Excess and Limiting Reagents WOO WOO EXCITMENT

What is UP my friends? Today we are going to learn something reallyyyy special. It's called STOICHIOMETRY OF EXCESS QUANTITIES.
So whats going to happen is I want you to clear your mind of eeeeverything you've learned so far about stoichiometry. (okay well not really....you actually still have to use 99% of the information but clear out that 1%)


So far, the stoichiometry calculations in the previous section just ASSUME that a given reactant is completely used up during a reaction. PSHT that is soooo lame. It's like when you see a fat woman and ASSUME she is pregnant. If you're wrong, run.....just run

In reality, most reactions are carried out in such a way that one or more of the reactants are present in EXCESS amounts. Some reasons this happens is:
1. When you add and excess of a reactant to make sure the second reactant is completely used up, because the second reactant may be too expensive to waste or too harmful to the environment if left unreacted
2. When there is a limiting amount of one reactant so having an excess of another reactant is unavoidable

One reactant is the EXCESS QUANTITY and some of it will be left over. The second reactant is used up completely, and is the LIMITING REACTANT

Now let's have an example equation shall we? : )

If 234 grams of CS2 is reacted with 619 grams O2 what is the limiting reactant? How many grams of the excess reactant remain after the reaction is complete?
                                        CS2 (l) + 3 O2 (g) ---> CO2 (g) + 2 SO2 (g)

Step 1: Randomly select one product to use. In this case, I'll randomly select CO2

Step 2: Convert both CS2 and O2 into grams of the same product, In this case, grams of CO2
234g CS2 x 1 mol CS2  x 1 mol CO2  x 44.0g CO2 = 135.12g CO2
                     76.2g CS2      1 mol CS2    1 mol CO2
619g O2 x 1 mol O2   x  1 mol CO2   x 44.0g CO2   = 283.71g CO2
                  32.0g O2      3 mol O2           1 mol CO2

Step 3: Now you can see that CS2 is the limiting reactant since it is the smaller amount. Now you must find out the grams of the excess reactant remaining. You do this by taking the ORIGINAL grams of the limiting reactant (CS2) from the question, and change that number into grams of the excess reactant (O2)
234g CS2 x 1 mol CS2  x 3 mol O2   x 32.0g O2 = 294.80g O2
                     76.2g CS2     1 mol CS2     1 mol O2

Step 4: After, you take the grams of the limiting reactant from the ORIGINAL equation, and subtract the grams you found in the equation above
619g O2 - 294.80g O2 = 324.2g O2 excess

Now for a joke because all the seriousness is starting to make me nauseous


If you need any further help, you can watch this video

http://www.youtube.com/watch?v=Vaiz0zLesHk&feature=related

Stoichiometry with Molarity + Gases (STP)

Stoichiometry's basic concept is simple. The thing that makes it challenging sometimes is organizing your information. Sometimes there are calculations that need to be done to find the information you need to actually do the stoichiometry part. For example, there are questions that won't tell you how many moles or grams of a reactant you have. Instead, it may only tell you the volume and molarity of it. In which case, you would have to use the molarity formula to determine the moles of the substance in question. 

Molarity = mol

                ------

                   L

Remember that this formula can be rearranged to solve for any of the 3 pieces of information-- as long as you have the other two pieces. 

There are also gas stoichiometry questions that will require knowledge of STP as well. Recall that...

                        One mole of any gas at STP is 22.4L gas            1 mole

                                                                     ---------------   OR ------------

                                                                      1 mole                 22.4 L gas

Use that STP rule to convert from moles to volume or volume to moles, depending on what the question requires you to do.

Molarity and STP are both concepts that you have learned earlier this year. Now it is time to apply these concepts to slightly more complicated situations... stoichiometry! 

We trust that you should be able to handle questions like these with no problem,  but here is an example anyways:

Given:

CaCO3 -> CO2 + CaO

How many grams of Calcium Carbonate are needed to produce 9.0L of Carbon Dioxide measured at STP?

9.0L CO2    1mole CO2       1mole CaCO3       100.1 g CaCO3

               x -----------------  x   ---------------------- x  ------------------------ = 4.0 x 10^1 g CaCO3 required

                    22.4 L CO2      1 mole CO2          1 mole CaCO3

(The bolded part of the work shown above is where STP comes into play.)

Stoichiometry (Pronounced: stoi'kē-ŏm'ĭ-trē)

The main idea of stoichiometry is to measure the amounts of elements and compounds that are involved in a reaction. (Ex/ the relationship between the amounts of reactants and products in a reaction)

Mole Ratio is something that will be used in pretty much ever stoichiometry related question that you will need to solve this year. It can be found by looking at the coefficients of a balanced equation.

Consider:

4NH3 + 5O2 -> 6H2O + 4NO

Mole ration of this equation:

4 : 5 : 6 : 4

If a question asks you to use the information provided in the equation to find the grams of NO produced if 3.0 moles of O2 were combined with excess 4NH3...

3.0 mol O2          4 mol NO       30.0 g NO

                       X -------------- X -------------- = 72 g NO should be produced

                            5 mol O2       1 mol NO

**The bolded part of the work shown above is where mole ratios come in.**

Breakdown:

• We start with 3.0 mol O2 because that's only solid piece of information we are given. 
• We use mole ratio to determine the amount of O2 and NO needed for a reaction to occur successfully. We do this by looking at the balanced equation. We see that the coefficient of O2 is 5 and the coefficient of NO is 4. That is the ratio of O2 : NO. 
• Add that to your work as the next step. Doing this has allowed us to figure out the moles for NO required for 3.0 moles of O2 to react successfully in this particular situation. We now need to convert moles of NO into grams, because that what the question asked us for. 
• Using the periodic table, we see that one mole of NO is 30.0g. Add this to the work, and calculate. 
• We have determined the grams of NO that should be produced if 3.0 moles of O2 were to be reacted. 

If you still don't understand how to do stoichiometry, have a look at these images below which outline the key points.

Let it be, and learn enthalpy!!!!!!

Oh hello there, it is so nice to see you : )

Guess what we're learning about today? Starts with enthalpy...YESS ENTHALPY (Mr. Hogaboam joke)

Now, in order to actually succeed in learning this foreign topic, I must tell you what enthalpy actually MEANS. Enthalpy, H, is the HEAT contained in a system.

ΔH = the change in enthalpy during a reaction a.k.a kJ/mol

The equation for the total change in enthalpy is:
ΔH = HProduct - HReactant

             HReactant = the heat energy contained in the reactant

          HProduct = the heat energy contained in the product

Now I bet you guys are just dying and so gosh darn excited to move on that you'd fly off a bridge. Well this is your lucky day!

Let us move on to exothermic and endothermic reactions shall we?

An EXOTHERMIC reaction GIVES OFF heat to its surroundings

A trick to remember this is: Heat EXits from the reactants

An ENDOTHERMICreaction ABSORBS heat from its surroundings
A trick to remember this is: Heat ENters the reaction
                           ENDOTHERMIC

                                                       




                        EXOTHERMIC

Energy of the activated complex = potential energy of the "transition state" between reactants and products

Activation Energy (Ea) = the energy that must be added to get the reaction to rpogress (reactants to activated complex)

Lets start with some example equations!!! : D

******Remember, when Kj is on the left side of the equation, it is endothermic and positive. When kJ is on the right side of the equation, it is exothermic and negative*****

Ex/ For the following reaction, calculate the energy released when 1.56 moles of H2O is produced?

CH4 + 2 O2 ----> CO2 + 2 H2O + 918kJ

1.56 mol H2O x   -918 kJ    = 716.04 kJ energy released = 716 kJ or -716kJ

                           2 mol  H2O

Ex/ Using CH4 + 2 O2 ----> CO2 + 2 H2O + 918kJ again, how many grams of CO2 would be needed to produce 2300 kJ of energy?

-2300 kJ x 1 mol CO2  x 44.0g CO2  = 110.2396514g CO2 = 110g CO2

                          - 918 kJ        1 mol CO2

If you still don't understand after my BEAUTIFUL explanatin, you can go to this video to learn what I just said again : )

http://www.youtube.com/watch?v=qtNDFu0EDCg

Lastly, I'll just put in a not so funny joke and hopefully you'll get it : )

This was sent in to us by Robert L. LaDuca (well not really I just found it)

"Yesterday when talking about energy units/enthalpy I drew a man, woman, and child on the board. I put a "J" on each of them. Then I kept on going,without paying it any attention. I let them figure it out, a minute or so later got groans and laughter. Yup, the "Family Joule". ;)"

haw haw haw haw haw haw. Yea this is only funny if you've watched the show "Family Jewels" starring Gene Simmons and his family (wife Shannon Tweed and their kids Nick and Sophie). OHH NOW YOU GET IT? ;)

Energy. Bow chicka wow wow

Despite the fact that our title here today lacks flamboyancy, this topic definitely does not. While the concept is generally simple, occasionally you may get thrown away by your stupidity. Jokes.

In general, there are two types of reactions: An exothermic and an endothermic reaction. I understand you are now facing the conundrum, but do not fear, MJH is here!

The best and easiest interpretation of energy is thinking of it as heat. Heat is a form of energy that is most commonly referred to in reactions.

An exothermic reaction is a reaction that releases energy. Therefore, it breaks bonds. The burning of firewood is an example, because it gives off heat into the atmosphere.

An endothermic reaction is a reaction that absorbs energy. Therefore, it combines bonds. The melting of ice is an example of an endothermic reaction, because you are adding heat to the ice cube, making it absorb the heat and melt.

Comrende? I hope y'all said si. Now here is where we get funky. Here is an example of an energy diagram.

This here, my beautiful friends is an energy diagram for an exothermic reaction. This looks very complicated, but after we explain this to you, we guarantee you will be well on your path to enlightenment.

First off, your y-axis is always your potential energy (in kJ) and your x-axis is your time elapsed. In this diagram it says "reaction pathway" but in our cases, it would be the time in seconds. You first start off with your reactants. Your reactants will always start and the end of your graph will be your products. Okay, so review what we just learnt:
Step 1: Label reactants and products. Reactants first, then products. This should be pretty straightforward.

The top of the little gooseneck that we have is our activated complex. Labelled on the graph above, is Ea. That is the activation energy. The activation energy is basically how much energy you need in order to get the reaction up and going. Review time once again:
Step 2: Label your activated complex whatever it is.

The triangle with the H is "delta ayche". Basically the change in energy/heat. For those of you who take physics, please do not be confused with delta Q. The scientific name for it is enthalpy. Which is basically the change in energy. They just simply had to give it a fancy schmancy name for it. Typical chemists.
Delta H can be calculated using the following formula: Eproducts - Ereactants= Delta H.
This is awfully just logical, however please do not be confused with difference.

If delta H is positive then the reaction is endothermic, since there would be more energy, therefore, the reaction would be absorbing energy.
If delta H is negative, then the reaction is exothermic, since the reactant would be losing energy, therefore the reaction would be releasing energy in some form or another.

Ms Chen said apparently next class we're going to be writing equations using this... Oh joy. In the meantime, please just enjoy this video.

This is an example of an exothermic reaction since it is giving off heat.

And here is a rather cool, if I may say so myself, example of an endothermic reaction.

Ps; I know y'all miss our videos, but since life has been taking a toll on us lately--yes, we're making it personal--it has been hard to find the time to make WONDERFUL and INTERACTIVE chemistry videos. But please to stay tuned, as Savannah, Harriet, and Jia Liu will be back in no time.

Sorry for the inconvenience caused,
-MJH

Lab of Joy

Hello all. Today in class, we all performed a lab. If you are a confused soul who has no idea what I'm talking about- refer to lab 5B (page. 52 in your lab text book).

Anyways, this lab was all about seeing all the different types of chemical reactions. These exist in four main categories:

-Synthesis (A + B -> AB)
-Decomposition (AB -> A + B)
-Single Replacement (AB + X -> A + XB) *switch non-metals with non-metals, or metals with metals
-Double Replacement (AB + XY -> AY + XB)

There were seven unique reactions for you to test out in this lab, and hopefully you all recorded your results on the chart that was supposed to be copied and completed. This lab should have expanded your knowledge with reactions, because you were actually able to see what it looked like.

All 4 types of reactions listed above were present in the lab, and if you did not realize it, your head must've been in the clouds. Be sure to reread the lab and make sure you understand what you were doing step by step.

You should now be more comfortable working with lab equipment and various chemicals + solids. If you didn't dispose of your waste properly (down the sink with lots of water, or in the special waste containers) remember to keep that in mind for our next lab.

The lab write up is due next class, so be sure to have that completed and ready to go. A thorough understanding of chemical reactions in general is needed to answer the required lab questions.

Good luck!

If this happened during your lab, you did it wrong. :)

-MJH

Continuation of the AW YEAH CHEMICAL REACTIONS YEAH.

This class, we learned three new types of reactions: Double Replacement, Combustion, and Neutralization

Double Replacement

     Bee Butterfly + Sun Leaf -> Bee Leaf + Sun Butterfly

Yea I know that example made about...no sense at all but bear with me.

A double replacement reaction is a reaction between two ionic compounds usually in solution. The ions switch partners.
The general formula is:
AB + CD -> CB + AD

Ex / 2 K3PO3 +  3 Cu(NO3)2 ->  1 Cu3(PO3)2 +  6 KNO3

Now, like the single replacement, we actually have to figure out whether a reaction actually occurs. How do you do that? You use the "Table of Solubilities"
If the reactants change state during the reaction, there is a reaction occurring. If there is no change of state, then there is no reaction.

How to use the table:
Step 1: Find your negative ion on the left hand column
Step 2: Look for the positive ion in the list in the 2nd column
Step 3: Follow that to the next column and it may say either soluble or not soluble

If it is soluble, the compound is aqueous = (aq)
If it is not soluble, the compound is solid = (s)

Ex/  2 BeI2(aq) +  1 Sn(NO3)4(aq) ->  2 Be(NO3)2(aq) +  1 SnI4(s)
The reaction above has a reaction because a precipitation occurs.

Combustion

A combustion reaction is a reaction where the reactants are the chemical to be burned and the oxygen that it reacts with. The oxygen atoms usually end up combined with more than one type of atom as products.
The general formula is:
AB + O2 -> AO + BO

Ex/ C4H8 + 6 O2 -> 4CO2 + 4H2O

Neutralization

A neutralization reaction is a double replacement reaction where acids react with bases to produce water and ionic salts.

The acids have an H and the bases have OH. Both should be aqueous solutions.
The general formula is:
HA + BOH -> H2O -> H2O + BA

Ex/  1 C3H8(g) + O2(g) ->  3 CO2 +  4 H2O

http://www.youtube.com/watch?v=t4iJM84WHRQ&feature=relmfu
This is a tutorial video on how to predict double replacement reactions. The video only gets useful around 1:40