Acids & Electrolysis Topic 3 Flashcards
What does a strong acid contain
Lots of H+ ions
What is pH a measure of
How many H+ ions are present in the solution
The more H+ ions the lower the pH ( more acidic)
What does a low pH mean
Acidic
What does a high pH mean
Alkali
Metal + acid =
Salt + hydrogen
When an ACID is added to LITMUS INDICATOR what colour change is seen
Dark red
When an ALKALI is added to LITMUS INDICATOR what colour change is seen
No colour change - stays dark blue
When an ACID is added to PHENOPHTHALEIN INDICATOR what colour change is seen
No change - stays colourless
When an ALKALI is added to PHENOPHTHALEIN INDICATOR what colour change is seen
bright pink
When an ACID is added to METHYL ORANGE INDICATOR what colour change is seen
Red
When an ALKALI is added to METHYL ORANGE INDICATOR what colour change is seen
Yellow
What is a concentrated acid
More H+ ions per volume of solution
What is a dilute acid
Less H+ ions per volume of solution (more water)
What is a base
Substances that neutralise acids to form a salt and water only
Give the generic equation for the neutralisation of an acid using a base
Metal oxide + acid ——> salt + water
Describe what happens during a neutralisation reaction with a metal oxide
During neutralisation hydrogen ions in the acid combine with oxide ions to form water. This removes the hydrogen ions and so the pH increases (becomes more neutral)
Give the products of the neutralisation reaction :
Tin oxide + hydrochloric acid
Tin chloride + water
How many times more hydrogen ions are there in an acid with a pH of 4 compared to an acid with a pH of 2
100 (every jump in pH is x10)
When obtaining dry crystals of copper sulphate why is the copper oxide added in excess
To make sure all the acid has reacted
What is the final step to get the dry salt crystals (neutralisation reaction)
Scrape out onto filter paper and blot them dry
Metal + acid =
Salt + hydrogen
Metal carbonate + acid =
Salt + carbon dioxide + water
What is titration a method of
Neutralisation
How do you obtain a soluble salt from an alkali (neutral solution left over from titration)
By crystallisation
Are nitrates soluble in water
Yes all of them
Are chlorides soluble in water
Yes most
apart from silver and lead chlorides (insoluble)
Are sulphates soluble in water
Most
apart from lead, barium and calcium sulphates (insoluble)
Are carbonates soluble in water
Most are INSOLUBLE
apart from sodium, potassium and ammonium carbonates
Are hydroxides soluble in water
Most are IBSOLUBLE
Apart from sodium, potassium and ammonium hydroxides
Are salts soluble in water
Yes all common sodium potassium and ammonium salts are
What is the method to form an INSOLUBLE SALT from two soluble salts
Mix the two soluble salts in a beaker and then filter.
Rinse beaker with distilled water and pour through funnel
Pour distilled water over precipitate in funnel
Carefully remove filter paper and dry in warm oven
What is the method to form a soluble salt
Core practical
Add excess metal oxide to acid
Gently warm in a water bath to speed up the reaction
Filter to remove the unreacted solid from the solution
Heat the solution to evaporate water and concentrate the salt solution
Leave to crystallise
Concentration =
no.moles ÷ volume (dm^3)
Why must titration be used to obtain a solution of a salt and water only, when using an acid and an alkali?
Because there is no insoluble excess reactant that could be removed by filtration.
Describe the method of a titration
Use the pipette and pipette filler to ADD a MEASURED VOLUME OF ALKALI TO a CONICAL FLASK
ADD a few DROPS of INDICATOR and put the CONICAL FLASK ON a WHITE TILE
FILL the BURETTE with ACID and NOTE the start VOLUME.
SLOWLY ADD ACID from the burette TO THE ALKALI in the conical flask, SWIRLING to mix.
STOP WHEN the indicator first PERMENANTLY CHANGES COLOUR). NOTE the FINAL VOLUME.
REPEAT steps 1 to 5 until you get concordant titres (2dp).
More accurate if added drop by drop.
What criteria do the products need to fulfil if a precipitate is to form
If AT LEAST 1 of the products meets the insoluble rule a precipitate will form
What are ionic compounds made out of
Charged particles (anions & cations)
When can ionic compounds conduct electricity and why
They can conduct electricity only when molten or dissolved
Can’t conduct when in a solid state as their ions are in fixed positions
What charge does the cathode have
Negative
What charge does the anode have
Positive
Would Chlorine be attracted to the anode or the cathode
Anode because chlorine is negatively charged and so would be attracted to positively charge anode
What is electrolysis
Electrolysis is a process in which electrical energy, from a direct current supply, decomposes electrolytes.
At which of the electrodes will an oxidation reaction occur
At the positively charged ANODE
(OILRIG)
Oxidation is loss of electrons
At which of the electrodes does a reduction reaction occur 
At the negatively charged CATHODE
(OILRIG)
Reduction is gain of electrons
When the substance is electrolysed IN SOLUTION what does it mean
That an extra OH- and H+ are produce (water)
What is the rule at the cathode for electrolysis in solution
Cation that is less reactive is reduced
Eg. If metal is lower than hydrogen in the reactivity series you get the metal
If hydrogen is lower then you get hydrogen instead of the metal
What is the rule at the anode for electrolysis IN SOLUTION
If you have halides (group 7 ions) the halogen is produced
With other common negative ions you get oxygen
What do you have to remember about group 7 molecules when balancing half equations
They are DIATOMIC (eg. Not Cl but Cl2)
What are the 3 elements less reactive than hydrogen on the reactivity series
Copper
Silver
Gold
(These are the only 3 elements that would win and be part of the half equation for electrolysis in solution)
Describe the method for electrolysis using copper electrodes
Pour copper sulphate solution into beaker
Measure and record mass of 2 copper (foil) electrodes
Attach one to the positive terminal and one to the negative terminal (making sure you know which one correspond to which mass)
Turn on power supply & leave for 20 mins
Remove 1 of the electrodes, rinse with distilled water and then dip it into propanone. Then lift it out and allow liquid to evaporate until dry.
Repeat with other electrode
Record mass of each and then calculate change in mass
You can repeat this experiment at different currents
Which of the copper electrodes gains mass?
Cathode (negative electrode) COPPER IS DEPOSITED
Which of the copper electrodes loses mass?
Anode (positive electrode) COPPER DISSOLVES
For the electrolysis with copper electrodes describe the change in mass as the current is increased
As the current is increased the change in mass of the electrodes becomes greater
Why are copper electrodes used in electrolysis
To purify copper
