4.5: Acids and Bases. SCH3U: Solutions and Solubility. Properties of Pure and Aqueous Substances. Arrhenius Theory. Acid:

Transcription

1 4.5: Acids and Bases Properties of Pure and Aqueous Substances Arrhenius Theory Acid: Acids are sour tasting and corrosive. They react readily with active metals such as Zn, Ca, Mg and carbonate-based rock like limestone and marble. They also cause natural and synthetic dyes to change colour and they burn organic materials such as skin or paper. Ionization: Base: Bases tend to have a slippery feel, taste bitter, and also affect the colour of dyes. Strong bases are very corrosive and chemically breakdown organic substances such as fats, hair and vegetable matter. For this reason they are used in drain cleaners. Dissociation:

2 Type of Substance Empirical Definition Theoretical Definition Acids Bases Neutral

3 Strong and Weak Acids Different acidic solutions have different electrical conductivity. This has been proven scientifically with laboratory evidence. There seem to be two fairly distinctive classes of acids. If we were to test the electrical conductivity of a variety of acids at equal concentration and temperature, we could classify the acids according to a high or low conductivity reading as a strong or weak acid. Strong Acid: Weak Acid: Percentage ionization: Revising Arrhenius Definitions The Arrhenius definitions of acids and bases cannot always explain whether a substance is an acid or base. Na 2 CO 3(aq) 1. Compounds of hydrogen polyatomic ions NaHCO 3(aq) and NaHSO 4(aq) 2. Oxides of metals and non-metals CaO (aq) and CO 2(g) 3. Compounds that contain no hydrogen Al(NO 3 ) 3(aq) 4. Compounds that are bases but contain no hydroxides NH 3(aq) and All experiments testing acids and bases take place in aqueous solutions Collisions occur with water molecules It is very unlikely that the hydrogen ion, H +, actually exists in an acidic solution Hydrogen ions present in solution react with highly polar water molecules to form Strong bases generally ionize to increase the hydroxide ion concentration All ionic hydroxides are strong bases (100% ionization)

4 Weak base molecules or ions partially react with water molecules to produce hydroxide ions E.g. Note that the numbers of atoms and charge are conserved in the balanced equation The Brønsted-Lowry Concept Even the revised definitions of Arrhenius acids and bases are too restrictive to explain reactions of acids and bases that do not involve water According to famous chemists Johannes Brønsted and Thomas Lowry, hydrogen chloride (an acid) donates a proton (H + ) to a water molecule, and ammonia (a base) accepts a proton from a water molecule Acid Reaction Base Reaction Overall NOTE: In this example, hydronium ions present in the hydrochloric acid solution can react directly with dissolved ammonia molecules A substance can only be classified as a Brønsted-Lowry acid or base A substance may gain protons in one reaction and lose protons in another Substances that can act as either a Brønsted-Lowry acid or base, depending on the reaction, are called

5 Conjugate Acid/ Base Pairs According to Brønsted-Lowry, all acid-base reactions require proton transfers to occur. The two molecules or ions that are transferring protons are called conjugate acid-base pairs E.g. *Note: Every acid has a conjugate base and every base a conjugate acid

6 Worksheet 4.5: Acids and Bases 1. What evidence is there that ionic compounds exist as ions in their pure state while molecular compounds, including acids, exist as molecules in their pure state? 2. Based upon their chemical formulas, classify the following chemicals as acid, base, or neutral. (a) H 2 SO 3(aq) (b) NaOH (aq) (c) CH 3 OH (aq) (d) HC 3 H 5 O 2(aq) (e) NaC 2 H 3 O 2(aq) (f) Ba(OH) 2(aq) 3. Acids are molecular compounds, but they don t behave quite like other molecular compounds. What properties make acids unique? 4. According to the Arrhenius theory, what causes the change in colour of litmus paper in a basic solution? in an acidic solution? 5. Write an empirical and a theoretical definition of an acid. 6. For each of the following compounds, indicate whether they dissociate or ionize in aqueous solution. Write ionic equations to represent the dissociation or the ionization. (a) sodium hydroxide (drain cleaner) (b) hydrogen acetate (vinegar) (c) hydrogen sulfate (battery acid) (d) calcium hydroxide (slaked lime) 7. Which of the following acids should be handled particularly carefully? (Use the table of Concentrated Reagents in Appendix C.) Give your reasons. (a) hydrochloric acid (concrete etching) (b) carbonic acid (carbonated beverages) (c) sulfuric acid (car battery acid) (d) acetic acid (vinegar) (e) nitric acid (copper etching) (f) phosphoric acid (rust remover) 8. Pure water is a molecular compound. Can you be electrocuted if you are standing in pure water? Provide your reasoning. 9. Distinguish between a strong and weak acid using the concept of reaction with water.

7 10. What class of substances are strong bases? Explain their properties. 11. What are the properties of a weak base? Explain these properties. 12. Write appropriate chemical equations to explain the acidic or basic properties of each of the following substances added to water. (a) hydrogen bromide (acidic) (b) potassium hydroxide (basic) (c) benzoic acid, HC 7 H 5 O 2(aq) (acidic) (d) sodium sulfide (basic) 13. Theories in science develop over a period of time. Illustrate this development by writing theoretical definitions of an acid, using the following concepts. Begin your answer with, According to [name of concept], acids are substances that (a) the Arrhenius concept (b) the revised Arrhenius concept (c) the Brønsted-Lowry concept 14. Repeat question 13, defining bases. Refer to both strong and weak bases in your answer. 15. According to the Brønsted-Lowry concept, what happens in an acid base reaction? 16. Use the Brønsted-Lowry definitions to identify each of the reactants in the following equations as acids or bases. (a) HCO 3- (aq) + S 2- (aq) HS - (aq) + CO 3 2- (aq) (b) H 2 CO 3 (aq) + OH - (aq) HCO 3- (aq) + H 2 O (l) 17. Complete the following chemical equations to predict the acid base reaction products. (a) HSO 4 - (aq) + PO 4 3- (aq) (b) H 3 O + (aq) + HPO 4 2- (aq) 18. Some ions can form more than one conjugate acid base pair. List the two conjugate acid base pairs involving a hydrogen carbonate ion. 19. Identify the two acid base conjugate pairs in each of the following reactions. (a) H 3 O + (aq) + HSO 3 - (aq) H 2 O (l) + H 2 SO 3(aq) (b) OH - (aq) + HSO 3 - (aq) H 2 O (l) + SO 3 2- (aq)