3.1 Acids Flashcards
pH scale
The pH scale goes from 0 – 14 (extremely acidic substances can have values of below 0).
All acids have pH values of below 7, all alkalis have pH values of above 7.
The lower the pH then the more acidic the solution is.
The higher the pH then the more alkaline the solution is.
A solution of pH 7 is described as being neutral.
Indicator
An indicator is a substance that is used to test if a solution is acidic or alkaline buy turning different colours. universal indicator is a mixture of several different indicators. Whereas most indicators can just show whether a substance is acid or alkali, universal indicator can indicate the strength. The strength is measured on the PH scale.
Hydrogen ions and pH
The more hydrogen ions the stronger the acid, but the lower the pH.
The higher the concentration of hydroxide ions in a solution the higher the pH.
The pH scale is logarithmic, meaning that each change of 1 on the scale represents a change in concentration by a factor of 10.
Therefore an acid with a pH of 3 has ten times the concentration of H+ ions than an acid of pH 4.
Acid solutions
An acid is a solution that contains H+ ions and has a pH below 7.
Solutions of acid are made by dissolving certain covalent molecules in water. The water causes the covalent molecules to break apart in a way that generates hydrogen ions.
Acid concentration
A solution is formed when a solute is dissolved in a solvent.
A concentrated solution of an acid is one that contains a high number of acid molecules per dm3 of solution.
A dilute acid solution is therefore one that has much fewer acid molecules per dm3 of solution.
Acid strength
Acids can be either strong or weak, depending on how many ions they produce when they dissolve in water.
When added to water, acids ionise or dissociate to produce H+ ions.
Strong acids such as HCl and H2SO4 dissociate completely in water, producing solutions with a high concentration of H+ ions and thus a very low pH.
Weak acids such as ethanoic acid, CH3COOH only partially ionise in water, producing solutions of pH values between 4 – 6.
For weak acids there is an equilibrium set-up between the molecules and their ions once they have been added to water.
Ethanoic acids: CH3COOH ⇌ H+ + CH3COO–
The ⇌ symbol indicates that the process is reversible, as the products can react together forming the original reactants.
Phenolphthalein
Acid - no colour
Neutral - no colour
Alkaline - pink
Methyl orange
Acid - pink
Neutral - orange
Alkaline - yellow
Bases
Bases are substances which can neutralise an acid, forming a salt and water.
A base which is water-soluble is referred to as an alkali.
Bases are usually oxides, hydroxides or carbonates of metals.
Alkalis
Alkalis have pH values of above 7.
The presence of the OH- ions is what makes the aqueous solution an alkali.
Like acids, alkalis can be concentrated or dilute, weak or strong.
Strong alkalis like sodium hydroxide will completely dissasoiciate In water to give a high concentration of OH- ions.
Weak alkalis like ammonia will only partially react with water to give a low concentration of OH- ions.
Reaction of acids with metals
Reactive metal + acid ——- metal salt + hydrogen
Reaction of acids with oxides & hydroxides
When an acid reacts with an oxide or hydroxide, a neutralisation reaction occurs.
Metal oxides and metal hydroxides act as bases.
Base (alkali) + acid ——- metal salt + water
Reactions of Acids with Metal Carbonates
These reactions are easily distinguishable from acid – metal oxide/hydroxide reactions due to the presence of effervescence caused by the carbon dioxide gas.
Metal carbonate + acid ——– metal salt + water + carbon dioxide
Ammonia and acids
Ammonia reacts with acids to form ammonium salts - which contain the ammonium ion (NH4)+.
Ammonia + acid —- ammonium salt
Neutralisation
When an acid reacts with an alkali - a neutral solution is formed. this type of reaction is called a neutralisation reaction.
Acid + base —– salt + water
Ionic equations are used to show only the particles that chemically participate in a reaction.
The other ions present are not involved and are called spectator ions.
HCl + NaOH ⟶ NaCl + H2O
H+ + Cl- + Na+ + OH- ⟶ Na+ + Cl- + H2O
H+ + OH- ⟶ H2O
The H+ ions come from the acid and the OH– ions come from the base, both combine to form the product water molecules.
Test for hydrogen
The test for hydrogen consists of holding a burning splint held at the open end of a test tube of gas.
If the gas is hydrogen it burns with a loud “squeaky pop” which is the result of the rapid combustion of hydrogen with oxygen to produce water.
Be sure not to insert the splint right into the tube, just at the mouth, as the gas needs air to burn.
Test for carbon dioxide
The test for carbon dioxide involves bubbling the gas through an aqueous solution of calcium hydroxide (limewater).
If the gas is carbon dioxide, the limewater turns milky or cloudy.
Prepare a salt by titration
If salts are prepared from an acid and a soluble reactant then a titration technique must be used.
In a titration, the exact volume of acid and soluble reactant are mixed in the correct proportions so that all that remains is the salt and water.
Procedure: Use a pipette to measure the alkali into a conical flask and add a few drops of indicator (phenolphthalein or methyl orange).
Add the acid into the burette and note the starting volume.
Add the acid very slowly from the burette to the conical flask until the indicator changes to appropriate colour.
Note and record the final volume of acid in burette and calculate the volume of acid added (starting volume of acid - final volume of acid).
Add this same volume of acid into the same volume of alkali without the indicator.
Heat to partially evaporate, leaving a saturated solution.
Leave to crystallise decant excess solution and allow crystals to dry.
Solubility rules
Compounds of sodium, potassium, and ammonium.
All nitrates.
All chlorides, except silver and lead (Il).
All sulphates except barium, calcium, and lead (Il)
Sodium, potassium, and ammonium carbonates.
Sodium, potassium, and ammonium hydroxides.
Deciding which methods of producing salts
If the salt is insoluble precipitation method is used (two solutions mixed together).
If the soluble salt is a sodium, potassium or ammonium salt then the titration method is used (appropriate acid with appropriate hydroxide or carbonate).
If the salt is not a sodium, potassium or ammonium salt then an excess bass method is used (equal proportions of an acid with an excess of an insoluble base - oxide hydroxide and carbonate).
Preparing an insoluble salt (Precipitation)
Procedure - Measure out 25 cm3 of 0.5 mol dm3 soluble salt1 solution and add it to a small beaker.
Measure out 25 cm3 of 0.5 mol dm3 of soluble salt2 add it to the beaker and mix together using a stirring rod.
Filter to remove the precipitate from mixture.
Wash the filtrate with distilled water to remove traces of other solutions
Leave in an oven to dry.
Preparing an soluble salt (excess base)
Procedure - Measure a specified amount of the insoluble compound and place it into a suitable container.
Slowly add an excess amount of a base solution while stirring continuously. The base reacts with the compound to form a soluble salt complex.
Continue stirring until the compound completely dissolves. Record any changes in appearance, temperature, or texture.
Practical 2 (investigating pH)
Procedure - Use a pipette to measure a fixed volume of dilute HCl into a conical flask.
Add one spatula of calcium oxide or calcium hydroxide to the flask and swirl
When all the base has reacted record the pH of the solution.
If using U.I. paper use the glass rod to extract a sample from the flask.
Repeat for different numbers of spatula (1-10) of solid but the same volume of HCl.
Record your results neatly in table format.
Practical 3 (copper sulphate)
Procedure - add 50 cm3 dilute acid into a beaker and warm gently using a Bunsen burner.
Add the copper(II) oxide slowly to the hot dilute acid and stir until the base is in excess (i.e. until the base stops dissolving and a suspension of the base forms in the acid).
Filter the mixture into an evaporating basin to remove the excess base.
Gently heat the solution in a water bath or with an electric heater to evaporate the water and to make the solution saturated.
Check the solution is saturated by dipping a cold glass rod into the solution and seeing if crystals form on the end.
Leave the filtrate in a warm place to dry and crystallise.
Decant excess solution and allow the crystals to dry.
