[4.2] acid-base titrations

Question 1

what are the 6 equations that can create a salt?

Answer 1

1. metal + acid -> salt + hydrogen
2. metal oxide + acid -> salt + water
3. metal hydroxide + acid -> salt + water
4. metal carbonate + acid -> salt + water + carbon dioxide
5. hydrogencarbonate + acid -> salt + water + carbon dioxide
6. ammonia + acid -> an ammonium salt

Question 2

what is a salt?

Answer 2

the product of a reaction in which the H+ ions from an acid are replaced by metal or ammonium ions

Question 3

what is the difference between a strong acid and a weak acid?

Answer 3

• strong acids completely dissociate in aqueous solutions by releasing all its hydrogen atoms into solution as H+ ions
• weak acids only partially dissociate in aqueous solutions

Question 4

what is the equation for the dissociation of HCl, CH₃COOH and NaOH?

Answer 4

• HCl (aq) -> H+ (aq) + Cl- (aq)
• CH₃COOH (aq) ⇌ H+ (aq) + CH₃COO- (aq)
• NaOH (aq) -> Na+ (aq) + OH- (aq)

Question 5

what is an alkali?

Answer 5

a base that dissolves in water by releasing hydroxide ions into the solution

Question 6

what is neutralisation and what is the equation for it?

Answer 6

• in neutralisation of an acid, H+ (aq) ions react with a base to form a salt and neutral water
• H+ (aq) + OH- (aq) -> H₂O (l)

Question 7

what is a titration?

Answer 7

a technique used to accurately measure the volume of one solution that is needed to neutralise another solution

Question 8

what 3 things can titrations be used for?

Answer 8

• finding the concentration of a solution
• identification of unknown chemicals
• finding the purity of a substance

Question 9

what is a standard solution?

Answer 9

a solution of known concentration

Question 10

how do you prepare a standard solution?

Answer 10

1. weigh solid accurately to 2dp
2. dissolve solid in beaker using less distilled water than will be needed to fill the volumetric flask
3. transfer solution to volumetric flask
4. rinse the last traces of the solution into the flask using distilled water
5. fill the flask to the graduation line by adding distilled water carefully until the bottom of the meniscus lines up exactly with the mark
6. invert the volumetric flask, slowly, several times to mix the solution thoroughly

Question 11

what is the procedure for an acid-base titration?

Answer 11

1. add 25cm³ of one solution to a conical flask using a pipette
2. add the other solution to a burette, and record the initial burette reading to the nearest 0.05cm³
3. add a few drops of an indicator to the solution in the conical flask
4. run the solution in the burette into the solution in the conical flask, swirling the conical flask throughout to mix the two solutions
5. the indicator permanently changes colour at the end point of the titration. this indicates the volume of one solution that exactly reacts with the volume of the second solution
6. record the final burette reading
7. a quick, trial titration is carried out first to find the approximate titre
8. repeat the titration, adding the solution dropwise towards the endpoint until concordant results, which agree to within 0.10cm³, are achieved

Question 12

what is a titre and how is it calculated?

Answer 12

the volume of solution added from the burette, which is calculated by subtracting the initial from the final burette reading

Question 13

how do you calculate the mean titre?

Answer 13

only use the concordant titres, which agree within 0.10cm³, and reject the inaccurate titres

Question 14

what colour is phenolphthalein and methyl orange in acid/alkali?

Answer 14

• phenolphthalein is colourless in acidic conditions and pink in alkaline conditions
• methyl orange is red in acidic conditions and yellow in alkaline conditions