Acids, bases, titrations and salt preparations

1st Solubility Rule (alkali metal salts)

1st Solubility Rule (alkali metal salts)

All alkali metal salts (group 1) and NH4+ are soluble.

2nd Solubility Rule (Nitrates)

All nitrate salts are soluble (NO3-)

3rd Solubility Rule (chlorine, bromine, iodine)

Most Cl-, Br-, and I- salts are soluble.

Except Ag and Pb2+

4th Solubility rule (sulfates)

Most sulfate salts are soluble (SO42-)

Except Ba, Pb2+ (CaSO4 is only slightly soluble.)

5th Solubility Rule (Hydroxides)

The only hydroxide salts (OH-) that are soluble are NH4OH, KOH, and NaOH.

Ca(OH)2 is only slightly soluble.

6th Solubility Rule (Carbonates)

The only carbonate (CO32-) salts that are soluble are sodium carbonate, potassium carbonate and ammonium carbonate.

Acid

• Acids dissociate in water to produce an H+ ion. → the presence of this H+ ion makes the solution acidic.

• Acids are proton donors.

• They have a pH of less than 7.

• Acids are a source of hydrogen ions in an aqueous solution.

Alkalis

• Alkalis dissociate in water to produce an OH- ion. → the presence of this OH- ion in a solution makes it alkaline.

• Alkalis are proton acceptors.

• Alkalis are bases that dissolve in water.

• They have a pH of more than 7.

• Alkalis are a source of hydroxide ions in an aqueous solution.

Base

• A base is a substance that can neutralize an acid. They are proton acceptors.

• Not all bases contain OH:

  • Metal carbonates

  • Metal oxides

  • Ammonia

Bases in terms of neutralization

• The reaction between an acid and a base (or an acid and an alkali) is called neutralization. (H+ + OH- → H20)

• The reaction can be seen in terms of proton transfer. → the acid donates protons, which are accepted by the base.

General equation for neutralization reaction

acid + base → salt + water

Reactions of acids

• Salts form when acids react with bases.

• During a neutralization reaction, a salt (ionic compound) is formed.

• The type of salt depends on the acid used.

• Metal oxides, metal hydroxides, and ammonia can act as bases.

Acid + metal oxide →

salt + water

Acid + metal hydroxide →

salt and water

Acid + ammonia →

Ammonium salt

Hydrochloric acid

• Reacts with metals, bases and carbonates forming a chloride salt.

example of: metal + acid → metal salt + hydrogen

Copper + Sulfuric acid → copper sulfate + hydrogen

Sulfuric acid

• Reacts with metals, bases and carbonates to form a sulfate salt.

Example of: metal oxide + acid → metal salt + water

copper oxide + sulfuric acid → copper sulfate + water

Nitric acid

• Reacts with metals, bases and salts, forming a nitrate salt.

Example of: metal carbonate + acid → metal salt + water + carbon dioxide

Calcium carbonate + hydrochloric acid → calcium chloride + H2O + CO2

Describe an experiment to prepare a sample of pure, dry hydrated copper(II) sulfate crystals starting from copper(II) oxide.

(page 179 textbook)

1. Heat

2. Add a spatula of black copper (II) oxide and continue heating.

   (if all the copper oxide disappears, add more copper oxide until there is some left in the beaker.)

   (When there is excess copper oxide, the acid has been neutralized.)

3. Filter off the excess copper oxide and transfer the blue, copper sulfate solution to an evaporating basin.

4. Heat the solution to boil off some of the water.

5. Heat until a saturated solution is formed. Crystals will begin to form.

6. Stop heating and allow it to cool so that larger crystals can form.

7. Blot the crystals with a paper towel.

Equation for making copper sulfate crystals

CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)

Describe and experiment to produce a pure, dry sample of a soluble salt, starting from an insoluble reactant.

Salts from INSOLUBLE metals and bases

1. Measure out the acid into a beaker.

2. Add the solid base (metal oxide, metal hydroxide, or a metal carbonate.)

3. Add a little at a time until no more will dissolve.

4. Warm if necessary to speed up reaction.

5. Filter the mixture to remove the unreacted base and collect the solution of the desired salt.

6. concentrate the solution by heating over a Bunsen, tripod and gauze. This is to make the solution saturated (evaporate some of the water.)

7. Allow the solution to cool, and crystallize slowly at room temperature.

8. Filter any remaining liquid, and dab dry with a paper towel.

Describe an experiment to prepare a pure, dry sample of a soluble salt, starting from an acid and alkali.

Salts from soluble metals and bases

TITRATION METHOD USING AN INDICATOR

1. Measure a known volume of acid into a beaker.

2. Add a suitable indicator.

3. Add a known volume of the alkali a little at a time from a burette, until the indicator just changes color.

4. Mix the known volumes of acid and alkali BUT with no indicator. (the indicator would make the salt impure.)

5. Concentrate the solution by heating over a Bunsen, tripod and gaze. This is to make the solution saturated (evaporate some of the water.)

6. Allow the solution to cool, and crystallize slowly at room temperature.

7. Filter any remaining liquid, and dab dry with a paper towel.

Describe an experiment to prepare a pure, dry sample of an insoluble salt, starting from two soluble reactants

Insoluble salts

1. Take samples of solutions containing 2 compounds, each containing one of the ‘halves’ (ions) in the required salt.

2. Mix together.

3. Filter the mixture to separate the solid precipitate of the required salt from the solution. (The precipitate is the residue in the filter paper.)

4. Wash the solid with distilled water (removes traces of the other substance left in the solution.)

5. Spread the filter paper out to allow the solid to dry (in a lab or warm oven.)

Reactions between metals and acids

• Only metals above hydrogen in the reactivity series will react with dilute acids.

• The more reactive the metal then the more vigorous the reaction will be.

• Metals that are placed high on the reactivity series such as potassium and sodium are very dangerous and react vigorously with acids

• When acids react with metals they form a salt and hydrogen gas:

  metal + acid ⟶ salt + hydrogen

Reactions between metal carbonates and acids

• Acids will react with metal carbonates to form the corresponding metal salt, carbon dioxide and water.

• These reactions are easily distinguishable due to the presence of fizzing, caused by carbon dioxide gas.

Reactions between metal hydroxides/ metal oxides and acids

• A salt and water are produced when acids react with metal hydroxides.

• Metal hydroxides are bases because they neutralize acids.

  acid + metal hydroxide → salt + water

Describe the use of universal indicator to measure the approximate pH value of an aqueous solution 

• Universal indicator can be used to measure how acidic or alkaline a solution is.

• Universal indicator is added to a colorless solution and will change color according to the pH scale.

Describe the use of litmus to distinguish between acid and alkaline solutions

• Litmus is a synthetic indicator.

• Purple = neutral

• Red = acidic

• Blue = alkali

  Litmus paper is usually more reliable, and comes as red and blue litmus paper:

• RED LITMUS - Acid, stays red. Neutral, stays red. Alkali, turns blue.

• BLUE LITMUS - Acid, turns red. Neutral, stays blue. Alkali, says blue

Describe the use of phenolphthalein to distinguish between acid and alkaline solutions.

• Phenolphthalein is a synthetic indicator.

• Colorless = acidic

• Pink = alkali

Methyl orange

• Red/orange = acid

• Yellow = alkali

What is the issue with litmus, methyl orange, and phenolphthalein?

They don’t tell you HOW acidic or alkaline a solution is.

Titration

A way to measure volumes precisely.

• Titrations require an indicator to show the end of the experiment.

• The end point of a titration is usually indicated by a sudden change in pH. However, universal indicator gives a continuous color change.

Rules for choosing an indicator in a titration experiment

A suitable indicator must be chosen when carrying out different combinations of acids & alkalis.

• Strong acid + strong alkali = any indicator

• Weak acid + strong alkali = phenolphthalein

• Strong acid + weak alkali = methyl orange

Litmus

• red in acidic solutions

• purple in neutral solutions

• blue in alkaline solutions.

  To see the color change, you need to use blue paper to test acidic solutions and red paper to test alkaline solutions.

What do litmus, methyl orange and phenolphthalein have in common?

What do litmus, methyl orange and phenolphthalein have in common?

Describe how to prepare a sample of pure, dry lead (II) sulfate

1. mix lead nitrate solution with sodium sulfate solution.

2. stir

3. filter mixture to separate lead sulfate precipitate.

4. wash precipitate with de-ionised water.

5. place lead sulfate (on the filter paper) in warm oven to dry.