Final Exam Questions From All Y all How do you know when valence electrons are relevant to solving a question? Well, obviously if it directly asks about electrons or their configuration. Otherwise, the valence electrons are the thing that controls what charge something forms in ionic compounds, and how many bonds it makes in covalent compounds. Anything relating to those topics ultimately comes down to valence electrons. When are you expected to know the state of matter of the products? If it s a precipitation reaction (in particular) or any other reaction that makes ionic products and has water around, you should be able to predict the state based off solubility rules. For covalent products, I expect that people can be reasonable (carbon dioxide is a gas, water is a liquid unless it s being made in a combustion reaction), but would typically give them to you. What is point of Lewis Structures? They show how molecules are put together. The major point here is that the formula alone does not determine what a compound is, or how it reacts. Two isomers (compounds with the same formula but different structures) can be wildly different. Like, one toxic and the other not kind of different. Predicting that from the structure is beyond the scope of this class, but that s why they exist. Where s Waldo? Right here How do you know when to set qs equal to each other, or when to set them equal to zero? You never set them equal to each other. They always add up to zero. You can sometimes solve problems correctly by setting them equal to each other, but only if you get all the signs right. Don t try; just set the sum of them to zero. How do you know how many physical moles can occupy a volume? Density. For liquids, that s pretty much constant (though it changes a bit with temperature). For solids it s constant if you have a solid block of the material, but variable if you powder it up. For gasses, it s completely changeable by temperature and pressure ( n/v = P/RT ) When can we assume a compound is in excess in a reaction? Combustion: if oxygen is limiting, you have bigger problems. Things reacting with water: very rarely would you ever have water be the limiting reagent. However, generally don t assume; go by what the data says. How do you recognize acids and bases? An acid is any covalent compound with hydrogen that can act as though it were ionic. For that to be true, you have to be able to pull off H + and leave behind something sensible. Like HCl would leave Cl -, which is a regular thing, and can therefore happen and HCl can be an acid. H 4 C would leave C 4-, which isn t normal, so it isn t an acid.
A base on the other hand (for our purposes) must contain either OH -, O 2-, or H -. They must be negative, which means that only ionic compounds need apply. CH 3 OH doesn t have a hydroxide; it has an oxygen and a hydrogen bonded to stuff. What is the point of molarity? To be a chemically-useful measurement of concentration, which is often more important than just amount. For example, essentially no amount of 0.000001 M HCl will kill you if you drink it (at least, not any faster than just drinking that much water would), whereas a small amount of 6 M HCl could. Secondly, many of our compounds come already dissolved in solution, so you couldn t weigh them even if you wanted to. What is the Bronsted definition for acids and bases? An acid is a thing that gives an H + ; a base is a thing that gets an H +. How does the laser thermometer work? The laser in the name is a bit of a misnomer the laser is just for aiming the thing. All objects emit what s called blackbody radiation, which is electromagnetic radiation that comes out mostly in the infrared range (though if things get hot enough, it can be visible, hence red hot ). The thermometer mostly just measures how much infrared light comes off of something, though it can get thrown off by things that reflect IR light or that emit less or more per surface. Why is a mole 6x10 23? Because we defined it that way, basically. To make gram to mole conversions work well, we basically define that 1 mole of protons/neutrons weighs 1 g, then figure what that number works out to be. That s why it s not 6 either, but actually 6.02214085774x10 23 (though we re not too sure about the last two digits yet). We re constantly getting better and better measurements of it. To be or not to be? Can Entropy be reversed? Yes, locally. But in order to do so, you make it go up even more somewhere else. For example, I can clean up my desk and make it more orderly (less entropy), but in doing so I burn calories and convert solid compounds in my body into radiant heat and gasses, and make even more entropy elsewhere. Should we avoid rounding until the final number? You should avoid rounding too much, but don t need to keep all 10 digits. If your answer should have three sig figs, keep 5 or 6 for the math, then round your answer to three.
Where is a good place to find practice problems? The textbook (look for the ones labeled marathon or challenge problems. The class website. Google. Me. How do you know what to do with amounts (either moles or concentrations) when you mix things? When you mix things, there are three possibilities (I ll stick to solutions, but the principles apply to others): * You mixed two solutions of the same thing. The concentration ends up between them. You can add moles here because they are the same thing. * You mixed two different things that don t react. Both have now been diluted. There are two separate molarities one for each compound. * You mixed two different things that reacted. One will be gone completely. One will probably have been in excess and there is some left over (use ratios to find how much was used). There will also be some products, in amounts determined by ratios and limiting reagent. YOU CAN NEVER ADD MOLES OF DIFFERENT THINGS!* (*except the ideal gas law, because the assumptions behind the ideal gas law say that all gasses are the same except for mass) What is the relationship between molarity and ph of a solution? Depends. If it s not an acid or a base, nothing; the ph is 7. If it is, ph = -log[h + ], where [H+] is the molarity of hydrogen ions. What is R anyway? Is the constant universal? R is the amount of energy that a mole of particles have at 1 Kelvin. You can think of it as a connection between heat and temperature. It is generally believed to be universally constant, though many of the fundamental constants of the universe may not have been completely constant under crazy conditions like the first picoseconds after the Big Bang. The value is just determined experimentally, and can change depending on the units. How much energy would it take to blow up Earth with a Death Star? According to http://what-if.xkcd.com/141/ the total energy of the sun would seem to do the job. The sun puts out 3.8x10 26 J per second. One second would probably be enough to sterilize the planet, but you d likely need several minutes or even an hour s worth to completely blast it apart. What is the difference between heat and temperature? Heat is the total thermal energy of a substance, while temperature is the average energy. Something can be blazing hot because every molecule has oodles of energy, but it won t have much heat if there are only a couple molecules. For example, why do you take small bites of something when it s hot? To minimize the amount of heat put into the roof of your mouth by keeping the mass small. How do you know when to use mc T + mc T = 0 vs mc T + H*moles = 0, etc? Write the qs first and label them. For example, if I do a reaction in solution and put a hot rock in, I would write: q reaction + q solution + q rock = 0
The solution and the rock are objects, so they get mc T. The reaction is a process (reactions and phase changes), so it gets H*moles. How do you find the ph of a mixture of an acid with a base? By looking at what s left over, and how much. The products are neutral, so what you need to know is how much excess acid or base there is, then find the molarity of that and do normal ph calculations. How do you know when to put a q setup (C cal * T) into a heat calculation? If it s given in the problem, or something specifically asks about it. If not mentioned, perfect heat transfer is assumed. Why does a heat transfer equation with a reaction use H*moles? Because reactions actively make new heat from chemical potential energy, or suck away heat and turn it into chemical potential energy. mc T only works for passive things objects that gain or release heat by contact with something of a different temperature. Objects just move around heat that is already there; reactions change the amount of heat. Phase changes use H*moles just like reactions because it s a process that takes in heat (for boiling) or gives it off (for condensing) without changing temperature. A gaseous molecule of water at 100 o C has more energy than a liquid molecule of water at 100 o C, so it must take heat to go between them, but there s no T. How do you know what a reasonable number of sig figs is? Three. The trick is in recognizing that 0.003 is NOT three significant figures. If your answer is 134.637 g, then call it 135 g (remember this is rounding and not chopping numbers off). If your answer is 0.000134637 g, then call it 0.000135 g. You don t have to be perfect on this--i ll take 130 g or 134.6 g--but three is a good number. I will NOT take 134.637 g unless all the measurements that went into it were super precise. Does having a H vaporization for water change how you solve a problem? Yes and not. You still solving things completely normally, but you need to include a q for boiling the water (which will get H vap *moles substituted in for it). So you would likely have at least three qs: one for the source of heat (either an mc T or H*moles depending on whether your source was a reaction or hot object), one for heating the water up to 100 o C, and one for boiling the water. How do you solve titrations with an unknown acid or base? This is the point of titrations; one solution is known and the other unknown. It doesn t matter which is which. When you hit the equivalence point (ph 7), the moles of acid and moles of base are exactly equal, and therefore both go away completely leaving only a neutral solution. If you know the molarity and volume of one of them, you know how many moles that is. By using that many moles and the volume of the other one, you can find the other concentration. What makes something a strong acid? The ability for the H + to come off completely. 100% of molecules lose theirs. In contrast, a weak acid mostly stays together (often 90% or more). The strong acids we ll use are HCl, HNO 3, and H 2 SO 4.
What s the difference between ionic and covalent compounds when they dissolve? Ionic compounds come apart into individual ions. Covalent compounds are bonded together so they molecule stays intact each molecule just comes apart from the neighbors. How does valence electrons relate to chemical behavior? Everything wants to have 8 valence electrons. Metals achieve this by losing valence electrons (to get back to the previous full level) and making ionic compounds. Non-metals achieve this either by gaining electrons and making negative ions (oxygen has 6 valence so it gains two to become -2) OR by sharing electrons and making bonds (oxygen has 6 valence so it makes two bonds). When to you subtract from 14 to find ph? When you found poh, not ph. If you have a base, when you take the logarithm of concentration, that is the poh, because you took the log of [OH - ]. ph and poh always add up to 14; you just don t need that fact with acids (for example, 0.01 M HCl has a ph of 2 and a poh of 12, but we don t care about the poh). How do you determine the percentage of each element in a compound such as Na 3 PO 4? Using atomic masses. For convenience sake, let s assume we have 1 mole of sodium phosphate. That means we have 163.97 grams. 69 of those grams are sodium, so sodium is 69/163.97 = 42.1%. Phosphorous is 30.97/163.97 = 18.9%. And so on. Will we need the 7f orbitals for electron configuration? The currently largest elements go up to the 7 th row, which is the 7s, 6d, 5f, and 7p orbitals. You should be able to extrapolate a bit beyond the current ones (like, what would the configuration of element 120 be?), but not that far. What are the rules for sig figs? Don t worry so much about the rules as about rounding reasonably. Make sure you are rounding, not cutting off numbers.