Chapter 2 - Salt Preparation Flashcards
What is a salt?
- an ionic compound
- formed when the hydrogen ion, H+ from an acid is replaced by a metallic ion or an ammonium ion (NH4+)
What is water of crystallisation?
Water molecules that combine with salts to form crystals
Hydrated salts are ________ to the touch
dry
What are hydrated salts?
Salts that contain water of crystallisation
What are anhydrous salts?
Salts that do not contain water of crystallisation
Soluble salts are in ________ state at room temperature
aqueous (aq)
Insoluble salts are in ________ state at room temperature
solid (s)
Soluble salt, soluble reactants → ________
Titration
Soluble salt, insoluble reactants → ________
Acid with excess insoluble metal/carbonate/base
Insoluble salt, soluble reactants → ________
Ionic precipitation
Method 1: Acid with excess insoluble metal/carbonate/base
- React excess insoluble base/metal/carbonate with acid. Some heating may be required. Stir until no more solids dissolve (in excess).
- Filter the mixture to remove excess solids.
- Collect the filtrate.
- Heat the filtrate until the solution is saturated.
- Cool and crystallize.
- Filter away excess solution.
- Wash and dry.
Why must the metal/carbonate/base be added in excess?
To ensure that all the acid has been reacted
Why must the metal/carbonate/base be insoluble?
If the reactant used was soluble, one would not be able to tell if it has been added in excess, and hence would not be able to form a pure salt
Acid with excess insoluble metal is suitable for ________ metals
moderately reactive
Acid with excess insoluble metal is not suitable for ________ and ________
highly reactive metals: Group 1 metals, calcium
unreactive metals: copper, silver, gold
Method 2: Titration (part 1)
- Fill up a burette with dilute acid. Note the initial burette reading (V1cm3).
- Pipette 25.0cm3 of alkali solution into a conical flask.
- Add 1−2 drops of indicator to the alkali solution.
- Add dilute acid slowly from the burette until the solution just turns (turning point colour of indicator).
- Stop adding dilute acid. Record the final burette reading (V2cm3).
Method 2: Titration (part 2)
- Pipette 25.0cm3 of alkali solution into a beaker.
- Add (V1-V2)cm3 of dilute acid from the burette.
- Heat to saturate the solution.
- Cool and crystallise.
- Filter away excess solution
- Wash and dry.
Titration is suitable for ________
Sodium, potassium and ammonium salts
What is a titrant?
A solution of known concentration (in the burette)
What is an analyte?
A solution of unknown concentration (in conical flask)
Method 3: Ionic precipitation
- Mix aqueous solutions of 2 soluble salts.
- Filter the mixture to obtain the precipitate.
- Wash the precipitate (to remove impurities).
- Dry the precipitate.
What is solubility?
The maximum amount of solute that can dissolve in a given amount of solvent at a given temperature
Solubility ________ as temperature increases (________ gradient)
increases
positive
Solubility ________ as temperature decreases (________ gradient)
decreases
negative