Acids and Solutions

acid + base -->

salt + water

acid + metal oxide/ hydroxide -->

salt+ hydrogen

acid + carbonate -->

salt + water + carbon dioxide

How do you convert between dm³ and cm³?

What ion does acids contain?

hydrogen ions

when dissolved in water acid releases hydrogen ions as protons into solution

eg HCL (g) + aq → H+ (aq) + Cl- (aq)

Define strong acid

completely dissociates in aqueous solution - releases all of its H atoms into solutions as H+ ions

eg HCl (aq) → H+ (aq) + Cl- (aq)

Define weak acid

partially dissociates in aqueous solution - releases a small proportion of its available H atoms into the solution as H+ ions

eg. CH3COOH(aq) < -- > H+ (aq) + CH3COO- (aq)

What is a base?

neutralises an acid to form a salt

What is an alkali?

a base that dissolves in water releasing OH- ions into solution

eg NaOH (s) + aq → Na+ (aq) + OH- (aq)

Define neutralisation

H+ ions and OH- ions react to form H2O

acids react with bases to form salts

What is the purpose of a titration?

technique used to accurately measure the volume of 1 solution that reacts exactly with another solution

used for :

• finding concentration of a solution

• identifying unknown chemicals

• finding purity of a substance

What is a standard solution?

solution of known concentration measured using a volumetric flask

How do you prepare a standard solution?

1. solid weighed accurately

2. then dissolved into a beaker using less distilled water than volume needed to fill the volumetric flask to the mark

3. solution transferred to vol flask

4. last traces of solution are rinsed into flask with distilled water

5. flask filled to graduation line - add distilled water dropwise - until bottom of the meniscus lines up with the mark (view at eye level for accuracy)

6. volumetric flask inverted several times to mix solution well (without this titration results are likely to be inconsistent

How do you carry out an acid-base titration?

1. add a measured vol of 1 solution to conical flask using a pipette

2. add the other solution to burette - record initial burette reading to nearest 0.05cm³

3. add a few drops of indicator to flask

4. run burette solution into flask solution and swirl flask to mix solutions

5. indicator colour changes at END of titration - the end indicates the volume of 1 solution that exactly reacts with the volume of the 2nd solution

6. record final burette reading - the vol of solution added from burette is called the titre (= initial - final burette reading)

7. a quick trial titration can be carried out first to find approximate titre

8. titration can then be repeated accurately adding solution dropwise near end point

9. repeat titrations until 2 accurate titres are concordant (within 0.1 cm³)

What are concordant titres?

titres within 0.1cm³ of each other

use only concordant titres when calculating the mean titre

What can titration results be used to calculate?

• concentration and reacting volume of one of the solutions

• the reacting volume of the other solution

What are the measurements of volume?

• 1 cm³ = 1ml

• 1 dm³ = 1000 cm³ = 1000ml = 1 l

What is gas measured in?

volume

How do you convert between mol/dm³ and g/dm³?

What is a titre?

The volume added from the burette when the volume of one solution has exactly reacted with the other solution

Describe the process of identifying a carbonate

use the titration technique to identify unknown carbonate: eg X₂CO₃.

1. Prepare a solution of an unknown carbonate, X₂CO₃, in a volumetric flask

2. Using a pipette, measure 25.00 cm³ of your prepared solution into a conical flask

3. Using a burette, titrate this solution using 0.100 mol dm⁻³ hydrochloric acid.

4. Analyse your results to identify the carbonate.

see worked example

Tips for titration

Often a sample is a smaller volume taken from the larger volume in a volumetric flask

• If sample X is titrated against sample Y: • Sample X is in the conical flask.• Sample Y is in the burette

• Draw a storyboard of the:• Volumetric flask• Burette• Conical flask

• On the storyboard label:• solutions• volumes• moles• concentrations given