Chemistry 20A – 2002. Review of key concepts from high school physics and chemistry Outline: Physics • Atomic building blocks (subatomic particles): Electrons, protons, neutrons. Isotopes. • Relation between macroscopic and microscopic - mole, units. Chemistry • Molecular and Empirical formulas • Stoichiometry -------------------------------------------------------------------------------------------------------------------------------------------------------------------- Physics: While chemistry may often seem like it is "stamp collecting" it is really governed by a few simple principles. The most fundamental are mass conservation and energy conservation. Mass conservation is useful in balancing atomic equations. Energy conservation is very important since it tells us that energy goes somewhere. For example, in a reaction 2H2 + O2 ---> 2H2O, the potential energy in the reagents (what you start from -- the left side of the equations) is converted into a potential energy of the products (water), but some energy remains (i.e., the potential energy of the products is lower than the potential energy of the reactants). This extra energy is released in the form of kinetic energy, i.e., some heat is released (a more accurate statement would be formulated in 110A-- thermodynamics, but this is good enough for our present purposes). There is one extra principle, which we would not tackle at this stage, i.e., that entropy increases -- or in less fancy words, that disorder increases. Armed with these 3 principles, it is possible to understand (and sometimes to predict) the outcome (reagents remaining or products formed) of much of chemical reactions, and to understand the state that the products are in, i.e., whether they are gas, liquid, solid, etc. The total energy is made of two parts: Kinetic energy and potential energy: E = K.E. + V Where K.E. = 1/2 m v**2, while V is the potential energy. The electrons are spread out over a region (more or less spherical ) of size 1- 3 Å ()1Å = 1 Angström = 10-10 m . Quantum Mechanics (later) triumphed in predicting this size - we can now see that it is true using special microscopes that can sample a single molecule. So: The spread-out electrons determine size of atoms. The electrons are very light compared to protons 1836pemm≅and me ~ 10-30 kg On a microscopic scale, electromagnetic forces are much stronger than gravitational forces. But gravity always has the same sign (always attracts) while electrostatic forces can cancel (attract and repel). For this reason gravity is stronger on a macroscopic scale.Each of the subatomic particles has a different role in the chemistry. The identity of an element is determined by Z, which denotes the number of protons. The number of neutrons may vary; this leads to the phenomena of isotopes: an element whose nuclei have the same number of protons, but vary in the number of neutrons. The effect of the neutrons is not felt beyond the nucleus; that’s why they don’t directly influence the chemistry (but they can be useful tools). Examples: Hydrogen: Z=1, but Nn = 0, 1, 2, 1H : ()~ 99% p 2H : ()D Used in nuclear reactors. n p 3H : ()T Radioactive trace. Hydrogen bombs. p n n Carbon: Z=6, Nn = 6, 7, 8 12C (mot common) 13C (important medically for MRI’s) 14C (radioactive dating) This is what you need to know now about subatomic particles. Moles: Macroscopic samples contain 1020 or more particles. We therefore count these “things” (atoms or molecules) using the concept of a mole (~ 6 • 1023 particles), since we don’t want to carry factors of 1020 in the equations. Note the motivation for this rather arbitrary choice of 6 • 1023, and not, say, 1024 or 8 • 1023: 1 gm of H atoms (which exist as separate atoms and not as H2 molecules only at high temp) contains 1 mole of atoms We define ~ 6 • 1023 as Avogadro’s Number, and say that a mole contains Avogadro’s number of things. I.e., 1 gm of H atoms contains 1 mole of atoms. Question: how many moles does one gram of hydrogen contains at room pressure and temperature ? Answer – only ½ mole! (since at room pressure hydrogen is in H2 form!) Atomic and Molecular Masses Units: macroscopically gram (g) and kilograms (kg). []Don’t confuse g with gravitational acceleration! . But in real life it is much easier to use 1 amu, defined (approximately) as the mass of one proton (or one hydrogen) 1 amu = 1AvoggN~ 1.7 • 10-24 g Now measuring masses is much more convenient: m()H2O = 2 • 1 + 16 = 18 amu (not exactly, but almost) We can rewrite this as m()H2O = 18 gmol = 18 g mol-1 High school chemistry - formulas and stoichiometry Now we’ll review some simple aspects of high school chemistry (which you do need to know!): Molecular formulas: Tells the number of atoms in a molecule. Example: Methanol. The molecular formula is CH4O. This formula is not very useful, since it does not indicate the structure. In practice, chemists would therefore often write the formula in a more transparent way: (for methanol it would be: CH3OH). With practice it is possible to get the structure for many molecules from this type of written formula. 2Empirical Formulas and percentage compositionEmpirical formula is the simplest formula for a compound, but, like molecular formula, it lacks structure details. Example: hydrogen peroxide H2O2 : molecular formula HO : empirical formula For water: H2O : is both the empirical and molecular formula The empirical formula is very easy to determine - just pick a sample of the compound and weight how many moles of each compound there are. Example: We find a gas sample containing 36g of C and 96g of O. Determine: • percent C by mass • percent C by moles • Empirical formula • Molecular formula Answer • Percent C by mass: 3696 36ggg+= 36132= 0.27 = 27% • Percent C by moles: First, use MC = 12 a.m.u. and MO = 16 a.m.u.: n ()C = 36g12 g/mol = 3 mol n ()O = 9616 /ggmol= 6 mol The ratio is: n ()Cn ()O = 0.5 = 1/2 And, Percent C ()by moles = 33 + 6 ≅ 0.33 • Thus, the empirical formula is: CO2 • Molecular formula cannot be determined on this basis Stoichiometry Balancing chemical equations. Example: C2H4 + xO2 → yCO2 + zH2O ()We pick for convenience