Friday, October 20, 2017

Limiting Reagents

If you mix two chemicals together as reactants it is unlikely that both reactants will be used up completely when forming products. One will be used to completion, this one is the limiting reactant, and will limit how much product can be made. The other will be used up until the other reactant runs out and there will be some left over, so it is an excess reagent. 


If you have these supplies, which one is limiting? Which one will you run out of first?

In this example it is easy to "see" which is the limiting and which is excess. When looking at quantities of chemicals it is not as easy to "see."

To determine which chemical is limiting, convert from the given reactants to a product (it does not matter which). Whichever reactant produces the least amount of product is limiting. 

Wednesday, October 18, 2017

Theoretical and Percent Yield

Theoretical yield is how much product you can make with given reactants. To determine theoretical yield, do a normal stoichiometry calculation. 



Actual yield is how much is actually produced when the reaction is performed. 

Percent yield is how well you do. Percent yield calculates how close to the theoretical yield you are. A high percent yield means that your actual yield was close to the theoretical yield, the reaction worked the way it was supposed to it, and it was efficient and accurate. 

Why doesn't theoretical yield equal actual yield very often? ERROR!

Error can include impure substances, uncalibrated equipment, improper procedure... all kinds of things.

Percent error measures the amount of error. Small percent error means things went  well and the actual yield was close to the theoretical yield. 

Percent yield plus percent error should equal 100. 

Monday, October 16, 2017

Intro to Stoichiometry


Stoichiometry is the most important part of chemistry and why chemistry is so useful in real life. With a balanced equation, stoichiometric conversions can be used to calculate how much product will be made, or how much reactant is needed to produce a certain amount of product.

Stoichiometry uses the three mole conversions that students are familiar with from unit one, plus the mole/mole conversion. A mole/mole conversion uses the coefficients from a balanced equation to convert from one chemical to another. You can only compare elements or chemicals when they are both in mole form. 

Using this equation N2 + 3 H2 --> 2 NH3 the following calculations can be made using stoichiometry. 
 
Check out this video from CrashCourse if you need some help!

Wednesday, October 11, 2017

Physical VS Chemical


We talked about the Law of Conservation of mass and how matter cannot be created or destroyed. If you burn a log, the mass of all the ashes, smoke, gases, and everything that is burned off and left behind EQUALS the mass of the original log.

Today students discussed physical vs. chemical properties and changes. They've heard all of this before I am sure, but it doesn't hurt to go over it again. Then we did a challenge to see if they really knew their stuff.

Need to practice identifying chemical and physical properties? Check this out!

Need help identifying types of matter and whether they are heterogeneous or homogeneous? Check this out!

Here's a helpful video lecture --> HERE

Tuesday, October 10, 2017

Entropy - Disorder

Entropy is a chemistry word for disorder. An increase in entropy is spontaneous. By looking for four things in a reaction, students can determine whether a reaction is spontaneous or nonspontaneous by looking for an increase in entropy. 

Exothermic reactions are spontaneous and show an increase in entropy. 

Gases are messier than solids, so a reaction that forms a gas shows an increase in entropy. 

More molecules show an increase in entropy. Count the coefficients on either side of a balanced equation. If the products have more molecules then there is an increase in entropy and the reaction could be spontaneous. 

A decrease in the size of molecules (count atoms making up the molecule) is an increase in entropy. 

Students look for all four things and decide whether the overall reaction would lead to an increase in entropy and be spontaneous.

Monday, October 9, 2017

Le Chatlier - shifting equilbrium to reduce stress

Students learned about reaction rates and how to increase them. They also learned about reversible reactions and how Le Chatlier's principle influences shifts of equilibrium in reversible reactions.

Basically as you apply a stress to a system, the system will shift in response to the stress. If you add one of the molecules it will shift away from that molecule. If you take away a molecule, it will shift towards it to make more. Heat works the same way.

Pressure is the tricky one. If pressure is applied to an equilibrium, then the reaction will shift to the side that has the least amount of molecules (count the coefficients).

Friday, October 6, 2017

Reaction Rate Basics

Reaction Rates are affected by a few things. Without telling them the point, the students had a quick demo where they had to dissolve sugar cubes the fastest. The things that speed up reactions are: 
  • Temperature - warmer is faster
  • Surface Area - small pieces have more surface area
  • Concentration - the more water, the faster sugar will dissolve
  • Catalyst - lowers the activation energy and speeds up the reaction
  • Agitation - shaking or stirring increases the frequency of collisions

Friday, September 29, 2017

Reaction Types


We started by talking about the simple definition of the terms, what the probably products and reactants are and went over a basic formula for the reaction types the students need to be familiar with.

Reaction Types include:
  • synthesis
  • decomposition
  • singe replacement
  • double replacement
  • combustion
  • endothermic
  • exothermic
  • oxidation-reduction
  • neutralization
  • nuclear
After discussing the basics, we drew cartoons of stick men and women going on dates to show how atoms move around in the simpler reactions. The picture posted is someone else's version of single replacement (see the one guy switches with the other). For more help with this, check here.



Can you guess what type this is?

Thursday, September 28, 2017

Balancing Reactions

Students are learning to balance equations. Today they learned that reactants are what you start with and are on the left side of the equation. Products are on the right side of the arrow and are what is made by process of a chemical change.

Because of the Law of Conservation of Mass, the number of atoms have to be equal on both sides. To balance an equation, the coefficients are changed. Coefficients are the big numbers in front that tell you how many molecules there are. The subscripts (the little lower numbers) are not allowed to be changed because those are there to make neutrally bonded molecules (what we learned in the last unit.

By changing the coefficients and counting the number of atoms on both sides of the arrow, balancing can be achieved.

Monday, September 18, 2017

VSEPR - Valence Shell Ectron Pair Repulsion Theory

Valence Shell Electron Repulsion Theory

Electrons do not like each other and when looking at molecular structures - electrons and unshared electrons (the two dots paired together) will space out evenly so they are as far apart as possible.

Most of the names of the shapes of hints like tri, tetra, planar, etc. Students need to memorize these shapes and be able to visualize them for given formulas.



For help with VSEPR - read this.