Unit 4 Review

     In unit 4, we learned all about toxins. The definition of a toxin is a substance that interacts with a living thing and causes harm. Toxins can enter the body and will often react with water. They can be molecular, ionic, or metallic-bonded substances.
     When we first started unit 4, we learned what it means to interpret a chemical equation. That includes basically looking at an equation and the phase changes (the italicized letters next to chemical compounds) in it and writing in words what's happening. This could possibly sound like "solid ___ reacts with an aqueous ___solution to form solutions of ___ and ___." The italicized letters can act as clues to whether a reaction is a physical or chemical change.
     A huge part of this toxins unit was balancing equations. It's crucial for a chemical equation to be balanced on both sides, otherwise the reaction will be unstable. A lot of reactions will produce toxic substances. Not only do equations need to be balanced for the safety of the chemists, but also to abide by the law of conservation of mass, which states that no atoms can be lost or gained. Balancing equations and predicting products go hand in hand; there are four different types of reactions: combination, decomposition, single exchange, and double exchange. If ever an atom moves places between the reactant side and the product side, you have to make sure it's present in equal quantities on both sides.
     Obviously, we also learned about toxicity in this unit. All substances can be toxic if ingested in large enough qualities, and the amount of substance needed to kill an organism or poison it is called the LD-50. LD stands for "lethal dose". To calculate how much substance you need to harm an organism of a certain weight in pounds, convert those pounds to kilograms and multiply by the LD-50.
     pH is important to consider when dealing with contaminated substances or substances which you don't know the toxicity of. On a logarithmic scale, anything with a pH of lower than 7 is an acid, and anything above 7 is a base. 7 is neutral.  PH is determined by the concentration of hydrogen and hydroxide ions in a solution. An acid or base can be diluted with water, and certain mL of water can decrease the acidity or basicness by tenfold.
     The final, and possibly hardest, skill that we learned in this unit was stoichiometry, which uses coefficients in BALANCED chemical equations to determine the number of moles of a substance. By constantly converting moles to grams and back to moles, you can find out how much reactant is needed to make a certain amount of product and vice versa. You can also figure out limiting reactant, a reactant that puts a limit on how much product can be made.

Problems: 

Jogkvsjdajfsk (to be added soon)

Disappearing Spoon, Ch. 15-17

In chapter 15, I read about William Crookes and his discovery of selenium, an element that, like most in the periodic table, is toxic in large doses. It can cause physical harm including but not limited to fevers and sores, and it can give one a kind of high if ingested. There were farmers and ranchers in Crookes' time whose cattle were prone to eating weeds rich in selenium compound. As one could guess, this harmed the cattle and the animals had to be monitored carefully. Go back to the mentioning of selenium giving highs; it was thought in those times that people could go mad if they took in a lot of selenium. People assumed that this was the cause of Crookes' "madness" that he displayed by abandoning his science to pursue a religious life path. His choice was made after the death of  his brother. While it's possible that his work with selenium could have poisoned William, I think that this was a good example of what grief over death can do to a person. I also read about William Röntgen, who discovered x-rays.

In chapter sixteen, Kean talked about a man named Robert Scott who took a few of his buddies with him to the South Pole. The explorers had quite a bit of tin with them when they were on their way, and as they got closer to Antarctica, the tin began to rust. Unlike iron rust, tin rust is white. However, like anything else that can rust, the tin grew fragile. One could imagine that things began falling apart on the journey, perhaps malfunctioning. A kerosene leak along the way claimed Scott's life, and his buddies had to turn back without him. Cold conditions do strange things to metals. An example I used to relate to this chapter is how cold temps drain battery life. It was highly likely that some disaster would intervene with Scott's goal.

Chapter 17 went into greater detail about the story behind a man whose name I've actually heard a few times in school. Supposedly, Donald Glaser invented a "bubble chamber" while sitting in a pub, and the actions that he performed with his invention apparently helped him see tiny, tiny things called "kaons", "muons", and "pions". I admit, I have no idea what those are. This assumption about Glaser is false, nothing more than a tall tale. However, his bubble chamber did make one thing clear. I read in this chapter that bubbles form around the imperfections of something like a glass. Regardless of carbonation, a glass of wine or champagne will be bubbly because there are microscopic cracks in the wine glass.

Unit 4, Lesson 26

Reactants hardly ever combine in the ratios that we see them in in chemical reactions. It's almost like the textbook writers made up problems for students to solve! Haha, fancy that. No, back to serious business, ususually there's more of one reactant than is required to burn up the other. Because of this, chemical reactions can be short-lived. The amounts of products depends on how much reactant is present. You can only have as much product as your limiting reactant will allow. The amount of product that a limiting reactant comes up with is called the % yield.

Extra notes / Practice Problems:

Unit 4, Lesson 25

In order to determine the mass of product produced by a certain mass of reactant (and vice versa), it is necessary to convert mass to moles and then back to mass. This process is called "the mole tunnel". The balanced chemical equations are written in moles--how is that known? Because the coefficients represent the number of moles of reactant. Sometimes a problem tells us how many grams of each reactant we have, and we have to find the mass of the precipitate or product. Sometimes, we know the mass of the product, and we have to find the mass of one of the reactants. This is where I, and a lot of others, found out that stoichiometry isn't all that easy!

Practice Problems:

Unit 4, Lesson 24

One can remove harmful substances from a water source by precipitating harmful ions from it. How does one do that, exactly? It's more of a question of how much of two reactants are needed to achieve a solid precipitate, and to answer that question, mole ratios come into play. The ratios can be identified by coefficients, the big numbers before compounds in a chemical equations. Those coefficients tell you how many moles of a certain substance are present in a reaction. They can be read by "for every __ mole(s) of reactant 1, there are __ mole(s) of reactant 2." You can also say "for every __ mole(s) of reactant one (or 2), there are __ mole(s) of product one (or two)." Sometimes we aren't given equal amounts of reactants in a reaction. In this case, one runs out first, and that reactant is called the limiting reactant. The one left over is the excess reactant.

Practice Poblems:

A convenient photo has appeared!

Unit 4, Lesson 23

In a precipitation reaction, an aqueous solution reacts with a solution that isn't very soluble, and when combined, a solid product is formed. Precipitation reactions are double exchange reactions and their reactants are usually ionic compounds. Compounds that aren't very soluble will precipitate from aqueous solutions and show up in the product half of a chemical equation. There is a chart that says what compounds are soluble (represented by an 'S') and which compounds are not ('N'). Insoluble compounds are precipitates. There are precipitates in our own bodies: bones, teeth, and kidney stones. All three of these are made from some kind of calcium compound.

Problems: 

3.) Explain what a spectator ion is. A spectator ion is an ion that is noted in a chemical equation but doesn't exactly contribute to the equation itself. More, it just kind of floats around. Or, as Ms. McDowell would say, it "chills in the beaker" with the other compounds.

4.) Which ionic solids are soluble in water? (answers in bold)

LiNO3   KCl    MgCl2   Ca(OH)2   RbOH  

CaCO3  Li2CO3   PbCl2   AgCl

Disappearing Spoon, Chapters 13-14

Chapter 13:

Metals that we might find petty today were very valuable in the past. Two such metals are aluminum and bronze. Bronze stole a bit more of the spotlight when Prince Midas of Turkey discovered the secret to forging it from tin and copper. (Tin? Sounds weak, doesn't it? Maybe that's why bronze metals are the most unappreciated ones.) Perhaps because bronze was so closely associated with Midas, many considered it to be the most valuable and sought-after metal in the world. They were rather excited, as well, when the prince discovered to different colors of lead--ooh~~~~! After the fall of bronze, aluminum rose up and made people's eyes sparkle. Chapter 13 talked about gold a lot, even though it's been mentioned throughout the book several times already. Anyone who's paid a lick of attention in history already knows that gold was made famous partially because of gold rushes worldwide. A lot of people seem to think that gold is one of those motherloads, like oil, that hides in the crevices of hard rocks. Gold ore? Well, actually, since gold doesn't bond with any element other than tellurium, it's typically found solidly by itself. That's why people panned for it. When it does bond with tellurium, metals with strange names result.

Chapter 14:

A man named Johann Wolfgang von Goethe might make you think of a composer. But he was a writer, and like anyone who can put thoughts on paper, some of his inner workings were quirky. He proposed a lot of statements about colors. He claimed that color group AB, when added to group CD, would create the reaction AB + CD --> AD + BC. Look familiar? Not sure if it exactly ties into what we're learning in chem. right now, but that smells like a double exchange reaction to me! Maybe Goethe's mind was just very colorful, or maybe he just wasn't looking into anything that anyone cares about. I say this because Sam Kean doesn't seem very, well, keen on Goethe's story. He doesn't consider him legitimate. Chapter 14 also talked about pens, which at one time were extremely valuable. Like the apple products of today, and how there always seems to be a new one, many pens came out in a consecutive row and people felt compelled to buy them.

...The day I see a pen with a touch screen, I will be astonished.

Unit 4, Lesson 22

Titration is the process of measuring the concentration of a strong acid in a water solution by using an indicator and adding a base to neutralize it. As stated before, neutralization reactions result in a salt and water being produced.  After titration, when a point of equality is reached, there are equal moles, equal volumes, and equal molarities of the acid and the base. Get this: if you really know what you're doing, you can completely neutralize an acid with a base (or vice versa) and get a solution safe enough to drink. Granted, no one would recommend you try that at home, or even in a chemistry classroom.

If the molarity of the acid or the base is known, the molarity of the other can be found. It's crucial to find out the concentration of H+ ions and know that that number is equal to the concentration of OH- ions. Those numbers can then be divided by the volume (in L) of the substance with the unknown molarity. (If that doesn't make much sense, see the problems below)

Problems:

3.) How many mL of 0.1 M NaOH would be needed to neutralize 2.0 L of 0.050 M HCl? 
First, know that 2.0 L = 2,000 mL.

0.050 M / 2,000 mL = 0.1 M / x mL
0.050 M( x mL) = 0.050x
2,000 mL( 0.1 M) = 200
200 / 0.050 = 4,000 mL

It would take 4,000 mL of NaOH to neutralize the HCl. 
Another way to look at this is to realize that the molarity of the HCl is 1/2 the molarity of the NaOH solution. This means you can convert the HCl volume into mL (2,000 mL) and multiply that by 2 to get 4,000 mL.

5.) A student mixes 100 mL of 0.20 M HCl with different volumes of 0.50 M NaOH. Are these final solutions acidic, basic, or neutral?
a.) 100 mL of 0.20 M HCl + 20 mL of 0.50 M NaOH Acidic (because there's more of the solution with the lower molarity and that overpowers the base)
b.) 100 mL of 0.20 M HCL + 40 mL of 0.50 M NaOH Neutral (there is enough base to neutralize the acid, despite their different molarities)
c.) 100 mL of 0.20 M HCl + 60 mL of 0.50 M NaOH Basic (if 40 is the neutral point, anything above that as far as the NaOH is concerned should be basic.)

Unit 4, Lesson 21

A neutralization reaction, also considered a double exchange reaction, occurs when a base is used to render an acid neutral, or when an acid is used to neutralize a base. When an acid and base are mixed, an aqueous, ionic compound (a salt) and H2O (water) are produced. In pure water kept at 25 degrees C, when there are equal amounts of acid and base, there are equal amounts of H+ ions and OH- ions.

There are weak acids and bases, and strong acids and bases. Strong acids and bases dissociate (break apart) completely in water and completely disperse their ions. They are also the most dangerous acids and bases and don't mean good news when they come into contact with skin, your insides, your clothing, etc. Weak acids and bases, however, do not dissociate completely. They're generally safer. Some typical weak acids are vinegar and citric acid, which is found in fruit. Ammonia, a household cleaner, is a weak base (but, like any kind of soap or cleaner, shouldn't be ingested. Not that smart Chem Honors kids need to be reminded of that!)

Problems: 

5.) Suppose you mix 1 mol of sulfuric acid, H2SO4, with 1 mol of sodium hydroxide, NaOH. Why does the pH of the solution remain below 7? There are not enough H+ ions in NaOH to neutralize the H+ ions in the sulfuric acid, which has 2 hydrogen. There are also not enough oxygen atoms. Either you need more sodium hydroxide, or a stronger base to neutralize the H2SO4.

8.) What combination of reactants would result in a neutralization reaction with sodium nitrate, NaNO3, as one of the products? (Mg(NO3)2 + NaOH, HNO3 + NaOH, CH3OH + NaOH, HNO3 + NaCl) Combination D, which is HNO3 + NaCl.

Unit 4, Lesson 20

The process of "watering down" an acidic or basic solution is called dilution. It simply entails adding water to an acid or base to, in an acid's case, raise the pH toward 7, or in a base's case, lower the pH toward 7. 7 is considered "neutral" for any substance. Diluting a substance makes it weaker, or less concentrated, since adding water lowers the molarity. When the molarity of an acid or base is lowered, the pH increases by 1. This is because the pH scale is logarithmic, and a tiny change in pH makes for a huge change in concentration. For example, changing an acid from a pH of 3 to a pH of 4 makes that acid 10x less acidic. Changing it to pH 5 is 100x more acidic. pH 6, 1000x more acidic, and so on.

Though diluting an acid or base is meant to make it more neutral, one cannot achieve full neutrality. Rather, they can get very close. An acid can only be diluted to a pH between 6 and 6.9. A base can only be diluted to a pH around 7.1 (or 8, because any pH above 7 is basic).

Problems: 

2.) Explain why you can't turn an acid into a base by diluting with water. You can't eliminate the acid completely--you can only neutralize it, because water is neutral. However, if you were diluting with a base, you could change the acid, because the pH would gradually raise until it was high enough to be a base.

4.) How much water do you need to add to 10 mL of a solution of HCl with a pH of 3 to change it to a pH of 6? 999 mL of water.
pH of 3 = .0010 M
pH of 4 = .00010 M (10x less acidic) (+9 mL of water)
pH of 5 = .000010 M (100x less acidic) (+99 mL of water)
pH of 6 = .0000010M (1,000x less acidic) (+999 mL of water)

...I don't know if that's correct. I'm a bit fuzzy on this myself.