Salts

It's Cinco De Mayo!!!!!!!!!!


We started this class with a look at our packet to look at topics such as conjugate acids and bases to determine whether a solution is
acidic, basic, or neutral.

Let's take this reaction, for instance...
H20(l) + HF(aq) -------- H3O+(aq) + F-(aq)

We know HF is a weak acid and water can be both an acid or a base (though weak). In this case however, we know water would be the base because H3O+ is its conjugate acid - meaning it wants to give up a proton (it has that + charge it really wants to get rid of). We know the F- would be the conjugate base of HF, because F- wants to accept an extra proton (it has that - charge it really wants to get rid of). At this point you might be asking why I'm going on about this basic stuff (ba dum tss) when I could be talking about how salts dissolve. Isn't that what we discussed in class today? Why, yes it is. I am getting there.

I'll give you two examples and go through them step by step:

1. NH4Br
2. KClO4 (both of these are salts)

An aquoeous solution of a salt in water will be either acidic, basic, or neutral due to hydrolysis: either in the cation, anion, or both. Hydrolysis is when an ion reacts with water. These are ions that DO NOT hydrolize:

A. anion of a strong acid (negative charge=anion)
B. cation of a strong base (positive charge=cation)

ions that DO hydrolize:

A. anions and cations of weak acids and bases

Let's look at NH4Br again. It consists of NH4+ and Br-. Br- would not hydrolize because it is the anion of a strong acid (HBr). Therefore, you disregard it. Since NH4+ would react with water, because it is an ion of a weak acid/base, the formula would look like this:

NH4+(aq) + H20(l) -------- NH3(aq) + H3O+(aq)

We still must determine if this salt is acidic, basic, or neither. We now look at H2O. Notice how it is acting as the base in the relationship (because NH4+ is an acid). That means H3O+ would be the conjugate acid of H2O, and the conjugate acid of a weak base (such as water) would mean the salt NH4Br would form an ACIDIC solution in water.

Moving on to the next one, we see
that KClO4 would be neutral, because K+ is the cation of a strong base (KOH) and ClO4- is the anion of a strong acid (HClO4). Therefore, you can think of them as "canceling out".

You may use this logic to solve any other issue of this type you may run into. What I want you to absorb is not only that notecard you've been sleeping on, but also this:

1. Conjugate base of weak acid: basic solution
2. Conjugate acid of weak base: acidic
3. Conjugate base of strong acid or conjugate acid of strong base: neither

This is also helpful (this may be the exact same thing, but I'm not sure):

1. If cation comes from strong bas
e, and anion comes from strong acid, salt is neutral.
2. If cation is from strong base, and anion is from weak acid, salt is basic
3. If cation is from weak base, and anion is from strong acid, salt is acidic
4. If cation and anion are both weak, then you can't tell without knowing relative strengths of acid and base.

It's very much like an arm-wrestling match.


Next, we discussed how to do ICE tables for such salt equations. We were introduced to a new concept called the Kw (which is 1e-14). Basically, to find Ka or Kb of salt, you do Kw/Ka or Kb (given, opposite of what you need). You then complete the ICE table as normal. Remember that notecard with all the important conversions on it! I won't dive to deep into this because this isn't such a new concept (the notecard or the ICE table).

Then, we completed our day with a lab. Basically, this day was all about salts.


Not as unlucky as previously thought.

Monday, October 18, 2010

Who is Amadeo Avogadro? Why, only the most famous scientist ever! Why is he so famous? He thought of the mole, that's why. The mole is a scientific unit. It is used in measurement, mostly in chemistry. The mole is abbreviated as "mol". But why is this significant? Today, we started Unit 3. Unit 3 focuses on stoichiometry, or the science that deals with the quantitative relationships that exist between products and reactants. (conversion factors) The number mole, is mathematically written as 6.022 x 10 to the 23rd

To begin our class today, Mr. Henderson passed back our grade reports, with the Unit 2 Test inscribed upon its face. Mr. Henderson also told us about the National Mole Week recently declared by President Obama. With that, we began class.

We took some notes and filled out a leaf of paper in our Unit 3 Packet. It focused on finding the number of moles of a certain atom in some situations and and finding the number of atoms in others. These are two very different ideas. Using conversion factors we learned earlier, we were able to make short work of these four problems we were given. Mr. Henderson also showed us how to find the number of atoms in a certain mass of an element. For example, let's take 12.01 grams of carbon. The number of atoms for this amount of carbon would be 6.022e23. (1 mole) How did we get that? The average atomic mass of carbon is 12.01. If you have an atom, with its average atomic mass in grams, just put down one mole as your answer. However, if the mass in grams is slightly smaller or larger than your given atom's AAM, you take the 6.022 from the mole and divide or multiply accordingly. For example, let's say you had 20 grams of calcium. The AAM for calcium is about 40. Because this is half, you would take 6.022 and divide by two. Your answer shall be 3.011 x 10 to the 23rd. (you keep the last part of the number)

To end the class, we started our MR1 lab. (after getting new lab groups and moving to new seats) This is the H2O challenge lab. We have to count how many molecules there are in a sample of water. Since we cannot see molecules, we must count them in a different way. By using the mol, of course. We add the AAM of two hydrogen atoms to the AAM of one oxygen atom. However, not everyone finished this lab in class today and Mr. Henderson will let us finish it tomorrow. For anything else check his website.

Wednesday, September 1, 2010

Today's chemistry class was educational, yet different from day to day events. The large, red thing I have above this text is an illustration of a water molecule. H2O - the large hydrogen atom in the center, and the two oxygen atoms on the outside. But this isn't just any ordinary chem "mumbo jumbo"; we performed an exercise in class today based on molecules such as this one.

At the beginning of class, we completed the preliminaries. We discussed Shidan's blog entry, and Mr. Henderson stated we are to be doing a computer activity that day. Entering Room 339, we started immediately.

To begin our work, we proceeded to log on to our computers. Next, we all created ChemThink accounts like Mr. Henderson has shown us in the class. After this, we proceeded through an "atom tutorial". In this tutorial, we learned about certain terms. Included were: pure substance, solution, mixture, atom, molecule, element, and compound. These will be discussed in later paragraphs.

For example, a solution would be much like the water molecule above. It is a compound that fits together nice and smoothly. All the atoms blend into a common molecule. A mixture is different. A mixture contains many different particles, but they do not fuse together in one convenient molecule. Our own drinking water is filtered through the process of distillation; as many non-water particles get taken out as possible.

In the tutorial, we also learned the definition of atoms and molecules. An atom is the basic building block of matter. Everything in existence is composed of atoms. Every element on the periodic table is composed of a single type of atom. A molecule is a series of atoms bonded together. A molecule may be a compound, different elements together, or you could even have a molecule of the same element (a pure substance).

After passing the tutorial, the class took a small quiz on the content. It was necessary to pass the quiz by answering ten questions correctly without getting three wrong in a row. The students could have taken this quiz an unlimited amount of times.

The homework besides that was to complete pages 3-4 in our packet. It was basically a review of what we had studied earlier that day. We had to illustrate atoms and molecules based on their chemical formulas. The pages tested us on whether or not we could distinguish between a solution, a mixture, or a pure substance.

To summarize, our period 3 chemistry class had a productive day today. I personally found the activity very interactive and fun. I may now say I have learned at least something about atoms and their chemical properties.

Blog written by Dmitriy Kalin

Picture taken from Wikimedia Commons