WOLPA/AP CHEMISTRY
Classifying Chemical Reactions: Analyzing and Predicting Products
Background
A chemical reaction is defined as any process in which one or more substances
are converted into new substances with different properties. Chemical reactions
change the identity of the reacting substance(s) and produce new substances.
Observing the properties of the reactants and products is therefore a key step in
identifying chemical reactions. Some of the observations that may be associated
with a chemical reactions in: (1) release of a gas; (2) formation of a precipitate;
(3) color changes; (4) temperature changes; (5) emission or absorption of light.
Chemical reactions may be classified by considering the number of reactant and
products in the reactions, the physical or chemical nature of the reactants and
products, and the rearrangement of atoms in the conversion of the reactants into
products.
Classification of Chemical Reactions
Type of
Reaction
General Description and Example(s)
Combination
Two reactants combine to form a single product. The reactants may be
elements or compounds. Also called a synthesis reaction.
Zn(s) + I2(s)  ZnI2 (s)
CaO(s) + H2O(l)  Ca(OH) 2 (s)
One reactant, a compound, breaks down to give two or more products
2H2O2 (aq)  2H2O(l) + O2 (g)
An element reacts with a compound and replaces one of the elements in
the compound. Metals replace hydrogen or other metals; nonmetals
replace nonmetals.
Decomposition
Single
Replacement
Double
Replacement
Combustion
Zn(s) + 2HCl(aq)  H2 (g) + ZnCl2 (aq)
Cu(s) + 2AgNO3(aq)  2Ag(s) + Cu(NO3) 2 (aq)
Cl2 (aq) + 2NaI(aq)  2NaCl(aq) + I2(aq)
Tow ionic compounds (or compounds that break apart to form ions in
solution) exchange ions in form new compounds. Examples include
precipitation reactions (driving force is formation of a precipitate), acidbase reactions (driving force is formation of water), and gas-forming
reactions (driving force is evolution of a gas).
NaCl(aq) + AgNO3(aq)  AgCl(s) + BaNO3(aq)
H2SO4(aq) + 2NaOH(aq)  Na2SO4(aq) + 2H2O(l)
Na2SO3(aq) + 2HCl(aq)  2NaCl(aq) _ H2O(l) + SO2(g)
A compound burns in the presence of oxygen, producing energy in the
form of heat and light. The combustion of organic compounds produces
carbon dioxide and water.
C4H8(l) + 6O2 (g)  4CO2 (g) + 4H2O(g)
Activity Overview
The purpose of this experiment is to study a variety of chemical reactions to
identify patterns in the conversion of reactants into products. The properties of
the reactions will be analyzed to classify the chemical reactions into different
groups.
Experimental
Reaction Procedure
1
2
3
4
5
6
7
A piece of magnesium ribbon is held in a burner flame
A piece of magnesium ribbon is placed in a solution of hydrochloric
acid
Ammonium carbonate is heated in a test tube
Calcium carbonate is added to a solution of hydrochloric acid
A piece of zinc is placed in a solution of copper (II) chloride
A solution of copper (II) chloride is added to a solution of sodium
phosphate
Sodium hydroxide is placed in a small test tube with phenolphthalein
(an acid base indicator). Hydrochloric acid is added drop wise to the
sodium hydroxide mixture
Data
Reaction Appearance of Reactants(s), Evidence of Chemical Reaction,
and Properties of Product(s)
1
Magnesium metal ribbon is silver and shiny. When placed in a flame,
it burns slowly with a yellow flame. It then bursts into an intense,
bright white flame. The product of the reaction is a dull, grayishwhite powder that crumbles easily.
2
Rapid bubbling occurs with magnesium is added to the acid solution.
The test tube feels warm. When a burning wood splint is placed in
the mouth of the test tube, a large “pop” is heard, the flame foes
out, and condensation is then seen on the inside of the test tube.
The final solution is clear and colorless.
3
Ammonium carbonate is a white solid with a strong ammonia odor.
Heating release more ammonia. Moistened litmus paper turns blue
when exposed to the ammonia vapor. Burning split is extinguished
by invisible gas inside the test tube. A clear liquid condenses at the
mouth of the test tube.
4
Adding hydrochloric acid to solid calcium carbonate produces
vigorous bubbling. The reaction is instantaneous and is over in less
than a minute. When a burning wood splint is placed in the test
tube, the flame is extinguished. The products are a gas and a clear
solution.
5
The zinc is shiny gray and turns dark when added to copper (II)
chloride solution. The color of the solution change from blue to
greenish blue and then pale gray. The zinc acquires a spongy, dark
red coating, which builds up quickly and then crumbles or flakes off.
The test tube feels hot. The final solution is pale gray, almost
colorless.
6
The copper (II) chloride solution is blue-green. Adding a colorless
solution of sodium phosphate produces a bulky turquoise precipitate.
The initial blue color of the solution fades and the final products
consist of a turquoise solid and a pale blue liquid.
7
Sodium hydroxide solution is colorless but turns bright pink with
phenolphthalein is added. The color of the solution changes back to
colorless after 21-22 drops of hydrochloric acid have been added.
The test tube feels slightly warm to the touch.
Questions
Write a balanced chemical equation for each of the seven reactions. Classify
each reaction using the information provided in the background section.
Reactions #1
Reactions #2
Reactions #3
Reactions #4
Reactions #5
Reactions #6
Reactions #7