b) To become familiar with some of the observable signs of these reactions,
c) To identify the products formed in each of these reactions,
d) To write balanced chemical equations for each double displacement reaction studied.
During a chemical reaction both the form and composition of matter are changed. Old substances are converted to new substances, which have unique physical and chemical properties of their own. Some of the observable signs that a chemical reaction has occurred include the following:
· A metallic deposit appears
· Bubbles appear
· A temperature change occurs
· A color change occurs
· A precipitate (cloudy, tiny particles) appears
Note that there are many other observable signs for chemical reactions, but these are the ones most likely to be encountered in this lab.
All double displacement reactions have the general form: AB + CD ® AD + CB
Reactions that can be classified as double displacements include precipitation reactions, neutralization reactions and gas forming reactions.
Here AB and CD are usually aqueous ionic compounds (or acids) consisting of aqueous ions (A+ and B-, C+ and D-). When a double displacement reaction occurs, the cations and anions switch partners, resulting in the formation of two new ionic compounds AD and CB, one of which is in the solid state. This solid product is an insoluble ionic compound called a precipitate. To determine whether a product ionic compound will be soluble or insoluble, consult the Solubility Rules provided at the end of the Background section. Note that if both of the predicted products are soluble, a precipitation reaction will not occur.
Example1: aqueous lead(II) nitrate + aqueous sodium chloride
The predicted products are lead(II) chloride (insoluble) and sodium nitrate (soluble). Since one of the predicted products is insoluble, a precipitation reaction is will occur.
Reaction Equation: Pb(NO3)2 (aq) + 2 NaCl (aq) ® 2 NaNO3 (aq) + PbCl2 (s)
Here AB is an acid (consisting of H+ and X- aqueous ions) and BC is a base (consisting of M+ and OH- ions). When a double displacement reaction occurs, the cations and anions switch partners, resulting in the formation of water and a new ionic compound (or salt), which is usually soluble. Neutralization reactions are exothermic, and are generally accompanied by a noticeable release of heat.
Example 2: sulfuric acid + aqueous lithium hydroxide
The predicted products are water and lithium sulfate.
Reaction Equation: H2SO4 (aq) + 2 LiOH (aq) ® Li2SO4 (aq) + 2 H2O (l)
Gas Forming Reactions
In these reactions one of the products (AD or CB) after the double displacement is in the gaseous state, such as hydrogen sulfide (H2S) or ammonia (NH3). One of the products could also be carbonic acid (H2CO3) or sulfurous acid (H2SO3). Both carbonic acid and sulfurous acid are unstable and will decompose to form carbon dioxide and sulfur dioxide gases, respectively:
H2CO3 (aq) ® H2O (l) + CO2 (g)
H2SO3 (aq) ® H2O (l) + SO2 (g)
Example 4: nitric acid + aqueous sodium sulfite
The predicted products are sulfurous acid and sodium nitrate. However sulfurous acid decomposes to sulfur dioxide and water:
Reaction Equation: 2 HNO3 (aq) + Na2SO3 (aq) ® 2 NaNO3 (aq) + H2SO3 (aq)
Final Equation: 2 HNO3 (aq) + Na2SO3 (aq) ® 2 NaNO3 (aq) + H2O (l) + SO2 (g)
· Write the correct formulas for each reactant and place a yield arrow (®) after the last reactant.
· Identify the reaction type – precipitation, neutralization or gaseous, using the guidelines above.
· If you determine that a reaction will occur, write the correct formula(s) of the products after the arrow. If you determine that a reaction will not occur, simply write “no reaction” after the arrow.
· Balance the equation (to ensure mass conservation).
· Be sure to include the physical states of all reactants and products in your final equation.
1. Alkali metal compounds, acetates, nitrates, and ammonium compounds are all soluble.
2. Hydroxides of alkali metals and NH4+1, Ca+2, Sr+2, and Ba+2 are soluble. All others are insoluble.
3. All halides (chlorides etc.) are soluble except for those containing Ag+1, Pb+2, and Hg2+2.
4. Most sulfates are soluble, except for BaSO4, SrSO4, Ag2SO4, PbSO4, and CaSO4.
5. Most phosphates, carbonates, chromates and sulfides are insoluble (except those of the alkali metals and ammonium).
6. In addition, all acids are soluble!
Be especially cautious when using the 6M HCl, 3M H2SO4 and 6M NaOH as they can burn your skin. Also be aware that skin discoloration will result from contact with AgNO3. If you feel any tingling sensations or see any color changes on your skin, flush with water immediately for a minimum of 15 minutes. Inform your instructor of any chemical contact as soon as possible.
Materials and Equipment
Solids: solid sodium hydrogen carbonate
Solutions: 6M sodium hydroxide, 3M sulfuric acid, 6M hydrochloric acid; all other solutions are 0.1M and include silver nitrate, sodium chloride, iron(III) chloride, ammonium hydroxide, sodium carbonate, cobalt(II) nitrate, sodium phosphate, copper(II) sulfate, potassium nitrate, nickel(II) nitrate, barium chloride.
Equipment: 9 small test tubes, plastic test tube rack
· Use approximately 3-mL quantities of all solutions. A good estimate is to use three full dropper squirts of each chemical.
· When finished, complete the data sheet by writing the balanced equation for each reaction.
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