CHAPTER 8
CHEMICAL REACTION
Physical change is when the chemical composition of the substance
remains constant while for chemical change, the chemical composition does
not remain constant.
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EVIDENCE OF CHEMICAL REACTION
A gas is released in form of 1) bubbles 2) fizzing evidence. 3) carbon
dioxide gas released.
An insoluble solid produced (precipitate) : aqueous solution is formed
when a substance dissolves in water. When we add two solutions
together, we may observe solid particles in solution.
Permanent color change : many chemical reactions involve a
permanent color change. Some are obvious while in some, like acid/
base reaction, an indicator is used( a substance that change color). The
indicator enables us to indirectly follow a reaction that would
otherwise not be visible.
A heat energy change is noted : In chemical reactions there is often a
change in temperature. A reaction that releases heat is said to be
Exothermic reaction. A reaction that absorbs heat is said to be
Endothermic reaction.
WRITING CHEMICAL EQUATIONS.
A chemical equation describes a reaction using formulas and symbols.
Example, A(aq)+ B(g)
C(S)+ D(aq)
A and B are known as REACTANTS while C and D are known as
PRODUCTS. The physical states are specified as follows;
Aq = aqueous solution, g = gas phase, s = solid phase, l = liquid
A catalyst is a substance that speeds up a reaction without being
consumed or permanently altered. The presence of a catalyst is indicated
by placing its formula above the arrow.
Naturally occurring Diatomic molecules are H2, N2, Cl2, Br2, and I2. This
is how they appear in chemical reactions.
1
Symbol
+
Fe
(s)
(g)
(aq)
NR
Interpretation of Chemical Equation Symbol
Produces, yields, gives(separates reactants and
products)
React with, added to, plus(separates two or more
reactants or products)
The reactants are heated.
Metallic iron catalyst is added to the reaction
Solid substance or precipitate
Liquid substance
Aqueous solution
No reaction
BALANCING CHEMICAL EQUATIONS
More often, the number of atoms of each element in the reactant and
products are not the same; therefore, it is necessary to balance the
number of atoms on both sides. This is done by putting whole numbers
called coefficients in front of each substance but the subscripts are
always unaltered. Coefficients always multiply all subscripts in chemical
formula that follows.
Example, 3H2O ------ 6 hydrogen atoms and 3 oxygen atoms.
4NH3 ----- 4 nitrogen atoms and 12 hydrogen atoms
Subscripts are never changed, because this alters the chemical
formula.
H2(g) + 1/2O2(g)
H2O(l)
2H2(g) + O2(g)
2H2O(l)
CLASSIFYING CHEMICAL REACTIONS
1) Combination Reaction: also known as synthesis reaction, simpler
substances combine to form complex substance.
A+B
AZ
2) Decomposition Reaction: a simple compound is broken down into
two or more simpler substances. Heat or light is usually applied to
decompose the compound.
AZ
A+Z
3) Single-replacement Reaction: One element displaces another element
(that is less reactive) from a compound or aqueous solution.
A + BZ
AZ + B
4) Double-replacement reaction: two compounds exchange anions.
2
AX + BZ
AZ + BX
5) Neutralization Reaction: an acid and a base react to form a compound and
water. HX + BOH
BX + H2O
Combination Reactions
Metal + O2
metal oxide
2Mg(s) +O2
2MgO(s)
Zn(s) + O2
2ZnO
4Cr + 3O2
2Cr2O3
Nonmetals and Oxygen
The nonmetals oxides are also formed but they are not predictable like
the metals and they vary with temperature and pressure. An example is when
nitrogen reacts with oxygen gas to give, NO, NO2, N2O, N2O3, N2O4 and
N2O5 .The name or the formula of the nonmetal oxide formed must be given.
DECOMPOSITION REACTIONS
Metal Hydrogen Carbonates
Baking soda decomposes from the heat from fire and releases carbondioxide, since CO2 is denser than air, it smother a fire by excluding oxygen
from the flame.
Metal hydrogen
Metal carbonate + water + carbon dioxide
carbonate
2NaHCO3
Na2CO3(s) + H2O(g) + CO2(g)
Metal Carbonates
Metal Carbonate, heated will give, metal oxide and carbon dioxide.
Example, NiCO3(s)
NiO(s) + CO2(g)
Oxygen-containing Compounds
2Hg(s)
Hg(l) + O2(g)
THE ACTIVITY SERIES CONCEPT
When a metal undergoes a replacement reaction, it displaces another
metal from a compound or aqueous solution. The reactivity series of a metal
is a measure of its ability to compete in a replacement reaction.
Activity series (electromotive series) is the sequence of metals arranged
according to their ability to undergo reaction. Metals that are most reactive
appear first while the less reactive ones appear last. Although, H is not a
metal, it is included as a reference on the series. Metals that precede (H) in
the series react with an aqueous acid; metals that follow (H) in the series do
not react with acids (Cu, Ag, Hg, and Au do not react with acids)
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Li> K> Ba> Sr> Ca> Na> Mg> Al> Mn> Zn> Fe> Cd> Co. Ni> Sn> Pb>
(H)> Cu> Ag> Hg> Au
Fe(s) + CuSO4(aq)
FeSO4(aq) + Cu
Cu(s) + FeSO4(aq)
NR
ACTIVE METALS
Few metals are so active that they react directly with water at room
temperature- Active Metals
They include group IA and group IIA metals. Specifically, Li, Na, K,
Rb, Cs, Ca, Sr,and Ba react with water. The most reactive being Li> K> Ba>
Sr> Ca> Na.
Na(s) + 2H2O(l)
2NaOH
Activity series of nonmetals is as follows:
F> Cl> Br> I
Cl2(g) + NaBr(aq)
NaCl(aq) + Br2(l)
Cl2(g) + NaF(aq)
NR
SINGLE-REPLACEMENT REACTION
1) Metal and Aqueous solution,
Metal1 + aqueous solution1 give metal 2 + aqueous solution 2
Cu(s) + AgNO3(aq)
Cu(NO3)2(aq) + Ag(s)
Ag(s) + Cu(NO3)2(aq)
NR
2) Metal and Aqueous Acid solution
Metal + aqueous acid give aqueous solution + hydrogen gas
Fe(s) + HS2O4(aq)
FeSO4(aq) + H2(g)
3) Active metal and water.
Metal + water give metal hydroxide + hydrogen gas
Ca(s) + 2H2O(l)
Ca(OH)2(aq) + H2(g)
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SOLUBILITY RULES
Solubility rules for ionic compounds
SOLUBLE
INSOLUBLE
a. Alkali metal ions and ammonium Carbonates Ion, CO32- (except group
ion.
a)
b. Acetate ion, C2H3O2
Chromate ion, CrO42-(except group
a)
c. Nitrate ion, NO3
Phosphate ion, PO43-(except group a)
d. Halides, Cl-, Br-, I- (except Ag,
Sulfide ion, S2- (except Ca, Sr, Ba
Hg2,Pb)
and group a)
e. Sulfate, SO42- (except Sr, Ba, Pb)
Hydroxide ion, OH- (except,Ca, Sr,
Ba and group a)
DOUBLE-REPLACEMENT REACTION
This type of reaction is also known as precipitation reaction.
AX + BZ
AZ + BX
If both AZ and BX are soluble, there is no precipitate formed, hence no
chemical reaction.
AGNO3(aq) + Na2CO3(aq)
Ag2CO3(S) + 2NaNO3(aq)
NEUTRALIZATION REACTIONS
Acid + Base
aqueous ionic compound +water
Acid is a substance that releases hydrogen ion.
Base is a substance that releases hydroxide ion.
The resulting ionic compound is called SALT which composes of cation
from the base and anion from the acid.
HX + BOH
BX + H2O
HCl(aq) + NaOH(aq)
NaCl (aq)+ H2O(l)
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