Observing Chemical Reactions

Introduction

Chemical reactions occur when atoms of same or different elements exchange electrons to form new products. The exchange of electrons or atoms is usually invisible and can be seen in form or chemical changes. The changes represents evidence of a chemical reaction. Such changes include formation of a solid precipitate from aqueous solutions, heat changes detected by use of a thermometer, production of a gas as effervesces as well as color changes detected by use of an indicator. Common examples of chemical reactions include burning of a candle, burning of charcoal, evaporating water, decomposing hydrogen peroxide, dissolving solids in liquids and reacting an acid with an anti-acid. Such chemical reactions can be represented in word and chemical equations which obey the law of mass conservation (atoms in the left hand side balancing those in the right hand side of a stoichiometric equation). Elements on the left hand side are usually termed as reactants which combine in a certain ratio to form products-those on the right hand side.

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Reactant products

A + B C + D

Chemical reactions can be grouped into single or double replacement with respect to the number of atoms or ions substituted in a chemical reaction. Single displacement involves a more reactive element replacing a weaker one from its salt as seen a reaction between metals and acids while double displacement involves two cations exchanging anions of their salts to resulting to production of a precipitate as one of the products as seen in neutralization reactions.

A + BY AY + B (single replacement) and AB + CD AD + BC (double replacement)

The purpose of this experiment was determine the type of reactions as well as the salts formed in double, single and neutralization type of reactions. Because salts are produced during chemical reactions, it was hypothesized that soluble and insoluble salts are produced in double displacement reactions and that more reactive elements displaces less reactive one in a chemical reaction. To determine whether soluble or insoluble salts are produced in double displacement reactions, the table below was used for inferencing.

Table 1: Solubility guidelines for ionic compounds in water

Stable ionic compounds of

Important expression

NO3-

None

CH3COO-

None

Cl-

Ag+, Hg2+, Pb2+

Br-

Ag+, Hg2+, Pb2+

I-

Ag+, Hg2+, Pb2+

SO42-

Sr2+, Ba2+, Hg2+, Pb2+

Insoluble ionic compounds of

Important expression of

S2-

NH4+, alkali metals cations, Ca2+, Ba2+

CO32-

NH4+, alkali metals cations

PO43-

NH4+, alkali metals cations

OH-

Alkali metal cations, Ca2+,Sr2+, Ba2+, NH4+

Evaluation of the results was based on observing solid deposits (precipitates) formed on the lower parts of the test tubes. The type of reactions and the salts produced was determined based on the reactants used.

Methods

Using labels test tubes were labeled DD1 to DD6 (DD standing for double displacement) and SS1 to approximately 1ml of each solution was used which made about ¼ inch of solution in the bottom of the smaller test tubes in the lab drawer. The reactions were then given ample time to occur

Double displacement reaction

In test tube labelled DD1, 0.1M NaOH solution was added dropwise to 0.1M MgCl2 until equal volumes of NaOH was added. In test tube labelled DD2, 2ml of NaOH was quickly added to equal volumes of 0.1M H2SO4. In DD3, 0.1M ZnCl2 was added to an equal volume of 0.1M Na2S dropwise. In test tube labeled DD4, solid CaCO3 was added to 0.1 M HCl dropwise and in test tube DD5, solid CaCO3 was added to 0.1M H2SO4 dropwise and test tube labelled DD6, 0.1M NH4OH was added dropwise to 0.1M CuCl2 while shaking and making observations after each drop.

Single displacement reactions

To test tube labeled SD1, zinc metal was dropped into 3.0M H2SO4. To test tube SD2, zinc metal was added to 0.1M CuSO4. \

Results

The results were based on the solubility rules of anions as per the reference table 1. Observation was based on evolution of gases, precipitates, heat, colorless liquid produced and temperature changes.

Table 2: Showing names of reactant, products and expected observations

REACTION NUMBER

REACTANTS NAMES AND FORMULAS

PRODUCT NAMES AND FORMULAS

OBSERVATIONS

DD1

Sodium hydroxide, NaOH

Magnesium chloride, MgCl2

Magnesium hydorxide, Mg(OH)2

Sodium chloride, NaCl

White precipitate

DD2

Sodium hydroxide, NaOH

Sulfuric acid, H2SO4

Sodium sulfate, Na2SO4

Water, H2O

Heat

Colorless liquid

DD3

Zinc chloride, ZnCl2

Sodium sulfide, Na2S

Zinc sulfide, ZnS

Sodium chloride, NaCl

White precipitate

DD4

Calcium carbonate, CaCO3

Hydrochoric acid, HCl

Calcium chloride, CaCl2

Water, H2O

Carbon (IV) oxide, CO2

Effervence, gas production

DD5

Calcium carbonate, CaCO3

Sulfuric acid, H2SO4

Calcium sulfate, CaSO4

Water, H2O

Carbon (IV) oxide, CO2

Effervences

DD6

Ammonium hydroxide, NH4OH

Calcium chloride, CaCl

Ammonium chloride, NH4Cl

Calcium hyroxide, Ca(OH)2

Temperature changes

SD1

Zinc, Zn

Sulfuric acid, H2SO4

Zinc sulfate, ZnSO4

Water, H2O

Colorless liquid and temperature changes

SD2

Zinc, zn

Copper sulfate, CuSO4

Zinc sulfate, ZnSO4

Copper, Cu

Blue-green deposit

Table 3: Balanced molecular equations

REACTION NUMBER

BALANCED MOLECULAR EQUATION

DD1

2 NaOH(aq) + MgCl2 (aq) 2NaCl (aq) + Mg(OH)2 (s)

DD2

2NaOH(aq) + H2SO4 (aq) Na2SO4(aq) + 2H2O (l)

DD3

ZnCl2 (aq) +Na2S (aq) ZnS (s) + 2 NaCl (aq)

DD4

CaCO3 (aq) + 2HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l)

DD5

CaCO3 (aq) + H2SO4 (aq) CaSO4 (aq) + CO2 (g) + H2O (l)

DD6

2NH4OH (aq) + CuCl2 Cu (OH)2(s) + 2NH4Cl(aq)

SD1

Zn(s) + H2 SO4 (aq) ZnSO4 (aq) + H2O(l)

SD2

Zn(s) + CuSO4 (aq) Cu(s) + ZnSO4 (aq)

Additional procedural details are available in the SOU General Laboratory Manual.

Discussions

DD1 white solid formed due to the formation of magnesium hydroxide precipitate.

2 NaOH (aq) + MgCl2 (aq) 2NaCl (aq) + Mg(OH)2 (s)

This is because this type of reaction is a double displacement in which sodium and magnesium ions exchange their cations to form sodium chloride and magnesium hydroxide. However, magnesium hydroxide is slightly soluble in water hence the white precipitate formed.

DD2 heat was produced as the reaction was highly exothermic

2NaOH (aq) + H2SO4 (aq) Na2SO4(aq) + 2H2O (l)

This is neutralization type of reaction which results to the production of salt and water as the only products. Most neutralization reactions are exothermic reactions hence the heat produced during the reaction.

DD3 white precipitate observed due to the formation of a white precipitate Zinc sulfide

ZnCl2 (aq) +Na2S (aq) ZnS (s) + 2 NaCl (aq)

Zinc sulfide is insoluble in water. It forms a white precipitate in aqueous solutions. When zinc chloride and sodium sulfide react, zinc and sodium exchanges their anions resulting to the formation of zinc sulfide which is seen as a white precipitate.

DD4 gas was produced due to production of carbon (IV) oxide gas

CaCO3 (aq) + 2HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l)

The bubbles seen was a result of production of carbon (IV) oxide. This because the carbonate ions in the calcium carbonate reacts with the hydrogen ions in the hydrochloric acid to form carbon (IV) oxide and water. The carbon (IV) oxide is seen as bubbles in the test tube.

DD5 white precipitate was produced accompanied by slight increase in temperature

CaCO3 (aq) + H2SO4 (aq) CaSO4 (aq) + CO2 (g) + H2O (l)

DD6 a green precipitate was formed accompanied by loss in temperature.

2NH4OH (aq) + CuCl2 Cu (OH)2(s) + 2NH4Cl(aq)

SD1 effervesces was produced

Zn(s) + H2 SO4 (aq) ZnSO4 (aq) + H2O(l)

Zinc displaces hydrogen ions from the sulfate ions in sulfuric acid. The displaced hydrogen ions react with oxygen to form water which is seen as colorless liquid on the cooler parts of the liquid. Such a reaction is described as a single replacement reaction.

SD2 Zinc turned black as depicted in the color changes observed.

Zn(s) + CuSO4 (aq) Cu(s) + ZnSO4 (aq)

This type of reaction is single replacement. Zinc replaces copper ion in the copper sulphate salt. This is because zinc is more reactive than copper hence displaces copper from its ions as seen in the reaction. Consequently, copper is left as metal as zinc displaces it from the sulfate ions forming a black solid deposit of copper.

Chemical reactions occur when atoms of same or different elements exchange electrons to form new products. Chemical reactions can be categorized as single replacement where a reactive cation replaces a less reactive action resulting to deposition of the less reactive element, double displacement where cations change anions of their salts forming a precipitate and neutralization reaction involving the reaction between a base and an acid to produce salt and water as the only products.

References

1- Observing Chemical Reactions/Writing Chemical Equations: General Chemistry Experiments: A Manual for Chemistry 204,206, and 206, Department of Chemistry, Southern Oregon University: Ashland, OR, 2016; pp 34- 36.

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