Experiment 16-Acids, Bases and ph

Transcription

1 Definitions acid-an ionic compound that releases or reacts with water to form hydrogen ion (H + ) in aqueous solution. They taste sour and turn litmus red. Acids react with certain metals such as zinc, aluminum and magnesium to generate hydrogen gas (H 2 ) and metal ions. They also react with carbonates (CO 3 2 ) and bicarbonates (HCO3 ) to form carbon dioxide gas (CO2 ), and they react with bases to neutralize their properties. base-an ionic compound that releases or reacts with water to form hydroxide ions (OH ) in aqueous solution. Bases taste bitter and turn litmus blue. They react with many metallic cations, such as Mg 2+, Fe 3+ and Cu 2+, to form insoluble hydroxide precipitates such as Mg(OH) 2, Fe(OH) 3 and Cu(OH) 2. salt-an ionic compound that can be formed from the reaction of an acid and a base. Most ionic compounds between a metal and a nonmetal other than O 2 or OH are salts. ph-a logarithmic method of measuring hydrogen ion concentration. The symbol [H + ] means concentration of hydrogen ions (in moles per liter), [OH ] means concentration of hydroxide ions. ph = log[h + ] [H + ] = 10 ph ph indicator-a substance that changes color in response to a change in ph. Examples include: Indicator ph Color Change litmus red in acid, blue in base grape tannins red in acid, green in base phenolphthalein colorless in acid, pink in base multiple range ph indicator various different colors at different phs. Acidity, Alkalinity, Neutrality of Solutions neutral solution: [H + ] = [OH ] = 10-7 M; ph = ~7 acidic solution: [H + ] > [OH ], [H + ] > 10-7 M, ph < 7. basic solution: [OH ] > [H + ], [H + ] < 10-7 M, ph > 7. (also called an alkaline solution) Strong and Weak Acids and Bases strong acid-an acid that is a strong electrolyte and has a ph < 3. e.g. H 2 SO 4, HCl, HBr, HI, HNO 3 weak acid-an acid that is a weak electrolyte or an ionic compound that partially reacts with water to form hydrogen ions in aqueous solution. It will have a ph greater than 3 but less than 7. e.g. H 2 S, H 3 PO 4, HC 2 H 3 O 2, H 2 CO 3, ammonium salts, acetic acids strong base-a hydroxide that is a strong electrolyte and has a ph > 11. e.g. NaOH, KOH, Ba(OH) 2, alkali metal oxides weak base-a hydroxide (OH ) that is a weak electrolyte or a compound that partially reacts with

2 102 water to form hydroxide ions in aqueous solution. Its ph will be less than 11 but greater than 7. e.g. carbonates, bicarbonates, ammonia (ammonium hydroxide), phosphates, insoluble and slightly soluble metal oxides and hydroxides. salt-an ionic compound produced by reacting an acid and a base. It will have a ph close to 7. TABLE 1: ph Scale -a table of ph values and corresponding hydrogen ion and hydroxide ion concentrations. ph [H + ] [OH ] Acid-Base Common example Rating M M Acidic Swimming pool acid M M Acidic Battery acid M M Acidic Stomach acid, lemon juice M M Acidic Vinegar, soft drinks M M Acidic Apple juice, acid rain M 10-9 M Acidic Urine M 10-8 M Acidic Unpolluted rainwater M 10-7 M Neutral Pure water M 10-6 M Basic Fresh egg white, bile M 10-5 M Basic Baking soda solution M 10-4 M Basic Milk of magnesia M 10-3 M Basic Ammonia solution M 10-2 M Basic Washing (laundry) soda M 10-1 M Basic Dilute lye solution M 10-0 M Basic Strong lye solution Materials: test tubes and beakers, various foods, natural materials and consumer products, red and blue litmus paper multiple range ph indicator paper, drinking straws Solutions provided in dropper bottles for ph determination: 0.1 M hydrochloric acid 0.1 M acetic acid 0.1 M sulfuric acid carbonic acid (carbonated water) 0.1 M sodium chloride 0.1 M potassium sulfate 0.1 M sodium hydroxide 0.1 M ammonium hydroxide 0.1 M potassium hydroxide 0.1 M sodium bicarbonate Solutions for steps 4-8: 6 M HCl 3 M H 2 SO 4 6 M HC 2 H 3 O 2 Mg ribbon NaHCO 3 powder CaCO 3 chips Phenolphthalein indicator solution 0.5 M CuCl 2 1 M NaOH 1 M HCl Hypothesis: What is the ph of various solutions?

3 103 PROCEDURE: 1. Referring to the standard ph color scale for universal indicator paper (shown on the package), test the ph of all the solutions of strong and weak acids and bases provided in lab by placing several drops of the solution on the indicator paper (while holding the paper over the sink!). Record the results in the data table. 2. Test the litmus reaction of all the laboratory acids and bases provided by placing several drops of the solution on the litmus paper (while holding the paper over the sink!). Record the results on your data table. Classify each solution as a strong acid, weak acid, strong base, weak base, or neutral. 3. Use strips of universal indicator paper to test the ph of 10 common materials or consumer products. Record the results on your data table. Solid materials can be crushed and then mixed with a little water. The entire sample of solid need not dissolve. If there are acids or bases present, a sufficient amount will dissolve to give a ph reading. Deeply colored materials may make it difficult to read the ph indicator paper. Examples of substances may include soil, the materials found on the ph scale list, cosmetics, foods, beverages, shampoos, cleaning products, deodorants, juices, health products, or anything else that can, at least in part, dissolve in water. Oily and greasy products do not work well, because they do not dissolve in water. 4. (a) Place about 5 ml of 6 M HCl (hydrochloric acid), 3 M H 2 SO 4 (sulfuric acid) and 6 M acetic acid (HC 2 H 3 O 2 ) into 3 consecutive test tubes. (b) Place a small strip (0.5 inches) of magnesium (Mg) ribbon into each tube of acid, one at a time. After bubbles form in the tube for 1 minute, test the identity of the gas being evolved by holding a lit splint over the top of the tube. If the bubbles form slowly, place a stopper VERY LOOSELY on the top of the tube for 1 full minute, then remove stopper and check with the lit splint. 5. (a) Cover the bottom of a beaker with a teaspoon or so of powdered NaHCO 3 (sodium bicarbonate). Now add 4 to 5 ml of 6 M HCl and immediately cover with a glass plate. Is the gas evolved the same as in part 4b? (You must figure out how to determine this on your own) (b) Repeat 5a, using marble (CaCO3) chips instead of NaHCO3. Is the gas evolved the same as in part 5a? 6. Add 10 drops of 0.1 M HCl (hydrochloric acid) to 25 ml of water in a beaker. Add 2-3 drops of phenolphthalein solution to the beaker and swirl. Slowly add 0.1 M NaOH (sodium hydroxide) dropwise to the beaker, swirling after each drop, until the solution definitely changes color. Then add 0.1 M HCl dropwise to the beaker in the same manner, until the solution changes color. 7. Add 1-3 drops of universal indicator solution to 25 ml of water in a beaker. With a straw, blow big lung-fulls of held breath through the straw into the solution. 8. Add 3 ml of 0.5 M CuCl 2 (copper chloride) solution to a test tube. Add 1 ml of 0.1 M NaOH (sodium hydroxide) and record your observations.

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5 105 Lab Report Name Section Lab partners: DATA TABLE: LABORATORY ACIDS, BASES AND SALTS Name of Substance 0.1 M hydrochloric acid Color of Indicator Paper ph Color of Litmus Paper Type of solution 0.1 M sulfuric acid 0.1 M acetic acid carbonic acid (carbonated water) 0.1 M sodium chloride 0.1 M sodium bicarbonate 0.1 M sodium hydroxide 0.1 M potassium hydroxide 0.1 M ammonium hydroxide 0.1 M potassium sulfate DATA TABLE: ph OF COMMON MATERIALS AND CONSUMER PRODUCTS Material ph Material ph

6 106 Observations and questions from steps 4-8: 4. Observations: Can you identify the gas in 4b? 5. Observations: Is the gas evolved the same as in part 4b? (You must figure out how to determine this on your own) 6. Observations: Record your observations, and write 1-2 sentences describing what is occurring in the beaker. 7. Observations: How many breaths until a color change occurs? Can you determine what gas is responsible for the color change? Does the solution become more acidic or more basic as you blow into it? Explain how you determine this. 8. Observations: What is the new compound that is formed? Write a balanced chemical equation for the reaction, and classify the reaction type.