Topic 3 - Chemical changes Flashcards
Charges of acids & alkalis in aqueous solutions
- acids produce H+ ions
- alkalis produce OH-
Colours of Phenolphthalein
Phenolphthalein
- Alkaline = pink
- Acidic = colourless
Colours of Methyl orange
Methyl orange
- Alkaline = yellow
- Acidic = red
Litmus solution and litmus paper
Litmus solution
-Alkaline = blue
- Acidic = red
○ Litmus paper
- Blue litmus paper goes red in acidic & stays blue in alkaline
- Red litmus paper goes blue in alkaline & stays red in acidic
What does does a strong acid and alkali mean ?
-higher concentration of H+ ions makes a strong acid
- higher concentration of OH- ions mean
solution is more alkaline, having a higher pH
Explain the terms concentrated and dilute
- concentrated is the larger amount of substance in a volume of a solution
▪ Dilute is the lesser amount of substance in a volume of a solution
acid + metal reaction
acid + metal -> salt + hydrogen gas
acid + metal oxide
acid + metal oxide -> salt + water
What is a weak acid?
- a weak acid partially dissociates – doesn’t produce many H+ ions, so reaction w/
What is a weak acid?
- a weak acid partially dissociates – doesn’t produce many H+ ions, so reaction w/ substance is slow
acid + metal hydroxide
acid + metal hydroxide -> salt + water
acid + metal carbonate
acid + metal carbonate -> salt + carbon dioxide
Test for hydrogen
- burning splint held at open end of a test tube of the gas
- Creates ‘squeaky pop’ sound
Test for carbon dioxide
- Bubble the gas through the limewater (calcium hydroxide solution) and will turn milky (cloudy)
Neutralisation reaction equation
H+ (aq) + OH- (aq) -> H20 (l)
Explain how to make copper sulfate crystals
- add excess of copper oxide (insoluble) to acid
- use filter and filter paper to filter off any copper oxide that hasn’t reacted
- evaporate off water by placing final solution in water bath
how to carry out acid-alkali titration, using burette, pipette
and indicator, to prepare salt
- Wash burette using acid and
- Fill burette with acid
- Use pipette to add alkali into conical flask, drawing alkali using a pipette filler
- Add phenolphthalein to conical flask
- use white tile
- Add acid from burette to alkali carefully and swirl till reaches endpoint (turns light purple)
- Repeat experiment to gain more precise results
- warm salt solution in water bath to evaporate the water– crystals will form
What is titre (titration experiment)?
-difference between first and second readings on burette
solubility of common sodium, potassium and ammonium
in water
- all are soluble in water
solubility of common chlorides in water
- are soluble except silver and lead,
solubility of common sulfates in water
-are soluble except lead, barium and calcium,
solubility of common carbonates and hydroxides are
-insoluble except sodium, potassium and ammonium
how to prepare pure, dry sample of an insoluble
salt
- mix two solutions needed to form salt
- filter mixture using filter paper, which insoluble salt will be left on
- wash salt using distilled water
- leave salt to dry on filter paper
What is an electrolyte?
-When an ionic substance is melted or dissolved & ions are free to move about within liquid or solution
What is electrolysis?
- process which electrical energy, from a direct current supply decomposes electrolytes
- Passing current through substances that are molten or solution means that solution can be broken down into elements– that’s electrolysis, and substance being broken down is the electrolyte
What is the movement of ions in electrolysis?
- positively charged ions (cations) move to negative
electrode (cathode) - negatively charged ions (anions) move to positive
electrode (anode). - Ions then discharged at electrodes producing elements
Explain what happens at the cathode during electrolysis?
- at cathode (-):
○ hydrogen (from H+ in water) is produced UNLESS positive ions in ionic compound are from a metal less reactive than hydrogen
○ if the metal is less reactive, it will be produced instead
What happens at anode during electrolysis?
- at anode (+):
○ oxygen (from OH- in water) will be produced UNLESS ionic compound contains halide ions (Cl-, Br-, I-)
○ if there are halide ions, the halogen will be produced instead (e.g. Cl2)
What is a halide ion ?
- halogen atom w/ a negative charge
List of halide ions
- fluoride, chloride, bromide, iodide and astatide
Electrolysis of copper chloride solution?
- Cu+ ions got to the cathode
- Cl- ions go to anode
Electrolysis of sodium chloride solution
H+ ions go to cathode
- Cl-ions go to anode
Electrolysis of Water acidified with sulfuric acid
- H+ to cathode,
- OH- to anode
Electrolysis of Molten lead bromide
- Pb2+ to cathode, Pb is produced (not in solution so these are the only + ions present)
- Br- to anode
What is the reactivity series (most reactive to least)
- potassium sodium calcium magnesium aluminium carbon zinc iron tin lead hydrogen copper silver gold platinum
Half equation of a negative electrode
X+ + e- -> X, electrons gained, so positive ions are reduced
Half equation of positive electrode
X- -> e- + X, electrons are lost, so negative ions are oxidised
where does oxidation and reduction occur?
- oxidation at anode – loss of electrons
- reduction at cathode – gain of electrons
How can electrolysis purify copper (experiment)?
-anode is made of impure copper
○ cathode is made of pure copper
○ the solution is copper sulfate
- Cu2+ ions from anode move to cathode, where they gain electrons and discharged as pure copper
○ impurities form as sludge below the anode
- cathode will increase in mass as it gains pure copper
– anode will lose mass as copper ions are lost – replace ones from CuSO4 solution & so are impurities
Why isn’t universal indicator used in titration experiments
- doesn’t show clear/ distinct change in colour
