Chemical Reactions Test Review – Test Thursday, January 5, 2012
Note: This review is intended to help you review major concepts presented in class discussions, notes and labs.
The test is not limited to the information or the format presented on this review. Be sure to study your notes
and go over assignments, particularly labs, as you study for your test.
1. List some evidence of a chemical reaction. Fizzing, bubbling, color change, energy change, formation of
a precipitate, release of light, rust
2. Study the following:
2 H2 + O2 2 H2O
a. List the reactants. What are the reactants of a chemical reaction?
H2 and O2
substances that undergo a change in a chemical reaction
b. List the products. What are the products of a chemical reaction?
H2 O
substances that result from a chemical reaction
c. Label a coefficient. What information is given by a coefficient?
(see above in GREEN) # of MOLECULES
d. Label a subscript. What information is given by a subscript?
(see above in PURPLE) # of ATOMS
e. Label a chemical formula. What is a chemical formula?
H2 O2 or H2O
element symbols and numbers showing the types and number of atoms
in a substance
f. Label the chemical equation. What is a chemical equation? What does the stand for?
(the entire thing is a chemical equation) a written form of a chemical reaction
“yields”
g. Is the chemical equation balanced? Explain.
Yes…. It has the same types and numbers of atoms on both sides ( 4 H and 2 O)
h. What happens to the atoms in a chemical reaction?
They are rearranged into different substances
3. What is the law of conservation of mass? How does it apply to chemical reactions? How was the law of
conservation of mass applied in the lab “Aggie Fire Starter” (This was the Steel Wool demo)? The law of
conservation of mass states that mass (or matter) cannot be created or destroyed.
In chemical reactions the sane types AND numbers of atoms must be present before in the reactants and
after the reaction in the products
In the Aggie Fire Starter, before the reaction the iron in the steel wool and the oxygen in the air were
separate but after the reaction, they were combined in a synthesis reaction, explaining the increase in
mass of the products compared to the reactants. The total mass of the reactants has to be equal to the
total mass of the products.
4. The illustration shows a conservation of mass experiment. How can you explain the loss of mass in the
beaker following the reaction? Heat is a form of energy and has NO mass so the reaction must have
produced and released a product in the form of a gas, which accounts for the loss of mass in the products.
This is similar to the reaction where we placed Alka-Seltzer in water with and without the glove,
measuring the mass of each.
5. Why do chemical reactions have to be balanced? What “numbers” have to be used and why? To follow the
Law of Conservation of Mass chemical reactions must be balanced. We use coefficients to add more
molecules and NOT subscripts because changing subscripts changes the entire compound.
6. Balance and identify reaction type:
a. __1__Mg + __2__ H(OH) __1__Mg(OH)2 + __1__ H2
Reaction Type
single displacement
b. __1__Pb(NO3)2 + ___2__ KI __1__ PbI2 + __2__ K(NO3)
double displacement
c. __8___ Fe + ___1__S8 __8___ FeS
synthesis
d. __1___C10H8 + ___12__ O2 __10___CO2 + __4___ H2O
combustion
e. ___1__ Na(OH) + __1__K(NO3) ___1__ Na(NO3) + __1___ K(OH) double displacement
f. __1___CH4 + __2___ O2 ___1__ CO2 + __2___ H2O
combustion
g. __1___Fe + __3___ NaBr __1___ FeBr3 + __3___Na
single displacement
h. __2__Al(OH)3 + __3__ H2(SO4) __6__H2O + ___1__Al2(SO4)3
double displacement
i. __1__ Na2(CO3) ___1__ Na2O + ___1__CO2
decomposition
7. What takes place in a synthesis reaction? How can a synthesis reaction be recognized? Give examples from
daily life. A larger molecule is formed from two smaller elements (A + B AB) 1 product
*photosynthesis, burning hydrogen gas in oxygen gas to form water, tarnish on copper and silver
8. What takes place in a decomposition reaction? How can a decomposition reaction be recognized? Give
examples from daily life. A large molecule breaks into two smaller molecules (AB A + B) 1
reactant *digestion, spoiling and rotting of foods, decaying plants and animals
9. How do single displacement and double replacement reactions differ? How can each be recognized?
In a single displacement reaction atoms of one element replace the atoms of another element in a
compound In a double displacement reaction positive ions from one compound replace positive ions
from another compound forming two different compounds
10. What takes place in a combustion reaction? How can a combustion reaction be recognized?
A hydrocarbon burns in the presence of oxygen hydrocarbon + oxygen yields carbon dioxide and water
11. What is the difference between complete and incomplete combustion? What are the additional products of
an incomplete combustion reaction? In complete combustion, all of the reactants are changed into carbon
dioxide and water. During incomplete combustion, some of the hydrocarbon is not completely changed
into carbon dioxide and water, leaving some black carbon residue or ashes and also forming deadly
carbon monoxide gas.
12. In an exothermic reaction, does heat enter or leave the reaction? How can you tell? Which has more
energy, the products or the reactants? Leaves, it gets hot, the reactants have more energy because energy
is being released as a result of the chemical change
Reactants Products + Energy released
13. In an endothermic reaction, does heat enter or leave the reaction? How can you tell? Which has more
energy, the products or the reactants? Enters, the products can feel cold because the energy was used by
the reaction, the products have more energy because the chemical reaction stored the energy in the
products
Reactants + Energy absorbed Products
14. List examples of exothermic and endothermic reactions.
EXOTHERMIC - fireworks, cellular respiration, burning wood, etc
ENDOTHERMIC - cold packs, photosynthesis
15. How can the rate of chemical reactions be changed? (4 ways)
1) amount of reactant
2) temperature
3) increase surface area (mix, crush, stir, spread out, chew, etc)
4) add a catalyst
16. What is an enzyme? A biological catalyst that speeds up reactions in living things ex: digestive
enzymes that speed up digestion