Answers:
1. a) 3 Br2 + S2O42- + 4 H2O → 2 SO42- + 6 Br- + 8 H+
Br2 + S2O42- + H2O → SO42- + Br- + H+
Break this into
Br2 → Br- and S2O42- → SO42-
Br2 → 2 Br- (balance Br) and S2O42- → 2 SO42- (balance S)
2 e- + Br2 → 2 Br- (balance e-) and S2O42- + 4H2O → 2 SO42- (balance O)
2 e- + Br2 → 2 Br- (balanced) and S2O42- + 4H2O → 2 SO42- + 8H+ (balance H)
2 e- + Br2 → 2 Br- (balanced) S2O42- + 4H2O → 2 SO42- + 8H+ + 6e- (balance e-)
6 e- + 3Br2 → 6 Br- (multiply by 3) S2O42- + 4H2O → 2 SO42- + 8H+ + 6e- balanced)
Add them together to get the equation above.
2. ii) and iv)
i) The uncertainty principle makes the exact position of an electron unobservable.
ii) The wave length, and hence the frequency, of the radiation can be found with a spectroscope.
iii) The path of the electron is unobservable for the same reason as in (i).
iv) The ionization energy of hydrogen, and of other atoms, is observable.
v) Since the path of an electron is unobservable, the radius of its orbit must be unobservable.
3. 1s2 2s2 2p6 3s2 3p6 4s2 3d5
4. Na2O2, MgO, NaF, XeF4 (or XeF2, XeF6), KO2 are the final products.
5. Hlatt = +787.5 kJ/mol for NaCl
6. (a) (b) (c) (d) (e)
7. C = [He]2s22p2, N = [He]2s22p3, O = [He]2s22p4
The electron affinity for C and O is substantial because Zeff is large (Zeff increases from left to right within a period). Both C and O seek more stable electron configurations. C can gain one electron and have a half filled 2p subshell. O can gain an electron and get closer to having a completed 2p subshell. Because a half filled shell is more stable than all other configurations except a filled shell, N does not want to gain an electron and lose stability.
8. a) Write a balanced net ionic equation for
2 AgNO3(aq) + Na2CrO4(aq) → Ag2CrO4(s) + 2 NaNO3(aq)
2 Ag+(aq) + CrO42-(aq) → Ag2CrO4(s)
b) Aqueous hydrochloric acid reacts with magnesium metal to yield hydrogen gas and aqueous magnesium chloride. Write a balanced net ionic equation for this process.
2H+(aq) + Mg(s) → H2(g) + Mg2+ (aq)
9. a) State which of the following combinations of quantum numbers are not allowed. Explain your answers.
a. n = 3, l = 0, ml = -1
not allowed. |ml| > l
b. n = 3, l = 1, ml = 1
fine
c. n = 4, l = 4, ml = 0
not allowed. l = n which is not allowed.
b) What would be a possible set for the four quantum numbers of the 53rd electron added to complete the ground state configuration of I (i.e., iodine)?
(5, 1, 0, ½) This electron should go into a p orbital so l = 1
10. a) State the number of unpaired electrons in O, Si, K and As. (2, 2, 1, 3)
b) Order the following atoms according to increasing atomic radius: S, F, O (F < O < S)
c) Do ionization energies have a positive or negative sign? Explain. +ve (endothermic) since you need to put energy into breaking off the electron. See number 5.
Bonus::: 64% Cl and Ca is the alkaline earth metal. (necessary to know the ratio).
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