Chemistry-Semester Exam
Review
Module 1
• What is the difference between
a scientific theory and law?
A scientific theory is a set of ideas that
attempts to explain why things happen the
way they do in the natural world. A law is a
generalization that describes a variety of
behaviors in nature, but does not attempt
to explain.
Examples: The theory of evolution and law
of gravity.
• What makes science different from other
fields of knowledge?
The scientific method which is a systematic
approach to problem solving. Science is about
observation to answer questions, unlike
philosophy which does not need to be based
upon observation.
• How many significant figures do each of
terms these have?
• 0.005670 cm
• 1.00 cm
• 100 cm
• 100.00 cm
Answers
• 0.005670 cm (4 sig figs)
• 1.00 cm (3 sig figs)
• 100 cm (1 sig fig)
• 100.00 cm (5 sig figs)
Five rules of sig figs.
1.
2.
3.
4.
5.
Five Rules of sig figs
1. Every non-zero is significant.
2. Any zero between non-zeroes is significant.
3. Any zero at the beginning of a number before
the first non-zero is NOT significant.
4. Any zero at the end of a number without a
decimal place is NOT significant.
5. Any zero at the end of a number with a
decimal place is significant.
Rules of sig figs for addition and subtraction.
• When adding or subtracting sig figs, you need
to find the lowest number decimal places and
then add or subtract and round your answer
to that number of decimal places.
• ex: 1.005 + 2.0074 =
• ex: 1.005 - 2.0074 =
Answers
• ex: 1.005 + 2.0074 = 3.0124 rounded to
3.012 (three decimal places)
• ex: 1.005 - 2.0074 = -1.0024 rounded to 1.002 (three decimal places)
When multiplying or dividing sig figs, you need to find
the lowest number of significant figures and then
multiplying and dividing and round your answer to that
number of significant figures.
• ex: 1.005 * 2.0074 =
• ex: 1.005 / 2.0074 =
Answers
• ex: 1.005 * 2.0074 = 2.017437 rounded to 2.017 (four
significant figures)
• ex: 1.005 / 2.0074 = 0.500647603865697 rounded to 0.5006
(four significant figures)
Unit Conversion.
• Can you convert 56.5 km to m?
Answer
Can you convert 56.5 km to m?
56.5 km
1000 m = 56500 m
1 km
Unit Conversion
• 500.5 ml to liters?
Answer
• 500.5 ml to liters?
500.5 mL
1 x 10-3 L
1 mL
= 0.5005
Chemistry Semester Exam Review
Module 2
Why do theories change and evolve?
A. No one believes the original theory.
B. New technology or experimentation leads to
revised ideas.
C. If one scientist makes a discovery that
challenges a theory it will change the theory.
D. If certain important groups of people think
that the theory is wrong, the theory will be
considered invalid.
Why do theories change and evolve?
A. No one believes the original theory.
B. New technology or experimentation leads to
revised ideas.
C. If one scientist makes a discovery that
challenges a theory it will change the theory.
D. If certain important groups of people think
that the theory is wrong, the theory will be
considered invalid.
What are models? Why do scientists use them?
A. Models are useful for scientists that can’t
collect real data
B. Models allow scientists to predict reactions
without making real life observations.
C. Models are used as visual representations of
something very small or very large. Something
that isn’t easy to see or observe.
D. Models help scientists study things without
having to conduct experiments or collect data.
What are models? Why do scientists use them?
A. Models are useful for scientists that can’t
collect real data
B. Models allow scientists to predict reactions
without making real life observations.
C. Models are used as visual representations of
something very small or very large. Something
that isn’t easy to see or observe.
D. Models help scientists study things without
having to conduct experiments or collect data.
• Describe the main points of Dalton’s atomic
theory and what points have changed as a
result of experiments performed later.
Include details of those experiments such as
their expected and observed results and the
scientists credited with those discoveries.
•
•
•
•
•
•
•
•
•
•
•
Dalton’s Theory:
All matter is composed of extremely small particles called atoms.
Atoms of a given element are identical in size, mass and other properties.
• Scientists now know that all atoms of a given element have the same number of
protons but may have different numbers of neutrons (isotopes).
Atoms cannot be subdivided, created, or destroyed.
• Scientists now know that atoms are divisible and made up of smaller, subatomic
particles. However, the law of conservation of mass still states that matter cannot be
created or destroyed in an ordinary chemical reaction.
Atoms of different elements can combine in simple, whole-number ratios to form chemical
compounds.
In chemical reactions, atoms are combined, separated, or rearranged.
Others/Changes:
The cathode ray experiments resulted in Thomson adding electrons to the atomic model.
Rutherford’s gold foil experiment disproved Thomson’s plum pudding model by demonstrating
that the mass of an atom is packed into a small, positive nucleus. He added the nucleus to the
model of that atom, surrounded by electrons.
Bohr’s observation of the line spectra led to his idea that electrons exist on orbits around the
nucleus, which he added to the model of the atom.
Schrodinger’s quantum model resulted from experiments that demonstrated the dual nature of
light and the electron. He added energy levels and orbitals to the electron cloud surrounding the
nucleus.
• A neutral atom has the atomic number 14 and
a mass number of 31, how many protons,
neutrons, and electrons are found within the
atom?
• A neutral atom has the atomic number 14 and
a mass number of 31, how many protons,
neutrons, and electrons are found within the
atom?
Protons: 14, neutrons : 17 (31-14 = 17), electrons : 14
• An atom is determined to contain 8 protons, 10
electrons, and 9 neutrons. What are the atomic
number, mass number and charge of this atom
or ion?
A. atomic number: 8 mass number: 17, charge: -2
B. atomic number 9, mass number: 27, charge:
neutral +1
C. atomic number 10, mass number: 17, charge: +2
D. atomic number 8, mass number: 27, charge: -2
• An atom is determined to contain 8 protons, 10
electrons, and 9 neutrons. What are the atomic
number, mass number and charge of this atom
or ion?
A. atomic number: 8 mass number: 17, charge: -2
B. atomic number 9, mass number: 27, charge:
neutral +1
C. atomic number 10, mass number: 17, charge: +2
D. atomic number 8, mass number: 27, charge: -2
• How many grams of carbon (C) are present in
a sample of 2.5 × 1024 atoms C?
• How many grams of carbon (C) are present in
a sample of 2.5 × 1024 atoms C?
• 49.9 g C
Which of the following contains the greatest number of
moles?
A. 6.02 × 1023 atoms Ag
B. 5.1 mol Ag
C. 0.3 mol Fe
D. 125 g Fe
Which of the following contains the greatest number of
moles?
A. 6.02 × 1023 atoms Ag
B. 5.1 mol Ag
C. 0.3 mol Fe
D. 125 g Fe
When an element is heated in a Bunsen
burner flame, the resulting color of the flame
changes can be used to identify the element.
What causes the colored light that is
observed?
When an element is heated in a Bunsen burner
flame, the resulting color of the flame changes
can be used to identify the element. What
causes the colored light that is observed?
When an electron moves from a higher to lower
orbital, energy is released as waves of light.
What occurs when an excited electron returns to its
ground state?
A. Protons are released
B. Energy is emitted as photons of light
C. It transfers to another atom
D. The electron is no longer able to bond
What occurs when an excited electron returns to its
ground state?
A. Protons are released
B. Energy is emitted as photons of light
C. It transfers to another atom
D. The electron is no longer able to bond
• How are the frequency, wavelength and energy
of an electromagnetic wave related?
• As frequency __________, energy __________
• As wavelength _________, energy __________
• As frequency _________, wavelength
_________
• How are the frequency, wavelength and energy
of an electromagnetic wave related?
• As frequency increases, energy increases
• As wavelength increases, energy decreases
• As frequency increases, wavelength decreases
•
What would be the most likely charge of an ion
formed from an atom with the electron
configuration: 1s2 2s2 2p5
A.
B.
C.
D.
+2
-2
+1
-1
•
What would be the most likely charge of an ion
formed from an atom with the electron
configuration: 1s2 2s2 2p5
A.
B.
C.
D.
+2
-2
+1
-1
Chemistry Semester One Exam
Review
Module 3
Periodic Trends
1). Atomic radius/atomic size.
2). Ionization energy
3). Electronegativity
Periodic Trends
1). Atomic radius/atomic size.
There is a gradual decrease in atomic radii from left to right across a period.
There is a general increase in atomic radii going down each group of
elements on the periodic table.
2). Ionization energy
Ionization energy has a general increase for elements across a period from
left to right.
The ionization energy of elements decreases going down a group because
the atomic radius of the atoms increases
3). Electronegativity
Electronegativity increases from left to right across a period because of the
increase in effective nuclear charge.
Electronegativity decreases down a group, as a result of increased distance
between the valence electrons and the nucleus (greater atomic radius).
Write the name of the following
compounds
• CO2
• Ba3(PO4)2
• CuO
• CaSO4 5H2O,
Write the name of the following
compuonds
• CO2
carbon dioxide
• Ba3(PO4)2 barium phosphate
• CuO copper (II) oxide
• CaSO4 5H2O calcium sulfate pentahydrate
Write the formula of the following
compounds
1). magnesium phosphide
2). iodine dichloride
3). nitric acid
4). tin (IV) nitrate
5). potassium nitrate tetrahydrate.
Write the formula of the following
compounds
1). magnesium phosphide Mg3P2
2). iodine dichloride ICl2
3). nitric acid HNO3
4). tin (IV) nitrate Sn(NO3)4
5). potassium nitrate tetrahydrate KNO3 3H2O
Which compound will experience hydrogen
bonding?
A. HCl
B. C6H6
C. HF
A. H2SO4
Which compound will experience hydrogen
bonding?
A. HCl
B. C6H6
C. HF
A. H2SO4
Draw Lewis Dot Structures for the following
compounds
• HCN
• H2O
• O3
Chemistry Semester One Exam Review
Module 4
Chemical Equations
Because of the principle of
the conservation of matter,
an equation must be
balanced.
It must have the same
number of atoms of the
same kind on both sides.
Lavoisier, 1788
Chemical Equations
Their Job: Depict the kind of
reactants and products and their
relative amounts in a reaction.
4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)
The numbers in the front are called
stoichiometric ____________
The letters (s), (g), and (l) are the
physical states of compounds.
Balancing Equations
When balancing a chemical reaction
you may add coefficients in front of the
compounds to balance the reaction, but
you may
not
change the
subscripts.
Changing the subscripts changes the
compound. Subscripts are
determined by the valence electrons
(charges for ionic or sharing for
covalent)
Steps to Balancing Equations
There are four basic steps to balancing a chemical equation.
1. Write the correct formula for the reactants and the products.
DO NOT TRY TO BALANCE IT YET! You must write the correct
formulas first. And most importantly, once you write them
correctly DO NOT CHANGE THE FORMULAS!
2. Find the number of atoms for each element on the left side.
Compare those against the number of the atoms of the same
element on the right side.
3. Determine where to place coefficients in front of formulas so
that the left side has the same number of atoms as the right
side for EACH element in order to balance the equation.
4. Check your answer to see if:
The numbers of atoms on both sides of the equation are
now balanced.
The coefficients are in the lowest possible whole number
ratios. (reduced)
Balancing Equations
___ H2(g) + ___ O2(g) ---> ___ H2O(l)
___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s)
Combination (Synthesis) Reactions
Two or more substances combine to form a
new compound.
A + X  AX




Reaction of elements with oxygen and sulfur
Reactions of metals with Halogens
Synthesis Reactions with Oxides
There are others not covered here!
Decomposition Reactions
A single compound undergoes a reaction that
produces two or more simpler substances
AX  A + X
Decomposition of:
Binary compounds
Metal carbonates
Metal hydroxides
Metal chlorates
Oxyacids
H2O(l )  2H2(g) + O2(g)
CaCO3(s)  CaO(s) + CO2(g)
Ca(OH)2(s)  CaO(s) + H2O(g)
2KClO3(s)  2KCl(s) + 3O2(g)
H2CO3(aq)  CO2(g) + H2O(l )
Single Replacement Reactions
A + BX  AX + B
BX + Y  BY + X
Replacement of:




Metals by another metal
Hydrogen in water by a metal
Hydrogen in an acid by a metal
Halogens by more active halogens
The Activity Series of the Metals
Lithium
Potassium
Calcium
Sodium
Magnesium
Aluminum
Zinc
Chromium
Iron
Nickel
Lead
Hydrogen
Bismuth
Copper
Mercury
Silver
Platinum
Gold
Metals can replace other metals
provided that they are above the
metal that they are trying to
replace.
Metals above hydrogen can
replace hydrogen in acids.
Metals from sodium upward can
replace hydrogen in water
The Activity Series of the Halogens
Fluorine
Chlorine
Bromine
Iodine
Halogens can replace other
halogens in compounds, provided
that they are above the halogen
that they are trying to replace.
2NaCl(s) + F2(g)  ??? 2NaF(s) + Cl2(g)
MgCl2(s) + Br2(g)  ???
No Reaction
Double Replacement Reactions
The ions of two compounds exchange places in an
aqueous solution to form two new compounds.
AX + BY  AY + BX
One of the compounds formed is usually a
precipitate, an insoluble gas that bubbles out of
solution, or a molecular compound, usually water.
Combustion Reactions
A substance combines with oxygen, releasing a large
amount of energy in the form of light and heat.
Reactive elements combine with oxygen
P4(s) + 5O2(g)  P4O10(s)
(This is also a synthesis reaction)
The burning of natural gas, wood, gasoline
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g)
Stoichiometry
“In solving a problem of this sort,
the grand thing is to be able to
reason backward. This is a very
useful accomplishment, and a very
easy one, but people do not practice
it much.”
Sherlock Holmes, in Sir Arthur Conan
Doyle’s A Study in Scarlet
Stoichiometry - The study of quantities of
materials consumed and produced in chemical
reactions.
Review: Chemical Equations
Chemical change involves a reorganization of
the atoms in one or more substances.
C2H5OH + 3O2  2CO2 + 3H2O
reactants
products
When the equation is balanced it has quantitative
significance:
1 mole of ethanol reacts with 3 moles of oxygen
to produce 2 moles of carbon dioxide and 3 moles
of water
Review: Chemical Equations
Mole Relations
Mole Relations from Chemical Equations
Calculating Masses of Reactants and
Products
1.
2.
3.
4.
Balance the equation.
Convert mass to moles.
Set up mole ratios.
Use mole ratios to calculate moles of
desired substituent.
5. Convert moles to grams, if necessary.
Working a Stoichiometry Problem
6.50 grams of aluminum reacts with an excess of
oxygen. How many grams of aluminum oxide are
formed.
1. Identify reactants and products and write
the balanced equation.
4 Al
+ 3 O2
2 Al2O3
a. Every reaction needs a yield sign!
b. What are the reactants?
c. What are the products?
d. What are the balanced coefficients?
Working a Stoichiometry Problem
6.50 grams of aluminum reacts with an excess of
oxygen. How many grams of aluminum oxide are
formed?
4 Al
6.50 g Al
+
3 O2  2Al2O3
1 mol Al
2 mol Al2O3 101.96 g Al2O3
26.98 g Al
4 mol Al
1 mol Al2O3
= ? g Al2O3
6.50 x 2 x 101.96 ÷ 26.98 ÷ 4 = 12.3 g Al2O3
Limiting Reactant
The limiting reactant is the reactant
that is consumed first, limiting the
amounts of products formed.
Limiting Reagents - Combustion
Limiting Reagents
Solving for Limiting Reactant
What is the limiting reactant if 6.50 grams of
aluminum reacts with 8.97 g of water to produce
14.5 g Al2O3? Show, or explain, all of the steps
that you used to solve this problem.
2Al
6.50 g Al
8.97 g H2O
+ 3H2O  Al2O3 + 3H2
1 mol Al
1 mol Al2O3 101.96 g Al2O3
26.98 g Al
2 mol Al
1 mol H2O
18.0 g H2O
1 mol Al2O3
1 mol Al2O3 101.96 g Al2O3
3 mol H2O
1 mol Al2O3
=
=
? g Al2O3
? g Al2O3
Solving for Percent Yield
Theoretical Yield
The maximum amount of a given product that
can be formed when the limiting reactant is
completely consumed
The actual yield (amount produced) of a reaction is
usually less than the maximum expected (theoretical
yield).
Percent Yield
The actual amount of a given product as the
percentage of the theoretical yield
Solving for Percent Yield
From the answers that you got from the problem
on limiting reactants, (what is the limiting
reactant if 6.50 grams of aluminum reacts with
8.97 g of water to produce 14.5 g Al2O3),
determine the theoretical yield and the percent
yield.