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.

Friday, September 15, 2017

Polar Vs NonPolar Covalent Bonds

Anyone who has ever had to share something with someone else knows that sometimes isn't exactly even. Covalent molecules or bonds are no different. 

If a molecules is nonpolar covalent, it is sharing its electrons equally. The best example of this is in diatomic molecules. Diatomic molecules are two of the same atom bonded together - so they would have exactly the same pull. Symmetrical molecules are also nonpolar.
Polar covalent bonds occur when electrons are not equally shared. One atom, usually more electronegative, has a stronger pull on the electrons and shares them unequally. The other atom that is less electronegative has a smaller hold on the electrons and is thus can be slightly positive. 

One way to remember this is... "Polar Bears do not share... equally."

Thursday, September 14, 2017

Covalent Bonding

If it is a - and -, the bond is covalent. The electrons are shared in the bond. To get the formula, you have to draw the Lewis Dot structures for the elements and connect the dots that don't have friends. You write the formula based on your drawing. To name it, use prefixes to indicate the number of atoms in the formula and the second one ends in -ide. For these it doesn't matter which element comes first.

There are 4 atoms that commonly form diatomic molecules with a covalent bond... and hydrogen is one of them! That weird atom!

Ionic Bonding

Students learned about ionic bonding. Ionic bonding happens between metals and nonmetals (positives and negatives). The electrons are given and taken in this ionic bond. 

To get the formula, you criss cross the charges. To name it, you say the name of the metal, then the name of the nonmetal with an -ide ending. If it is a metal from DForP block, then you use a roman numeral to indicate the charge of the metal. Basically everyone gets a Roman numeral except S-block, Boron, and Aluminum
After learning the basics, students practice. 

Ionic Bonds for Dummies

Here is a cool interactive where you can build models to simulate ionic bonding.

Tuesday, September 12, 2017

Formula Writing

Today we discussed how to write chemical formulas, and what the numbers associated with a chemical formula mean.

Coefficients are the big numbers in front and are distributed to the whole molecule (which means you may have to multiply). Coefficients tell you how many molecules are present. 
     3He = He He He :)

Subscripts are the little lower numbers and they indicate the number of atoms and only apply to the atom it is to the right of. Subscripts tell you how many of each atom are present. Students wrote their name as a chemical compound and thought it looked pretty interesting. Some students have long formulas, other short.

We then led into counting atoms for real chemical formulas using subscripts and coefficients.
Be careful....

Monday, September 11, 2017

Roman Numerals for metals

Time to learn about roman numerals.... Here is a handy clock if you are unfamiliar with them. Pretty much you need to know 1-7. 1 is represented with I, five with V and 10 with X.  4 and 6 and 7 is where it gets tricky. 4 is 1 before 5 - so its Roman numeral is IV. 6 is one after five so its roman numeral is VI.

Students also learned how to identify the charges of metals with more than one oxidation state using Roman numerals. Metals in the D, F, and lower P get Roman numerals - basically all metals but the S block, Aluminum and Boron get roman numerals. The roman numeral tells you the charge. We have to use this system because those odd metals can actually be found in more than one form - some with 2 possible charges - some with more than four!

Friday, September 8, 2017

Valence Electrons and Ions

Students learned about valence electrons. Valence electrons are the outermost electrons and are the electrons that are used for bonding and participate in reactions. Valence electrons are only found in the S and P blocks. The max number of valence electrons is 8. Students practiced counting valence electrons and drawing Lewis Dot Structures.

Students also practiced identifying which noble gas an element wanted to be like. All elements want to be like two noble gases - it is just a matter of figuring out which is closer. Elements want to be like noble gases because they have full outer electron shells, or full valences. This makes them stable and non reactive which is why noble gases are sometimes called the inert gases.

Today students learned how to use valence electrons and dot structures to determine the charge of an atom. Atoms either want to gain electrons or lose electrons to become like those noble gases they envy.
  • Ions are atoms or molecules that have a net charge, either positive or negative. There are two kinds of ions:
  • Anions are negatively charged ions because they have negative net charges. This means that there is a greater number of electrons (-) than protons (+). For example, the anion, fluoride (F 1-), has a one negative charge because it has a total of nine protons and ten electrons. Thus, the net charge for fluoride is 1 negative.
  • Cations are positively charged ions because they have  positive net charges. This is due to these ions having more protons (positive charges) than electrons (negative charges). For example, calcium (Ca 2+) is a cation ion with 20 protons and 18 electrons. The net charge for Calcium is 2 positive. (from here)

Wednesday, September 6, 2017

Organic Chemistry Primer

Organic Chemistry is the study of carbon chemistry! Carbon has four valence electrons and thus always likes to make four bonds. 

The basics for naming organic molecules include: counting the number of carbons, looking at the number of bonds between carbons, and checking for functional groups. 

Prefixes are based on the number of carbons. 1=meth, 2=eth, 3=prop, 4=but, and after that the prefixes mimic the covalent prefixes. The suffix identifies the bond type. A single is an alkane and thus the suffix is -ane. Doubles - alkene; triples - alkyne. 

  Click on the image to make it larger.

Compound Interest's original post can be found here.

There are many organic functional groups, but at this level of chemistry, students just need to be familiar with alcohols, aldehydes, and carboxylic acids. 

Monday, September 4, 2017

Periodic Trends

It is all about the electrons! Always!

Electro- negativity is how badly atoms want electrons. The most electronegative atoms are Fluorine, Chlorine, and Oxygen. Everyone wants to be a Noble Gas... and halogens are the closest so they are the most electronegative.

Ionization energy is how difficult it is to remove electrons. It is difficult to remove electrons from atoms that are electronegative.

Atomic radius increases as you move down the periodic table because atoms have more mass, but actually decreases from left to right because atoms are holding on to their electrons tighter (because they are more electronegative).

Shielding has to do with how protons are blocked by the electron shells - the more shells there are, the more blocking there is. So something in period 5 (with 5 shells) has more shielding than an element in period 2 with two electron shells. Sheilding is constant across the periodic table because the number of shells is constant.

Compound Chemistry has an infographic on the Periodic Trends here

Friday, September 1, 2017

Electronic Configuration

Battleship, a classic game by Milton Bradley, is a game easily adaptable to learning electronic configuration.

Electronic Configuration is an intense mathematical calculation proposed by Schrodinger & Heisenberg as a way to predict where to find an electron around the nucleus in the electron cloud model. 

There are four main parts of the periodic table known as orbitals. The S block, P block, D and F orbitals. Within each block, you just count over how many spaces it is. There are seven energy levels that are loosely based on the period that an element is (the D & F blocks are exceptions to energy levels). The D block is dumb and that's why it starts with one number lower. Really they just have less energy and have the same amount of energy as the S and P block in the 3rd period. The F block are failures and that's why they are 2 lower... or they have a lot less energy.

So to identify Hydrogen you would say 1s2 because it is in the first period or first energy level, in the s block, and the first member of the first block. Carbon is a 2P2 because it is in the 2nd period, in the P block, and the 2nd one over in the P block.

Students learned the pattern of electronic configuration and how to use it. Basically its like giving directions to an element on the PT using set landmarks. It is a bit confusing, but once you get the pattern, its not too bad.

Students practiced a bit and then they played Battleship to practice some more. The Periodic Table became the game board and students hid their ships on it, then guessed hits using the electronic configuration of the atoms. I think they really got the hang of it because I did not field many questions at that point.

Saturday, August 26, 2017

Dudes! (Chemistry Scientists)

There are several "dudes," famous chemistry folks that students need to be familiar with. These dudes (Democritus, Dalton, JJ Thompson, Millikan, Rutherford, Bohr, Heisenberg, De Broglie, and Planck) all did experiments and came up with different and improved atom models. The current model is the elctron cloud or quantum mechanical model which was formulated by Heisenberg and De Broglie.

Heisenberg and De Broglie came up with the current electron cloud model, but we draw Bohr's planetary model the most often because it easier to count the electrons. There's a TedEd video about the progression - HERE.

Electrons are tricky because they move constantly and at high speeds. Heisenberg's Uncertainty Principle states that you cannot know both the speed and location of an electron - you can only know one - because measuring either one, changes the other. DeBroglie's wave theory helps explain why electrons sometimes act like particles and sometimes are compared to waves.

For more information about the evolution of the atomic model, check out this link. And here is a video!

Here are some Dude Quizlet Flashcards to help you out.

Friday, August 25, 2017

Atoms and the PT

Atoms, or elements, are the smallest unit of matter. They retain their identity in chemical reactions and are combined to form compounds and everything in the universe.

Atoms have some basic parts. Protons and Neutrons are found in the nucleus and make up the atomic mass. To find the number of neutrons, you subtract the atomic number (number of protons) from the atomic mass number (protons plus neutrons).

Electrons are so tiny that they do not influence the atomic mass. They are found orbiting the nucleus in shells or orbitals. Atoms are neutral so the number of protons equals the number of electrons.

How small is an atom? Watch this entertaining TedEd video!

Mendeleev designed the periodic table by looking at the properties of elements on cards and arranging them different ways until he got a system that worked. No one told him how to do it, he just did it until it worked. He even left spaces for elements that were discovered in his lifetime. (More info about Mendeleev) His periodic table was set up according to atomic mass number. The current table, altered slightly by Moseley, is organized by atomic number (number of protons). This is an AMAZING Video about Mendeleev and his PT!

Next we discussed regions of the periodic table, colored them, and labeled them. Periods are horizontal rows (periods go at the end of a sentence) and there are 7 periods. There are 18 groups or families (vertical columns) and a few of them have special names. This a pretty excellent diagram. This website gives a lot of helpful information.

Tuesday, August 22, 2017

Percent Composition

Percent composition is just like determining your grade - the amount you got divided by the the whole amount. 

With compounds, you find the mass of a particular element and divide it by the mass of the whole compound. So if you wanted to know the percent composition of oxygen in water, you would take the mass of oxygen and divide it by the mass of water. 

We practiced some basics and the students measured the amount of sugar found in DubbleBubble bubble gum. 

Students will each have a piece of gum and observe the gum by weighing it, drawing it, and smelling it. The students chew the gum for ten minutes. While they are waiting we will watch How Its Made on bubblegum.

After ten minutes, students will do more observations and re-weigh the gum. The gum weighed less... why? Because the sugar dissolved and was lost. Using this weight difference, students determined the percent composition of sugar in the gum they chewed. They also can convert the grams to moles and determine how many moles of sugar were in the gum.

Monday, August 21, 2017

What is a Mole? What does it have to do with chemistry?

Moles are used to count atoms. There are 22,000,000,000,000,000,000 quintillion atoms in a grain of sand and even counting grains of sand is a pain. Because atoms are so tiny, we use the mole to estimate.

There are 6.02 x 10 ^23 molecules in one mole. That's a whole lot. This is our new favorite number because it needs to be memorized. We will practice converting from moles to molecules.

Next we discussed molar mass. Molar mass = 1 mole and it also equals atomic mass from the periodic table. To find the molar mass of carbon dioxide you find the mass of carbon and two oxygens and add them together. Finding molar mass is not difficult unless the molecule has tricky subscripts (which we have been practicing).

The third thing about moles is that "one mole of any gas will occupy 22.4 Liters." 22.4 is another favorite number. We can convert from moles to liters and from liters to moles. 

Just how big is a mole? There's a TedEd talk on that! Watch it here!