acids, alkalis and titrations Flashcards
acids are substances that turn litmus paper what colour
red, yellow or orange
acids have a pH
below 7
bases turn litmus paper what colour
sark green, blue or purple
what are bases called if they dissolve in water
alkali’s
bases react with _____ in acids
H+ ions
use of universal indicator
measure pH of a solution
colour of universal indicator with acid
red
colour of universal indicator with alkali
blue
use of phenolphalein indicator
determine the end point of a strong or weak acid
TITRATIONS
what is an alkali
a soluble base
colour of phenolphthalein indicator in acid
colourless
colour of phenolphthalein indicator in alkali
pink
colour of methyl orange in acid
red
colour of methyl orange in alkali
yellow
how is universal indicator used
a drop of it is added to the solution and colour is matched to the colour chart which indicates the pH
use of methyl orange indicator
determine the end point of a strong acid- strong alkali or strong acid- weak alkali
TITRATIONS
When acids are added to water they form
positive hydrogen ions
what makes a solution acidic
presence of hydrogen ions
when alkalis are added to water, what is formed
negative hydroxide ions
what makes a solution alkaline
hydroxide ions
what happens when an acid and an alkali react
a neutralisation reaction
Ionic equation of neutralisation ( H)
H⁺ + OH⁻ —-> H₂O
4 common indicators
litmus
universal
phenolphthalein
methyl orange
the point at which an indicator changes colour is known as the
end point
neutralisation is
a reaction between a base/alkali and an acid which forms a salt and water
what is a base
a proton acceptor ( they are metal oxides, hydroxides or carbonates)
strong acids in do what water
completely ionize
properties of strong acids (3)
produce high concentrations of H+ ions in solution
- have a low pH 0-3
- react more vigorously with metals, carbonates and bases
weak acids do what in water
partially ionize
if weak acids only partially ionize in water, what does that mean about the H+ions
only some of their molecules release H+ions
properties of weak acids (3)
- produce lower concentration of H+ions in solution
- have a pH of 4-6
- react less vigorously compared to strong acids
carboxylic acids are what type of acids
weak acids
carboxylic acids contain a functional group of what atoms (3)
and how are they arranged
carbon , hydrogen and oxygen atoms
COOH
Examples of carboxylic acids
methanoic ( HCOOH)
ethanoic ( HC₃ COOH)
How do bases neutralise acids
by combining with the hydrogen ions in them
word reaction of a base neutralising an acid
acid + base —> salt + water
acids in aqueous solution are a source of what ions
positive hydrogen ions
alkalis in aqueous solution are a source of what ions
negative hydroxide ions
titrations are a method of
analysing the concentration of solutions
acid-base titrations determine
exactly how much alkali is needed to neutralise a quantity of acid – and vice versa
Titrations can also be used to prepare
salts
PRACTICAL- HOW TO CARRY OUT AN ACID- BASE TITRATION
- Place exactly 25cm³ sodium hydroxide in conical flask
- Place the conical flask on a white tile so the tip of the burette is inside the flask
- Add a few drops of a suitable indicator to the solution in the conical flask
Perform a rough titration by taking the burette reading and running in the solution in 1 – 3 cm3 portions, while swirling the flask vigorously
Quickly close the tap when the end-point is reached (sharp colour change) and record the volume, placing your eye level with the meniscus
Now repeat the titration with a fresh batch of sodium hydroxide
As the rough end-point volume is approached, add the solution from the burette one drop at a time until the indicator just changes colour
Record the volume to the nearest 0.05 cm3
Repeat until you achieve two concordant results (two results that are within 0.1 cm3 of each other) to increase accuracy
