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7/17/2019 Homework 4
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HOMEWORK 04
1. What is a Lewis dot symbol? To what elements does the
symbol mainly apply?It is a way to represent chemical formulas, you use the symbol of the
element and the electrons of valence to represent the relationships
among the elements that form a compound. It is mainly applied for
Alkaline, alkaline earth, metals of transition non-metals and noble
gases. (Chang, 2!"
2. Use the second member of each group from Group 1A to
Group A to show that the number of !alence electrons on an
atom of the element is the same as its group number.
#he number of
electrons of valence has increased according to the respective group
number". Write Lewis dot symbols for atoms of the following elements#
$a% &e' $b% (' $c% )a' $d% Ga' $e% *' $f % &r' $g% +' $h% ,' $i% As' $ -%
.
/. According to the following isoelectronic serie# +a01' g02' Al0"
. Which ion is the biggest?$a%! is the biggest because the atomic rate decreases according to
the families from left to right. (Chang, 2!"
. 34plain what an ionic bond is.It is the electrostatic attraction between two elements, that involves a
complete transference of electrons of valence, ending electricallycharged. #heir most common structure are crystals, (such as common
salt". #hey tend to ioni&e in presence of water producing a'ueous
solutions. (Chang, 2!"
5. 34plain how ioni6ation energy and electron a7nity determine
whether atoms of elements will combine to form ionic
compounds. #he elements with low ioni&ation energy are more likely to form
cations, and a high electron anity determines if an element could
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receive electrons to form anions, then, the electrons transference is
easier in element with low ioni&ation energy and high electron anity.
(Chang, 2!"
. +ame 8!e metals and 8!e nonmetals that are !ery li9ely toform ionic compounds. Write formulas for compounds that
might result from the combination of these metals and
nonmetals. +ame these compounds.$a, i3 +e, Ca, 4r5, Cl, I, 4, #e$a5 sodium 6uoride, iCl lithium chloride, +e4, beryllium sulphide,
4rI2 strontium iodide, Ca#e calcium telluride.:. +ame one ionic compound that contains only non;metallic
elements.Ammonium chloride $17Cl
<. +ame one ionic compound that contains a polyatomic cation
and a polyatomic anion($17"2C*89 Ammonium Carbonate18:*79 :hosphoric Acid
1=. 34plain why ions with charges greater than " are seldom
found in ionic compounds. #he elements with high ioni&ation energy are unlikely to form ions, so
the transference of more than three electrons, involves an
e;penditure of high 'uantities of energy and the compound tends to
be more unstable. (Chang, 2!"
11. What is the ad!antage of using the term >molar mass
when we discuss ionic compounds?Ionic compound doesn<t form really moles, so we cannot talk about
molar mass, but about mass of formula because they are only ions
electrically e;tracted. (Chang, 2!"!2.,n which of the following states would +a)l be electrically
conducting? (a" 4olid, (b" molten (that is, melted", (c" dissolved in
water. 0;plain your answers.=hen an ionic compound is dissolved in water it is divided in its ions,
which are electrically charged and can conduct electricity. 4o the
answer is C.
1". &eryllium forms a compound with chlorine that has the
empirical formula &e)l2. @ow would you determine whether it
is an ionic compound? $The compound is not soluble in water%. #he subtraction between their radioactive values is generally a way to
determine if this compound is ionic or covalent, where if the result is
>2 it is an ionic compound. #he solid structure also, in which, the
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ionic compound is common presented in a cubic form. (Chang,
2!"1/. Gi!e the empirical formulas and names of the
compounds formed from the following pairs of ions# $a% b0
and ,;' $b% )s0 and B*/ 2;' $c% Br20 and +";' $d% Al"0 and B2;.
I ubidium Iodide, Cs24*7 Cesium sulphate, 4r8$2 4trontium $itride,
Al248 aluminium sulphide.!?./se ewis dot symbols to show the transfer of electrons between the
following atoms to form cations and anions9 (a" $a and 5, (b" @ and 4,
(c" +a and *, (d" Al and $.
15. Write the Lewis dot symbols of the reactants and
products in the following reactions. $irst balance the
eCuations.%
1. or each of the following pairs of elements' state
whether the binary compound they form is li9ely to be ionic or
co!alent. Write the empirical formula and name of the
compound# $a% , and )l' $b% g and .a" Iodine chloride9 Cll, covalentb" g529 magnesium 6uoride, ionic
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1:. or each of the following pairs of elements' state
whether the binary compound they form is li9ely to be ionic or
co!alent. Write the empirical formula and name of the
compound# $a% & and ' $b% ( and &r.
a" +5 +oron tri6uoride9 Covalentb" @+r :otassium +romide9 Ionic
1<. What is lattice energy and what role does it play in the
stability of ionic compounds?attice energy is the measure that shows how stable are ionic
compounds, also dened as the necessary energy to divide a ionic
bond, then, the energy used to ioni&e a compound and the energy
used by the anion when it capture the electrons must be lower than
the lattice energy to obtain a stable compound, that is the reasonbecause elements cannot lose more or less than the electrons they
should, producing the ionic compound that we actually know. (Chang,
2!"2=. 34plain how the lattice energy of an ionic compound
such as ()l can be determined using the &orn;@aber cycle. *n
what law is this procedure based? #he +orn-1aber cycle is based on the 1ess law, and we can use it to
calculate lattice energy, relating it with ioni&ation energy, electron
anity, among other properties.5irst measure ne energy re'uired to sublimate the @, second the
energy necessary to ioni&e the molecular Cl, third, the resultant
energy of ioni&e the @, fourth the necessary energy to Boin the
electrons to the anion, this total we subtract from the enthalpy of the
overall reaction, and the result is the lattice energy. (Chang, 2!"
2!.Calculate the lattice energy of calcium chloride given that the heat of
sublimation of Ca is !2! k)mol and D1Ef (CaCl2" F - GH? k)mol. (4ee #ables .2 and .8 for other data."!. Cas Cag F!2! k)mol2. Cl2 2Cl F272. k)mol8. Ca Ca% % e- F?H.? k)mol
Ca% Ca2% % e- F!!7? k)mol
7. 2(Cl % e- Cl- " F-JH k)mol
?. Ca2% % 2 Cl- FK D1Eoverall F-GH? k)mol!%!%8%7%?F D1Eoverall?F2!H? k)mol
22. Use an e4ample to illustrate each of the following terms#lone pairs' Lewis structure' the octet rule' bond length.
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ewis structure represents the valence electrons of chemical
elements and how it could get related to others.
Atoms get stability when their more e;ternal layer is complete with
electrons (with e;ception of 1e and 1" so they form diLerent types of
bond to gain that stability.
In a molecule the distance between the nuclei of each atom is called
bond length and it may vary.
=hen only an electron is shared by each atom it is called lonely pair
(Chang, 2!"2". )ompare the properties of ionic compounds and
co!alent compounds. #he structures Boined by ionic bond are solid, have high melting
points, present cubic assemblies, their unions are strong and can form
strong electrolytes, while the molecules composed by covalent bonds
can be li'uid, gas or solid at room temperatures, do not have the
same structure, have low melting points and are weak or non-
electrolytic substances. (Chang, 2!"
27.The following Lewis structures for $a% @)+' $b% )2@2' $c% Bn*2'
$d% &"' $e% @*' $f % @)*' and $g% +" are incorrect. 0;plain
what is wrong with each one and give a correct structure for the
molecule. (elative positions of atoms are shown correctly."
A9 $itrogen octet incomplete.+9 1ydrogen cannot form double
bonds
C9 4n octet was incomplete.
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M9 + does not have ? electrons of valence
09 6uorine cannot receive 2 electrons
59 o;ygen octet is not complete
N9 $ has ve electrons of
Oalence (Chang, 2!"
2. 34plain why the bond enthalpy of a molecule is usually
de8ned in terms of a gas;phase reaction. Why are bond;brea9ing processes always endothermic and bond;forming
processes always e4othermic?In the gas phase is seldom that the surrounding molecules could
interfere in the division of a molecule of the product and alter the net
energy necessary to divide it. #he molecule structure needs energy to be broken, otherwise could
be easily destroyed, ergo, an e;tra energy will be necessary to
destroy a chemical bond. And based on the rst thermo dynamical
law, if the destruction involves absorption of energy the creation must
releases it. (Chang, 2!"
25. @ow is the geometry of a molecule de8nition and why is
the study of molecular geometry important? #he geometry of a molecule is dened by its three-dimensional
structure, specically, the position of the atoms in it, as the length
and angles among them. It is important because the molecule
structure outline how its properties will be. (Chang, 2!"2. De8ne dipole moment. What are the units and symbol
for dipole moment?It is the 'uantity to measure the polarity of a bond, it is e;pressed by
the formula M =Q∗r
=here is the product of the charges and r the distance between
them. Its unit is the Mebye. (Chang, 2!"
&ibliography
Chang, . (2!". Chemistry (! 0dition". $ew Pork9 cNraw-1ill.
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