I got a 680 on sat II chem practice?
so uh i have a few questions
you dont need to answer them all
1. A conjugate acid/base pair w/ diff. spectral absorbencies
a) chemical pH indicator
b) acid/base buffer
the book says A, but why not also B? isnt a buffer made from conjugates?
2. Is 0 for a crystalline solid that is elementally pure at 0K
b) entropy
c) enthalpy
I understand why the book says entropy, but why isnt enthalpy also right? at 0 K, shouldnt enthalpy be 0?
3. A volatile covalent liquid at 25C and 1 atm
a) N2
b) KI
c) CCl4
d) AgNO3
e) CaCO3
I blanked out here...answer is c. any explanations?
4. Choose the best answer below that accurately describes the correct molecular shape for the molecule, XeOF4
a) tetrahedral
b) trigonal pyramidal
c) trigonal bypyramidal
d) square pyramidal
e) flat
when it asks for molecular shape, do i take into account lone pairs or not? and what is the term for the opposite (when i do count lone pairs)
5. highest ionization energy
b) C
c) Cl
ans is C, plz explain
Answer:
Number 1) buffers are made from weak acids and their conjugate bases, it doesn't specify that you are using a weak acid, so I would hesitate to think it is a buffer for that very reason. Also, spectral absorbancies means there is color involved in there somewhere - which is what we use an indicator for - it will change color for you!
2.) Enthalpy is always measured from a difference, entropy measurements are absolute values. The definition of a perfect crystal is one whose entropy is zero at 0K. At oK, nothing is moving so there is no disorder. By definition, delta H values are measured differences. The only species that have 0 delta H values are elements in their natural state
3.) N2 is a gas (know your gases, H2, N2, O2, F2 and thte noble gases. KI is an ionic compound, so it isn't covalent (ionic = metal + non-metal or metal + polyatomic ion) CCl4 is covalent (two non-metals bonded together) AgNO3 is ionic (metal + polyatmoc nitrate ion) and CaCO3 is also ionic (Carbonate ion) so the answer has to be C
Covalent species: non-metals bonded together
Ionic: metal + nonmetal (e.g. NaCl) or metal + polyatomic ion (like carbonate, sulfate, nitrate, and so on, know your polyatomic ions!)
4.) Xe brings 8 valence electrons (group 8), F brings 7 valence electrons (and there are 4 of t hem so a total of 28 electrons) and O bring 6 valence electrons
Put Xe in the middle. Surround the Xe with the 4 F's (no F-F bonds, no O-F bonds) and then put the O on the Xe. Fill in with l.p electrons around each F and O (all will have one bond and 3 lone pairs. Even if you double bond the O to the Xe, you will still have 1 lone pair also needing to be added to the central Xe atom in order to shove 42 electrons on the structure. That means that Xe has 6 electrons domains. But one of them is a lone pair. 6 electron domains with 1 l.p. means octahedral (VSEPR shape) and square pyramidal for molecular shape.
Ionization energy means the ability to lose an electron. The harder it is to lose and electron, the higher the ionization energy. The more protons there are, the more electrons. The more electrons there are, the farther those outer electrons are from the nucleus and the easier it is to remove the outer electron. C's electrons are a LOT closer to the nucleus than are Cl's so it is easier to take an electron from Cl or harder to take one from C
1) Although a buffer is indeed made up of an acid and it's conjugate base a buffer's components DO NOT have to have different spectral absorbencies. This, however, is vital for a pH indicator. You see, as the pH goes up or down, the concentration of the acid/base will change and therefore the color of the solution will change since the acid and base have different color absorbencies.
2) Third law of thermodynamics - for any pure crystalline solid, at absolute zero (0 K) there is no disorder (entropy)
3) N2 (nitrogen) is a gas, calcium carbonate (CaCO3) is a solid, and neither KI (potassium iodide) nor AgNO3 (silver nitrate) are volatile. CCl4 (chloroform) is a volatile organic solvent.
4) Shape does account for lone pairs
5) Chlorine has more total electrons, so the electrons in the outermost shell are farther from the nucleus and thus the electrostatic interactions between the electrons and the nucleus are stronger. Therefore, it requires more energy to pull an e- from chlorine than it does from carbon.
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