APChem Big Idea 1 – Flashcards
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Unlock answers| Ratio of Masses in a Pure Sample |
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Question: A 4.5 gram sample of which of the following would have the greatest mass percent of oxygen? Li2O (molar mass = 30 g/mol) MgO (molar mass = 40 g/mol) SrO (molar mass = 104 g/mol) Na2O (molar mass = 62 g/mol) |
Answer: A. 16/30 x 100 = 53% B. 16/40 x 100 = 40% C. 16/104 x 100 = 15% D. 16/62 x 100 = 26 % |
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Impurities in a substance can change the percent composition by mass If more of a certain element is added from an impurity, then the percent mass of that element will increase andvice versa When heating a hydrate, the substance is heated several times to ensure the water is driven off Then you are simply left with the pure substance and no excess water |
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Percent mass can be used to determine the composition of a substance % mass can also be used to find the empirical formula The empirical formula is the simplest formula of a substance It is a ratio between the moles of each element in the substance Quick steps to solve!
The molecular formula is the actual formula of a substance It is a whole number multiple of the empirical formula |
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How many moles of carbon are in 88 grams of propand, C3H8?
Molar mass = 44g/mol 2moles of propane in 88g 3 moles of C in 1 mole of propane 2*3 = 6 moles of C |
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1stIonization Energy (IE) indicates the strength of the coulombic attraction of the outermost, easiest to remove, electron to the nucleus: X(g) + IE X+(g) + e– 1st IE generally increases across a period and decreases down a group
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2nd & subsequent IE’s increase as coulombic attraction of remaining e–’s to nucleus increases X+ + IE X2++ e– X2++ IE X3++ e–
Large jump in IE when removing less-shielded core electrons |
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1st Ionization Energy Energy (IE) decreases from Be to B and Mg to Al Electron in 2p or 3p shielded by 2s2 or 3s2 electrons, decreasing coulombic attraction despite additional proton in nucleus. Same effect seen in 3d10-4p, 4d10-5p and 5d10-6p 1st Ionization Energy decreases from N to O and P to S
np4 contains first paired p electrons, e–-e– repulsion decreases coulombic attraction despite additional proton |
| Chemical Reactivity Using Periodic Trends |
Nonmetals have higher electronegativities than metals --> causes the formation of ionic solids Compounds formed between nonmetals are molecular Usually gases, liquids, or volatile solids at room temperature Elements in the 3rd period and below can accommodate a larger number of bonds The first element in a group (upper most element of a group) forms pi bonds more easily (most significant in 2nd row, non-metals) Accounts for stronger bonds in molecules containing these elements Major factor in determining the structures of compounds formed from these elements Elements in periods 3-6 tend to form only single bonds
Reactivity tends to increase as you go down a group for metals and up a group for non-metals. |
Question: Of the elements below, _____ reacts the most violently with water. A. Na B. Ba C. Ca D. Cs E. Mg |
| Cesium--largest atomic radius, 1 v.e. most loosely held (weak coulombic attraction) |
Question: Which of the following groups from the periodic table would be the easiest to oxidize? A. Halogens B. Transition Metals C. Alkali Metals D. Alkaline Earth Metals |
Answer: Alkali metals--oxidation involves the loss of electrons, so the group of elements that has the lowest ionization energy would be the easiest to oxidize.
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Group 1 metals more reactive than group 2 metals Reactivity increases as you go down a group Metals on left form basic oxides Ex. Na2O + H2O > 2 NaOH Nonmetals on right form form acidic oxides Ex. SO3 + H2O > H2SO4 Elements in the middle, like Al, Ga, etc can behave amphoterically
If SiO2 can be a ceramic then SnO2 may be as well since both in the same group |
[image] Classic Shell Model vs. Quantum |
Developed by Schrodinger and the position of an electron is now represented by a wave equation Most probable place of finding an electron is called an ORBITAL (90% probability) Each orbital can only hold 2 electrons with opposing spins (S, P, D & F orbitals) Evidence for this theory: Work of DeBroglie and Planck that electron had wavelike characteristics Heisenberg Uncertainty Principle - impossible to predict exact location of electron- contradicted Bohr
This new evidence caused the Shell Theory to be replaced by the Quantum Mechanical Model of the atom |
[image] Shell Model consistent with IE data |
The patterns shown by the IE graph can be explained by Coulomb’s law As atomic number increases, would expect the ionization energy to constantly increase Graph shows that this is NOT observed. WHY NOT? The data implies that a shell becomes full at the end of each period Therefore the next electron added must be in a new shell farther away from the nucleus.
This is supported by the fact that the ionization energy drops despite the addition positive charge in the nucleus |
Using Spectroscopy to measure properties associated with vibrational or electronic motions of molecules [image] |
IR Radiation - detects different types of bonds by analyzing molecular vibrations UV or X-Ray Radiation Photoelectron Spectroscopy(PES) Causes electron transitions Transitions provides info on electron configurations |
Beer's Law-used to measure the concentration of colored solutions [image] |
A = abc A = absorbance a = molar absorptivity (constant for material being tested) b = path length (cuvette = 1 cm) c = concentration (Taken at fixed wavelength) |
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Answer: Based on Beer's Law, for small concentration ranges there is a linear relationship between absorbance and concentration. For an absorbance of 0.4, the corresponding concentration is 0.28M. |
Stoichiometry g--> mole: use the periodic table mole-->mole: use the BALANCED equation mole--> g: use the periodic table |
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Gravimetric Analysis [image] |
Buchner Filtration Apparatus How much lead (Pb2+) in water? Pb2+(aq) + 2Cl-(aq) > PbCl2 (s) By adding excess Cl- to the sample, all of the Pb2+ will precipitate as PbCl2 Solid product is filtered using a Buchner Filter and then dried to remove all water Mass of PbCl2is then determined
This can be used to calculate the original amount of lead in the water |
