Topic 3 - Chemical Changes Flashcards
Explain what acids and alkalis are sources of
acids in solution are sources of hydrogen ions and alkalis in solution are sources of hydroxide ions
What is the effect of acids and alkalis on indicators, including litmus, methyl orange and phenolphthalein
LITMUS: acid - red, alkali - blue
PHENOLPHTHALEIN: acid - colourless, alkali - pink
METHYL ORANGE: acid - red, alkali - yellow
Explain the effect of a higher concentration of hydrogen ions
the higher the concentration of hydrogen ions in an acidic solution, the lower the pH; and the higher the concentration of hydroxide ions in an alkaline solution, the higher the pH
Explain the difference between concentrated and dilute and how to change between them
For a given volume:
- a concentrated solution has a greater amount of dissolved solute particles than a dilute solution
You can change a concentrated solution into a dilute solution by adding more water to it. You can change a dilute solution to a more concentrated solution by:
- dissolving more solute in it
- evaporating some of the water
Explain the effect of concentration of hydrogen ions on the pH
as hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1
Explain the method of how to investigate the change in pH when calcium hydroxide, a base, is added to a fixed volume of dilute hydrochloric acid
Add some dilute hydrochloric acid to the beaker. Measure and record the pH of the contents of the beaker. Add a small mass of calcium hydroxide powder, stir, and then measure and record the pH again. Repeat until the pH no longer changes
Explain precautions needed to be taken when investigating the change in pH when calcium hydroxide, a base, is added to a fixed volume of dilute hydrochloric acid
- calcium oxide is corrosive but calcium hydroxide is an irritant , so calcium hydroxide is safer to use
- you should choose appropriate apparatus for practical work. If using a measuring cylinder
Explain the terms weak and strong acids
Hydrochloric acid and sulfuric acid are strong acids because they fully dissociate into ions in their solution. All their molecules release H+ (aq) ions
Ethanol acid is a weak acid because it partially dissociates into ions in solution. Only a few molecules release H+ (aq) ions
Explain what a base is
A base is any substance that reacts with an acid to form a salt and water only.
Bases are metal oxides and metal hydroxides. In general:
Base + acid —> salt + water
Describe an alkali in its base form
An alkali is a SOLUBLE base
Describe the chemical test for:
- hydrogen
- carbon dioxide (limewater)
HYDROGEN - a lighted splint ignites hydrogen with a ‘pop’. Gas is collected in upturn tube
CARBON DIOXIDE - carbon dioxide turns limewater milky or cloudy white
Explain the general reactions of aqueous solutions of acids with metals
Reactive metals react with acids to produce a salt and hydrogen ONLY. In general:
Metal + acid —> salt + hydrogen
Explain the general reactions of aqueous solutions of acids with metal carbonates
Metal carbonates react with acids to produce a salt, water and carbon dioxide. In general:
Metal carbonate + acid —> salt + water + carbon dioxide
Describe a neutralisation reaction
Neutralisation is the reaction between an acid and a base. In an acid-alkali neutralisation, hydrogen ions from the acid react with hydroxide ions from the alkali to form water
Explain how are soluble salts are prepared from an acid and a soluble reactant using titration
- Use an acid-base titration to find the exact volume of the soluble base that reacts with the acid
- Mix the acid and soluble base in the correct proportions, producing a solution of the salt and water
- Warm the salt solution to evaporate the water - this will leave crystals of the salt behind
Explain the method to make a sample of pure, dry hydrated copper sulfate crystals using copper oxide and sulfuric acid
- Add excess base to the acid
- Filter to remove unreacted copper oxide
- Crystallise the copper sulfate solution by heating it or leaving it to stand in a warm place
Explain the apparatus needed to make a sample of pure, dry hydrated copper sulfate crystals using copper oxide and sulfuric acid
- eye protection
- beakers
- spatula
- stirring rod
- Bunsen burner
- heat-resistant mat
- tripod and gauze mat
- filter funnel and filter paper
- evaporating basin
- dilute sulfuric acid
- copper oxide powder
Describe how to carry out an acid-alkali titration, using burette, pipette and a suitable indicator, to prepare a pure, dry salt
- put acid into a burette
- use a pipette to put a known volume of alkali into a conical flask
- put a few drops of a suitable indicator solution, such as phenolphthalein or methyl orange, into the alkali
- record the burette start reading
- add acid to the alkali until the colour changes - the end-point
- record the burette end reading
Describe the solubility rules
SOLUBLE:
- all common sodium, potassium and ammonium salts (This is why solutions in practicals are often sodium compounds)
- all nitrates
- common chlorides
- common sulfates
- sodium hydroxide, potassium hydroxide, ammonium hydroxide
- sodium carbonate, potassium carbonate, ammonium carbonate
INSOLUBLE:
- silver chloride, lead chloride
- lead sulfate, barium sulfate, calcium sulfate
- common hydroxides
- common carbonates
Predict, using solubility rules, whether or not a precipitate will be formed when named solutions are mixed together, naming the precipitate if any
A precipitate will form - silver chloride
Describe the method used to prepare a pure, dry sample of an insoluble salt
- Mix solutions of two substances that will form the insoluble salt
- Filter the mixture. The insoluble salt will be trapped in the filter paper
- Wash the salt with distilled water
- Leave the salt to dry on the filter paper. It could be dried in an oven
Describe what an electrolyte is
An electrolyte is an ionic compound in the molten state (liquid) or dissolved in water
Describe what electrolysis is
Electrolysis is the process that splits ionic compounds into the different elements that they are made of. Electrolysis happens using a solution that conducts electricity.
Explain the movement of ions during electrolysis
Positively charged cations migrate to negatively charged cathode
Negatively charged anions migrate to positively charged anode
Explain the formation of the products in the electrolysis, using inert electrodes, of some electrolytes, including:
a copper chloride solution
b sodium chloride solution
c sodium sulfate solution
d molten lead bromine
COPPER CHLORIDE SOLUTION - copper at the cathode, chlorine at the anode (copper less reactive than hydrogen, so copper is produced, chloride ions are present, so chlorine is produced)
SODIUM CHLORIDE SOLUTION - hydrogen at the cathode, chlorine at the anode (sodium is more reactive than hydrogen, so hydrogen produced instead of sodium)
SODIUM SULFATE SOLUTION - hydrogen at the cathode, oxygen at the anode (halide ions aren’t present, so oxygen is produced when OH- ions are discharged
MOLTEN LEAD BROMINE - produces lead and bromine, electrons gained (reduction) at negative electrode and electrons lost (oxidation) at positive electrode
Predict the products of electrolysis of other binary, ionic compounds in the molten state
Eg:
Zinc chloride (and lead bromine) is a binary ionic compound. This means that it consists of two elements only. You must be able to predict the products of electrolysis of such compounds in the molten state.
In molten zinc chloride zinc forms at the negative electrode and chlorine at the positive electrode
METAL AND HYDROGEN FORM AT THE CATHODE
NON METLA FORMS AT THE ANODE
Explain reduction and oxidation
OXIDATION - happens at the positive electrode (anode), gaining oxygen but losing electrons
REDUCTION- happens at the negative electrode (cathode), losing oxygen but gaining electrons
Write half equations for reactions occurring at the anode and cathode in electrolysis
Eg (molten zinc chloride):
Negative electrode: Zn 2+ + 2e- —> Zn
Positive electrode: 2Cl- —> Cl 2 + 2e-
Electrons are shown as e- in these equations
Explain the formation of the products in the electrolysis of copper sulfate solution, using copper electrodes, and how this electrolysis can be used to purify copper
Copper can be purified by the electrolysis of copper sulfate solution using copper electrodes.
The cathode gains mass as copper ions gain electrons:
Cu 2+ (aq) + 2e- —> Cu (s) - reduction
The anode loses mass as the copper atoms lose electrons:
Cu (s) —> Cu 2+ (aq) + 2e- - oxidation
Copper ions move to copper electrode, gain electrons and are discharged as pure copper.
Ions are replaced by copper ions from the impure copper anode
