Acids and Bases

Today we started discussing acids and bases.

Acids have H+ and donate them, they have the smaller numbers 0-6.9, they turn litmus red, and have sour tastes. 

Bases have OH- and want H+ to make water, they have Bigger numbers, they have a pH between 7.1 and 14, they turn litmus Blue, and they taste Bitter. 

If you mix an acid and a base together, then there is a neutralization reaction. A neutralization reaction is also a double replacement reaction. In neutralization, the acid and base combine to form a water and a salt. The water and the salt are neutral (hence the name).

pH measures the concentration or molarity of H (Hydrogen ions) in a solution. That's why the H is capitalized.

pH + pOH = 14. So if you have the pH, it is easy to get the pOH... just subtract from 14. There are some fun interactives for acids and bases linked at the bottom of this webpage. Check out the Alien Juice Bar.

Getting Ready for the Chemistry SOL

The End of Course Chemistry Assessment is coming!

Look over all notes, study guides, and tests.

Practice Questions are available at J-Lab. Be sure to choose Chemistry - and if you go under "more options" you can focus the questions somewhat by topic.

Released SOL practice tests at Poquoson.

Need a re-teaching? Ask Tyler DeWitt to help you out!

More info on Acids Bases, and Nuclear

For acids and bases, use the universal indicator test, from this link. Be sure to make your predictions first.

And here is a TED-Ed on the strengths and weaknesses of acids and bases.

Click here for the Online radiation quiz from Hanford and OMSI. Be sure to write down the numbers that apply to you.

There are lots of Ted Talks on Nuclear topics... check them out. 

People often ask about Chernobyl and here is a TEDtalk about the people who refused to relocate. There is also a lot of information here at National Geographic.

Matter!

Today we discussed matter. Everything in the universe is made up of matter. Matter can be classified as substances of mixtures. A substance is pure and is either made up of elements, the simplest form of matter found on the periodic table, or compounds - two or more atoms chemically combined that can be represented with a formula (ie. H2O).

A mixture is two or more things (substances or anything really) in the same place at the same time. Salt water is a tricky one because we know we can write a formula for salt (NaCl) and water (H2O), but it is a mixture because when mixed in solution, these compounds do not combine.

Mixtures can be heterogeneous of homogeneous based on how the solute is spread through the mixture. Heterogeneous mixtures are not mixed evenly and each sample could be different - like salad, chocolate chip cookie dough ice cream, air, and soil. Homogeneous mixtures are the same throughout like creamy peanutbutter, vanilla ice cream, pure air, sugar, and kool-aid.


One important thing we discussed today was the Law of Conservation of Matter proposed by Lavoisier. Basically matter cannot be created or destroyed... it is conserved or recycled or moved somewhere else, but it cannot magically appear or disappear!

Specific Heat

Heat capacity is the amount of energy needed to raise a substance 1*C. Heat capacities depend on the amount of the liquid and how it is contained.

Specific heat is the amount of heat needed to raise 1 gram of a substance 1*C. Specific heat is measured using a formula with J/gC. (Apparently that now stands for Jancaitis gone crazy instead of Joules per grams Celsius.)

We re-learned endo and exothermic reactions.We discussed them in the reaction unit, but are revisiting the topic because now we are also going to be calculating heat changes and specific heat values.

Endothermic reactions absorb heat and get warmer (End Up).

Exothermic reactions lose or release heat and get colder (Exit down). 

To test this out, students in groups were given a whack-a-pack and asked to make observations. The pack starts off at room temperature and when you hit it, the reaction occurs. This is a chemical reaction for a few reasons - one you can hear it fizzing. Two it blows up so a gas is being formed (1 of the 4 ways you know a chemical reaction has occurred). And Three there is a temperature change (another of the four ways). The pack gets really cold which means it is releasing heat and this is an exothermic reaction.

Watch this little video to see how it works. These are available at Dollar Tree at Valentine's Day if you are interested.

After this lab demo, students answered questions and then worked on math practice for heat changes and specific heat.

Solutions and Solubility

Today in class we discussed solutions. Solutions are homo- geneous mixtures comprised of solutes dispersed in a solvent. Water is the universal solvent, but not the only solvent. For example, a marshmallow is a solid (sugar) solute dispersed in a gaseous solvent (air).

Solubility is how well something dissolves. Some things are very soluble, and some are insoluble (do not dissolve).

Solutions are said to be saturated if they are holding all the solute that they can. When the solute starts to build up on the bottom, you know a solution is definitely saturated (like the dark blue solution on the right). Solutions are unsaturated if they can dissolve more solute (like the two light blue solutions on the left).

Solutions can be super-saturated if they are heated because they can hold more solute than normal. Even if you cool these solutions back down, they will still hold this additional solute in solution. Sweet Tea and all candies are made by first making super-saturated solutions and then cooling them.

For an excellent website about all of these topics and others regarding solutions that have and will be covered in this unit - check out this useful website.

Properties of Solutions

Next we discussed colligative properties. If you add solute to a solution, like salt to water, it changes the properties of the solution, particularly the boiling point or freezing point. We put salt on the roads to lower the freezing point of water so ice does not form on the roads. 

Electrolytes can conduct electricity because the solute breaks up into ions and the ions can carry the electric current. Pure water does not conduct electricity - but water with solutes in it can. We did an in-class demo similar to this one to test some solutions. Salt water does conduct electricity, but sugar water does not because of the carbon. Gatorade conducts electricity but barely because of the high sugar amount in the drink.

Finally we talked about Molarity. Molarity is moles/Liters and is a quantitative way to measure concentration. Molarity describes with numbers if a solution is dilute or concentrated. It is a pretty easy formula so students zoomed through it. Molarity changes with the amount of solute OR the amount of solvent (liquid).

The formula is M1V1=M2V2. These questions will mention molarity twice and volume twice and will not mention moles. Three numbers are given and the fourth needs to be solved for by isolating it through division.

When diluting substances the molarity (concentration) decreases or becomes more dilute because more water is added to the original solution. The number of moles of solute stays the same, but the molarity changes because the volume increases.

Density

 

We have made sure that everyone had a good handle on density in regards to definition, math and formulas, and what it actually means.

Density measures matter in a given volume or the amount of stuff in a space. It can also refer to the amount of space between molecules. We looked at some diagrams and discussed scientifically why a person cannot walk through walls, why moving through air is easy, and why moving through water is slightly more difficult.

Density is how close together the particles are in a substance. If they are close together the substance is more dense. If the particles are far apart, the substance is less dense. I do not float in Lake Anna, but I do float in the ocean - therefore I am more dense than Lake Anna and less dense than the ocean.

Things that are more dense-sink, things that are less dense-rise to the top, things with similar densities-mix. If you were to pour liquids in a random order layers form because of the differences in density. Here is a photo of a demo.

Phases and Phase Changes

We discussed solids, liquids, and gases. We talked about the amount of energy the particles had and how the particles move. We discussed whether they had definite or indefinite volumes and shapes. We talked about why exactly we can't walk through walls, but why we can walk through gases and liquids.

Next we discussed the phase changes that happen between solids, liquids, and gases. These changes can be represented on one of two graphs. We talked about the graphs, labeled them, and and answered questions about them. 

Volatile and NonVolatile Substances

•Volatile substances are more likely to vaporize.
•Examples – Acetone, Alcohols, Smelly Liquids
•They have high vapor pressures, weak intermolecular forces, and low boiling points
•Tend to be flammable and more reactive
 
•Nonvolatile substances are more stable and less likely to vaporize.
•Examples – Water, Salts, Liquid Mercury
•They have low vapor pressures, strong intermolecular forces, and high boiling points 

Combined Gas Law

The combined gas law combines the work of Charles, Boyle, and Gay-Lussac.

PV = PV
nT     nT

Basically, memorize one formula and then use only the variables you need, so sometimes you need PV = PV, and sometimes V/T = V/T.

This will help you with placement and deciding whether you should multiply or divide.

Ideal Gas Law

Ideal gases do not actually exist, but we pretend they do and use the Ideal Gas Formula of PV=nRT.

One of these variables will not be given to you and you have to solve for it. This does not seem difficult after stoich, so students dove in and did well.

Beginning Gases and Partial Pressure

Today we started learning about the behavior of gases and the factors that affect them. Gases are lightweight fast moving particles that generally have a lot of empty space between them. Because of this, they are easily compressible (pictured left). If not contained, gases can spread (or diffuse) to fill any size and shape container.

Gases are affected by pressure, volume, number of moles, and temperature. Changing any one of these variables, changes all the others.

Today we also learned the formula for the Law of Partial Pressure. Basically partial pressures add up to form total pressure. If the total pressure is given then you subtract the partial pressures.

Vapor Pressure

For notes today we discussed vapor pressure and boiling point. The boiling point of a liquid is when the vapor pressure equals the external pressure. When the pressures are equal it is easier for liquids to boil and vaporize into gases and steam away. We discussed definitions and answered questions about vapor pressure graphs. STP is 101.3kpa, 1 atm, or 760 torr.

Vapor pressure is measured with a manometer. A "normal" manometer is when the levels in the U are even. A "HOT" manometer has increased vaporization and increased particle movement so the liquid levels in the U are pushed away.

Limitng Reagents - MORE

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 8 cars and 48 tires, 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. 

4 NH3(g) + 5 O2(g)4 NO(g) + 6 H2O(g)

If you have 2.00 grams of ammonia and 4.00 grams of oxygen gas which is limiting?

Because the oxygen produced less of the product nitrogen monoxide, oxygen is limiting.

Limiting reagents

Today we discussed limiting reagents - basically the ingredient that limits how much products you can make. A reagent is a fancy word for reactant.

For a S'more, you need a graham cracks (to break in half), a marshmallow, and two squares of chocolate. If you have 10 marshmallows, then you can only make 10 sandwiches no matter how many graham crackers you have.

In a chemistry problem, you have to take what's given to you and convert to the products (so two sets of conversions) and then determine if what you NEED is what you HAVE. Whichever produces the least amount of product is the limitng reactant. It's really a thought process applied to stoichiometry and unfortunately a necessary evil.

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.