Topic 3 – Chemical Changes Flashcards
Explain the meaning of weak acid
• partially ionised
• concentration of H+ ions lower
Define electrolytes
Ionic compound in molten state/dissolved in water
Define electrolysis
A process in which electrical energy/ electricity (from a direct current supply) decomposes electrolytes
Cations (positively charged ions) move towards the…
Cathode (negative electrode)
• REDUCED (gains electrons) so mass increases
Anions (negatively charged ions) move towards the…
Anode (positive electrode)
• OXIDISED (loses electrons) so mass decreases
What ions are present at the cathode?
H+ and metal ions
When is hydrogen produced at the cathode?
Hydrogen produced if metal is more reactive than hydrogen (e.g. sodium)
When are metals produced at the cathode?
Metal produced if it’s less reactive than hydrogen (e.g. copper, silver)
What happens when OH- & halide ions are present at the anode?
• if ionic compound contains halide ions, the halogen is produced (Cl2)
What happens when OH- & halide ions are NOT present at the anode?
• Oxygen produced if no halide ions are present (Cl, Br, I)
What are the products formed when a sample of molten potassium bromide is electrolysed?
Potassium & bromine
• Potassium (more reactive than Hydrogen)
• Bromine (halide ion)
Test for Hydrogen
• Lit splint
• Burns with a squeaky pop
State why it is important that the electrodes are inert
inert electrodes do not react with the electrolyte
Explain, in terms of structure, how graphite conducts electricity
• electrons
• move (through graphite)
• are delocalised/ free to move
Explain why mass decreases at anode
• Atoms become ions
• Atoms are oxidised (lose electrons)
• ions move into solution
Explain why mass increases at cathode
• Ions become atoms
• Ions are reduced (gain electrons)
• Ions move out of solution
Molten lead bromide can be electrolysed to form molten lead and bromine gas. Explain how a student can set up an apparatus to carry out this electrolysis (2)
• Use a crucible (instead of beaker)
—> so it doesn’t break (when heated strongly)
Explain what should be done to copper electrodes before their masses are determined
• Electrodes should be cleaned (using emery paper)
• to remove impurities
Explain how Hydrogen gas is formed at the cathode
• hydrogen ions attracted to cathode
• hydrogen ions are reduced (gain 2 electrons)
State what equipment could be used to measure the volume of a gas
• measuring cylinder
• burette
• gas syringe
State what could be added into the circuit to show a current is flowing during electrolysis
• ammeter
• light bulb/lamp
State how hydrochloric acid and base can be measured accurately
• HCL - Burette, pipette, measuring cylinder
• Base - balance, scales, weigh out amount
Colour change of phenolphthalein indicator
Start: colourless
End: pink
Explain why pH increases during neutralisation
• H+ ions neutralised/reacted
• more concentration of OH- ions
Test for carbon dioxide
• Bubble gas through limewater
• limewater turns milky/cloudy
Test for oxygen
• glowing splint
• relights
Describe what you would see when copper carbonate powder is added to a beaker of dilute sulfuric acid
• powder disappears/dissolved in acid
• effervescence/fizzing/bubbles
• blue solution forms
Describe how pure, dry crystals are obtained from a solution (2 marks)
• Heat solution/leave water to evaporate
• Filter off/dry crystals formed
Describe how a sample of pure, dry crystals can be obtained from the filtrate
• heat solution (to concentrate)
• pat dry between filter papers
• dry in oven
Copper carbonate + sulfuric acid —> copper sulfate + carbon dioxide + water. State two observations that would show the reaction had finished
• no more fizzing/bubbles
• no further change in colour
• solid (copper carbonate) remains at bottom of flask / no more solid dissolves
Explain why dilute hydrochloric acid is described as a strong acid
• fully dissociates/ionises
• to form H+ ions
Explain why the new mixture was evaporated rather than the original mixture from the titration (2)
• solution from titration contains an indicator
• therefore second solution used with no indicator
• original mixture was contaminated by indicator so doesn’t form a pure salt (2)
Equipment that can measure pH
• pH meter
• pH probe
Explain how pH changes as magnesium hydroxide is added to dilute hydrochloric acid
• pH increases
• until pH above 7
• magnesium hydroxide neutralises the acid
• so hydrogen ions concentration is reduced/all H+ ions reacted/there is an excess of hydroxide ions
The hydrogen ion concentration in a solution is decreased by a factor of 10.
State how the pH of this solution changes.
pH increases by 1
Explain why litmus paper would not be a suitable indicator to use
• litmus paper only shows if solution is acidic/alkaline
• does not show how acidic / alkaline solution is
Suitable indicators for titrations
• phenolphthalein
• methyl orange
NOT UNIVERSAL INDICATOR
Why is universal indicator not a suitable indicator to use in an acid-alkali titration?
• does not show sharp colour change at end point
• not known which colour change gives correct end point
Describe how to obtain a pure, dry sample of the precipitate of copper hydroxide from the reaction mixture
• Filter mixture using filter paper
• residue (solid) is rinsed/washed (using distilled water)
•(scrape copper hydroxide onto fresh filter paper)
• dry in oven
What element promote plant growth?
• Nitrogen
• Phosphorus
• potassium
What colour is methyl orange in acid, neutral & alkaline?
ACID - red
NEUTRAL - yellow
ALKALINE - yellow
What colour is phenolphthalein in acid, neutral & alkaline?
ACID - colourless
NEUTRAL - colourless
ALKALINE - pink
What colour is litmus in acid, neutral & alkaline?
ACID - red
NEUTRAL - purple
ALKALINE - blue
Acid + Metal Oxide produces…
Salt + water
Acid + Metal Hydroxide produces…
Salt + water
Acid + Metal produces…
Salt + Hydrogen
Acid + Metal Carbonate produces…
Carbon dioxide + Salt + Water
What 3 salts are soluble?
• sodium
• potassium
• ammonium
Are all nitrates soluble / insoluble?
Soluble
Name 2 insoluble chlorides
• Silver chloride
• Lead chloride
Name 3 insoluble sulfates
• Lead sulfate
• Barium sulfate
• Calcium sulfate
Name 3 soluble carbonates & hydroxides
• Ammonium carbonate/hydroxide
• Sodium carbonate/hydroxide
• Potassium carbonate/hydroxide
