Chemical changes Flashcards
What does the pH scale measure?
How acidic or alkaline a solution is
What is an indicator?
A dye that changes colour depending on whether it’s above or below a certain pH
Other than indicator, how can you test the pH of a solution?
A pH probe attached to a pH meter which measures the pH electronically, giving it a numerical value
What is an acid?
A substance that forms an aqueous solution with a pH less than seven and forms H+ ions in water
What is a base?
A substance with a pH greater than seven
What is an alkali?
A base that dissolves in water to form a solution with a pH greater than seven and forms OH- solutions in water
What will a neutralisation reaction produce?
Acid + base –> salt + water
What can titrations be used for?
To find out exactly how much acid is needed to neutralise a quantity of alkali
How do you increase the accuracy of a titration experiment?
Use the first titration as a practice (do not include in results), repeat the experiment a few times (making sure the same answer is got each time) and calculating a mean of your results, ignoring any anomalies
Why should a single indicator be used for a titration?
It allows you to see a sudden colour change at the end-point, rather than a gradual change
Give three examples of a single indicator
Phenolphthalein, litmus or methyl orange
What is the difference between a strong and a weak acid?
Strong acids ionise completely in water (all acid particles dissociate to release H+ ions) and weak acids do not fully ionise in solution (only a small proportion of particles dissociate to release H+ ions)
Will a strong acid or a weak acid be more reactive and why?
A strong acid because reactions of acids involves the reaction of H+ acids with substances so as the concentration of H+ ions is higher, the rate of reaction will be faster
What is the rule for the concentration of H+ ion son the pH scale?
Factor H+ ion concentration changes by = 10^-x
Is a high concentration acid and a strong acid the same thing?
No, acid strength tells you what proportion of the acid molecules ionise in water and concentration measures how much acid there is in a certain volume of water
What is produced from the neutralisation of an acid and an alkali?
All metal oxides and metal hydroxides (alkaline compounds) react with acids to form a salt and water
Give three examples of reactions between metal oxides or hydroxides and acid
hydrochloric acid + copper oxide –> copper chloride + water, sulfuric acid + potassium hydroxide –> potassium sulfate + water, nitric acid + sodium hydroxide –> sodium nitrate + water
What does a reaction between an acid and a metal carbonate (a type of base) produce?
Acid + metal carbonate –> salt + water + carbon dioxide
Give two examples of acid and metal carbonate reactions
hydrochloric acid + sodium carbonate –> sodium chloride + water + carbon dioxide, sulfuric acid + calcium carbonate –> calcium sulfate + water + carbon dioxide
What is the reactivity series?
Potassium, sodium, lithium, calcium, magnesium, carbon, zinc, iron, hydrogen and copper
How is reactivity determined for metals?
How easy they lose their electrons, forming positive ions
What will how metals react with acids tell you about their reactivity?
The speed of the reaction is indicated by bubbles of hydrogen being given off or the temperature change; the more reactive a metal, the faster a reaction will go
What will the reaction between a metal and water produce?
Metal + water –> metal hydroxide + hydrogen
Which metals on the reactivity series will react with water?
The only ones reactive enough are potassium, sodium, lithium and calcium
What is an oxidation reaction?
When a metal reacts with oxygen to form an oxide (gain of oxygen)
What is a reduction reaction?
A reaction that separates a metal from its oxide (loss of oxygen)
Give an example of an oxidation reaction and a reduction reaction (in symbols)?
2Mg +O2 –> 2MgO
2CuO + C –> 2Cu + CO2
What is the most common of extracting metals?
By using reduction with caron: the ore is reduced as oxygen is removed from it and the carbon is oxidised as it gains oxygen
Which metals in the reactivity series can be reduced with carbon and why?
Metals below carbon as carbon can only take oxygen away from metals that are less reactive than carbon itself
What are redox reactions in terms of electrons?
A loss of electrons is oxidation and gaining electrons is reduction (OIL RIG)
Why is it called a redox reaction (give an example)?
Reduction and oxidation are happening at the same time e.g. Fe + 2H^+ –> Fe^2+ + H2: the iron atoms lose electrons whilst the hydrogen ions gain electrons
What is the rule of displacement?
A more reactive metal will displace a less reactive metal from it’s compound
In a displacement reaction, what is reduced and what is oxidised?
The metal ion always gains electrons (reduced) and the metal atom always loses electrons (oxidised)
What does electrolysis mean?
Splitting up with electricity
How does electrolysis work?
A current is passed through an electrolyte (molten or dissolved ionic compound). The ions move towards the electrodes, where they react and the compound decomposes.
What happens at the cathode?
As it is negative, the positive ions will move towards it where they gain electrons
What happens at the anode?
As it is positive, the negative ions will move towards it and lose electrons
What are the pros and cons of using electrolysis to extract metals from their ores?
It is a way of extracting metals if they are too reactive to be reduced with carbon, however, it can be very expensive as it requires lots of energy to melt the ore and produce the current
Describe in detail the process of extracting aluminium
It is extracted as aluminium oxide from bauxite, then as it has a very high mp, it is mixed with cryolite to lower the mp. As a molten mixture, it contains free ions so can conduct electricity. The positive ions are attracted to the cathode where they pick up three electrons and turn into aluminium atoms.
Give the two electron equations and the overall equation for the electrolysis of aluminium
Al^3+ + 3e^- –> Al
2O^2- –> O2 + 4e^-
2Al203 –> 4Al + 302
At the cathode, what happens if the metal ions form an elemental metal that is more or less reactive than hydrogen?
If it is more, hydrogen gas will be produced and if it is less, a solid layer of the pure metal will be produced instead
How can you test for any gases present in an electrolysis experiment?
Chlorine bleaches damp litmus paper, hydrogen makes a squeaky pop with a lit splint and oxygen will reight a glowing splint
