topic 4 chem

what happens as pH lowers

more acidic

what happens when PH increases

more alkaline

how to measure pH of a solution

Using an indicator - a dye that changes colour depending on whether it’s above or below a certain pH

what are wide range indicators (universal)

an indicator contain a mixtures of dyes that means they gradually change colour over a broad range of pH

useful for estimating pH of a solution

how to measure pH electronically

using a pH probe attached to a pH meter

the probe is placed in the solution u are measuring and the pH is given on the digital display as a number, so more accurate

what do acids and bases do to eachother

neutralise eachother - pH 7

what do acids form in water

H+ ions

what is a base

any substance that will react with an acid to form a salt

what is an alkali

a base that dissolves in water to form a solution with pH higher than 7.

form OH- ions in water

acid + base

salt + water

H+ + OH-

H2O

what are titrations used for

allow you to find out exactly what vol of acid is needed to neutralise a measured vol of alkali (or vise versa)

how to do a titration

to find out the conc of some alkali:

1. using a pipette and pipette filler and add a set vol of the alkali to a conical flask, and add 2-3 drops of indicator

2. use a funnel to fill a burette w some acid of known conc, then record initial vol of the acid in the burette

3. use the burette to add the acid to the alkali abit at a time and swirling the solution often, ensure to slow even more when you think the end-point is near

4. the indicator changes colour when all the alkali has been neutralised

5. record the final vol of acid in the burette and calculate the vol of acid used to neutralise the alkali

6. to improve accuracy, find a mean value

what type of indicator should u use for titrations

single indicators so u see a sudden colour change rather than a gradual

examples of indicators and colour changes

phenolphthalein - colourless(acids) →pink(alkalis)

litmus - red →blue

methyl orange - red →yellow

what do acids produce in water

H+ ions (protons)

bc they ionise in aqueous solutions - producing hydrogen ions

what do strong acids (sulfuric, hydrochloric and nitric acids) do in water

ionises completely

all acid particles dissociate to release H+ ions

what do weak acids (ethanoic, citric and carbonic acids) do in water

small proportion of acid particles dissociate to release H+ ions

what is the pH of an acid or alkali a measure of

H+ ions

what happens to H+ ions for every decrease of 1 on pH scale

conc of H+ increases by a factor of 10

so for a decrease of 2 H+ ions increase by factor of 100

what does conc of an acid tell u

meausres how much acid there is in a certain vol of water

baso how watered down ur acid is

acid + metal (hydr)oxide →

salt + water

acid + metal carbonate →

salt + water + carbon dioxide

how to make soluble salts using an insoluble base

1. gently warm the dilute acid using a bunsen burner

2. add the insoluble base to the acid a little at a time until no more reacts (base in excess and sinks to bottom of flask)

3. then filter out the excess solid to get the salt solution

4. to get the pure, solid salt crystals: gently heat w water bath to evaporate some of the water n make it more concentrated then stop and allow leave to cool

5. crystals of the salt should form, which can be filtered out of the solution and then dried (crystallisation)