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# Thermochemistry and Periodic Properties of Elements

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1. State which atom is larger according to periodic trends. Explain the difference for each of the pairs.
a) Br, F
b) Ca, Mg
c) C, Si
d) He, Ar

2. State which atom has a smaller first ionization energy according to periodic trends. Explain the difference in ionization energy for the pairs.
a) O, S
b) K, Na
c) Cd, Zn
d) Al, Ga

3. Which of the following is smaller? Explain the trend.
a) Mg, Mg2+
b) Al, Al3+
c) Cs, Cs+

4. Arrange in order of increasing size and explain the trend.
a) Fe, Fe 2+, Fe3+
b) Ti, Ti3+, Ti4+

5. Arrange in order of increasing size:
Br-, Ca2+, Cl-, F-, Li+, Mg2+, N3-, Na+, O2-, Rb+, S2-

6. Given the following data:
2NO(g) ---> N2(g) + O2(g) (delta)H= -180.6 kJ
N2(g) + O2(g) + Cl2(g) -----> 2NOCl(g) (delta)H= +103.4 kJ
Find the ?H for the following reaction:
2NOCl(g) ----> 2NO(g) + Cl2 (g)

7. During the discharge of a lead-acid storage battery, the following chemical reaction takes place:
Pb + PbO2 + 2H2SO4 ----> 2PbSO4 + 2H2O
Using the following two reactions:
Pb + PbO2 + 2SO3 ------> 2PbSO4 (delta)H(degrees)= -775 kJ
SO3 + H2O ---> H2SO4 (delta)H(degrees)= -113 kJ

Determine the enthalpy of the reaction for the discharge reaction above.

*The delta triangle symbol and the degrees symbol are written out.

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The solution examines thermochemistry and periodic properties of elements.

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1.
If the atoms share the same group (column) in the periodic table, the larger the atomic number (the number of protons), the larger the atom.
(a)
Br (Bromine) atom is larger than F (Fluorine) atom.
Bromine has 35 protons in its nucleus (and 35 electrons around it)
Fluorine has 9 protons and 9 electrons.
They both share the same column in the periodic table (17), and as we go down (increasing the number of the period) the size of the atoms increases.

(b)
Ca (Calcium) atom is larger than Mg (Magnesium) atom.
Calcium has 20 protons in its nucleus.
They both share the same column in the periodic table (2).

(c)
Si (silicon) atom is larger than C (carbon) atom.
Carbon has 6 protons in its nucleus.
Silicon has 14 protons.
They both share the same column in the periodic table (14).

(d)
Ar (Argon) atom is larger than He (Helium) atom.
Helium has 2 protons in its nucleus.
Argon has 10 protons
They both share the same column in the periodic table (18)

2.
First ionization energy is related to the outer shell (valence) electrons affinity. it measures how easy it is to remove an electron from the atom.
As the number of the group (column) increases, so is the electrons affinity increases, so it is harder to pull an electron out.
Alkali metals (group 1) have only one valence electron and they readily give it away
Noble gasses (group 18), on the other hand, have 8 electrons in ...

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