SC3: chemical changes Flashcards
what do acids form in water?
acidic solutions & H+ ions
what do alkalis form in water?
alkaline solutions & OH- ions
litmus paper colour changes:
-red in acid
-blue in alkali
-purple in neutral
methyl orange colour changes
-red in acid
-yellow in alkali & neutral
phenolphthalein colour changes:
-colourless in acid & neutral
-pink in alkali
universal indicator colour changes:
-red/orange in acid
-purple/blue in alkali
-green in neutral
acid + carbonate
salt + water + carbon dioxide
acid + metal
salt + hydrogen
acid + metal oxide (base)
salt + water
acid + metal hydroxide (base)
salt + water
acid + base
neutralization; salt + water
what is a base?
a substance that reacts with an acid to neutralise it and produce a salt
what are soluble bases?
alkalis, bases that are alkalis are defined by neutralising acids and dissolving in water
are all bases soluble in water?
no
explain why all alkalis are bases, but not all bases are alkalis
-alkalis are soluble bases, but only some bases are soluble in water
-bases that dissolve in water are alkalis
neutralisation
reaction of an acid with a base, forming a salt and water
what happens with ions in neutralisation?
hydrogen ions from the acid react with hydroxide ions from the alkali
how can pH change be measured?
-with a pH probe and meter
-indicator
core practical investigating neutralisation (equipment)
-measuring cylinder
-beaker
-glass rod
-universal indicator paper
-spatula
-white tile
neutralisation steps
- use a measuring cylinder to add dilute hydrochloride acid to a beaker
- dip a clean glass rod into the contents of the beaker, use it to transfer a drop of liquid to a piece of universal indicator paper on a white tile, wait 30s and record the estimated pH
- add a spatula of calcium hydroxide powder to the beaker
- stir thoroughly, estimate & record pH of mixture
- repeat steps 3 & 4 till there are no more pH changes
what axis should the independent and dependant variable be on?
-independent on x
-dependant on y
what is a solution?
solute dissolved in a solvent
concentration
the mass of solute in a given volume of solution
what do strong strong acids do in water?
completely dissociate in water
what do weak acids do in water?
partially dissociate into ions in water
dissociation
the breaking up of a molecule into ions when dissolved in water
what happens when the concentration of OH- or H+ in a substance is timed by 10
the pH of the substance decreases by 1 (acids/hydrogen)
the pH of the substance increases by 1 (alkalis/hydroxide)
what does a high concentration of H+ mean?
lower pH & vice versa
what does a higher amount of
OH- mean?
a higher pH and vice versa
how are soluble salts made?
acids reacting with solid insoluble substances/bases (eg: metals, metal oxides, carbonates)
how do chemists usually make salts?
by reacting a metal oxide or a metal carbonate with an acid
naming salt
first part from metal, second from acid
making copper sulfate crystals practical (equipment)
-conical flask
-water bath
-spatula
-glass rod
-evaporating basin
-filter
-bunsen burner
making copper sulfate crystals - core practical
- place some sulfuric acid in a conical flask and warm it in a water bath
- add a spatula of copper oxide powder to the acid and stir with a glass rod
- continue adding copper oxide powder until it is in excess
- filter the mixture to remove the excess copper oxide
- pour the filtrate (the copper sulfate solution) into an evaporating basin
- heat the copper sulfate solution to evaporate half of the water
- pour the solution into a watch glass and leave to allow all of the water to evaporate
why is sulfuric acid warmed in the salt core practical?
warm acid reacts faster
titration practical equipment
-a pipette
-a conical flask
-a burette
-a suitable indicator
making salts from acids and alkalis (titration)
- use the pipette and pipette filler to add a measured volume of alkali to a clean 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 starting volume
- slowly add the acid from the burette to the alkali in the conical flask, swirling to mix
- stop adding the acid when the end-point is reached (when the indicator first permanently changes colour). note the final volume reading
- repeat steps 1 to 5 until you get concordant titres. more accurate results are obtained if acid is added drop by drop near to the end-point
what is solubility?
a measure of the maximum mass that will dissolve in a given volume of solvent, at a particular temperature
what do substances that are very soluble have?
high solubility and vice versa
what is a precipitate?
an insoluble solid formed by a reaction of two solutions
how can the precipitate in a reaction be predicted?
if the product is soluble no precipitate forms, if the product isn’t soluble, a precipitate forms
always dissolve
SNAP
s- sodium
n- nitrate
a- ammonium
p- potassium
sometimes dissolve
most chlorides = soluble, lead chloride & silver chloride = insoluble
most sulphates = soluble, lead, barium & calcium sulphates = insoluble
SNAP carbonates & hydroxides = soluble, most carbonates & most hydroxides = insoluble
method of making insoluble salts
- mix two suitable solutions together
- use filtration to separate the precipitate from the solution
- wash the precipitate distilled water while it is in the filter funnel
- leave the washed precipitate aside or in a warm oven to dry
what is electrolysis?
-electrical energy, from a direct current (dc) supply, decomposes electrolytes
-the free moving ions in electrolytes are attracted to the oppositely charged electrodes, which connect to the dc supply
what are electrolytes?
-ionic compounds that are in the molten state (heated so they become liquids), or dissolved in water
-under these conditions, the ions in electrolytes are free to move within the liquid or solution
cathode
negative electrode
anode
positive electrode
what happens to cations at the cathode?
cations gain electrons (metals)
what happens to anions at the anode?
anions lose electrons (non-metals)
why can water conduct electricity?
a small proportion of its molecules dissociate into H+ and OH- ions
during the electrolysis of water:
-H+ ions are attracted to the cathode, gain electrons and form hydrogen gas
-OH- ions are attracted to the anode, lose electrons and form oxygen gas
what does an electrolyte formed by dissolving an ionic compound contain?
two parts of negative & positive ions (positive H+ from water & positive ions from compound, negative OH- from water & negative ions from compound)
what is produced at the cathode in electrolysis of dissolved ionic compounds?
the metal is produced at the cathode if it is less reactive than hydrogen
mnemonic for reactivity series
please - potassium
stop - sodium
calling - calcium
me - magnesium
a - aluminium
careless - carbon
zebra - zinc
instead - iron
try - tin
learning - lead
how - hydrogen
copper - copper
saves - silver
gold - gold
what is produced at the anode in electrolysis of dissolved ionic compounds?
if halide ions are present, then the nonmetal/halide forms
electrolysis of copper sulfate (using inert electrodes)
- pour some copper sulfate solution into a beaker
- place two graphite rods into the copper sulfate solution. attach one electrode to the negative terminal of a dc supply, and the other electrode to the positive terminal
- completely fill two small test tubes with copper sulfate solution and position a test tube over each electrode
- turn on the power supply and observe what happens at each electrode
- test any gas produced with a glowing splint and a burning splint
- record your observations and the results of your tests
observations of the negative and positive terminals of copper sulfate electrolysis using inert electrodes:
-brown/pink solid forms (copper) at the cathode
-there are bubbles of colourless gas (gas relights a glowing splint - oxygen)
electrolysis of copper sulphate using copper electrodes
- pour some copper sulfate solution into a beaker
- measure and record the mass of a piece of copper foil. attach it to the negative terminal of a dc supply, and dip the copper foil into the copper sulfate solution
- repeat step 2 with another piece of copper foil, but this time attach it to the positive terminal
- make sure the electrodes do not touch each other, then turn on the power supply
- adjust the power supply to achieve a constant current
- after 20 minutes, turn off the dc supply
- carefully remove one of the electrodes. gently wash it with distilled water, then dip it into propanone. lift the electrode out and allow all the liquid to evaporate. do not wipe the electrodes clean. measure and record the mass of the electrode.
- repeat step 6 with the other electrode
- make sure you know which is which. repeat the experiment with fresh electrodes and different currents
how are the results of copper sulfate electrolysis with copper electrodes recorded?
with a table:
- calculate changes in mass
- plot a graph the show the change mass at both electrodes
evaluation for copper sulfate electrolysis:
-the gain in mass by the negative electrode is the same as the loss in mass by the positive electrode
-the copper deposited on the negative electrode must be the same copper ions that are lost from the positive electrode
hazards, risks and precautions of copper sulfate electrolysis with copper electrodes:
- copper sulfate solution
-causes skin and serious eye irritation -> wear gloves and eye protection - propanone
-highly flammable liquid and vapour may cause drowsiness or dizziness -> keep away from naked flames - use it in a fume cupboard
purifying copper by electrolysis
- get a beaker with pure and impure copper rods dipped into copper sulfate solution
- the pure copper rod is connected to the negative terminal of a battery (cathode) and the impure rod is connected to the positive terminal (anode)
- the pure copper rod has increased in size while the impure rod has deteriorated, leaving a pool of anode sludge at the bottom of the beaker
during electrolysis, the anode loses mass as copper dissolves, and the cathode gains mass as copper is deposited
half equation
used to represent the reaction that happens at an electrode during electrolysis
reduction
gain of electrons
oxidation
loss of electrons
OILRIG
oxidation is loss, reduction is gain
in half equations:
-electrons are shown as e-
-the numbers of atoms of each element must be the same on both sides
-the total charge on each side must be the same (usually zero)
half equations at the cathode
positively charged ions gain electrons at the cathode (the reaction will look like an ion + e- -> atom)
half equations at the anode
negatively charged ions lose electrons at the anode (the reaction will look like an ion -> atom + e-)
