Second exam of the second term for 2nd ESO G
Name:
Date:
1. Balance the following chemical equations:
a) H2 + I2 → HI
b) Cr + O2 → Cr2O3
c) C2H4 + O2 → CO2 + H2O
d) N2O5 + H2O → HNO3
e) Na2O + H2O → NaOH
f) KClO3 → KCl + O2
The elements that appear in these chemical equations are:
H: hydrogen
I: iodine
Cr: chromium
O: oxygen
C: carbon
N: nitrogen
Na: sodium
K: potassium
Cl: chlorine
(3 points)
2. Hydroiodic acid (HI) reacts with magnesium hydroxide (Mg(OH)2) to yield magnesium iodide (MgI2)
and water (H2O), according to the following chemical reaction:
2 HI + Mg(OH)2 → 2 H2 O + MgI2
The elements that appear in this chemical equation are:
Mg: magnesium
(the rest appear in exercise 1)
When 145 g of Mg(OH)2 react with a certain amount of HI, we obtain 90 g of H2O and 695 g of
MgI2.What is the mass of HI that has reacted? What is the law that you are applying?
2 HI +
Mg(OH)2 →
2 H2O +
MgI2
?
145 g
90 g
695 g
(1 point)
3. Thanks to a process called “electrolysis” we can decompose water (H2O) in oxygen (O2 ) and hydrogen
(H2 ), according to the following equation:
2 H2O → 2 H2 + O2
If 600 molecules of H2O decompose:
a) How many molecules of H2 are we going to obtain? (0.5 points)
b) How many molecules of O2 are we going to obtain? (0.5 points)
4. Copper (II) oxide (CuO) reacts with hydrochloric acid (HCl) to yield copper (II) chloride (CuCl 2) and
water (H2O), according to the following chemical equation:
CuO + 2 HCl → CuCl2 + H2O
The elements that appear in this chemical equation are:
Cu: copper
(the rest appear in exercise 1)
If 159 g of CuO react with 146 g of HCl, we obtain 269 g of CuCl2 and 36 g of H2O.
If 318 g of CuO react:
a) What is the mass of HCl that it has reacted with? What law have you applied? (1 point)
b) What is the mass of CuCl2 that has been obtained? What law have you applied? (1 point)
c) What is the mass of H2O that has been obtained? What law have you applied? (1 point)
CuO +
2 HCl →
CuCl2 +
H2O
159 g
146 g
269 g
36 g
318 g
? (a)
? (b)
? (c)
5. Explain all you know about the destruction of the ozone layer. How is it caused? How does it affect the
planet? (1 point)
6. Humans use a chemical reaction called combustion in order to obtain energy. 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 2 nd ESO G (answers)
Name:
Date:
1. Balance the following chemical equations:
a) H2 + I2 → HI
b) Cr + O2 → Cr2 O3
c) C 2H4 + O2 → CO2 + H2O
d) N2O5 + H2O → HNO3
e) Na2O + H2O → NaOH
f) KClO3 → KCl + O2
The elements that appear in these chemical equations are:
H: hydrogen
I: iodine
Cr: chromium
O: oxygen
C: carbon
N: nitrogen
Na: sodium
K: potassium
Cl: chlorine
(3 points)
a) H2 + I2 → 2HI
b) 4Cr + 3O2 → 2Cr2 O3
c) C 2H4 + 3O2 → 2CO2 + 2H2O
d) N2O5 + H2O → 2HNO3
e) Na2O + H2O → 2NaOH
f) 2KClO3 → 2KCl + 3O2
2. Hydroiodic acid (HI) reacts with magnesium hydroxide (Mg(OH) 2) to yield
magnesium iodide (MgI2) and water (H2O), according to the following chemical
reaction:
2 HI + Mg(OH)2 → 2 H 2O + MgI 2
The elements that appear in this chemical equation are:
Mg: magnesium
(the rest appear in exercise 1)
When 145 g of Mg(OH)2 react with a certain amount of HI, we obtain 90 g of H2O and
695 g of MgI2.What is the mass of HI that has reacted? What is the law that you are
applying?
2 HI +
Mg(OH)2 →
2 H2O +
MgI 2
?
145 g
90 g
695 g
(1 point)
mreactants=mproducts (law of conservation of mass)
x+145=90+695
x=90+695-145=640 g of HI have reacted.
3. Thanks to a process called “electrolysis” we can decompose water (H 2O) in oxygen
(O2) and hydrogen (H2), according to the following equation:
2 H2O → 2 H2 + O2
If 600 molecules of H 2O decompose:
a) How many molecules of H2 are we going to obtain? (0.5 points)
b) How many molecules of O2 are we going to obtain? (0.5 points)
4. Copper (II) oxide (CuO) reacts with hydrochloric acid (HCl) to yield copper (II)
chloride (CuCl2) and water (H2O), according to the following chemical equation:
CuO + 2 HCl → CuCl2 + H2O
The elements that appear in this chemical equation are:
Cu: copper
(the rest appear in exercise 1)
If 159 g of CuO react with 146 g of HCl, we obtain 269 g of CuCl2 and 36 g of H 2O.
If 318 g of CuO react:
a) What is the mass of HCl that it has reacted with? What law have you applied? (1
point)
b) What is the mass of CuCl2 that has been obtained? What law have you applied? (1
point)
c) What is the mass of H 2O that has been obtained? What law have you applied? (1
point)
CuO +
2 HCl →
CuCl2 +
H 2O
159 g
146 g
269 g
36 g
318 g
? (a)
? (b)
? (c)
a)
I have used the law of definite proportions.
b)
I have used the law of definite proportions
c) 1st way:
I have used the law of definite proportions
2nd way:
mreactants=mproducts
318+292=538+x
X=318+292-538=72 g of H2 O
I have used the law of conservation of mass.
5. 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.
6. Humans use a chemical reaction called combustion in order to obtain energy. 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.