3rd exam (global) of the 2nd term for 3 rd ESO F
Name:
Date:
1. In the reaction of combustion of ethane (C2H6) we obtain carbon dioxide (CO2) and water (H2O):
C2H6 + O2 → CO2 + H2O
a) Balance the chemical equation. (0.25 points)
b) If we want 100 g of C2 H6 to react, what is the mass of O2 that we need? (0.75 points)
c) What is the mass of CO2 that we are going to obtain? (0.75 points)
d) What is the mass of H2O that we are going to obtain? (0.75 points)
Data: AM(C)=12 u; AM(H)=1 u; AM(O)=16 u.
2. Nitrogen (N2) reacts with hydrogen (H2) to yield ammonia (NH3).
N2 + H2 → NH3
a) Balance the chemical equation. (0.25 points)
b) If we want to obtain 2.5 moles of NH3, what is the number of molecules of N2 that we need? (0.6
points)
c) What is the number of atoms of N that participate in this chemical reaction? (0.5 points)
d) How many moles of H2 do we need? (0.5 points)
3. Answer the following questions:
 How many electrons is calcium (Ca) going to lose or to gain, if its atomic number is 20? Justify
your answer. Is calcium a metal or a non metal?
 How many electrons is bromine (Br) going to lose or to gain, if its atomic number is 35? Justify
your answer. Is bromine a metal or a non metal?
 What is the type of bond between Ca and Br? What is the empirical formula of the compound
they form? Justify your answer.
 What is the type of bond between the two atoms of Br in the molecule Br2? Justify your answer.
(1.75 points)
4. Explain all you know about the metallic bond: how it is formed, what type of atoms form this bond and
if in this bond crystals, molecules or both structures are formed. (0.75 points)
5. Explain all you know about the covalent bond: how it is formed, what type of atoms form this bond and
if in this bond crystals, molecules or both structures are formed. (0.75 points)
6. Write the formula of the following compounds:
a) aluminium hydroxide
c) lead (IV) oxide
e) hydrogen selenide
(1.2 points)
b) chromium (III) chloride
d) manganese trihydride
f) phosphine
7. Name the following compounds in the nomenclature that is indicated. If nothing is indicated, it means
that we have studied only one way.
a) HF (aq)
b) AuBr3 (stoichiometric)
c) CuH2 (stock)
d) Pt(OH)4 (stock)
e) SiH4
f) SnO (stock)
(1.2 points)
3rd exam (global) of the 2 nd term for 3 rd ESO F (answers)
Name:
Date:
1. In the reaction of combustion of ethane (C 2H6) we obtain carbon dioxide (CO2) and
water (H2O):
C2H6 + O2 → CO 2 + H2O
a) Balance the chemical equation. (0.25 points)
b) If we want 100 g of C2H6 to react, what is the mass of O2 that we need? (0.75 points)
c) What is the mass of CO2 that we are going to obtain? (0.75 points)
d) What is the mass of H2 O that we are going to obtain? (0.75 points)
Data: AM(C)=12 u; AM(H)=1 u; AM(O)=16 u.
a) 2C2H6 + 7O2 → 4CO2 + 6H2O
b) MM(C2H6)=2∙12+6∙1=30 g/mole
MM(O 2)=16∙2=32 g/mole
MM(CO2)=12+16∙2=44 g/mole
MM(H 2O)=1∙2+16=18 g/mole
c)
d)
Another way. We could have used the law of conservation of mass:
mreactants=mproducts
100+373.33=293.33+mH2O
mH2O=100+373.33-293.33=180 g
2. Nitrogen (N2 ) reacts with hydrogen (H2) to yield ammonia (NH3).
N2 + H2 → NH3
a) Balance the chemical equation. (0.25 points)
b) If we want to obtain 2.5 moles of NH3, what is the number of molecules of N 2 that
we need? (0.6 points)
c) What is the number of atoms of N that participate in this chemical reaction? (0.5
points)
d) How many moles of H2 do we need? (0.5 points)
a) N2 + 3H2 → 2NH3
b)
c) 1st way:
2nd way:
d)
3. Answer the following questions:
 How many electrons is calcium (Ca) going to lose or to gain, if its atomic
number is 20? Justify your answer. Is calcium a metal or a non metal?
 How many electrons is bromine (Br) going to lose or to gain, if its atomic
number is 35? Justify your answer. Is bromine a metal or a non metal?
 What is the type of bond between Ca and Br? What is the empirical formula of
the compound they form? Justify your answer.
 What is the type of bond between the two atoms of Br in the molecule Br 2?
Justify your answer.
(1.75 points)
Ca: 1s2 2s2 2p6 3s2 3p6 4s2
It loses 2 electrons to acquire the electron configuration of a noble gas.
Ca2+: 1s2 2s2 2p6 3s2 3p6 (this is the electron configuration of Ar).
As Ca loses electrons, it is a metal.
Br: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5
It gains 1 electron to acquire the electron configuration of a noble gas.
Br-: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 (this is the electron configuration of Kr).
As Br gains electrons, it is a non metal.
As Ca is going to lose electrons and Br is going to gain them, they form an ionic bond.
Each atom of Ca loses two electrons that go to two atoms of Br (one electron to each
atom) so the empirical formula is CaBr2 (for each atom of Ca there are two atoms of
Br).
In the molecule of Br2 both atoms of bromine want to gain one electron, so they share a
pair of electrons. They form a covalent bond.
4. Explain all you know about the metallic bond: how it is formed, what type of atoms
form this bond and if in this bond crystals, molecules or both structures are formed.
(0.75 points)
Metals tend to lose electrons. When a lot of atoms of a metal are together, they all lose
electrons: there is a crystal formed by cations with the lost electrons moving throughout
the crystal, forming a sea of electrons.
5. Explain all you know about the covalent bond: how it is formed, what type of atoms
form this bond and if in this bond crystals, molecules or both structures are formed.
(0.75 points)
Non metals tend to gain electrons, so when two non metals are together, they both can
gain electrons by sharing them. That´s the covalent bond: non metals sharing pairs of
electrons. The number of pairs of electrons that an atom shares is equal to the number
of electrons that it wants to gain. Covalent compounds can form molecules or crystals.
6. Write the formula of the following compounds:
a) aluminium hydroxide
b) chromium (III) chloride
Al(OH)3
CrCl3
c) lead (IV) oxide
d) manganese trihydride
PbO2
MnH3
e) hydrogen selenide
f) phosphine
H2Se
PH 3
(1.2 points)
7. Name the following compounds in the nomenclature that is indicated. If nothing is
indicated, it means that we have studied only one way.
a) HF (aq)
b) AuBr3 (stoichiometric)
hydrofluoric acid
gold tribromide
c) CuH2 (stock)
d) Pt(OH)4 (stock)
copper (II) hydride
platinum (IV) hydroxide
e) SiH4
f) SnO (stock)
silane
tin (II) oxide
(1.2 points)