chemical changes (1) Flashcards
what Ph numbers are acids, which are neutral and which are alkaline
0-6 acid
7 neutral
8-14 alkali
Measuring Ph
universal indicator changes colour depending on the Ph level (red= acid green= neutral dark blue = alkali)
Neutralisation reaction
acid + base -> salt + water
H+ + OH- -> H20
Titration RP
why are single indicators used
- Use a pipette and pipette filler to measure 25 cm³ of alkali into a conical flask.
- Add a few drops of indicator (e.g., phenolphthalein or methyl orange) to the alkali.
- Fill the burette with acid using a funnel and record the starting volume.
- Slowly add the acid from the burette to the alkali, swirling the conical flask continuously.
- As you approach the endpoint, add the acid drop by drop until the indicator changes colour (e.g., phenolphthalein turns colourless from pink).
- Record the final volume of acid in the burette.
- repeat to get concordant results and a mean
single indicators are used to clearly show when the titration has reached an endpoint
What is a strong acid
What is a weak acid
What is Ph
Strong acid- ionise completely in water releasing H+ ions
weak acid- partially ionise in water and release less H+ ions
Ph= concentration of H+ ions, each Ph level has 10x more ions than the last
acid concentration-
how much acid there is in a solution
Reaction of Metal oxides and hydroxides with acid
acid + metal oxide/hydroxide -> salt + water
Reaction of metal carbonates and acids
acid + metal carbonate -> salt + water + carbon dioxide
reaction of metal and water
metal + water -> metal hydroxide + hydrogen
reaction of metal and acid
salt + hydrogen
reactivity series
potassium
sodium
lithium
calcium
magnesium
aluminium
-carbon-
zinc
iron
tin
lead
-hydrogen-
copper
silver
gold
platinum
reaction of acid and metal
acid + metal -> salt + hydrogen
how can metals be separated by carbon
metals less reactive than carbon can be reacted with carbon and extracted
metals more reactive than carbon have to be separated by electrolysis
Oxidation=
reduction=
Oxidation= gain of oxygen, loss of electrons
reduction= loss of oxygen, gain of electrons
what is a redox reaction
where both oxidation and reduction happen in a reaction (electrons being transferred)
displacement reaction
where a more reactive metal takes the place of a less reactive metal in a compound
what is electrolysis
metals more reactive than carbon are extracted by electrolysis
it involves passing an electrical current through an electrolyte (molten or aqueous solution so ions can move)
Positive ions move to the negatively charged cathode and gain electrons
negative ions move to the positively charged anode and loose electrons
Electrolysis of an aqueous solution
electrolysis in water also has H+ and OH- ions
At the cathode, hydrogen forms if the metal is more reactive than hydrogen
otherwise a layer of the metal forms
At the anode if a halide is present molecules of the halogen will form, otherwise oxygen is produced
Molten electrolysis of aluminium
- Aluminium oxide is mixed with cryolite to lower its melting point and reduce energy costs.
- The mixture is heated until it becomes molten so ions can move and conduct electricity.
- Carbon electrodes are placed into the molten mixture, and a direct current power supply is switched on.
- At the cathode Aluminium ions are attracted and gain electrons (reduction) to form molten aluminium metal:
- At the anode Oxide ions are attracted and lose electrons (oxidation) to form oxygen gas:
- The molten aluminium collects at the bottom of the electrolysis cell and is tapped off.
- Oxygen gas is released at the anode, which reacts with the carbon electrodes, forming carbon dioxide
What is used to lower the melting points of metals in electrolysis
molten cryolite
Why does the positive anode need continually replacing
oxygen reacts with the carbon in the anode to form CO2 so it corrodes
What is a base
substance with a Ph greater than 7
What is an alkali
a base that dissolves in water to create a solution with a Ph greater than 7
Forms OH- ions
What is an acid
substance that forms aqueous solutions with a Ph less than 7
Forms H+ ions
