ClassAct SRS enabled.

1. ClassAct SRS enabled. • In this presentation you will: • Explore acids and bases, the pH scale and buffers.

2. Acids and bases are among the most common chemicals in our everyday lives, as well as being among the most important chemicals for laboratories and industry. For example, acids are used to unblock waste pipes in the home and to remove oxide from metals before they are painted. Bases are used as household cleaning products and the manufacture of both paper and artificial fibers. There is a huge range of different types of acids and bases, but each group has common properties. We will learn about some of these properties in this presentation. Next >

3. Water - + + Acid, BaseOr Salt Ions Theories of Acids and Bases In 1884, a Swedish chemist called Svante Arrhenius established the first comprehensible theory of the behavior of acids and bases in solution. He proposed that, when dissolved in water, acids, bases and salts split or dissociate into charged particles called ions, becoming electrolytes due to the good electric conduction of ion solutions. Next >

4. Water + hydrogen Acid - hydroxide Base + Acid Theories of Acids and Bases According to Arrhenius: Acids are considered to be electrolytes that produce the hydrogen ion, H+. Bases are substances which produce the hydroxide ion, OH-. Nowadays we know that acid solutions do not contain simple H+ ions but hydrated protons, H3O+ called oxonium ions. Next >

5. Question 1 What is the name given to the hydrated protons H3O+ that react in a acid-base reaction? A) Electrolyte ions B) Dissociated ions C) Hydrogenous ions D) Oxonium ions

6. + Theories of Acids and Bases Many acid-base reactions occur in solvents other than water, or even with no solvent at all. One such reaction occurs between hydrogen chloride (acid) and ammonia (base). These two gases react together to form ammonium chloride. HCI(g) + NH3(g) → NH4Cl(s) Next >

7. Theories of Acids and Bases In 1923, chemists J. N. Brønsted and T. M. Lowry defined acids as proton donors and bases as proton acceptors. Consequently, in the reaction between hydrogen chloride and ammonia, the hydrogen chloride molecule can be seen as the proton donor and the ammonia molecule as the proton acceptor. Transferred Proton NH3(g) + HCI(g) → NH4Cl(s) Next >

8. Question 2 How does the Brønsted-Lowry model defines acids substances? A) As electrolytes that produced the hydrogen ion H+ when dissolved in water. B) Acids are substances which act as proton donors.

9. Amphoteric Substances Some substances can react as both acids and bases depending what they are reacting with. An example of this is water, which behaves as an acid when it reacts to ammonia (aqueous solution), donating a proton. Transferred Proton H2O(l) + NH3(aq) → OH-(ac) + NH4+(aq) Conversely, it acts as a base with hydrogen chloride, receiving a proton in the reaction. Transferred Proton H2O(l) + HCl(aq) → H3O+(ac) + Cl-(aq) Substances behaving as a base in some reactions and as an acid in others are called amphoteric. Next >

10. Question 3 What is an amphoteric substance? A) An acid which reacts with substances other than water. B) A substance that can react both as an acid and as a base. C) A substance that does not react neither as an acid or as a base.

11. Lewis Model In 1923, G. N. Lewis proposed a new alternative model to explain acids–base reactions: An acid is a substance able to accept and share a pair of electrons and a base is a substance able to donate and share a pair of electrons. For example, ammonia shares a pair of nitrogen electrons when mixed with boron trifluoride. Lewis definitions expanded acids and bases models beyond both Arrhenius’s and Brønsted-Lowry’s models. + Acid Base Acid-Base Next >

12. Question 4 Is the following statement true or false? 'Lewis model proposes that an acid is a substance able to accept and share a pair of electrons and a base is a substance able to donate and share a pair of electrons'. Answer True or False.

13. Lewis Model These three theories, Arrhenius’s, Brønsted-Lowry’s and Lewis’s, explaining acid-base reactions are not contradictory but each expands the previous model by adopting a wider perspective. For example, the ion OH-, recognised as a base by Arrhenius, is a base too according to Brønsted-Lowry, since it is a proton receptor. Furthermore, this ion is a Lewis base since it is an electron pair donor. Next >

14. Question 5 Which of the following statements is NOT true? A) Arrhenius’s, Brønsted-Lowry’s and Lewis’s models are not contradictory. B) Arrhenius’s, Brønsted-Lowry’s and Lewis’s models each expands the previous model adopting a wider perspective. C) The latest theories, Lewis’s model, invalidates all previous definitions about acid-base reactions.

15. - - + + + Neutralization The reaction of an acid with a base, mixed in the right amounts, produces a substance which has lost the properties of the original substances. This reaction is called neutralization. Neutralization of an Acid and a Base Some acids react with bases to form salts and water. + These salts will be formed by the cation (positive ion) of the base and the anion of the acid. For example, hydrochloride acid reacts with potassium hydroxide, a base, to form water and potassium chloride, a salt. HCl + KOH → KCl + H2O Next >

16. Question 6 The reaction of an acid with a base mixed in the right amounts, produces a substance which has lost the acidic and basic properties of the original substances. What is this reaction called? A) Amphoteric B) Dissociation C) Donation D) Neutralization

17. Acids and Bases Strength Acids and bases are classified as strong and weak, depending on the degree in which they ionize in a solution. The strongest acids and bases ionize totally while weaker ones ionize partially. In a strong acid, the attraction between H+ and A- is low, resulting in complete ionization. Therefore, the strength of an acid can be measured by its dissociation when transferring a proton to water, producing the oxonium ion and the strength of a base can be measured by the degree of acceptance of a proton from water. In a weak acid, the attraction between H+ and A- is high, resulting in partial ionization. Next >

18. Acids Formula Present in Sulfuric acid H2SO4 Batteries Nitric acid HNO3 Fertilizers Hydrochloric acid HCI Gastric juices Citric acid C6H8O7 Lemon juice Acetylsalicylic acid C9H8O4 Aspirin Acetic acid C2H402 Vinegar Acids and Bases Strength Examples of strong acids are sulfuric acid H2SO4 and nitric acid HNO3. The terms strong and weak should not be confused with the terms concentrated and dilute. Strong and weak refer to the extent of dissociation of an acid or a base. Concentrated and dilute refer to the amount of acid or base in a given volume of water. Next >

19. Question 7 What does the strength of an acid or base depends on? A) The degree of dissociation in which they ionize. B) The amount of acid or base in a given volume. C) Their degree of neutralization.

20. pH values of some common substances H+ concentration moles per liter Household Lye Bleach Ammonia Milk of Magnesia Borax Baking Soda Sea Water Blood Distilled Water Milk Corn Boric Acid Orange Juice Vinegar Lemon Juice Battery Acid pH Scale Acids and bases can be measured with the pH scale which specifies the concentration of hydrogen ions in a solution, [H+]. A neutral solution like pure water has a pH of 7, the strongest acids will have a pH of 0 and the stronger bases a pH of 14. A change in the acidity or basicity of a solution of one whole pH unit, represents a change of actually 10 times in the concentration of hydrogen ions in the solution. Next >

21. Question 8 The concentration of hydrogen ions in a solution with a pH of 1 is...? A) Twice the concentration of a solution with a pH 2. B) Half the concentration of as a solution with a pH 2. C) Ten times the concentration of a solution with a pH 2. D) A tenth of the concentration of a solution with a pH 2.

22. Buffer Solutions Buffers are solutions with a pair of substances that resist a change in pH when small amounts of acid or base are added so that they keep a nearly constant pH. They contain a reservoir of the acidic component to neutralize OH- ions and a reservoir of basic component to neutralize H+ ions. Thus, it is essential that these two components must be able to coexist in a solution without completely neutralizing each other. Often buffers are made of weak acid-base conjugate pairs, for example, acetic acid, C2H4O2 with its acetate base C2H3O2-. Base C2H3O2 Acetic Acid C2H4O2 Next >

23. Question 9 What are buffer solutions? A) Solutions capable of maintain their pH when an acid or a base is added to them. B) Solutions which change drastically their pH when an acid or a base is added to them.

24. Summary After completing this presentation you should be able to: • Show knowledge and understanding about acids and bases and their models. • Show knowledge and understanding about neutralization. • Show knowledge and understanding about the strength of acids and bases. • Show knowledge and understanding of the pH scale. • Show knowledge and understanding of buffers. End >