CHEMICAL REACTIONS
A chemical reaction
is the transformation of one or more substances into one or more new substances.
In a reaction the atoms of
the reacting substances
will rearrange and bond (chemically combine) to give new substances. The
new substances will have
different physical
and chemical properties from the initial substances.
Chemical reactions
are symbolically represented by chemical equations. Equation 1 shows solid
sodium metal reacting with
liquid water to produce
aqueous sodium hydroxide and hydrogen gas.
2 Na(s) + 2 H2O(l) ----> 2NaOH(aq) + H2(g) (Equation 1)
REACTIONS OCCURRING
IN AQUEOUS SOLUTIONS
Of the several types
of reactions involving aqueous solutions, the three most common are:
1. Neutralization.
In a neutralization reaction, an acid reacts with a base to produce a salt
(or salt solution) and possibly water.
An example is indicated
by Equation 2.
H2SO4(aq) + 2 NaOH(aq) ---->Na2SO4(aq) + 2 H2O(aq) (Equation 2)
2. Precipitation. In a precipitation reaction, two aqueous solutions react to form an insoluble product. Equation 3 is an example of a precipitation reaction.
AgNO3(aq) + NaCl(aq) ----> AgCl(s) + NaNO3(aq) (Equation 3)
3. Oxidation. Chemical processes involving the transfer of electrons are called oxidation reactions (redox). Equation 4 shows a typical redox reaction. Please note that one reactant and one product are in elemental form.
Zn(s) + 2 HCl(aq) ----> ZnCl2(aq) + H2(g) (Equation 4)
OBSERVATIONS
Often when reactions
occur there are accompanying visible changes. The most common observations
are:
1. Evolution of a gas. When one of the products of a reaction is a gas, bubbles of the gas may appear in the solution or on the surface of a solid. When a gas is produced, foam may form, or if the gas evolution is rapid enough a "fizzing" sound may be heard.
2. The formation (or disappearance) of an insoluble solid called a precipitate. Some reactions result with the production of an insoluble solid called a precipitate, which normally settles to the bottom of the reaction mixture as a collection of particles. Describing a precipitate involves not only noting its color but also texture. Precipitates can exist in the form of crystals, gelatinous or milky suspensions, or granular solids. The disappearance of a solid can also indicate a reaction has taken place.
3. A color change. Both precipitation reactions and redox reactions can involve color change. Also, color change can be associated with reactions that result in the formation of a species referred to as a complex ion.
4. Evolution or absorption of heat. Energy is transferred in all reactions. Sometimes the vessel in which the reaction occurs may get warm or cold, depending on whether the reaction evolves or absorbs heat.
LITMUS PAPER
Litmus paper is an
indicator paper coated with a substance (litmus) which undergoes a color
change in the presence of acids and bases. Red litmus paper is used to
check for bases, blue litmus paper is used to check for acids. Dipping
red litmus paper into a basic solution turns it blue. Dipping blue litmus
paper into an acidic solution turns it red.
REACTION MATRICES
One effective method
for tabulating the observations from chemical reactions involves the construction
of a reaction matrix. An example of a reaction matrix is shown in Matrix
I. It provides a cell for entering the observation made from every possible
mixing of two solutions from a total of six different compounds.
Notice that just over half of the matrix cells are shaded. Cells where identical solutions are being mixed are shaded because nothing results from this type mixture (e.g. CELL B2 - AlCl3/AlCl3). Cells which duplicate a mixing of solutions (i.e. CELL B3 and CELL C2 result with the same two solutions being mixed.) are also shaded.
Matrix I
| AlCl3 | Ba(NO3)2 | NiSO4 | KOH |
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Matrix II is the completed matrix with legend. It provides the observations and information from the mixing of the 15 possible pairs. When recording observations made from chemical reactions, it is suggested that you devise a code such as the following. G = gas produced; Ppt = precipitate; Ppt(white) = white precipitate; H = evolution of heat; C = absorption of heat; NR = no reaction; D = dissolved.
The first pair of solutions to be considered in Matrix I shows AlCl3 mixed with Ba(NO3)2. The matrix shows that when these two solutions were mixed no reaction was observed, thus NR is entered into Cell C1. Cell D1 also has a NR entry because no reaction was observed when AlCl3 was mixed with NiSO4. Cell E1 indicates that when AlCl3 and KOH were mixed a precipitate formed initially but dissolved upon addition of additional KOH.
Matrix II
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EXPERIMENTAL PROCEDURE
There are two parts
to this lab.
1. Uses litmus
paper and test the different aqueous solutions to determine which are acids
and which are bases.
Recall that:
BASES turn RED litmus paper BLUE.
ACIDS turn BLUE litmus paper RED.
2.
Construct and complete a reaction matrix associated with the observations
from mixing the following 10 solutions.
1. Aluminum nitrate, Al(NO3)2
2. Ammonium chloride, NH4Cl
3. Ammonium hydroxide, NH4OH
4. Ammonium sulfate, (NH4)2SO4
5. Barium chloride, BaCl2
6. Hydrochloric acid, HCl
7. Potassium carbonate, K2CO3
8. Silver nitrate, AgNO3
9. Sodium hydroxide, NaOH
10. Sulfuric acid, H2SO4