Enthalpy Calculations

Well, in order to do enthalpy calculations, you should know what enthalpy is. Enthalpy is the measure of total energy in a system. This symbol (ΔH), indicates the change in the reaction in KJ/mole. That indicates the change in potential energy during the reaction. To calculate that you use this formula: (energy of products - energy of reactants).

So... this lesson is kind of hard to explain unless with an example. So.. let's start with an exampleee =)

*When the KJ is on the right side of an equation, it becomes a negative number as it is an exothermic reaction. Vice versa, when the KJ is on the left side, it is positive as the reaction is endothermic.

Example 1 /

2 NO2(g) + O2(g) --> 2 NO2 + 131 KJ

well... we can see that in this reaction, the KJ/moles can be expressed in these ways :

- 131 KJ       or  -131KJ___     which can reduce to  -65.5 KJ__     
  1 mol O2         2 mol NO2                                      1 mol NO2                                                                         
    and again... -65.5 KJ__
                       1 mol NO2             

Now that you know these, it will be much easier to calculate the energy release, and how many moles needed to produce a certain amount of energy.

1. Ex/ Calculate the energy released when 0.55 moles of NO2 is produced.

So... back to mole conversions...again =.=" We set up the equation...

0.55 moles NO2   x   -65.5 KJ__    =   -36KJ  
                               1 mol NO2   


So.. remember to do your sig figs, and because it is a negative number, it is releasing energy, thus it is a exothermic reaction.

2. Ex/ How many moles of O2 are needed to produce 1523KJ of energy?

1523 KJ   x  1 mol O2   =  - 11.63 moles = 11.63 moles O2
                    -131 KJ

* Moles cannot be a negative number, so when this case repeats itself, drop the negative sign.

SEE! isn't it easy? Yay .... well ya think u so smart? Why don'tcha try doing these practice problems first:

http://www.docstoc.com/docs/19490471/Enthalpy-Worksheet
http://spiepho.sbc.edu/worksheets/ ( click on the 2nd Chapter 6)

Well...If you still need help, i guess you can watch these videos:









YAY. all is well? :) ok good good...good stuff x)

Endothermic and Exothermic Reactions

What are endothermic and exothermic reactions?

All chemical reactions have something to do with energy. It just depends on whether energy is released or absorbed. If energy is released, it is exothermic. If energy is absorbed, it is endothermic.

For example, if you hold on to an ice pack, after a few minutes, the ice will start to melt. You can realize that this ice has absorbed energy (heat) and is an endothermic reaction.



In bombs and explosions, heat is released as things explode into pieces, making it an exothermic reaction.



In any kind of reaction, molecules stay together by chemical bonds, and we can add energy to break the bonds or release energy to make them join together.

Now, we can get a more detailed definition of endo and exothermic reactions.

If a reaction needs more energy to break bonds than it does to give off energy in order to form bonds, it is endothermic.

In contrary, if a reaction takes less energy to break bonds than it does to give off energy to from bonds, it is exothermic.

We can express this energy as enthalpy (H), which is the heat contained in the system.

Now, for us to tell whether a reaction is exothermic or endothermic, we can use energy diagrams.
The purpose of the energy diagram is to show potential energy of chemicals as they change from reactants to products vs. time.

Reactants start with a certain amount of energy, and as energy is added to start the reaction, the amount of energy at the end can be lesser or greater, depending on the type of reaction.



In the diagram above, the substrate is the reactant. We can call its starting point the energy of reactants. We can also call the end point of this graph the energy of products.

The energy of activated complex is the highest point of the entire graph, and this is usually located in the middle of the graph.

The activation energy is the amount of energy that must be added to start up the reaction. This can be calculated by Energy of Activated Complex - Energy of Reactants.

Finally, the change in energy between the substrate and the product is ΔH or change in enthalpy. This can be calculated by Energy of Products - Energy of Reactants.

Using the information, we can tell if a reaction is exothermic or endothermic. How?
Well, all we have to do is check if the change in enthalpy is positive or negative.

If it is negative, it means that the Energy of Products is lesser than the Energy of Reactants, or in other words, energy was released, so it is exothermic.



If it is positive, it means that the Energy of Products is greater than the Energy of Reactants, so energy was added or absorbed, and it is endothermic.



Now, after determining whether it was exothermic or endothermic, we can incorporate the amount of energy gained or lost into the chemical equation. If it is exothermic, you would add the energy onto the right hand side of the equation.

For example, 2 C₂H₆ + 7 O₂ --> 4 CO₂ + 6 H₂O + 800 kJ (this 800 is a made-up number, just to give an example. It can be any number, depending on the amount of energy.)

If you see the energy on the right hand side, it is exothermic. Contrarily, if the number in kJ is on the left hand side, it is endothermic.

Here is a video explaining energy diagrams in more detail:




Here is a worksheet for practice:
http://colgurchemistry.com/Chem12/pdfs/Microsoft%20Word%20-%20Ch%2012%20Worksheet%201-2_doc.pdf
http://gohs1.tvusd.k12.ca.us/TeacherWebs/science/JMacLean/IB-AP%20Chemistry/Chapter%2012/Potential%20Energy%20Diagram%20ws.pdf

Types of Reactions II: Double Replacement, Neutralization, Combustion

Last time, we looked at the three simple types of reactions: synthesis, decomposition, and single replacement. Now, we will talk about double replacement, neutralization (a special type of double replacement) and combustion.

In double replacement, the ionic compounds in the reaction switch partners like a dance.
In general: AB + CD --> CB + AD



For example, if we have Na₂SO4 + CaCl₂ as reactants, the products will be NaCl + CaSO₄. Now how can we predict if a double replacement reaction occurs? We know that for single replacements, we can use the activity series to predict if the reaction will happen.

In double replacements, we use the Solubility Table. First, find the negative charged ion in the very left hand column. Then, look for the positive charged ion in the middle column. Finally, the last column will tell you if it is soluble or not soluble.

Here is something very important: if it is soluble, the state of the compound will stay the same. In many cases, this is aqueous (aq), meaning dissolved in water. However, if it says not soluble, this means that a precipitate (a solid) was formed, meaning there WAS a reaction.

So in summary: if both products are soluble and did not change state in the reaction, there is no reaction. If at least one of the products has changed states, then it works!

Finally, if a reaction DOES happen, we will express the solid formed in a Net Ionic Equation.
Basically, all the aqueous ions that are equal on both sides will get cancelled (like algebra in math), and the last ones remaining, along with the solid compound will be your equation.

So, for example, if we use the  Na₂SO4 + CaCl_{2 --------->} NaCl + CaSO₄, and then we balance it first...we'd get


_{1 Na2SO4(aq) + 1 CaCl2(aq)--------->} 2 NaCl_(aq) + 1 CaSO_4(s) 
We see that CaSO₄ produces becomes a solid state, so we would have to write a net ionic equation.

This equation is basically only going to involve the ions that form the solid compound.

So we would get: 

Ca²⁺_(aq) + 1 SO₄^2-_(aq) -----> 1 CaSO_4(s)


And that is your net ionic equation.

In neutralization, the reactants are an acid + a base, producing water and a salt. 
Acids have a positive H cation, and bases have a negative OH anion. Both are aqueous solutions, and the water produced is usually liquid. 
+ = + salt


In general: HA + BOH -----> H₂O + BA 
For example, HCl (aq) + NaOH (aq) will produce H₂O (l) and NaCl (aq). 


Finally, the last type of reaction is combustion. This is the burning of a gas, usually in the form of organic compounds or hydrocarbons. 



The general formula is: AB + O2 ----> AO + BO
While it is believed that the products of combustion are always CO₂ and H₂O, this is false, as there may not be H's or C's in the reactants, so C's and H's should not appear in the products.


An example of combustion would be: 
CH₄ + O₂ -------> CO₂ + H₂O

Here is a video explaining double replacements more in-depth:

LAB 5B

Today, we did Lab 5B. And let me begin by saying that i really enjoyed this Lab. There were a lot of chemical reactions/experiments that we had to do, which was fun. Anyways our objective was to observe a variety of chemical reactions. In these reactions we saw synthesis, decomposition, single replacement, and double replacement reactions. We did 7 reactions in this Lab. Here is a summary of each reaction.

1. In this reaction we burned a piece of copper. The copper turned into a shiny silverish color.

2. Here we put a nail inside a tube and filled it with a chemical. The nail after 20 minutes turned pinkish brown.

3. This reaction was one of my favourites. We took the shiny blue powder and heated on a bunsen burner. After time the subtance solidified and turned into a cement like powder.

4. After reaction 3. We added drops of water to the cement like substance. This caused the substance to turn back to blue(it's original form).

5. In this reaction my partner and I added two clear liquids together. The liquids became milky white and began to precipitate.

6. Here we added a silver substance with a clear solution. after a few minutes the substance began to bubble and take on a more metallic color.

7. Similar to reaction 6 we added a colorless substance with a grey powder. The top of the solution began to bubble and the frey substance turned black. Also the gas evaporated out of the test tube.

Here is a video that shows reaction number 1 and reaction number 3