Second exam of the second term for 2nd ESO F
Name:
Date:
1. Balance the following chemical equations:
a) Al + Cl2 → AlCl3
b) C5H12 + O2 → CO2 + H2O
c) HBr → H2 + Br2
d) O2 + HCl → Cl2 + H2O
e) Mg + O2 → MgO
f) PbO + C → Pb + CO2
The elements that appear in these chemical equations are:
Al: aluminium
Cl: chlorine
C: carbon
H: hydrogen
O: oxygen
Br: bromine
Mg: magnesium
Pb: lead
(3 points)
2. Ammonia (NH3) reacts with oxygen in order to obtain nitrogen dioxide (NO2) and water (H2O),
according to the following chemical equation, which is already balanced:
4 NH3 + 7 O2 → 4 NO2 + 6 H2O
The elements that appear in this chemical equation are:
N: nitrogen
(the rest are elements that appear in exercise 1)
If 40000 molecules of NH3 react, what is the number of molecules of O2 they are going to react with?
(1 point)
3. Phosphorus tribromide (PBr3) reacts with water (H2O) in order to obtain hydrobromic acid (HBr) and
phosphorous acid (H3PO3).
PBr3 + 3 H2O → 3 HBr + H3PO3
(the chemical equation is already balanced)
The elements that appear in this chemical equation are:
P: phosphorus
(the rest are elements that appear in exercise 1)
We know that 270.7 g of PBr3 react with 54 g of H2O. If we obtain 242.7 g of HBr, what is the mass of
H3PO3 that you are going to obtain? What is the law you are using?
(1 point)
4. When calcium carbonate (CaCO3) decomposes, we obtain calcium oxide (CaO) and carbon dioxide
(CO2), according to the following chemical equation, which is already balanced:
CaCO3 → CaO + CO2
The elements that appear in this chemical equation are:
Ca: calcium
(the rest are elements that appear in exercise 1)
If 100 g of CaCO3 decompose, we obtain 56 g of CaO and 44 g of CO2.
If 200 g of CaCO3 decompose:
a) What is the mass of CaO that you are going to obtain? What is the law you have used to answer this
question? (1 point)
b) What is the mass of CO2 that you are going to obtain? What is the law you have used to answer this
question? (1 point)
5. Explain all you know about acid rain. How is it caused? How does it affect the planet? (1 point)
6. Explain all you know about the destruction of the ozone layer. How is it caused? How does it affect the
planet? (1 point)
7. In exercise 1b you have to balance a type of chemical reaction that is called combustion. In these types
of chemical reactions, we obtain energy (which we can use). A product of this reaction is carbon dioxide
(CO2). Explain the environmental problem that the excess of CO2 in the atmosphere causes to the planet.
(1 point)
Second exam of the second term for 2nd ESO F (answers)
Name:
Date:
1. Balance the following chemical equations:
a) 2Al + 3Cl2 → 2AlCl3
b) C5H12 + 8O2 → 5CO2 + 6H2O
c) 2HBr → H2 + Br2
d) O2 + 4HCl → 2Cl2 + 2H2O
e) 2Mg + O2 → 2MgO
f) 2PbO + C → 2Pb + CO2
The elements that appear in these chemical equations are:
Al: aluminium
Cl: chlorine
C: carbon
H: hydrogen
O: oxygen
Br: bromine
Mg: magnesium
Pb: lead
(3 points)
2. Ammonia (NH3) reacts with oxygen in order to obtain nitrogen dioxide (NO2) and water (H2O),
according to the following chemical equation, which is already balanced:
4 NH3 + 7 O2 → 4 NO2 + 6 H2O
The elements that appear in this chemical equation are:
N: nitrogen
(the rest are elements that appear in exercise 1)
If 40000 molecules of NH3 react, what is the number of molecules of O2 they are going to react with?
(1 point)
3. Phosphorus tribromide (PBr3) reacts with water (H2O) in order to obtain hydrobromic acid (HBr) and
phosphorous acid (H3PO3).
PBr3 + 3 H2O → 3 HBr + H3PO3
(the chemical equation is already balanced)
The elements that appear in this chemical equation are:
P: phosphorus
(the rest are elements that appear in exercise 1)
We know that 270.7 g of PBr3 react with 54 g of H2O. If we obtain 242.7 g of HBr, what is the mass of
H3PO3 that you are going to obtain? What is the law you are using?
(1 point)
mreactants=mproducts
270.7+54=242.7+x
x=270.7+54-242.7=82 g=mH3PO3
I have used the law of conservation of mass.
4. When calcium carbonate (CaCO3) decomposes, we obtain calcium oxide (CaO) and carbon dioxide
(CO2), according to the following chemical equation, which is already balanced:
CaCO3 → CaO + CO2
The elements that appear in this chemical equation are:
Ca: calcium
(the rest are elements that appear in exercise 1)
If 100 g of CaCO3 decompose, we obtain 56 g of CaO and 44 g of CO2.
If 200 g of CaCO3 decompose:
a) What is the mass of CaO that you are going to obtain? What is the law you have used to answer this
question? (1 point)
I have used the law of definite proportions.
b) What is the mass of CO2 that you are going to obtain? What is the law you have used to answer this
question? (1 point)
1st way:
I have used the law of definite proportions.
2nd way:
mreactants=mproducts
200=112+x
x=200-112=88 g
I have used the law of conservation of mass.
5. Explain all you know about acid rain. How is it caused? How does it affect the planet? (1 point)
Oxides of sulfur and nitrogen (pollution caused by humans) react with water vapour in the atmosphere
producing corrosive acids such as sulfuric acid (H2SO4) and nitric acid (HNO3) which can fall with the
rain, making it acid. Acid rain harms plants and animals.
6. Explain all you know about the destruction of the ozone layer. How is it caused? How does it affect the
planet? (1 point)
In the past we used compounds known as CFCs in spray cans that released atoms of chlorine in the
atmosphere. These atoms of chlorine caused the destruction of the ozone layer.
The ozone layer protects us from the harmful ultraviolet radiation from the sun. The destruction of ozone
caused “holes” in the ozone layer. Ultraviolet radiation can reach us if it passes through these holes,
causing us diseases such as skin cancer.
Even though we no longer use CFCs, their effect still remains.
7. In exercise 1b you have to balance a type of chemical reaction that is called combustion. In these types
of chemical reactions, we obtain energy (which we can use). A product of this reaction is carbon dioxide
(CO2). Explain the environmental problem that the excess of CO2 in the atmosphere causes to the planet.
(1 point)
Carbon dioxide (CO2) allows heat (from the sun) to enter in the Earth but doesn´t allow it to go back to
space. This effect is called greenhouse effect. Without greenhouse effect the Earth would be too cold for
us to live in it. But the increase of carbon dioxide (caused by reactions of combustion) is causing an
increase in greenhouse effect. Therefore more heat is retained in the Earth and global temperature is going
to rise. This is the global warming. Among other damaging effects, the global warming is making the
poles melt.