Topic 3 - Chemical Changes Flashcards
What is the pH scale?
The pH scale is how acidic or alkaline a solution is.
What is the pH of a neutral substance?
A neutral substance has a pH of 7.
What is an acid?
An acid is a substance with a pH of less than 7.
What ions do acids form in water?
H+ ions
What happens as the concentration of hydrogen ions increases in a solution?
It becomes more acidic - the pH decreases.
What happens to the concentration of hydrogen ions as the solution becomes more acidic and the pH decreases?
The concentration of hydrogen ions increases
What is a base?
A base is a substance that reacts with an acid to produce a salt and water.
What is an alkali?
An alkali is a base that is soluble in water.
What do all alkalis have a pH of?
All alkalis have a pH of more than 7.
What do alkalis form in water?
Hydroxide (OH ⁻) ions.
What happens to the concentration of hydroxide ions as the pH increases in a solution?
It increases.
What happens as the concentration of hydroxide ions increases in a solution?
The pH increases.
What is an indicator?
An indicator is a dye that changes colour depending on whether it’s above or below a certain pH.
What are the four indicators you need to know about?
Universal indicator
Litmus
Methyl orange
Phenolphthalein
What colour is litmus in acidic solutions?
Red
What colour is litmus in neutral solutions?
Purple
What colour is litmus in alkaline solutions?
Blue
What colour is Methyl orange in acidic solutions?
Red
What colour is Methyl orange in neutral solutions?
Yellow
What colour is Methyl orange in alkaline solutions?
Yellow
What colour is Phenolphthalein in acidic solutions?
Colourless
What colour is Phenolphthalein in neutral solutions?
Colourless
What colour is Phenolphthalein in alkaline solutions?
Pink
What is a reaction between an acid and an alkali called?
Neutralisation
What does neutralisation produce?
A salt and water
When an acid neutralises a base (or a base neutralises an acid) what pH are the products?
The products are neutral (pH 7).
At pH 7, what are the concentrations of hydrogen ions and hydroxide ions?
The concentrations of hydrogen ions and hydroxide ions are equal.
What are some examples of strong acids?
Sulfuric acid, hydrochloric acid, and nitric acid.
What does dissociate mean?
Ionise
Do strong or weak acids ionise completely in water?
Strong acids, weak acids do not fully ionise in solution.
What pH do strong acids tend to have?
Strong acids tend to have a pH of around 0-2.
What are some examples of weak acids?
Ethanoic acid, citric acid, and carbonic acid
What pH do weak acids tend to have?
Weak acids tend to have a pH of around 2-6.
Is the ionisation of a weak acid reversible or irreversible?
The ionisation of a weak acid is a reversible reaction.
What does acid strength tell you?
Acid strength tells you what proportion of the acid molecules ionise in water.
What does the concentration of an acid measure?
Concentration measures how much acid there is in a litre (1 dm³) of water.
What unit is concentration measured in?
g dm⁻³
or
mol dm⁻³
What does concentration describe?
Concentration describes the total number of dissolved acid molecules - NOT the number of molecules that produce hydrogen ions.
What happens as the number of grams (or moles)
of acid per dm cubed increases?
The acid becomes more concentrated.
What happens if the concentration of hydrogen ions (H+ ions) increases by a factor of 10?
The pH decreases by 1.
What happens if the concentration of hydrogen ions (H+ ions) decreases by a factor of 10?
The pH increases by 1.
Acid + metal oxide =?
Acid + metal oxide = salt + water
Acid + metal hydroxide =?
Acid + metal hydroxide = salt + water
Acid + metal =?
Acid + metal = salt + hydrogen
Acid + metal carbonate=?
Acid + metal carbonate = salt + water + carbon dioxide.
How can you test for hydrogen?
You can test for hydrogen with a lit splint (the squeaky pop test)
What sound does hydrogen make with a lit splint?
Hydrogen makes a squeaky pop with a lit splint.
Why does hydrogen make a squeaky pop with a lit splint?
Hydrogen makes a squeaky pop with a lit splint because the hydrogen is burning with the oxygen in the air to form water.
How can you test to see if a gas is carbon dioxide?
You can test it to see if a gas is carbon dioxide by bubbling it through with limewater.
What will happen to limewater if carbon dioxide is present?
If carbon dioxide is present the limewater will turn cloudy.
What is a precipitate?
A precipitate is a solid insoluble salt which is formed when two solutions react together.
Are common salts such as sodium, potassium and ammonium soluble or insoluble?
Soluble.
Are nitrates soluble or insoluble?
Soluble.
Are common chlorides soluble or insoluble?
Soluble - except silver chloride and lead chloride.
Are common sulfates soluble or insoluble?
Soluble - except lead, barium and calcium sulfate.
Are common carbonates and hydroxides soluble or insoluble?
Insoluble – except for sodium, potassium and ammonium ones.
What reaction do you use to make a pure, dry sample of an insoluble salt?
You can use a precipitation reaction.
How can soluble salts be made?
Soluble salts can be made by reacting an acid with an alkali in a titration.
Why can’t universal indicator be used in titrations?
Universal indicator can’t be used in titrations because it’s colour change is too gradual, Phenolphthalein or Methyl orange should be used instead.
What is electrolysis?
Electrolysis is the breaking down of a substance using electricity.
How does electrolysis happen?
Electrolysis occurs when an electric current is passed through an electrolyte causing it to decompose.
What is an electrolyte?
An electrolyte is a molten or dissolved ionic compound.
What happens in electrolysis?
In electrolysis, oxidation (loss of electrons) and reduction (gain of electrons) occurs.
What are positive ions called?
Cations.
What is the negative electrode called?
The negative electrode is called the cathode.
What are negative ions called?
Anions.
What is the positive electrode called?
The positive electrode is called the anode.
What happens to the positive ions in electrolysis?
The positive ions in the electrolyte move towards the cathode, and are reduced.
What happens to the negative ions in the electrolyte?
The negative ions in the electrolyte move towards the anode, and are oxidised.
What is produced as the positive ions move towards the cathode and the negative ions move towards the anode?
A flow of charge is produced through the electrode.
What happens as ions gain or lose electrons?
They form uncharged substances and are discharged from the electrolyte.
What are half equations?
Half equations show how electrons are transferred during reactions.
How do you write half equations showing what happens at each electrode during electrolysis?
1) Put one of the substances being oxidised or reduced on one side of an arrow, and the substance it gets oxidised or reduced to on the other side.
2) Balance the numbers of atoms as you would in a regular symbol equation
3) Add electrons (written e-) on to one side to balance the charges.
E.g.
Sodium is losing one electron to become a sodium ion : Na => Na+ + e-
Hydrogen ions are gaining electrons to become hydrogen : 2H+ + 2e- => H2
What is an electrochemical cell?
An electrochemical cell is a circuit which is made up of the anode, cathode, electrolyte, a power source, and the wires which connect the two electrodes.
Why can’t an ionic solid be electrolysed?
An ionic solid can’t be electrolysed because the ions are in fixed positions and are not free to move.
What are positive metal ions reduced to at the cathode during electrolysis?
Metal atoms
What are negative ions oxidised to at the anode?
Atoms or molecules
In aqueous solutions what will be present as well as the ions from the ionic compound?
Hydrogen and hydroxide ions from the water will be present.
At the cathode if hydrogen and metal ions are present what will be produced if the metal is more reactive than hydrogen?
Hydrogen gas
At the cathode if hydrogen and metal ions are present what will be produced if the metal is less reactive than hydrogen?
A solid layer of the pure metal will be produced.
At the anode if hydroxide and halide ions (Cl⁻, Br⁻, I⁻)are present then what will be formed?
Molecules of Chlorine, bromine, or iodine will be formed.
At the anode if hydroxide ions are present but there are no halide ions present then what will be formed?
Oxygen
What is electrolysis used for with copper?
Electrolysis is used to purify copper by using an electrochemical cell with copper electrodes.
How is copper purified with electrolysis?
When copper is purified using electrolysis the anode is initially a large lump of impure copper, the copper sulfate solution is the electrolyte, and the cathode is a thin piece of pure copper.
The impure copper anode is oxidised, dissolving in the electrolyte to form copper ions.
The copper ions are reduced at the pure copper cathode and add a layer of pure copper to it.
Impurities from the impure copper anode sink to the bottom of the electrochemical cell, forming a sludge.
How do you investigate the neutralisation reaction between calcium oxide (a base) and dilute hydrochloric acid?
1) Measure a set volume of dilute hydrochloric acid into a conical flask. Using a pipette or a measuring cylinder.
2) Measure out a fixed mass of calcium oxide using a mass balance.
3) Add the calcium oxide to the hydrochloric acid.
4) Wait for the base to completely react, then record the pH of the solution, using either a pH probe or universal indicator paper.
5) Continue adding fixed amounts of calcium oxide until all of the acid has reacted. You will know when you’ve reached this point when you get unreacted calcium oxide at the bottom of the flask (excess).
6) You can then plot a graph to see how pH changes with the mass of base (calcium oxide) added.
How do you make insoluble salts using precipitation reactions?
1) Add one spatula of a soluble salt to a test tube. Use deionised water to dissolve the soluble salt - deionised water ensures that there are no other ions in it.
2) Stir thoroughly to ensure that all the salt has dissolved. Then, in a separate test tube, do the same with one spatula of another soluble salt which will react with the first salt to form an insoluble salt.
3) Pour the two solutions into a beaker and stir well to make sure that it is throughly mixed. The insoluble salt produced should precipitate out.
4) Filter the solution using filter paper and a filter funnel.
5) Rinse the beaker with deionised water, and pour into the filter paper, this is to ensure that all the precipitate is removed from the beaker.
6) Also rinse the contents of the filter paper with deionised water to make sure that all the soluble salt solution has been washed away.
7) Then scrape the insoluble salt produced onto fresh filter paper and leave it to dry in an oven or desiccator.
How do you make soluble salts using an acid and an insoluble base?
1) You can make soluble salts by reacting an acid that contains one of the ions you want in the salt with an insoluble base that contains the other ion you need (often a metal oxide or metal hydroxide).
2) Start by heating the acid in a water bath - this speeds up the reaction between the acid and the insoluble base. Do this in a fume cupboard to avoid releasing acid fumes into the room.
3) Then add the base to the acid – the base and acid will react to produce a soluble salt (and water). You will know when the base is in excess and all the acid has been neutralised because the excess solid will just sink to the bottom of the flask.
4) Filter off the excess solid to get a solution containing only the salt and water.
5) Heat the solution gently, using a Bunsen burner, to slowly evaporate off some of the water. Leave the solution to cool and allow the salt crystallise. Filter off the solid salt and leave it to dry.
What is the ionic equation for neutralisation?
H⁺ (aq) + OH⁻ (aq) => H₂O(l)
What is an equation showing how HCl is a strong acid?
HCl => H⁺ + Cl⁻
What is an equation to show how ethanoic acid is a weak acid?
CH₃COOH ⇌ H⁺ + CH₃OO⁻
How do you make soluble salts using acid and alkali reactions?
By titration using an indicator.
How do you carry out titration using an indicator?
1) Measure out a set amount of acid into a conical flask using a pipette. Add a few drops of indicator.
2) Slowly add alkali to the acid, using a burette, until you reach the end point – this is when the acid has been exactly neutralised and the indicator changes colour.
3) Then, carry out the reactions using exactly the same volumes of alkali and acid but with no indicator, so the salt won’t be contaminated with indicator.
4) The solution that remains when the reaction is complete contains only the salt and water.
5) Slowly evaporate off some of the water and then leave the solution to crystallise. Filter off the solid and dry it – you will be left with a pure, dry salt.
How do you set up an electrochemical cell if your electrolyte is a solution?
1) Get to inert (unreactive) electrodes, e.g. graphite or platinum electrodes.
2) Clean the surfaces of the electrodes using some emery paper (or sandpaper).
3) From this point on, be careful not to touch the surfaces of the electrodes with your hands – you could transfer grease back onto the strips.
4) Place both electrodes into a beaker filled with your electrolytes.
5) Connect the electrodes to a power supply using crocodile clips and wires. When you turn the power supply on, a current will flow through the cell.
Does the voltage of the cell increase or decrease as the electrolysis continues and the reactants get used up?
Decrease.
How do you set up an electrochemical cell if your electrolyte is a molten ionic substance?
1) Put your solid ionic substance (which will become your electrolyte) into a crucible.
2) Heat the crucible with a Bunsen burner until the solid is molten. You should do this in a fume cupboard to avoid releasing any toxic fumes into the room.
3) Once the solid is molten, dip two clean, inert electrodes into the electrolyte.
4) Then, connect the electrodes to a power supply using wires and clips – you should get a current flowing through the cell when you turn the power on.
What does the electrolysis of copper sulfate with inert electrodes produce?
Oxygen
What will happen if you increase the current in electrolysis?
You will increase the rate of electrolysis
What does increasing the rate of electrolysis result in?
A bigger difference between the mass of the two electrodes after the same amount of time.
what is calcium oxide an example of?
a base
what does the ionisation of a weak acid (a reversible reaction) set up?
an equilibrium
where does the equilibrium of the ionisation of a weak acid lie? Why is this?
the equilibrium lies to the left because only a few of the acid particles release H+ ions
what is an acid with a large number of acid molecules compared to the volume said to be?
concentrated
what is an acid with a small number of acid molecules compared to the volume of water said to be?
dilute
what are salts?
ionic compounds
in general what salts does hydrochloric acid produce?
chloride salts
in general what salts does sulphuric acid produce?
sulfate salts
in general what salts does nitric acid produce?
nitrate salts
what are the products of nitric acid with metals?
a nitrate salt, nitric oxide, nitrogen dioxide and water
what is an example of soluble salts that you can use in a precipitation reaction? what would be the products of this reaction?
lead nitrate and sodium chloride, lead chloride and sodium nitrate
what is an example of an insoluble salt and an acid you can use to make soluble salts?
copper oxide and warm sulfuric acid to make copper sulfate and water, you can then filter off the water and leave the solution to crystallise, the result should be blue crystals which can be filtered off and dried.
why do titrations need to be used to make soluble salts using acid/alkali reactions?
when reacting an acid with an alkali you can’t tell when the reaction has finished and the acid has been neutralised.
you cant add an excess of alkali to the acid because the salt is soluble and would be contaminated with excess alkali.
the exact amount of alkali needed to neutralise the acid needs to be worked out by a titration.
how could you check that you have set up your electrochemical cell correctly?
put an ammeter or bulb in series with the circuit to check if you’ve set it up correctly
what are some examples of inert electrodes to use in an electrochemical cell?
graphite or platinum electrodes
what does inert mean?
unreactive
what is an example of a substance which can be separated by electrolysis?
lead bromide, the silver molten lead goes to the cathode because the ions are positive and the bromide gas goes to the anode because the ions are negative
what happens when you electrolyse copper sulfate solution with inert electrodes?
copper metal is produced at the cathode because copper is less reactive than hydrogen,
oxygen and water are produced at the anode because there aren’t any halide ions present.
if you use copper electrodes in electrolysis reactions with a solution of copper sulfate instead of non-inert electrodes, what happens to the masses of the electrodes as the reaction continues?
the mass of the anode decreases and the mass of the cathode increases, this is because the copper is transferred from the anode to the cathode
how long should you leave the electrochemical cell running for in an electrolysis reaction when using copper electrodes in copper sulfate solution in order to get a significant mass change of the electrodes?
around 30 minutes
how can you find how the mass of the electrodes has changed in an electrolysis reaction when using copper electrodes in copper sulfate solution?
by finding the difference between the initial and final masses of the electrodes before and after the experiment
why should you make sure that the electrons are dry before weighing them to find the change in mass of the electrodes in an electrolysis reaction when using copper electrodes in copper sulfate solution?
because if there is any copper sulfate solution on the electrodes then the masses may appear to be greater than they actually are
if you increase the current by adding batteries to the electrochemical cell what happens? what does this mean will happen?
the rate of electrolysis will increase so there will be a greater difference between the mass of the two electrodes after the same amount of time.
how does the electrical supply act by in electrolysis?
what does this mean and why?
by pulling electrons from the copper atoms at the anode and giving the electrons at the cathode to the nearby copper ions.
this means that the concentration of copper ions in solution is constant because the ions are produced and removed at the same rate.
