homework 4

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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homework 4

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