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Acids and Bases Acids and Bases Chapter 11 Acids and Bases in our Lives Acids and bases are important substance in health, industry, and the environment. One of the most common characteristics of acids is their sour taste. • Lemons and grapefruits taste sour because they contain acids such as citric and ascorbic acid (vitamin C). • Vinegar tastes sour because it contains acetic acid. Acids and Bases in our Lives •We produce lactic acid in our muscles when we exercise. •Acid from bacteria turns milks sour in the products of yogurt and cottage cheese. •We have hydrochloric acid in our stomachs to help digest food and we take antacids, which are bases such as sodium bicarbonate, to neutralize the effects of too much stomach acid. Acids and Bases in our Lives •In the environment, the acidity or pH of rain, water, and soil can have significant effects. •When rain becomes too acidic, it can dissolve marble statues and accelerate the corrosion of metals. •In lakes and ponds, the acidity of water can affect the ability of plants and fish to survive. •The acidity of soil around plants affect their growth. It can stop the plant from taking up nutrients through the roots Acids and Bases in our Lives •The lungs and kidneys are the primary organs that regulate the pH of body fluids, including blood and urine. •Major changes in the pH of the body fluids can severely affect biological activities within the cells. Buffers are present to prevent large fluctuations. Chapter 11 – Acids and Bases • 11.1 Acids and Bases • 11.2 Brønsted-Lowry Acids and Bases • 11.3 Strengths of Acids and Bases • 11.4 Dissociation Constants for Acids and Bases • 11.5 Dissociation of Water • 11.6 The pH Scale • 11.7 Reactions of Acids and Bases • 11.8 Acid-Base Titration • 11.9 Buffers 11.1 - Acids and Bases Describe and name acids and bases. Acids The term acid comes from the Latin word acidus which means “sour.” In 1887, the Swedish chemistry Svante Arrhenius was the first to describe acids as substances that produce hydrogen ions (H+) when they dissolve in water. Acids are Electrolytes Because acids produce ions in water, they are also electrolytes (can conduct electricity). Hydrogen chloride dissociates in water to give hydrogen ions, H+, and chloride ions, Cl- : It is the hydrogen ions that give acids a sour taste. Naming Acids Acids have two common formats: Binary acids: HnX Hn = some number of H’s x=nonmetals Examples: HCl, HBr, H, H2S… Polyatomic acids: HnXOm XOm = polyatomic ion Examples: H2SO4, H3PO4, HClO4… Naming Acids Binary acids: H X Change the ending of the nonmetal n to –ic and insert into the brackets. hydro[nonmetal –ic] acid hydro and acid do not change. HCl HBr H2S Polyatomic Ion Review 2- More O’s = -ate SO4 2- Less O’s = -ite SO3 Chlorine can form 4 polyatomic ions with oxygen: - ClO4 - ClO3 - ClO2 ClO- Naming Acids Polyatomic Acids: HnXOm [Polyatomic ion] acid -ate to –ic -ite to –ous H2SO4 H3PO4 HClO3 Bases • You may be familiar with some household bases such as antacids, drain cleaners, and oven cleaners. • According to the Arrhenius theory, bases are ionic compounds that dissociate into cations and hydrogen ions (OH-) when they dissolve in water. • They are electrolytes. Bases Most Arrhenius bases are formed from a metal from Groups 1 or 2 and one or more hydroxides (OH-) M(OH)n M=metal (OH)n = 1 or more hydroxide group Examples: LiOH, Ca(OH)2 The hydroxide ions give bases common characteristics such as a bitter taste or slippery feel. Naming Bases Bases have the same names that we used for ionic compounds. LiOH NaOH Ca(OH)2 Al(OH)3 Chapter 11 – Acids and Bases • 11.1 Acids and Bases • 11.2 Brønsted-Lowry Acids and Bases • 11.3 Strengths of Acids and Bases • 11.4 Dissociation Constants for Acids and Bases • 11.5 Dissociation of Water • 11.6 The pH Scale • 11.7 Reactions of Acids and Bases • 11.8 Acid-Base Titration • 11.9 Buffers 11.2 – Brønsted-Lowry Acids and Bases Identify the conjugate acid-base pairs for Brønsted- Lowry acids and bases. Arrhenius Acids and Bases The definitions we gave in section 11.1 for acids and bases were first described by Arrhenius. So we call acids and bases described by H+ and OH- as Arrhenius acids and bases. Arrhenius acid: substances that produce H+ in water. Arrhenius base: substances that produce OH- in water. Brønsted-Lowry Acids and Bases In 1923, a pair of scientists, J.N. Brønsted and T.M. Lowry expanded the definitions of acids and bases. The shortcoming of the Arrhenius definitions was that there were many molecules that didn’t have OH- groups that acted like bases. A new set of definitions describing Brønsted-Lowry acids and bases included a greater number of molecules. Brønsted-Lowry Acids and Bases Brønsted-Lowry acid: a substance that donates a hydrogen ion, H+ Brønsted-Lowry base: a substance that accepts a hydrogen ion, H+ Arrhenius acid: produces H+ Arrhenius base: produces OH- + + H = H3O • A free hydrogen, H+, does not actually exist in water. • Its attraction to polar water molecules is so strong that the H+ bonds to a + water molecules and forms a hydronium ion, H3O Brønsted-Lowry acid: donates H+ Brønsted-Lowry base: accepts H+ Brønsted-Lowry Acids + + - • HCl donates its H to water producing H3O and Cl • By donating the H+, HCl is acting as the acid in this reaction. • By accepting the H+, water is acting as a base in this reaction. Brønsted-Lowry acid: donates H+ Brønsted-Lowry base: accepts H+ Brønsted-Lowry Bases + + - • Water gives an H to NH3 forming NH4 and OH + • NH3 acts as the base by accepting the H • Water acts as the acid by donating the H+ Water: a B-L acid and base Water can act as both a Bronsted-Lowry acid or base depending on what it reacts with. Brønsted-Lowry acid: donates H+ Brønsted-Lowry base: accepts H+ Practice Identify the reactant that is a Bronsted-Lowry acid and the reactant that is a Bronsted-Lowry base: + - HBr(aq) + H2O(l ) H3O (aq) + Br (aq) Brønsted-Lowry acid: donates H+ Brønsted-Lowry base: accepts H+ Practice Identify the reactant that is a Bronsted-Lowry acid and the reactant that is a Bronsted-Lowry base: - - CN (aq) + H2O(l ) HCN(aq) + OH (aq) Conjugate Acid-Base Pairs According to Bronsted-Lowry theory, a conjugate acid-base pair consists of molecules or ions related by the loss of one H+ by an acid, and the gain of one H+ by a base. Every acid-base reaction contains two conjugate acid-base pairs because an H+ is transferred in both the forward and reverse directions. Conjugate Acid-Base Pairs When an acid such as HF loses one H+, it becomes F-. HF is the acid, and F- is its conjugate base. * The conjugate is always what is formed by donating or accepting H+. So it is always on the products side. Conjugate Acid-Base Pairs + + When the base H2O gains an H , its conjugate acid, H3O is formed. Conjugate Acid-Base Pairs Now if we combine the two previous examples: Conjugate Acid-Base Pairs Amphoteric Substances Water can act like an acid when it donates H+ or as a base when it receives H+ Substances that can act as both acids and bases are amphoteric. Water is the most common amphoteric substance and its behavior depends on the other reactant. Water will donate H+ when mixed with a base and will accept H+ when mixed with an acid. Amphoteric Substances - Another example of an amphoteric substance is bicarbonate, HCO3 . - + - With a base, HCO3 acts as an acid and donates H to give CO3 . - + With an acid, HCO3 acts as a base and accepts H to give H2CO3 Practice Identify the conjugate acid-base pairs in the following reaction: - + HBr(aq) + NH3(aq) Br (aq) + NH4 (aq) Chapter 11 – Acids and Bases • 11.1 Acids and Bases • 11.2 Brønsted-Lowry Acids and Bases • 11.3 Strengths of Acids and Bases • 11.4 Dissociation Constants for Acids and Bases • 11.5 Dissociation of Water • 11.6 The pH Scale • 11.7 Reactions of Acids and Bases • 11.8 Acid-Base Titration • 11.9 Buffers 11.3 – Strengths of Acids and Bases Write equations for the dissociation of strong and weak acids; identify the direction of reaction. Strong vs Weak In the process called dissociation, an acid or base separates into ions in water . + The strength of an acid is determined by the moles of H3O that are produced for each mole of acid that dissolves. The strength of a base is determined by the moles of OH- that are produced for each mole of base that dissolves. Strong acids and bases dissociate completely in water. Weak acids and bases dissociate only slightly, leaving most of the initial acid or base undissociated. Strong Acids Strong acids are examples of strong electrolytes because they donate H+ so easily that their dissociate in water is essentially complete. + When HCl (a strong acid) dissociates in water, H is transferred to H2O. + - The resulting solution contains essentially only H3O and Cl . • Thus one mole of a strong acid dissociates in water to yield one mole of H3O+ and one mole of its conjugate base. • We write the equation for a strong acid, such as HCl, with a single arrow. Weak Acids Weak acids are weak electrolytes because they dissociate slightly in water, forming only a small + amount of H3O ions. When acetic acid dissociates in water, it donates the H+ to water.
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