thermochemistry is the study of chemical reactions, and the energy and heat that they produce. Thermodynamics is how heat, energy, and work are related.

Basic Vocabulary for Thermochemistry:
-Energy= the ability to do work or transfer heat
-Work=energy that is used to move an object that has mass over some distance
-Potential energy= energy of position or chemical composition
-Kinetic energy= energy of motion
-Heat=one mode of energy, measured in joules
-Temperature= the measure of the kinetic energy of atoms and molecules in a system
-System= the molecules that are undergoing the specified change
-Surroundings= everything that is not undergoing change
-Enthalpy= total energy of a system, the change in potential energy and kinetic energy of the particles in a system
-Solvent= the liquid that is dissolving the substances
-Solute= the substance that is being dissolved
-Calorimetry=the measurement of heat transfer
-Specific heat= the amount of energy required to raise the temperature of a substance by 1 K or 1°C 
-Heat capacity= the energy needed to raise 1g of a substance by 1K
-State function= a property that depends on the present state of a system, not the path taken to arrive at that state
-Entropy= a statistical measure of a the number of distinguishable microstates (degrees of freedom) available to a system, the number of possible arrangements
-Exothermic reactions= heat flows out of system into surroundings
-Endothermic reactions= heat flows into system from surroundings
-Thermodynamically favorable= greater number of distinguishable microstates
-High heat capacity= maintains temperature with small condition changes, heats up slowly and cools down slowly, takes a larger amount of energy to notice a change in temperature
-Low heat capacity= small amount of energy can noticeably change the temperature, heats and cools quickly
 


We learned the Laws of Thermodynamics so we could understand the :

1st Law of Thermodynamics= no matter what mode energy takes or how it is transferred, the total energy before the reaction is equal to the total energy after the reaction. Energy is conserved.

2nd Law of Thermodynamics= the entropy in the universe is constantly increasing.

Our first major topic was Enthalpy.
Enthalpy

Enthalpy is heat at constant pressure. The equation for enthalpy is =m Cp T. Where m is the mass, Cp is the heat capacity heat transferred/ (mass)(temperature change), and T is (final temperature)- (initial temperature). 

When and T are both positive, this means that the system absorbed energy and the process is endothermic. When and T are negative, this means that the system released energy and the process is exothermic. We also know that - surroundings = system because of the 1st Law of Thermodynamics, energy can not be created or destroyed. 

Enthalpy of Fusion (H fusion) is the amount of energy needed to melt 1g of a substance. The equation is = (m) (H fusion) where m is the mass and H fusion is the heat of fusion. 

Enthalpy of Vaporization (H vap) is the amount of energy needed to boil 1g of a substance. The equation is =m(H vap), where m is the mass of the substance and H vap is the enthalpy of vaporization. 

Heat of reaction is the heat produced during a reaction. It can be found by subtracting H reactants from H products. =H products - H reactants

More about Enthalpy:

• Enthalpy is an extensive property meaning that it depends on the amount of a subtance
•   for a reaction is equal in size but opposite in sign to for the reverse reaction
• depends on the state (solid, liquid, gas) of the reactants and products

Bond Enthalpies:

Forming bonds causes the release of energy because the potential energy of the electrons decreases as the atoms move closer together.

Breaking bonds requires adding the same amount of energy that is released when bonds are formed. This is because as the atoms move away from each other the potential energy of their electrons increases.


Equation for Bond Enthapies
=∑BE(bonds broken)+∑BE (bonds formed)  For the bond energies of the bonds formed, make sure to input negative numbers.



Calorimetrey:
A process that uses the the energy change of the surroundings to find of the reaction. To do this the temperature is measured before and after the reaction takes place and this allows us to measure the heat transfer from the system to the surroundings and therefore figure out the heat of the reaction.
 This process is detailed in this equation: H rxn = -surroundings



We learned that another way to figure out is to use Hess' Law:

This is where you use chemical equations that have a given or known and you adjust the coefficients to create the reaction given as the problem. For example in the picture above, the coefficient 1/2 has been placed in front of the Cl2 molecule. This gives 2Cl2 molecules from the first equation and 1Cl2 molecules from the second equation which added together gives 3/2 Cl2 molecules. The Fe and the FeCl3 stay the same because they are on the right side of the equation and are needed for the equation we are trying to solve. The FeCl2 can be crossed out because it is present on both the left and the right of the equations and is not directly involved in the reaction. Now that the reaction is balanced, you can add the given for each part to get the  of the reaction.


Standard Enthalpies of Formation:
A hypothetical value that indicates how much heat would be lost or gained during the formation of on mole of the compound form from the elements in their standard states. 

The Hf value for any element in its standard state is zero. So O2(g), Ca(s), Na (s) all have heat of formations of 0. 

 
 The heat of formations can be used to calculate the enthalpy of a reaction .
rxn = ∑n f(products)-∑n f(reactants) where n is a stoichiometric coefficient.



 Next we learned about entropy.
 Entropy
We learned that entropy (S) is a statistical measure of the number of most probable distinguishable microstates or degrees of freedom available to the system. The greater the number of distinguishable microstates the more favorable the reaction will be, also called spontaneous. Entropy is a state function which means that the route that the system took to reach its present state is unimportant.
The equation for entropy is S=∑nS(products)-∑nS(reactants) where n is a stoichiometric coeffieient.

We found out that:

S is positive:
-Mealting
-vaporization
-reactions where thereactants are in the same phase as the products but contain more particles
-reactions that produce less ordered phases (solid -> liquid -> gas)
-making most soluitions
-adding heat

S is negative:
-making solutions of gas into liquids


We also talked about thermodynamically favored processes:
If a reaction is spontaneous that means that it will happen without assistance from outside the system. For example Fe will rust when exposed to diatomic oxygen and water.
If a reaction is un-spontaneous that means that in order to change, the system requires outside help. An example of this would be that water doesn't freeze at 45 degrees F.

The equation for entropy that is used to determine if a reaction is favorable or not is :
S universe= S system + S surroundings
This is because entropy is always increasing which is the 2nd Law of Thermodynamics. So when S universe is positive the reaction is thermodynamically favorable.

For Exothermic reactions S surroundings will be positive (increase) because heat is flowing out of the system
For Endothermic reactions S surroundings will be negative (decrease) because heat is flowing into the system

Gibbs Free Energy equation is G=H - TS where H is enthalpy in kJ, T is  temperature of system in K and S is entropy change in kJ/K.

If G is negative, the reaction is thermodynamically favored.

The last thing that we learned about was precipitation reactions. These are double replacement reactions that happen between aqueous ionic compounds. The reaction forms a solid from one of the pairs of ions and the other two ions remain aqueous. These solids form when the attractive forces between the cations and anions is stronger than the force of attraction between the water molecules and the ions.

Rules for determining the solubility of a compound:
-All salts containing Na+, K+, NH4+ or NO3- are soluble in water
-Nitrates, Hydrogen carbonates, and Chlorates are soluble in water
-Chlorides, Hydrogen, and Iodides (except with Pb, Ag, or Hg) are soluble in water
-Sulfates (except Ag, Sr, Ba, Pb, or Ca) are soluble in water
-Hydroxides (except group 1 and Ammonium) are not soluble in water.
-Carbonates, Phosphates, Chromates, and Sulfides (except group 1 and Ammonium) are not soluble in water

We also learned how to write a balanced net ionic equation.
1. Write out the reaction as stated, do the double displacement.
2. Look at the products and the reactants and make sure that the reaction is balanced. If not, add stoich coefficients.
3. Write the reaction as the separated cations and anions with charges.
4. Look at the solubility rules and figure out which compound is the precipitate. Write as a solid ionic compound in the products.
5. Cross of the ions that did not form the precipitate on both sides of the reaction. These are the spectator ions and are not what we are looking at in the net ionic equation.
6. Rewrite the equation using only the ions that are directly involved in creating the precipitate.

Reflection:
Over all I think that making this blog helped me to put some of the thermodynamic concepts together and figure out how they were all related. I think that I have a pretty good understanding of this material. I would give my understanding a 7/10. This is because only recently did I begin to understand the relationships and differences between enthalpy and entropy and the other thermodynamic values. I think that if I study a little bit more and clarify my questions I will be ready for the test at the end of the week. I also really like white boarding because it helps me work through the problems easier and helps me to see how others are thinking the problems through.