2C Acids, Alkalis, and titrations

Question 1

what is the range of the pH scale?

Answer 1

0-14

Question 2

give an example of a: strongly acidic, weakly acidic, neutral, weakly alkaline, strongly alkaline substance

Answer 2

0-3 strongly acidic- HCl,
4-6 weakly acidic-ethanoic acid (vinegar), 7 neutral - sodium chloride,
8-10 weakly alkaline-ammonia,
11-14 strongly alkaline substance- sodium hydroxide

Question 3

what is universal indicator made of? what can it be used as?

Answer 3

many different indicators, which change in a gradual range of pH values. it can be used as a solution or paper

Question 4

what are the most common acid-alkali solutions?

Answer 4

litmus, methyl orange, phenolphthalein, universal indicator

Question 5

what colour does litmus go?

Answer 5

acid- red
alkali- blue

Question 6

what colour does methyl orange go?

Answer 6

acid - red
alkali- yellow

Question 7

what colour does phenolphthalein go?

Answer 7

acid-colourless
alkali- pink

Question 8

what colour does univeral indicator go?

Answer 8

acid- red
alkali- blue

Question 9

describe the use of universal indicator.

Answer 9

to measure the approximate pH value of an aqueous solution

Question 10

what doe all acids contain?

Answer 10

hydrogen

Question 11

what are acids in an aqueous solution? why?

Answer 11

a source of hydrogen ions because when acids are in water they dissociate (break apart) to form hydrogen ions. (so when we are measuring pH we are actually measuring conc. of hydrogen ions in solution)

Question 12

what happens when an acid reacts with a base?

Answer 12

the acid is neutralised (the H is replaced)

Question 13

what are alkalis in an aqueous solution?

Answer 13

a source of hydroxide ions

Question 14

what do metal carbonates do in water?

Answer 14

form hydroxide ions

Question 15

what happens when bases such as metal oxides and acids react?

Answer 15

the acid is neutralised

Question 16

describe titration fully.

Answer 16

1. measure out 25cm^3 of HCl using a pipette and pipette filter
2. transfer it into a conical flask
3. fill the burette with sodium hydroxide solution. take the initial reading on the burette and record to 2 d.p
4. add a few drops of phenolphthalein into the acid.
5. add sodium hydroxide until indicator changes colour
6. take the final reading from burette
   the solution should be a permanent pale pink colour (for atleast 30 seconds)

We usually do a rough titration first in order to find out approximately how much alkali is required to neutralise the acid. Imagine we found out that we required 22.50 cm of alkali in the rough titration. We would then do the experiment again but this time running in about 20cm of the alkali quite quickly, swirling the conical flask all the time to mix the solutions. We would then add the alkali very slowly (dropwise) until one drop of alkali causes the indicator to change from its acid to its alkali colour. We then know the amount needed for neutralisation to within 1 drop; this is about as precise as we can be. In the above titration we could also have added the acid to the alkali but it is easier to see this indicator turning pink rather than going colourless.
When reading a burette it is important to remember that the numbers increase from the top to the bottom: O is at the top and 50 is at the bottom. We normally record readings from the burette oal places The sec, therefore all readings should be written down to 2 decima places. The second decimal place is given as “0’ if the level of solution is on the line and 5 if it is between the lines.

Question 17

how do you figure out how much alkali you used in titration?

Answer 17

volume of alkali = final reading on burette-initial reading

Question 18

why can’t we use other indicators for titration?

Answer 18

you can but not universal indicator because it has a range of colours which changes gradually so we would not be able to see a clear endpoint

Question 19

how does an acid and metal hydroxide react?

Answer 19

acid+metal hydroxide -> salt+water

Question 20

how does an acid and metal carbonate react?

Answer 20

acid+metal carbonate -> salt+water+ carbon dioxide

Question 21

how does an acid and metal react?

Answer 21

acid+metal -> salt+hydrogen

Question 22

what is an acid/base in terms of protons?

Answer 22

acid-proton (H+) donor
alkali-proton acceptor

Question 23

what are the solubility rules for ionic compounds in water?

Answer 23

Solubility rules for ionic compounds in water:
Common sodium, potassium and ammonium compounds are soluble
* All nitrates are soluble
* Common chlorides are soluble, except those of silver and lead (Il)
* Common sulfates are soluble, except for those of barium, calcium and lead (Il)
* Common carbonates are insoluble, except for those of sodium, potassium and ammonium
* Common hydroxides are insoluble except for those of sodium, potassium and calcium (calcium hydroxide is slightly soluble).