Types of Chemical Reactions
This week we will perform reactions that are examples of two extremely important types. of reaction: oxidation/reduction (single replacement) and acid-base neutralization (double replacement). As we will see, both of these reaction types are very useful in manufacturing and testing the things we use. We will also see that these reactions may also damage and destroy valuable and important objects.
I. Oxidation-Reduction reactions.
This type of reaction always involves the transfer of electrons from one chemical species to another. Most of the chemical reactions that occur in living organism are oxidation-reduction reactions. The oxidation-reduction reaction most commonly encountered in living systems is the metabolism of sugars, such as glucose (C6H12O6).
C6H12O6 + 6O2 à 6CO2 + 6H2O
There are however, many other oxidation-reduction reactions. We will perform two of them here.
A. The Oxidation of Copper.
In this reaction, Cu is oxidized (electrons are removed) to Cu2+, while Fe3+ is reduced (electrons are added) to Fe2+. The reaction equation is as follows:
Cu(s) + 2Fe3+(aq) à Cu2+(aq) + 2Fe2+(aq)
Since the Fe3+, Cu2+, and Fe2+ are all soluble in water, this reaction can be used to remove Cu metal from a surface. Such a capability is very handy when producing circuit boards for computers, calculators, and other electronic devices: the circuit is printed on a copper-coated plastic board so that only the copper that we wish to remove is exposed to the Fe3+ solution.
Procedure:
1. Obtain a piece of copper-clad board. Using the transfer letters supplied, transfer your initials to the copper side of the board.
2. Place 30 mL of FeCl3 solution into a clean 150 mL beaker.
3. Gently place the board copper face up into the beaker containing the FeCl3 solution.
4. Set the beaker aside for 40 minutes, while you proceed to the next part of the experiment. After 40 minutes, swirl the beaker and inspect the board. If no more copper is visible, remove the board and rinse it in tap water. If copper remains visible on the board, place it back in the FeCl3 solution and check it every 15 minutes until no copper remains.
5. When the board is etched (i.e., no copper is visible and only your initials remain), pour the FeCl3 solution down the drain and rinse with tap water.
6. Buff the initials lightly with a scouring pad on the board until all the black is removed and the copper underneath is bright and shiny.
7. Save the board and tape it to the space provided in your report.
B. Silver mirror
The silvering reaction performed here is very similar to that used to commercially produce mirrors. Many of the great astronomical telescopes consist of huge curved mirrors which magnify images of the stars and planets; these mirrors are silvered by this process. The Hubble Space Telescope uses a large concave mirror to focus light from distant stars.
Procedure:
1. We will perform our silver mirror reaction directly inside a clean Cracker Barrel syrup bottle. First, pour 18 mL of the 0.1 M AgNO3 solution into the syrup bottle (You may wish to wear gloves, since silver nitrate stains skin). Then add 15 M NH4OH solution a few drops at a time. A brown precipitate will form. After the brown precipitate is formed, continue adding NH4OH solution a few drops at a time, until the brown precipitate just dissolves. BE CAREFUL. Use the 15 M NH4OH.
2. Add 8 mL of 0.80 M KOH solution to the syrup bottle. If the brown precipitate forms again, add the 15 M NH4OH dropwise until the brown precipitate just dissolves once more.
3. Place 1.5 mL of the 0.25 M dextrose solution into the syrup bottle.
4. Stopper the bottle, holding the stopper firmly in place with your thumb, and shake the bottle vigorously, so that the liquid comes in contact with the entire inner surface. Continue to shake until a silver coating appears on the bottle. This may take from one to fifteen minutes to work, depending upon how carefully you added your reagents in steps 1 and 2. If you were very sloppy, the reaction will not work at all.
5. After the mirror has formed, pour the contents of the bottle into the large container marked “Silver Waste.” DO NOT leave the solution in your bottle; the solution could, in a few hours, produce a potentially explosive precipitate. Then rinse your bottle once or twice with distilled water from the distilled water tap, and pour that wash down the drain.
6. Return your rubber stopper to the stockroom, and you will be issued a small cork and a plastic screw cap. Firmly insert the cork into the bottle neck, and press the top of the cork against the edge of the bench top to push it in until it is flush with the top of the bottle. Then screw the plastic cap in place.
II. Acid-Base neutralization
Acid-base neutralizations do not involve the direct transfer of electrons from one chemical species to another. Rather, a hydrogen ion (H+) is transferred from an acid to a receiving substance, called a base. Often, the base contains the hydroxide (OH-) so that the neutralization has the pattern
Acid + Base à Salt + H2O
The H+ from the acid combines with the OH- from the base to create H2O.
Sometimes, however, H+ is transferred to a base that does not contain OH-. For example, hydrochloric acid (a component of stomach juices) can undergo acid-base neutralization with sodium bicarbonate:
HCl + NaHCO3 à NaCl + CO2 + H2O
Acid-base neutralizations are commonly accompanied by the generation of heat, consequently, a rise in temperature is one of the chief evidences for reaction in the case of acid-base neutralizations. However, chemists like visual evidence for reaction, so they often add an indicator to the reaction flask to track the reaction’s progress. An indicator is a compound that is one color in an acidic solution, and another color in a basic solution.
In this experiment, we will add a few drops of phenolphthalein indicator to the reaction mixtures. Phenolphthalein is pink in a basic solution and colorless in an acidic solution. (In addition to its use as an indicator, phenolphthalein is a powerful laxative — be careful!)
A. The Reaction of Hydrogen Ion with Hydroxide Ion
In this reaction, we will react hydrochloric acid (HCl) with sodium hydroxide (NaOH):
HCl + NaOH à NaCl + H2O
The hydrogen ion is transferred from the acid to the hydroxide ion of the base.
Procedure:
1. Place 20 mL of 0.1 M NaOH solution in a clean 100 mL beaker. Add 4 drops of phenolphthalein indicator. What do you observe?
2. Slowly add 20 mL of 0.1 M HCl solution to the beaker. What evidence do you see that a reaction has occurred?
3. If your solution is still pink, continue to add a little 0.1 M HCl until the color changes. How many mL of HCl did you add altogether?
B. Ester Formation: A Flavorful Experience
Esters are among the most important groups of organic compounds. An ester consists of two carbon/hydrogen parts linked by a structure made up of a carbon and two oxygens. An example is pentyl acetate:
Chemists are very interested in esters because many of them function as flavorings. in food. The pentyl acetate shown above has the flavor of banana; other flavors include octyl acetate (orange flavor), and methyl anthranilate (grape flavor).
One of the ways developed to synthesize esters uses an organic chemistry version of an acid-base neutralization. An organic acid, containing H+ bonded to the -C-O- group, reacts with an alcohol to produce the ester and water. For example, pentyl acetate is made by the reaction
Note that the alcohol consists of an -OH group bonded to a carbon/hydrogen group. Note also that H2SO4 serves as a catalyst, i.e., it speeds up the production of pentyl acetate.
In our experiment, we will make a very familiar ester, methyl salicylate, from methanol (wood alcohol) and salicylic acid (a relative of aspirin). A more familiar name for methyl salicylate is oil of wintergreen.
Procedure:
1. Place about 2 g of salicylic acid, 30 mL of methanol, and 20 drops of concentrated H2SO4 solution into a 150 mL beaker labeled with your name.
2. Heat the beaker with contents on a hot plate for about 10 minutes.
3. Remove the beaker form the hot plate, add 30 mL of water to the beaker, and allow the beaker to cool. Make observations of the appearance and odor of the beaker’s contents.
4. After you've made your observations, pour the contents of your beaker into the sink, followed by plenty of water.
Date ______Name ______
I. Oxidation-Reduction Reactions:
A. The oxidation of copper.
Write your observations, listing the characteristics of the board and solution both before and after the reaction.
Note those that are evidence that a chemical reaction is taking place with an *
Before reaction / After reactionCopperboard
FeCl3 solution
The copper atom has been changed to a copper ion. Did the atom gain or lose electrons?
______
The Fe+3 has been changed to Fe+2. Did the Fe+3 gain or lose electrons?
______
B. The silver mirror: Write your observations below, noting the changes that occur in the solutions as they are prepared and in the bottle as it is shaken.
Is the silver ion oxidized or reduced in this reaction? ______
What evidence (color, etc.) do you have for your answer?
II. Acid-Base Neutralizations
A. The reaction of hydrogen ion with hydroxide ion
1. What do you observe when phenolphthalein is added to the NaOH solution?
2. After the 20 mL of 0.1 M HCl is added to the beaker, what evidence do you see that a reaction has occurred?
3. How many mL of HCl did you need to add to make the solution change color? ______
B. Ester formation: a flavorful experience
1. Complete the chemical equation for the formation of methyl salicylate. Use the balanced chemical equation for the formation of pentyl acetate as a guide.
2. What substance or material that you are familiar with has a similar odor to the compound you just prepared (oil of wintergreen)?