Acids, Bases and Salts Flashcards
What is an indicator
an indicator is a substance that is one colour in acids and a different colour in alkalis
Litmus Solution
acid - red/orange
neutral - purple / no change
alkali - darker purple / blue
phenolphthalein
acid - colourless
neutral - colourless
alkali - pink
methyl orange
acid - pink / red
neutral - orange
alkali - yellow
what causes acidity
hydrogen ion
universal indicator is a measure of
concentration of hydrogen ions. 1 is 10x more conc. than 2, 100x more than 3 etc
salts are
ionic substances where hydrogen is replaced by other positive ions
acids are
substances that contain hydrogen and have a pH of less than 7 when dissolved in water
difference between hydrogen chloride gas and hydrochloric acid
gas - covalently bonded HCl molecules
acid - H+ and Cl- ions in solution
What happens when an acid is dissolved in water
Arrhenius acids dissolve in water to form hydrogen ions
Concentrated and dilute
amount of acid molecules dissolved in a volume of water
strong and weak
how many of the acid molecules have split into ions ( dissociation or ionisation )
Strong
all molecules have dissociated
weak
small proportion of molecules have dissociated ( shown by reversible arrow )
why is hcl more dangerous than methanoic acid
higher proportion of acid molecules dissolved have dissociated, forming a higher conc. of H+ ions
What are bases
substances that can neutralise acids. often hydroxides or metal oxides
what are alkalis
sub group of bases, all soluble in water, hydroxides.
Dissolve in water to form hydroxide ion, which causes alkalinity
Example of a weak alkali
ammonia
acid + reactive metal
metal salt + hydrogen
gold, platinum, silver and copper are unreactive so dont work this way
effervescence, exothermic, ‘dissolves’
base + acid
metal salt + water
solid base - dissolves on heating
soluble base - exothermic
metal carbonate + acid
metal salt + water + carbon dioxide
effervescence, ‘dissolves’
ammonia + acid
ammonium salt
exothermic
ionic equations
write balanced equation
rewrite equation separating ions ( not covalent )
cancel ions appearing on both sides
write out ionic equation
ionic equation metal + acid
metal + H+ = metal ion + hydrogen
ionic equation base + acid
OH- + H+ = H2O
O2- +2H+ = H2O
ionic equation metal carbonate + acid
CO3 + 2H+ = H2O + CO2
ionic equation ammonia + acid
NH3 + H+ = NH4
ionic equation acid + hydrogencarbonate
H+ + HCO3 = CO2 + H2O
What is a neutralisation reaction
acid + alkali
salts form
When to use excess base
soluble salt but no potassium, sodium, ammonium
base is insoluble but salt is soluble
excess base method
add excess base to alkali to ensure it all reacts
heat to speed reaction
filter out unreacted base
heat the filtrate to make a hot saturated solution
leave to cool and crystallise
use metal oxide and acid
how to test is solution is saturated
dip in glass rod and blow on it
crystals formed = saturated
Anhydrous v Hydrated
anhydrous = contains no water of crystallisation
hydrated = water molecules are ‘ locked’ in the solid structure
when to use titration
soluble salts of sodium, potassium and ammonium
titration method
measure 25. cm3 of alkali with glass pipette and place in conical flask and add indicator ( phenolphthalein )
put acid in biurette and record starting value ( aim for 0 )
add acid to alkali slowly and swirl conical flask to make sure they react
when the acid is nearly neutralised add the alkali a drop at a time until indicator changes colour
record volume of alkali used
repeat with no indicator with same volumes
heat mixture and evaporate until saturated
leave to crystallise
filter off crystals and leave to dry
use Naoh, KOH, nh3 + acid
when to use precipitation
insoluble salt
precipitation method
mix two soluble solutions together - one containing positive ion and one containing negative ion.
filter solution. residue is the insoluble salt.
rinse off impurities with distilled water
leave to dry
use metal nitrate + sodium salt
why does silver chloride change colour when left to dry
silver ions gain electrons in light, so change colour.
hence why silver solutions is kept in brown bottles
ionic equation for precipitation reaction
metal ion + eg. sulphate = metal eg.sulphate
three salts which are always soluble
sodium, potassium, ammonium
which chlorides are soluble
all except silver and lead
which sulphates are soluble
all except barium, lead and calcium
nitrates are always…
soluble
which carbonates and hydroxides are soluble
only sodium, potassium, ammonium
