Acids, bases and salts 1. Do introductory Activity on Acid and Bases. 2. How do we tell if a substance is an acid ? a) litmus paper - red acid - blue base b) react with magnesium to produce hydrogen 3. How do we tell from a formula if a substance is an acid, base or salt ? HCl - hydrochloric acid NaOH - base H2SO4 - sulfuric acid KOH - base HNO3 - nitric acid Ca(OH)2 - base CH3COOH - acetic acid Al(OH)3 – base 4. Arrhenius Definition of an acid and base: Acid: Any substance that increases the concentration of hydrogen ions in aqueous solution. HNO3(l) -- H+ (aq) + NO3 – (aq) The structure of an H+ ion: If you remember from chemistry 11, a hydrogen atom is composed of a proton and an electron. If we turn the atom into an ion, it loses its electron. The hydrogen ion, H+, can be referred to as a PROTON. A proton in water undergoes the following equilibrium : H+(aq) + H2O (l) <====> H3O+(aq) The chemical species H3O+(aq) is referred to as a HYDRONIUM ion. [H3O+] In the above equilibrium, Keq = ___________ [H+] The actual value for this Keq is 1.0 which means that the [H3O+] must equal the [H+]. For this reason, we will use the terms proton, hydrogen ion and hydronium ion interchangeably. Base: Any substance that increases the concentration of hydroxide ions in aqueous solution. KOH(s) -- K + aq) + OH- (aq) It looks as though that if the formula has an ‘OH’ in it, it will be a base and if it has an ‘H’ it will be an acid. This however is not entirely true : Water can act as a proton acceptor and proton donor. Compounds and ions that can exhibit this dual behaviour are called AMPHIPROTIC. From the activity, you noticed that not all compounds have structures allow obvious classification as an acid or a base. Demo : test a solution of Na2CO3 with litmus paper. It turns blue indicating a basic solution. Clearly we must have another definition for an acid and a base to show exactly what is happening in this situation. Acid and Base Chemistry, Intro Activity Acid indigestion, commercial antacids remedies for indigestion, pH-balanced shampoos- you don’t have to go very far to find references to acid or acidity. Many people think that all acids are corrosive, and therefore dangerous, because strong acids react with many substances. This may be a misconception. For example, your body produces hydrochloric acid in the stomach, and you may have used boric acid to wash your eyes. We want to develop a better understanding of what are acids and bases and where you would find these substances. Investigation: Testing Arrhenius’s Acid-Base Definitions The purpose of this investigation is to test Arrhenius’s definitions of acid and base. A number of common substances in solution are identified as acid, base, or neutral using one or more diagnostic tests. Problem: Which of the substances tested may be classified as acid, base, or neutral? How would you define an acid and a base? Material: - You must wear Lab apron and Safety goggles. - A series of solutions to be tested provided by the teacher. - Material and equipment used for diagnostic tests. (Described by teacher). Procedures: 1. Clean test tubes then obtain small amounts (1/3 full) of each sample. Use markers to label each test tube. Place a clean medicine dropper in each. 2. Use the Corning cell Wells to test each sample with the test chemicals outlined on the table below. Record observations. 3. There are samples in the front bench containing a pH meter. Measure the pH of each sample. 4. Dispose of each reagent as described by instructor. 5. Clean up apparatus, bench and wash your hands. Results: a) Standards: Chemical phenolph- methyl orange litmus paper magnesium PH talein metal Acid Base b) Samples: Chemical phenolph- methyl orange litmus paper magnesium pH talein metal hydrogen chloride (peptic juices ) sodium carbonate (soda ash) sodium hydrogen carbonate (baking soda) sodium hydroxide (lye) ammonia ( house cleaner) forms ammonium hydroxide hydrogen acetate (vinegar) Sprite Hydrogen carbonate lemon Juice Aspirin potassium hydroxide hydrogen sulphate Magnesium hydroxide Synthesis: 1. Write the chemical formulas of each compound tested. Write it on the table just below the name. Can you see any similarities in the chemical structure of the acids? The bases? Acids Bases 2. Based on your observations, how would you differentiate an acid from a base? 3. Using your textbook as reference, write out the Arrhenius definition of acid and bases. Conclusions: From the lab, we should be able to identify an acid and a base from its properties and its formula. 1) In short, the chemical properties of acids and bases are as follows: Acids Bases 2) Examining the chemical formula, we see that when an acid dissolves in water: and Bases dissolves in water: So we can define an Acid as_______________________________ and a Base as__________________________________________ 3) When an acid and a base react, the result is a __________________ and ____________________. This type of reaction is called _______________. The reason why is because the properties of the acid and the base are _____________________ by each other. eg. The net ionic equation for this reaction: For a neutralization reaction, what do we know about the number of moles of H + and the number of moles of OH-? Chemistry 12 Arrhenious Definition of Acids and Bases Neutralization Problems: Moles of H+ = mol OH - 1) What volume of 6.00 M HCl must be added to 125 mL of 1.59 M NaOH in order to neutralize it? 2) What volume of 0.10 M Sr(OH)2 is needed to neutralise 115.2 mL of 0.55 M H3PO4? Be very careful with this one! Watch the mole ratio of OH- 3) What volume of 0.23 M NaOH is needed to neutralize 100.0 mL of 0.10M HCl? 4) What volume of 0.30 M KOH is needed to neutralize 50.0 mL of 0.20 M H 2SO4? Incomplete neutralization! When moles of H + is different than OH -. 1) What is the concentration of hydroxide ion left in solution, when 25.0 mL of 0.203 M NaOH is mixed with 40.0 mL of 0.080 M HCl. 2) What is the concentration of H+ ion left in solution when 25mL of 0.20 M H2SO4 reacts with 35 mL of 0.20 M NaOH. 3. What is the concentration of OH – ion when 40.0 mL of 0.30 M Ca(OH) 2 is mixed with 30.0 mL of 0.35M HNO3? 4. A student mixes 30.0 mL of 0.20M Sr(OH) 2 with 50.0 mL of 0.23M HNO3. Is the mixture acidic or basic?