Topic 3 – Chemical Changes Flashcards by Fiza Ahmed

Explain the meaning of weak acid

• partially ionised
• concentration of H+ ions lower

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Define electrolytes

Ionic compound in molten state/dissolved in water

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Define electrolysis

A process in which electrical energy/ electricity (from a direct current supply) decomposes electrolytes

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Cations (positively charged ions) move towards the…

Cathode (negative electrode)
• REDUCED (gains electrons) so mass increases

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Anions (negatively charged ions) move towards the…

Anode (positive electrode)
• OXIDISED (loses electrons) so mass decreases

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What ions are present at the cathode?

H+ and metal ions

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When is hydrogen produced at the cathode?

Hydrogen produced if metal is more reactive than hydrogen (e.g. sodium)

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When are metals produced at the cathode?

Metal produced if it’s less reactive than hydrogen (e.g. copper, silver)

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What happens when OH- & halide ions are present at the anode?

• if ionic compound contains halide ions, the halogen is produced (Cl2)

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What happens when OH- & halide ions are NOT present at the anode?

• Oxygen produced if no halide ions are present (Cl, Br, I)

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What are the products formed when a sample of molten potassium bromide is electrolysed?

Potassium & bromine

• Potassium (more reactive than Hydrogen)
• Bromine (halide ion)

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Test for Hydrogen

• Lit splint
• Burns with a squeaky pop

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State why it is important that the electrodes are inert

inert electrodes do not react with the electrolyte

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Explain, in terms of structure, how graphite conducts electricity

• electrons
• move (through graphite)
• are delocalised/ free to move

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Explain why mass decreases at anode

• Atoms become ions
• Atoms are oxidised (lose electrons)
• ions move into solution

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Explain why mass increases at cathode

• Ions become atoms
• Ions are reduced (gain electrons)
• Ions move out of solution

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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)

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Explain what should be done to copper electrodes before their masses are determined

• Electrodes should be cleaned (using emery paper)
• to remove impurities

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Explain how Hydrogen gas is formed at the cathode

• hydrogen ions attracted to cathode
• hydrogen ions are reduced (gain 2 electrons)

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State what equipment could be used to measure the volume of a gas

• measuring cylinder
• burette
• gas syringe

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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