February 21st -- Ahmad Kilani

Enthalpy Calculations

Key Concepts

• The heat content of a chemical system is called the enthalpy (symbol: H)
• The enthalpy change (H) is the amount of heat released or absorbed when a chemical reaction occurs at constant pressure.
• H = H_(products) - H_(reactants)
• H is specified per mole of substance as in the balanced chemical equation for the reaction

The enthalpy change, or H,  for each reaction is unique. So what relationship exists between the heat produced or absorbed by a reaction and the amount of reacting substance?

The energy change in these reactions varies directly as the number of moles of substance reacted or formed. The factor that determines the amount of heat absorbed or released is the H of the reaction: 

Moles of compound reacted or formed	Varies directly with	Heat absorbed or released in the reaction
	<--------------------->
	H reaction (kJ)

The energy is a variable because it differs from one reaction to another. However, it is a given that it is going to be positive when it is absorbed (i.e LHS, or reactants side); and negative when it is produced (i.e RHS, or products side). 

Examine the following reaction:

2 C₄H₁₀ (l) + 13 O₂ (g) ---> 8 CO₂ (g) + 10 H₂O (g) + 5315 kJ

The equation tells us that 8 moles of CO2 (g) releases 5317 kJ , since the relationship is a direct one then 2 moles would release one quarter as much heat or 2 moles would release 1,329 kJ.

To calculate heat changes using equations, we will perform the following steps

1) calculate the number of moles of substance reacted or formed.
2) create a proportion using the balances and heat in the chemical equation.
3) solve for missing quantity.

Question:

Calculate the amount of heat released when 25.0 grams of C₄H₁₀ (l) is burned in oxygen using the equation above.

Solution:

.

Calculate the number of moles of C₄H₁₀ (l), and times it by the energy per one mole.

MM of  C₄ is  12x4= 48.0 g/mol

MM of   H₁₀  is  1x10=10.0 g/mol

Total MM is 58.0 g/mol

Therefore, 

the conversion is from grams to moles to energy (heat).

25.0 g  x  1 mole   x  2657.5  KJ     =  1145.47 KJ   ---> ( 3 sig figs) = 1150 KJ 

                 58.0 g           1 mole

So 1150 KJ  of heat would be released when 25 grams is burned.

February 16th - Ahmad Kilani

Endothermic and Exothermic

 So there are two types of chemical reactions; endothermic reactions and exothermic reactions:

- Endothermic reactions are reactions that require energy to take place, and it's usually two or more elements joining together, storing energy in the process, for example, the making of glucose in plants

6CO2 +  6H2O + light (energy) --> 1C6H12O6 + 6O2

Endothermic Graph

- Exothermic reactions are reactions that release energy when they take place, it's usually a big compound breaking down into smaller parts like in combustion; an example would be the burning of glucose;

1C6H12O6 + 6O2 --> 6CO2 + 6H2O

Exothermic Graph

Exothermic and Endothermic can also be processes, not just reactions, such as freezing, and melting.

Feb 4th - Nick Kim

Today we revised, how to do double replacement, Net ionic equations, and combustion
First, double replacement is a reaction between two ionic compounds. The positive ions switch their partners(negative ions) each others and form new ionic compounds. After balance the equations by writing numbers in front of forumla so that number of particles are same
Ex, 2Na3PO4(aq)+3Ca(NO3)2(aq)---6NaNO3(aq)+Ca3(PO4)2(s)
When double replacement is done, we use Net ionic equation if it is possible. We cancel out the same elements on each side, but they should have same number of particles and need to be (aq)

Ex,2Na3PO4(aq)+3Ca(NO3)2(aq)---6NaNO3(aq)+Ca3(PO4)2(s) example above
Na cancel each other since they have same number of particles and (aq)
Po4 won't cancel each other because right side is (s)
NO3 will work too because they both have same number of particles and (aq)
Therefore, new equation is 3Ca+2PO4---Ca3(PO4)2

Combustion is a reaction that involve burning in air.
General Formula:
AB+O2---AO+BO
Ex,
C4H8+6O2---4CO2+4H2O

Practice Questions(double replacement)
http://chemwiki.wikidot.com/double-replacement-reaction-problems

Practice Questions(Net ionic equation)
http://www.ccsf.edu/Departments/Chemistry/chem101a/resourc/net_ion2.htm

Videos that will help
http://www.youtube.com/watch?v=y-xLlY18_Mk
http://www.youtube.com/watch?v=I2liasSXCXM
http://www.youtube.com/watch?v=CqftRowoNKY

February 8th - Greg Sra

In thursday's class we did a lab learning about the reactions that occur when you react using decomposition double & single replacement, and synthesis.

First Reaction: We burned a piece of copper wire over a bunsen burner using crucible tongs to see it change to a molten state.

Second Reaction: We put a nail in copper(II)Sulfate, this reaction was Single Replacement. Later the nail wasn't as glossy and had been coated in a layer of copper.

Third Reaction: We heated copper(II)sulfate.

Fourth Reaction: We added water back into copper(II)sulfate.

Fifth Reaction: We poured Calcium Chloride into a Sodium Carbonate solution. We did this to show DOUBLE REPLACEMENT!

Sixth Reaction:  We put Zinc into Hydrochloric Acid.

Seventh Reaction: We got a glowing splint (fancy word for slightly burnt stick). We already had a Manganese Oxide solution that we added to Hydrogen Peroxide. Now with our "glowing splint" we put into the top of the test tube.

Below is what we did last class. 
In thursday's class we did a lab learning about the reactions that occur when you react using decomposition, double & single replacement, and synthesis...

Last class we reviewed double replacement and how to tell if the reaction actually occurred.

1) Use the table of solubilities, find out if it is aqueous.

2) Find an Anion (Negative) –––––––––––––-> Look at the next column see if it has a Cation.

3) Follow it's presence or absence to find if it is in fact aqueous or a solid.

4) Profit???!?!?!?!?!?

Net Equations

Well if you find that there are aqueous substances on both sides of the equation or on the same side, all you do it cross them out.

Chemical Reactions Part II- Ahmad Kilani

So we already discussed the first 3 types of reactions! Now we need to mention the remaining 3!


Double Replacement is one of them.

This is when the anions and cations of two different molecules switch places, forming two entirely different compounds. These reactions are in the general form:

 

AB + CD ---> AD + CB

One example of a double displacement reaction is the reaction of lead (II) nitrate with potassium iodide to form lead (II) iodide and potassium nitrate:

 

Pb(NO₃)₂ + 2 KI ---> PbI₂ + 2 KNO₃

 

The fifth reaction is Combustion. 

A combustion reaction is when oxygen combines with another compound to form water and carbon dioxide. These reactions are exothermic, meaning they produce heat.

An example of this kind of reaction is the burning of napthalene:

 

C₁₀H₈ + 12 O₂ ---> 10 CO₂ + 4 H₂O

 

And finally, the last reaction is Neutralization.

This is a special kind of double replacement reaction that takes place when an acid and base react with each other. The H⁺ ion in the acid reacts with the OH^- ion in the base, causing the formation of water. Generally, the product of this reaction is some ionic salt and water:

 

HA + BOH ---> H₂O + BA

One example of a neutralization reaction is the reaction of hydrobromic acid (HBr) with sodium hydroxide:

HBr + NaOH ---> NaBr + H₂O

Things to help you distinguish the difference between all of them:
Answer the following questions until you reach one with a "yes" answer:

1)  Does your reaction have oxygen as one of it's reactants and carbon dioxide and water as products? If yes, then it's a combustion reaction

 

2)  Does your reaction have two (or more) chemicals combining to form one chemical? If yes, then it's a synthesis reaction

3)  Does your reaction have one large molecule falling apart to make several small ones? If yes, then it's a decomposition reaction

4)  Does your reaction have any molecules that contain only one element? If yes, then it's a single displacement reaction

5)  Does your reaction have water as one of the products? If yes, then it's a neutralization reaction

6)  If you haven't answered "yes" to any of the questions above, then you've got a double displacement reaction

Sample Problems (the solutions are in the next section)

List what type the following reactions are:
 

1)  NaOH + KNO₃ --> NaNO₃ + KOH
2)  CH₄ + 2 O₂ --> CO₂ + 2 H₂O
3)  2 Fe + 6 NaBr --> 2 FeBr₃ + 6 Na
4)  CaSO₄ + Mg(OH)₂ --> Ca(OH)₂ + MgSO₄
5)  NH₄OH + HBr --> H₂O + NH₄Br
6)  Pb + O₂ --> PbO₂
7)  Na₂CO₃ --> Na₂O + CO₂

 

Solutions to the Sample Problems

1)  Double Resplacement
2)  Combustion
3)  Single Replacement
4)  Double Replacement
5)  Neutralization
6)  Synthesis
7)  Decomposition