Acids, alkalis and titrations Flashcards
what is an acid?
An acid is a substance that produces hydrogen ions, H+(aq), when dissolved in water and acts as a proton donor in neutralisation reactions
what happens to acids in aqueous solutions (when dissolved in water)?
when dissolved in water, acids become a source of hydrogen ions or a proton donor
what is an alkali?
an alkali is a base that is soluble in water and produces OH- ions when dissolved in water
what happens to alkalis in aqueous solutions (when dissolved in water)?
when dissolved in water, alkalis always produce OH- ions, if they don’t already contain them, they react with water to produce them
what is a base?
a base is a substance that reacts with an acid to neutralise it and is a proton acceptor
what is the difference between a base and an alkali?
a base is a substance that accepts protons and can neutralize acids in a neutralization reaction to produce salt and water, whereas an alkali is a water-soluble base that releases OH- ions when dissolved in water and so is a source of hydroxide ions in reactions
examples of acids
- all acids contain H in them
- nitric acid = HNO3
- sulfuric acid = H2SO4
- hydrochloric acid = HCl
examples of bases
- most bases tend to be hydroxides, carbonates or oxides
- sodium hydroxide = NaOH (both a base and alkali as its water soluble (SNAP))
- calcium hydroxide = Ca(OH)2 (both a base and alkali (clasp bag salt))
- copper oxide = CuO (insoluble base)
- ammonia = NH3 (both a base and alkali as it reacts w/ water to make OH- ions)
examples of alkalis
- all alkalis are bases but not all bases are alkalis
- NaOH = sodium hydroxide
- KOH = potassium hydroxide
- Ca(OH)2 = calcium hydroxide
- NH4OH = ammonium hydroxide
Why do not all bases release hydroxide ions?
not all bases are soluble in water (only alkalis are) therefore they can’t release hydroxide ions. Also bases like ammonia (NH3) don’t naturally contain hydroxides in them so have to react with water first to do so
how does the pH scale classify solutions as acidic, alkaline or neutral?
- The pH scale ranges from 0 to 14
- A pH values < 7 indicates an acidic solution, with lower values representing stronger acids.
- A pH value of 7 represents a neutral solution.
- A pH values >7 indicates an alkaline solution, with higher values representing stronger alkalis.
- Acidic solutions have higher concs of hydrogen ions (H⁺)
- alkaline solutions have higher concs of hydroxide ions (OH⁻)
- The pH scale measures the relative concentrations of these ions to determine how acidic or alkaline it is
why isn’t universal indicator good for a titration?
the colour change is too slow and not clear enough to accurately pinpoint the endpoint of an acidic solution, so doesn’t meet the crucial accuracy required for titrations
ionic equation for neutralisation
H+ (aq) + OH- (aq) => H2O (l)
define neutralisation reaction
when an acid react with a base to produce a salt and water
what colour is universal indicator in acidic, neutral and alkaline solutions?
- acidic = red to yellow (more acidic = more red)
- neutral = green
- alkaline = blue to purple (more alkaline = more purple)
what colour is methyl orange in acidic, neutral and alkaline solutions?
- acidic = red
- neutral = orange
- alkaline = yellow
what colour is phenolphthalein in acidic, neutral and alkaline solutions?
- acidic = colourless
- neutral = colourless
- alkaline = pink
what colours are red and blue litmus paper in acid, neutral and alkaline solutions?
- acidic = blue litmus turns red and red litmus stays red
- neutral solutions = red stays red and blue stays blue
- alkaline = red litmus turns blue and blue litmus stays blue
what are the 4 state symbols?
- solid = (s)
- liquid = (l)
- gas = (g)
- aqueous = (aq)
what does an (s) and an (aq) next to an ionic compound indicate?
- (s) = insoluble
- (aq) = soluble
SNAP salts are soluble meaning
all salts containing sodium, nitrate, ammonium and potassium are soluble
ClASPbAg meaning
all chloride salts are soluble except lead and silver
So Suft CaPbBa(ra) meaning
all sulfates are soluble except calcium, lead and barium
InCH(es) meaning
all carbonates and hydroxides are insoluble except SNAP salts
only metals that are more reactive than ………… will react with acids
only metals that are more reactive than hydrogen will react with acids
metal + acid => ?
metal salt + hydrogen
metal + sulfuric acid => ?
metal sulfate + hydrogen
metal + nitric acid => ?
metal nitrate + hydrogen
metal hydroxide + acid => ?
metal salt + water
metal oxide + acid => ?
metal salt + water +
metal carbonate + acid => ?
salt + water + carbon dioxide
ammonia + acid =>
ammonium salt
what type of oxides to metals always form?
metals always form basic oxides
what types of oxides do non-metals always form?
non-metals always form acidic oxides
define a salt
a salt is a compound that is formed when the hydrogen atom of any acid is replaced by a metal atom
what do bases always produce in reactions with acids? why is this?
- bases always produce water when reacting with acids in a neutralisation reaction
- this is because the hydrogen ions from the acid react with the OH- ions of the base, producing neutral water molecules
describe an experiment to prepare a pure, dry sample of an insoluble salt, starting with two soluble reactants (precipitation method)
- dissolve each solid in a beaker of water and stir well to ensure that it has completely dissolved
- there shouldn’t be an excess of either salt as this isn’t removable form the final salt (makes it impure)
- mix the solutions and stir well to combine
- continue stirring the solution until it becomes cloudy, indicating the formation of a precipitate
- filter the solution, keeping the precipitate
- rinse the precipitate with distilled water to remove any soluble impurities
- leave the precipitate in a warm place to dry
what method is used to make lead(II) sulfate?
precipitation method as lead is insoluble
describe an experiment to prepare a pure, dry sample of a soluble salt, starting from an acid and alkali (describe how to carry out an acid-alkali titration)
- rinse conical flask with distilled water (no residual chemicals from previous uses contaminate the solution being analysed), ensures the accuracy of the titration
- rinse pipette and burette with the solution that they will be filled with rather than water as this may dilute the conc.
- use accurate pipette to measure known volume or conc in conical flask
- add a few drops of indicator to the acid to show when endpoint is just reached, swirling between additions
- fill the burette with the unknown conc or volume and take an initial reading
- place conical flask under burette on a white tile so the colour change can be seen more easily
- open the burette tap, swirling between additions and adding dropwise near the endpoint
- add until the solution just changes colour and take a final reading
- repeat to obtain concordant (within 0.20cm3 of each other) results to identify anomalies and calculate a mean volume
- repeat using the mean volume without the indicator (this cannot be separated from the final solution) to make a pure salt
- heat solution gently to evaporate some of the water
- leave solution to cool and crystallise
- filter off crystals and leave crystals to dry in a warm place
describe an experiment to prepare a pure dry sample of a soluble salt, starting from an insoluble reactant
- gently warm the acid for 2-3 mins in a hot water bath to increase RoR
- add the solid in the acid in small portions, swirling between additions to combine
- continue until the solid stops disappearing so that the solid is in excess
- this ensures that all the acid is used up so that the final salt is pure
- filter off the excess solid and keep the filtrate
- gently heat the filtrate with a bunsen burner to evaporate some of the water
- leave the saturated solution to cool and crystallise
- filter out the crystals and leave them in a warm place to dry
concentration formula
conc = moles / vol
what is the units for volume?
dm3
how do you convert from cm3 to dm3?
divide by 1000
what is the unit for concentration? how is this obtained by its calculation?
- unit = mol/dm3
- C = n/V
- mol/dm3 = mol (moles unit) / dm3 (volume unit)
density formula
density = mass/vol
what is the unit for density? how is this obtained from its calculation?
- unit = g/dm3
- g/dm3 = g (mass unit) / dm3 (volume unit)
moles formulae
moles = mass/molar mass
OR
moles = conc x vol
molar mass formula
molar mass = moles/mass
what is the unit for molar mass? how is this obtained from its calculation?
- unit = g/mol
- g/mol = g (mass unit)/ mol (moles unit)
concentration formula (w density)
conc = density/ molar mass
what are the units for concentration? how can they be obtained from its calculation?
- unit = mold/dm3
- mold/dm3 = (g/dm3) (desnity unit) / (g/mol) (molar mass unit)
- cancel ou the g and u have g/mol
how to approach a moles concentration Q
- calculate what you can = moles using concentration x volume
- write a balanced equation for mole ratio
- use mole ratio
- calculate concentration using conc = mol/vol
What goes into the burette?
The solution with the known concentration
