ChemQuest 28
Name: ____________________________
Date: _______________
Hour: _____
Information: Introduction to Reactions
During a chemical reaction, new substances are formed. Reactants are transformed into different
products. Atoms are never created or destroyed, but they are rearranged. A chemical equation
represents what happens during a reaction. The following is an example of a chemical equation:
Example Equation: Ca + HNO3  Ca(NO3)2 + H2
This equation describes the reaction of calcium (Ca) with nitric acid (HNO3) to produce calcium
nitrate (Ca(NO3)2) and hydrogen gas (H2). You may notice that there are more total atoms on the
right side than there are on the left side of the equation. If this seems strange to you, don’t worry
about it now; we’ll fix this later.
Note in the above equation that hydrogen gas is written as H2 and not simply as H. There are a
few elements that exist as diatomic molecules. If a substance is diatomic then the substance
must always be bonded to something. A hydrogen atom is diatomic and so it must be bonded to
something else like in HCl or HNO3. If nothing is available for it to bond to, it will bond to itself
by forming H2. All of the diatomic substances are listed below:
Br
I
N
Cl
H
O
F
When by themselves these elements exist as Br2, I2, N2, Cl2, H2, O2, and F2. By the way, you can
remember these by recalling a made-up name: Mr. Brinclhof.
Critical Thinking Questions
1. Consider the bromine atoms in this reaction: LiBr + P  Li3P + Br2.
a) Why is bromine written as Br2 on the right side?
Bromine is a diatomic molecule and always needs to be bonded to something; even
bonding to itself works.
b) Why is it not necessary for LiBr to be written as LiBr2?
The Br is already bonded to something else (the Li). If it were written LiBr2 it would
not be a neutral compound.
2. What are the reactants in the example equation in the above information section?
Reactants are written on the left side of the equation and so the reactants are Ca and HNO3.
3. Answer the questions that follow based on this chemical equation: Na + MgCl2  NaCl +
Mg.
a) Why can’t NaMg be produced?
Na+ cannot bond with Mg2+ because they are both positive.
b) Why can’t NaCl2 be produced?
Na+ only requires one Cl-. NaCl2 is not a neutral compound.
c) Are NaCl and Mg the only products that can be produced?
Yes.
4. Given the following equation: Li + Ca3(PO4)2  Li3PO4 + Ca.
a) Why can’t CaLi2 be produced?
Ca2+ and Li+ won’t bond because they are both positive.
b) Why can’t Li3P be produced?
Although Li3P is neutral, there is no P3- in the equation. There is only PO43- and we will
almost never be breaking up polyatomic ions like PO43-. Don’t mess with polyatomic
ions!
c) Are Li3PO4 and Ca the only substances that can be produced?
Yes.
5. Write chemical equations for the following reactions.
a) Aluminum sulfate reacts with barium to produce barium sulfate and aluminum.
Al2(SO4)3 + Ba  BaSO4 + Al
b) Magnesium reacts with copper(I) nitrate to produce magnesium nitrate and copper.
Mg + CuNO3  Mg(NO3)2 + Cu
c) Sodium reacts with calcium phosphide to produce sodium phosphide and calcium.
Na + Ca3P2  Na3P + Ca
d) Phosphorus reacts with sodium chloride to produce sodium phosphide and chlorine.
P + NaCl  Na3P + Cl2
(Note that chlorine is diatomic!)
6. Each of the reactions you wrote in question 5 follows a similar pattern. The same pattern is
followed by the equations in questions 3 and 4. Describe this pattern.
A single atom reacts with a compound and replaces one of the atoms in that compound.
7. a) How are reactions 5c and 5d different?
Na forms a positive ion (Na+) and replaces another positive ion, but P forms a negative ion
(P3-) and replaces another negative ion.
b) How are reactions 5c and 5d similar?
In both reactions a single atom replaces another atom from a compound that it is reacting
with.
8. Complete the following reactions:
a) NaCl + Ag  AgCl + Na
write
(note: order is not important, so you could also
Na + AgCl)
b) Li + Ca3(PO4)2  Li3PO4 + Ca
(again, order is not important)
Information: Single and Double Replacement Reactions
Each of the equations that you looked at in the above section is called a single replacement
reaction. Notice that in each of them, a single atom replaces an ion from another reactant. Study
what happens in the following reactions. They are called double replacement reactions.
MgCl2 + NaF  NaCl + MgF2
Al2S3 + Na2O  Al2O3 + Na2S
Ca(NO3)2 + Na3P  NaNO3 + Ca3P2
Critical Thinking Questions
9. What is the difference between single replacement reactions and double replacement
reactions?
In single replacement reactions, one of the reactants is made of only one kind of atom. In
double replacement reactions, both reactants contain more than one kind of atom.
10. Complete the following reactions by providing the formulas for the missing compound(s).
a) Cu(NO3)2 + _____NaCl________  NaNO3 + CuCl2
b) ZnI2 + ______Al2(SO4)3________  ZnSO4 + AlI3
c) K2O + MgBr2  ____MgO_______ + ______KBr_______
11. Name the two products in the reaction between calcium phosphate and sodium iodide.
Calcium iodide and sodium phosphate
(The reaction is Ca3(PO4)2 + NaI  CaI2 + Na3PO4)
12. Explain why when you mix the following reactants, no reaction occurs: Na2SO4 + NaCl 
Both reactants contain sodium. Therefore, if the sodium atoms replaced each other, the same
products would be formed—no change would occur.
Information: Combustion, Synthesis, and Decomposition Reactions
Another type of reaction is a combustion reaction. During combustion, a hydrocarbon reacts
with oxygen. The products for complete combustion are always the same—water and carbon
dioxide and energy. The following equation is an example of the combustion of a hydrocarbon.
C3H8 + O2  CO2 + H2O
Two other types of reactions are synthesis and decomposition. During a synthesis reaction,
several reactants combine to make a single product. During a decomposition, one reactant
decomposes into two or more products. The following table shows some examples of these types
of reactions.
Synthesis
H2 + O2  H2O
Na + Cl2  NaCl
Decomposition
H2O  H2 + O2
NaCl  Na + Cl2
Critical Thinking Questions
13. Categorize each of the following reactions as single replacement (SR), double replacement
(DR), synthesis (S), decomposition (D) or combustion (C).
__S__ a) Ca + O2  CaO
__SR_ b) Mg(NO3)2 + Cu  CuNO3 + Mg
__C__ c) C4H10 + O2  CO2 + H2O
__D__ d) Al2O3  Al + O2
__SR_ e) SrCl2 + F2  SrF2 + Cl2
__DR_ f) BaF2 + Na2O  BaO + NaF
14. Write an equation for the combustion of C3H6.
C3H6 + O2  CO2 + H2O
(Combustion reactions always involve combining with O2 to form CO2 and H2O.)
15. Write an equation for the decomposition of calcium oxide.
CaO  Ca + O2
(Oxygen is diatomic, so it is written as O2 when by itself.)
Practice Problems
1. Complete the following reactions.
a) Na2CO3 + AlN  Na3N + Al2(CO3)3
(Remember that the order is never important.)
b) BaCl2 + F2  BaF2 + Cl2
c) CuNO3 + Ag  AgNO3 + Cu
2. Fill in the blanks for the missing reactant or product and then in the blank to the left of each
equation indicate whether the reaction is a single replacement (SR), double replacement
(DR), synthesis (S), decomposition (D) or combustion (C).
__DR_ a) LiCl + ___Zn(NO3)2____  ZnCl2 + LiNO3
__SR_ b) ____Na______ + CaBr2  NaBr + Ca
__S__ c) K + Cl2  ____KCl_______