Unit 4: TOXINS Stoichiometry, Solution Chemistry, and Acids and Bases Living By Chemistry Unit 4: TOXINS Stoichiometry, Solution Chemistry, and Acids and Bases

In this unit you will learn: how toxins are defined how chemists determine toxicity the mechanisms by which toxic substances act in our bodies and what this has to do with chemical reactions

Section IV: Acidic Toxins Lesson 17 Heartburn Lesson 18 Pass the Proton Lesson 19 pHooey! Lesson 20 Watered Down Lesson 21 Neutral Territory Lesson 22 Drip Drop

Lesson 17: Heartburn Acids and Bases

ChemCatalyst Countless products are advertised on TV with the promise of reducing acid indigestion. What is acid indigestion? What does acid have to do with your stomach? How do you think antacids work?

Key Question What are the properties of acids and bases?

You will be able to: identify acids and bases based on general observable properties explain how an indicator is used to determine whether a solution is acidic, basic, or neutral

Prepare for the Lab Work in groups of four. Indicator: An indicator is a molecular substance that changes color when it comes into contact with an acid or a base. Acids and bases are corrosive. Do not get solutions on skin. In case of a spill, rinse thoroughly with water. Wear safety goggles at all times.

Discussion Notes The substances on the left side of the number line (from 0 to 7) are all acids. The substances on the right side of the number line (from 7 to 14) are called bases. The substances located in the middle of the number line (around 7) are called neutral substances.

Wrap Up What are the properties of acids and bases? Acids and bases are solutions that are classified according to their observable behavior. Acids and bases change the color of indicators. Substances that are not acids or bases are considered neutral.

Check-in An unknown substance is mixed with cabbage juice, and the solution turns purple. The substance does not react with calcium carbonate. Is it an acid, a base, or a neutral substance? Explain.

Lesson 18: Pass the Proton Acid-Base Theories

ChemCatalyst Which of these four solutions conduct electricity: 0.10 M HCl (hydrochloric acid), 0.10 M CH3COOH (acetic acid), 0.10 M NaCl (sodium chloride), 0.10 M C12H22O11 (sugar)? Explain.

Key Question How are acids and bases defined?

You will be able to: define Arrhenius and Brønsted-Lowry acids and bases explain the behavior of acids and bases on a particulate level explain the difference between strong and weak acids and bases

Prepare for the Activity Work in groups of four.

Discussion Notes Acids are substances that add H+ to solution. Bases are substances that add OH- to solution. Neutral substances do not add H+ or OH- to solution. The definitions of acids and bases have changed over time.

Discussion Notes (cont.) Arrhenius Definition of Acids and Bases: An acid is any substance that adds hydrogen ion (H+) to solution. A base is any substance that adds hydroxide ion (OH-) to solution.

Discussion Notes (cont.) Brønsted-Lowry Definition of Acids and Bases: An acid is a proton donor. A base is a proton acceptor. Acids and bases that break apart (dissociate) completely in solution are called strong acids and strong bases. Acids and bases that do not dissociate completely in solution are called weak acids and weak bases.

Wrap Up How are acids and bases defined? According to the Arrhenius definition, an acid is a substance that adds hydrogen ions, H+, to an aqueous solution. According to the Arrhenius definition, a base is a substance that adds hydroxide ions, OH–, to an aqueous solution.

Wrap Up (cont.) Some substances add OH– by removing H+ from water. Brønsted and Lowry define an acid as a proton (H+) donor and a base as a proton (H+) acceptor. Strong acids and strong bases dissociate completely into ions, while weak acids and weak bases do not.

Check-in Which substances do you expect will conduct electricity: hydrocyanic acid, HCN; magnesium hydroxide, Mg(OH)2; methanol, CH3OH? Explain your thinking.

Lesson 19: pHooey! [H+] and pH

ChemCatalyst Pure water has an H+ concentration of 1.0 x 10–7 M and an OH– of 1.0 X 10–7 M. What does this mean?

Key Question How is pH related to the acid or base concentration of a solution?

You will be able to: explain the mathematical relationship between the H+ and OH– concentrations in a solution define pH and explain the relationship between H+ concentration and pH determine the H+ concentration of a solution given the [OH–], and vice versa

Prepare for the Lab Work in groups of four. Wear safety goggles at all times. Acids and bases are corrosive. Do not get any on skin or near eyes. In case of a spill, rinse with large amounts of water.

Discussion Notes The pH scale is a logarithmic scale that describes the concentration of H+ ions in solution. pH is related to [H+] by the formula pH = -log [H+]

Discussion Notes (cont.) More acidic H+ concentration OH– concentration pH 1.0 X 100 M 1.0 X 10–14 M 1.0 X 10–1 M 1.0 X 10–13 M 1 1.0 X 10–2 M 1.0 X 10–12 M 2 1.0 X 10–3 M 1.0 X 10–11 M 3 1.0 X 10–4 M 1.0 X 10–10 M 4 1.0 X 10–5 M 1.0 X 10–9 M 5 1.0 X 10–6 M 1.0 X 10–8 M 6 1.0 X 10–7 M 7 8 9 1.0 X 10–0 M 14 neutral More basic

Discussion Notes (cont.) In any solution, the product of the hydrogen ion, H+, concentration and hydroxide ion, OH–, concentration is a constant. Water dissociates into H+ and OH– ions.

Wrap Up How is pH related to the acid or base concentration of a solution? The pH scale is a logarithmic scale that describes the concentration of hydrogen ions, H+, in solution: pH = -log [H+]. The H+ concentration is related to the OH– concentration: [H+][OH–] =10–14. So, as [H+] increases, [OH–] decreases, and vice-versa. The pH of water is 7. In water the H+ concentration is equal to the OH– concentration. Thus, water is neutral.

Check-in If you know the pH of a solution, what else do you know?

Lesson 20: Watered Down Dilution

ChemCatalyst Imagine that a sample of drinking water has been contaminated with an acid. The pH is now 4.0. What do you think would happen to the pH if you added more water to the sample? Do you think you could ever get the pH back to 7? Explain why or why not.

Key Question How does dilution affect acids and bases?

You will be able to: complete a serial dilution of a solution explain the effect of dilution on the acidity or basicity of a solution Do simple calculations involving dilution

Dilutions Often in the lab, you need to prepare a solution of a certain molarity by combining a stronger solution with distilled water This is called dilution The amount of “stock” solution and of water you need can be calculated using the equation M1 x V1 = M2 x V2 Where M1 equals the molarity of the original solution M2 equals the new molarity required V1 is the volume of original solution required V2 is the TOTAL volume of the new dilute solution So you must subtract to get the volume of water!

Practice Problems How many mL of .500M KCl solution would you need to dilute to make 100.0 mL of .100 M KCl You have 150 mL of 1.2M HCl. You add 500mL of water. What is the new molarity? You begin with 10 mL of a solution that is pH 3 due to HNO3. If you add 990 mL of distilled water, what is the new pH?

Discussion Notes Adding water to an acid or a base dilutes the solution, making it less acidic or less basic. An acid can never be turned into a base by diluting it with water, and a base can never be turned into an acid by diluting it with water.

Wrap Up How does dilution affect acids and bases? Dilute solutions are less acidic and less basic than more concentrated solutions. Dilution of an acid or a base results in a solution that is increasingly neutral. It is not possible to go past a pH of 7 when diluting an acid or a base.

Check-in If you get lemon juice in your eye, you should flush the eye with lots of water. Why?

Lesson 21: Neutral Territory Neutralization Reactions

ChemCatalyst Excess stomach acid, HCl, can cause extreme discomfort and pain. Milk of magnesia, Mg(OH)2, is often taken to reduce stomach acid. What products do you think are produced when Mg(OH)2 and HCl are mixed? What products do you think are produced when HNO3 and HCl are mixed?

Key Question What happens when acids and bases are mixed?

You will be able to: write a chemical equation for an acid-base neutralization reaction describe how the pH changes when acids and bases are mixed

Prepare for the Lab Work in pairs. Wear safety goggles at all times. Acids and bases are corrosive. Do not get any on skin or near eyes. In case of a spill, rinse with large amounts of water.

Discussion Notes A neutralization reaction between a strong acid and a strong base in aqueous solution produces an ionic compound (salt) and water. A neutralization reaction can be described as a double exchange reaction in which the two compounds exchange cations.

Wrap Up What happens when acids and bases are mixed? A neutralization reaction between a strong acid and a strong base produces an ionic salt and water. When strong acids and bases are mixed, the pH of the product approaches 7 at 25 °C.

Check-in Sulfuric acid, H2SO4(aq), reacts with magnesium hydroxide, Mg(OH)2(aq). Write a balanced equation for the reaction that occurs.

Lesson 22: Drip Drop Titration

ChemCatalyst A student mixes 100 mL of 0.10 M HCl with different volumes of 0.10 M NaOH. 1: 100 mL of 0.10 M HCl + 50 mL of 0.10 M NaOH 2: 100 mL of 0.10 M HCl + 100 mL of 0.10 M NaOH 3: 100 mL of 0.10 M HCl + 150 mL of 0.10 M NaOH a. Which solution is the least toxic? b. Are the final solutions acid, basic, or neutral?

Key Question How can a neutralization reaction help you figure out acid or base concentration?

You will be able to: explain and complete a titration procedure use titration data to determine the molarity of a solution whose concentration is unknown

Prepare for the Lab Work in pairs. Wear safety goggles at all times. Acids and bases are corrosive. Do not get any on skin or near eyes. In case of a spill, rinse with large amounts of water.

Discussion Notes The chemical equation for the neutralization reaction shows the ratio in which the substances combine. HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)

Discussion Notes (cont.) A titration is a procedure in which a neutralization reaction is monitored with an indicator allowing you to calculate the unknown concentration of an acid or base. When the equivalence point is reached in a titration between a strong acid and a strong base, the number of moles of H+ ions equals the number of moles of OH– ions.

Wrap Up How can a neutralization reaction help you figure out acid or base concentration? A titration is a procedure that allows you to calculate the unknown concentration of an acid or a base using a neutralization reaction. During a titration, an indicator is used to signal when the equivalence point has been reached.

Check-in A beaker has 50 drops of HCl, along with a drop of phenolphthalein indicator. After 100 drops of 0.10 M NaOH are added, the color changes from clear to bright pink. What is the concentration of the original HCl solution?