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GCSE Chemistry > Topic 4- Chemical Changes > Flashcards

Topic 4- Chemical Changes Flashcards

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1
Q

How can you measure how acidic or alkaline a substance is?

A

This can be measured on the pH scale.

The pH scale goes from 0 to 14.

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2
Q

What range of pHs show that a substance is an acid and alkali?

A

-Anything that forms a solution with a pH of LESS than 7 is an ACID.
The LOWER the pH of a solution, the MORE ACIDIC it is.

-Anything that forms a solution with a pH GREATER than 7 is an ALKALI.
The HIGHER the pH, the MORE ALKALINE it is.

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3
Q

If a solution is neutral what pH will it have?

A

NEUTRAL substances have a pH of EXACTLY 7.

Pure water is an example of a neutral substance.

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4
Q

How can you measure the pH of a solution?

A

There are 2 ways of measuring pH, using an indicator or using a pH probe attached to a pH meter.

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5
Q

What is an indicator?

A

An indicator is a DYE that CHANGES COLOUR depending on whether it’s ABOVE OR BELOW A CERTAIN pH.

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6
Q

What is a wide range indicator and why is it useful?

A

Some indicators contain a MOXTURE OF DYES that means they GRADUALLY CHANGE COLOUR over a broad range of pH. These are called WIDE RANGE INDICATORS and they’re useful for ESTIMATING the pH of a solution.

A UNIVERSAL INDICATOR is a wide range indicator.

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7
Q

Describe how you could measure the pH of a solution using a pH probe…

A

A pH PROBE attached to a pH METER can also be used to measure pH ELECTRONICALLY.

The probe is placed in the solution you are measuring and the pH is given on a digital display as a NUMERICAL VALUE, meaning it’s more accurate than an indicator.

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8
Q

What are the characteristics of an acid?

N.b.-Ions are electrically charged particles formed when atoms lose or gain electrons. This loss or gain leaves a complete highest energy level

A
  1. An ACID is a substance that forms AQUEOUS SOLUTIONS with a pH of LESS THAN 7.
  2. Acids form H+ IONS in WATER.
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9
Q

What is a base?

A

A BASE is a substance with a pH GREATER THAN 7.

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10
Q

What are the characteristics of an alkali?

A
  1. An ALKALI is a base that DISSOLVES IN WATER to form a solution with a pH GREATER THAN 7.
  2. Alkalis form OH- IONS in WATER.
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11
Q

What type of reaction occurs when an acid and alkali react and what is formed?

A

An ACID will react with an ALKALI to form a SALT and WATER- this is called NEUTRALISATION.

acid + alkali —–> salt + water

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12
Q

Other than a word equation, how else can neutralisation reactions be shown?

A

Neutralisation reactions between acids and alkalis can also be shown in terms of H+ ions and OH- ions.

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13
Q

Explain how a neutralisation reaction occurs? (what happens to the ions)

A

During neutralisation reactions, hydrogen ions (H+) from the acid react with hydroxide ions (OH-) from the alkali to produce WATER.

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14
Q

What is the ionic equation showing neutralisation?

A

H+ (aq) + OH- (aq) ————–> H2O (l)

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15
Q

What is the pH of the solution formed from a neutralisation reaction?

A

When an acid neutralises an alkali (or vice versa), the solution that’s formed is NEUTRAL- it has a pH of 7.

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16
Q

How can you tell if a neutralisation reaction is over?

A

An indicator can be used to show that a neutralisation reaction is over.

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17
Q

What is a titration?

A

A TITRATION is an experiment that lets you see what volume of reactant is needed to react with a certain volume of another reactant.

E.g. titrations can be used to find out EXACTLY how much acid is needed to NEUTRALISE a quantity of alkali. You can then use this data to work out the CONCENTRATION of the acid or alkali.

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18
Q

Explain how to do a titration to accurately find the volume of a strong acid needed to neutralise a measured amount of alkali…

A

1) Use a PIPETTE and a PIPETTE FILLER, measure a set volume alkali.
2) Put the alkali in a conical flask. Add a two or three drops of INDICATOR. Place the conical flask on a WHITE TILE so you can see a COLOUR CHANGE more clearly.
3) Using a FUNNEL, fill a BURETTE with some acid of a KNOWN CONCENTRATION.

(Make sure you do this BELOW EYE LEVEL- you don’t want to be looking up if some acid spills over. You should wear safety glasses too).

Run a small amount through the tap, then turn the tap off.
Place the flask containing the alkali under the burette.

4) Take a READING of the volume of acid in the burette by reading off the value where the bottom of the meniscus touches the scale.
5) The FIRST titration you should do, should be a ROUGH TITRATION. This helps to make your final results more ACCURATE by giving you an APPROXIMATE idea of how much acid is needed to neutralise the alkali. Add the acid to the alkali a bit at a time, giving the flask a regular SWIRL.
6) The indicator CHANGES COLOUR when ALL the alkali has been NEUTRALISED, e.g. phenolphthalein is PINK in ALKALINE conditions, but COLOURLESS, in ACIDIC conditions.
7) Record the volume left in the burette.
8) Calculate the amount of acid that was needed to neutralise the alkali by SUBTRACTING the INITIAL READING of acid in the burette from the FINAL READING of acid in the burette. This is your rough TITRE.
9) Now do an accurate titration. Take an initial reading of how much acid is in the burette. Then run the acid in to within 2㎤ of the end point. Continue to add drop by drop- you need to spot exactly when the colour of the indicator changes for your result to be accurate.
10) Repeat step 8 and write the measurement down.
11) REPEAT the process of making an accurate titration a few times, until you have at least 3 results that are within 0.10 ㎤ of each other.
12) Calculate the MEAN volume of acid needed to neutralise the alkali, ignoring the rough titre and any ANOMALOUS RESULT.

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19
Q

How could you make sure that your results are valid/ accurate?

A

Doing the experiment more than once lets you see if your results are REPEATABLE or not.

Collecting lots of results and calculating a mean will reduce the amount of RANDOM ERRORS in your results as well.

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20
Q

What is UI used for?

What colour is an acid and alkali in universal indicator?

Why isn’t universal indicator a suitable indicator to use in a titration?

A

Universal indicator is used to ESTIMATE the pH of a solution because it can turn a VARIETY OF COLOURS. Each colour indicates a NARROW RANGE of pH values.

It’s made from a MIXTURE of different indicators. The colour GRADUALLY changes from red in acidic conditions to violet in alkali solutions.

You can’t use UI in an acid-alkali titration though, because the COLOUR CHANGE is too GRADUAL. You need an indicator that will give you a DEFINITE colour change so you can see the exact point where the alkali is neutralised by the acid (the end-point).

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21
Q

What type of indicator should you use in a titration?

A

You need to use a SINGLE INDICATOR, such as METHYL ORANGE (yellow in alkalis and red in acids).

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22
Q

What colour does this indicator go in an acid and an alkali?

A

Methyl orange is YELLOW in alkalis but RED in acids, so if there’s methyl orange and alkali in the flask they’ll be yellow at the beginning of the titration. When all the alkali has reacted with the acid but added the indicator will turn from yellow to red, and you know you’ve reached the end-point.

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23
Q

What is the first step before carrying out a titration?

A

Before carrying out a titrations you’ll need to do a risk assessment to identify any HAZARDS. You’ll then need to take precautions to make sure that they don’t cause any harm.

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24
Q

Explain the risk assessment taken when carrying out a titration…

Suggest 2 safety precautions that you should take when carrying out a titration using an acid an alkali…

A

The main hazards in titrations are the acid and alkali.

Acids are often irritants or corrosive. Alkalis are often corrosive. So you need to wear SAFETY GOGGLES to protect your eyes when using them. Sometimes concentrated acids or alkalis are DILUTED before they’re used to stop them damaging the skin if they come into contact with it.

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25
Q

What is an ion?

A

Ions are electrically charged particles formed when atoms lose or gain electrons.

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26
Q

How do acids react in aqueous solutions?

A

All acids IONISE in aqueous solutions (they form H+ ions).

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27
Q

Give the definition of a strong acid?

Name 3 examples of strong acids…

A

Strong refers to the degree to which an acid will ionise (form H+ ions).

A strong acid is completely ionised in aqueous solution.
strong acids are hydrochloric, nitric and sulfuric acids.

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28
Q

Give the definition of a weak acid and name 3 examples..

A

A weak acid is only partially ionised in aqueous solution.

Examples of weak acids are ethanoic, citric and carbonic acids.

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29
Q

Explain in terms of chemical structure, how weak acids react with water…

A

Weak acids only PARTIALLY ionise in water-if you put a sample of a weak acid in water, only some of the acid molecules will ionise and release H+ ions.

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30
Q

Why do weak acids only partially ionise in water?

A

The ionisation of a WEAK acid is a REVERSIBLE REACTION, which sets up an EQUILIBRIUM between the UNDISSOCIATED and DISSOCIATED ACID.
Since only a few of the acid particles release H+ ions, the position of EQUILIBRIUM lies well to the LEFT.

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31
Q

Define what a concentrated acid is?

A

Concentrated in chemistry refers to the amount (number of moles) in a particular volume of liquid; a concentrated acid means there are a lot of acid molecules crammed into a small volume of liquid.

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32
Q

What is a dilute acid?

A

Dilute acids can be considered to be watered down versions of the real stuff-i.e. lots of water and hardly any acid.

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33
Q

What is the effect of acid strength on reactivity?

A

Reactions of acids involve the H+ ions reacting with other substances. If the concentration of H+ ions is HIGHER, the RATE OF REACTION will be faster, so STRONG ACIDS will be MORE REACTIVE than WEAK ACIDS of the same concentration.

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34
Q

What is pH?

A

The pH of an acid or alkali is a measure of the CONCENTRATION of H+ ions in the solution.

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35
Q

What is the effect of acid strength on pH?

A

Remember acid strength (i.e. strong or weak) tells you what PROPORTION of the acid molecules IONISE in water.

The lower the pH, the higher the concentration of H+ ions and so the more acidic the solution is. The pH of a strong acid will be lower than a weak acid if both acids are at the same concentration. This is because strong acids dissociate more than weak acids resulting in a larger concentration of H+ ions.

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36
Q

How can you calculate the change in H+ ion concentration?

A

You can calculate the change in H+ ion concentration with the equation:

Factor H+ ion concentration changes by = 10-x

(N.b this is 10 to the power - x. x is the difference in pH. So if pH falls from 7 to 4 the difference is -3, and the factor the H+ ion concentration has increased by is 10-(-3)= 10 to the power 3).
As the pH decreases by one unit, the hydrogen ion concentration of the solution increases by a factor of 10.

When the concentration of hydrogen ions is expressed using powers of 10, the pH is equal to the negative of the power number.

Example:
In a solution of pH 2, what is the concentration of H+(aq)?
pH of solution = 2
[H+(aq)] = 10-2 mol l-1

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37
Q

Useful tips…

A
  • Acids don’t produce hydrogen ions until they meet water. So, for example hydrogen chloride gas isn’t acidic.
  • An H+ ion is just a proton.
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38
Q

Describe the difference between the strength and the concentration of an acid…

A

1) Acid STRENGTH (i.e. strong or weak) tells you WHAT PROPORTION of the acid molecules IONISE in water.
2) The CONCENTRATION of an acid is different. Concentration measures HOW MUCH ACID there is in a certain volume of water. Concentration is basically how WATERED DOWN your acid is.
3) The larger the amount of acid there is in a certain volume of liquid, the MORE CONCENTRATED the acid is.
4) So you have a DILUTE (not very concentrated) but STRONG acid, or a CONCENTRATED BUT WEAK acid.
5) pH will DECREASE with INCREASING acid concentration REGARDLESS of whether it’s a strong or weak acid.

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39
Q

What is a salt?

A

A salt is an ionic compound made from any metal and any non-metal.

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40
Q

What is a soluble salt and how can they be made?

A

Soluble salts dissolve in water and can be made by reacting an appropriate acid with either:

1) An ALKALI (metal hydroxide-soluble in water e.g. NaOH).
2) A BASE (insoluble metal hydroxide or metal oxide-not soluble in water e.g. MGO).
3) A METAL CARBONATE e.g. CaCO3.
4) A suitably reactive metal e.g. Na or Zn.

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41
Q

What is a base?

A

Substances that can react with acids and neutralise them to make a salt and water are called bases. All metal oxides and hydroxides are bases. These soluble compounds are ALKALIS. For example, copper oxide and sodium hydroxide are bases.

Soluble metal hydroxides are alkalis because they dissolve in water to form OH- ions. The products of these neutralisations are a salt and water.

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42
Q

How do acids and bases (insoluble metal hydroxides or metal oxides) react?

What type of reaction is this?

A

acid + base = salt + water

These are neutralisation reactions. The H+ ions of the acid react with the 02- of the alkali.

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43
Q

Which salt is formed from an acid and a base?

A

The name of the salt produced depends on the metal ion in the oxide or hydroxide and the acid that is used. The first part of the name of the salt is the metal ion in the ion/hydroxide and the second part of the name comes from the acid that is used.

44
Q

How do acids and alkalis react?

A

acid + alkali ͢ salt + water

45
Q

What type of reaction occurs with an acid and an alkali?

A

These are all neutralisation reactions. The H+ ions of the acid reacts with the OH- of the alkali.

46
Q

How do acids and metal carbonates react?

A

acid + metal carbonate ͢ salt + water + carbon dioxide

47
Q

How can you make soluble salts?

A

Soluble salts can be made from ACIDS by reacting them with SOLID INSOLUBLE SUBSTANCES, such as metals, metal oxides, hydroxides or carbonates. The solid is added to the acid until no more reacts and the excess solid is filtered off to produce a solution of the salt.
Salt solutions can be crystallised to produce solid salts.

48
Q

Describe the process of making soluble salts…

A

If you are making a soluble salt by adding an insoluble reagent to an acid, you can add it in excess and separate it out at the end of the reaction using filter paper. Many metal oxides handsome metal carbonates are insoluble, so this is the method you’d use if you’re using any of those. E.g. if you want to make copper chloride, you could mix hydrochloric acid and copper oxide.

  1. Gently WARM the dilute acid using a BUNSEN BURNER, then turn off the Bunsen burner.
  2. Add the INSOLUBLE REACTANT/BASE to the ACID a bit at whilst stirring it- it will form a soluble product in the acid as it reacts.
  3. Keep adding the insoluble reactant until it is in excess. (until no more reacts). You’ll know when this is because even after stirring, the excess solid won’t react, it will just SINK to the bottom of the flask. This shows that all the acid has been neutralised and the reaction has finished.
  4. Then FILTER out the EXCESS solid to get the salt solution. This is done using FILTER PAPER and a FUNNEL.
  5. You’ll be left with a salt solution at the end of the filtration. You can convert this into pure, solid crystals of salt using crystallisation. To do this you first need to heat the solution using a WATER BATH or an ELECTRIC HEATER to evaporate some of the water and make the solution more concentrated. Then, stop heating it and leave the solution to cool. CRYSTALS of the salt should form, which can be FILTERED out of the solution and then dried..
49
Q

How can you tell how reactive a metal is?

A

When metals react with other substances the metal atoms loses an electron to the nn-metal to form a full outer shell and to form positive ions. The reactivity of a metal is related to its tendency to form POSITIVE IONS..

A positive ion can also be called a CATION.

So a metal that is more able to lose electrons to become a positive ion is much more reactive than a metal that is happy to hold onto its electrons.

50
Q

How can you see how reactive metals are?

A

You can see how reactive metals are by monitoring the rate of hydrogen production when they react with an acid. The more reactive the metal, the faster the reaction will go.

51
Q

How can you determine the speed of a reaction?

A

The speed of the reaction is indicated by the rate at which bubbles of hydrogen are given off- a speedy reaction is shown by bubbles being produced rapidly.

52
Q

How do the metals potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper react?

A
  • Very reactive metals like Potassium, Lithium, and Calcium react EXPLOSIVELY.
  • Less reactive metals such as Magnesium, Zinc and Iron react LESS VIOLENTLY.
  • Copper won’t react with cold, dilute acids.
53
Q

How can you detect the presence of hydrogen in a reaction?

A

The production of hydrogen can be detected using the burning splint test. This involves putting a lit splint at the mouth of the tube containing the metal and the acid.

If hydrogen is there, you’ll hear a ‘squeaky pop’. The more reactive a metal, the ore hydrogen is produced in a certain amount of time and the louder the ‘squeaky pop’.

54
Q

How can metals be arranged?

A

Metals can be arranged in order of their reactivity in a reactivity series. The metals potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper can be put in order of their reactivity from their reactions with water and dilute acids.

55
Q

What is the reactivity series?

What is the order of the metals in the reactivity series?

A

The REACTIVITY SERIES is a list of metals that are arranged in order of how reactive they are.

The most reactive metals are at the top and the least reactive metals are at the bottom.

  1. Potassium K
  2. Sodium Na
  3. Lithium Li
  4. Calcium Ca
  5. Magnesium Mg
  6. CARBON C
  7. Zinc Zn
  8. Iron Fe
  9. Copper Cu
56
Q

Which two non-metals are in the reactivity series and why?

A

Even though CARBON and HYDROGEN are non-metals, they are often included in the series so you can compare where metals are in relation to them.

57
Q

How was the order of the reactivity series found?

A

It was found by comparing the relative reactivity of different metals with either an ACID or WATER, and then putting them in order from the most reactive to the least reactive, the order you get is the reactivity series.

The higher a metal is in the reactivity series, the more easily it reacted with the water or acid.

58
Q

Other than measuring a metals reactivity to water or acid, how else can you investigate the reactivity of metals?

A

You can investigate the reactivity of metals by measuring the TEMPERATURE CHANGE of the reaction with an acid or water over a set time period.

If you use the same mass and surface area of metal each time, then the more reactive the metal, the greater the temperature change should be.

59
Q

Why is it useful to know the order of reactivity of metals?

A

By knowing the order of reactivity than cientists could begin to predict exactly what new substances would be formed and use this knowledge to develop a wide range of different materials and processes.

*A more reactive metal can displace a less reactive metal from a compound.

60
Q

How do some metals react with dilute acids?

Which metals react in this way?

A

-Dilute acids react with some metals to produce salt and hydrogen.

Metal + Acid ͢ Salt + Hydrogen

This reaction can be remembered as MASH.

Metals more reactive than HYDROGEN react with dilute acids.

61
Q

Which salt is formed?

A

The first part of name comes from the METAL and the second part comes from the ACID that is used.

The way to remember which acid produces which salt is in the name. HydroCHLORIC acid produces CHLORIDE salts and SULFURIC acid produces SULFATE salts.

62
Q

How does potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper react with dilute acids?

A
  • Potassium and Sodium react EXPLOSIVELY.
  • Calcium reacts VIOLENTLY to give hydrogen gas.
  • Magnesium reacts RAPIDLY to give hydrogen gas.
  • Zinc reacts MODERATELY FAST to give hydrogen gas.
  • Iron reacts SLOWLY to give hydrogen gas.
  • Copper NO reaction occurs.
63
Q

How do metals react with water?

Which metals react in this way?

A

Most metals do not react with cold water. However, those above magnesium will react, to form a metal hydroxide (an alkaline) and hydrogen gas:

Metal + Wate ͢ Metal hydroxide + Hydrogen

64
Q

How does potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper react with water?

A
  • Potassium reacts VERY VIOLENTLY. The HYDROGEN gas burns with a LILAC FLAME.
  • Sodium reacts reacts VIOLENTLY. The HYDROGEN gas burns with a YELLOW FLAME
  • Calcium reacts STEADILY.
  • Magnesium reacts VERY SLOWLY.
  • Zinc, Iron and Copper do NOT react.
65
Q

How do metals react with oxygen?

A

Metals react with oxygen to produce metal oxides.

Metal + Oxygen ͢ metal oxide

66
Q

What type of reaction is metal and oxygen?

A

The reaction are oxidation reactions because the metals gain oxygen.

67
Q

Define oxidation…

A

Oxidation can be defined as the gain of oxygen by an element or compound.

68
Q

Define reduction…

A

Reduction can be defined as the loss of oxygen from a compound.

69
Q

What are displacement reactions?

A

Displacement reactions involve one METAL REPLACING another one out of a COMPOUND.

A more reactive metal will displace a less reactive metal from its compound.

70
Q

Name a metal which is found in the earth as the metal itself?

A

Unreactive metals such as gold are found in the Earth as the metal itself.

71
Q

What structure are most metals found in?

A

Most metals are found as COMPOUNDS that require chemical reactions to extract the metal.

72
Q

How are most metals extracted?

N.b a metal ore is a rock which contains enough metal to make it profitable to extract the metal from it. In many cases the ore is an oxide of the metal.

A

The rest of the metals we get by extracting them from METAL ORES, which are mined from the ground.Most metals require chemical reactions to extract the metal.

73
Q

What is reduction?

A

A reaction that separates a metal from its oxide is called REDUCTION.

*Reduction involves the loss of oxygen from an element or compound.

74
Q

Which metals can be extracted by reduction with carbon?

Explain in terms of oxidisation and reduction what occurs during this reaction…

A

A metal below carbon can be extracted from it ore by reducing it in a reaction with carbon.

In this reaction, the ore is reduced as oxygen is removed from it and carbon gains oxygen so it is oxidised.

75
Q

How are metals below carbon in the reactivity series extracted?

A

Metals BELOW CARBON in the reactivity series can be extracted by REDUCTION using CARBON.

This is because carbon can only take the oxygen away from metals which are less reactive than carbon itself.

76
Q

How are metals higher than carbon in the reactivity series extracted?

A

Metals higher than carbon in the reactivity series, or that react in different ways with carbon, have to be extracted using ELECTROLYSIS, which is expensive.

77
Q

What can oxidation and reduction also refer to?

A
  • Oxidation is the LOSS of electrons.

- Reduction is the GAIN of electrons.

78
Q

What does the term OILRIG mean and why is it used͢

A

To remember the electron definition of oxidation and reduction we use OILRIG.

O oxidation
I is
L loss

R reduction
I is
G gain

-REDuction and Oxidation happens at the SAME TIME- hence the term REDOX.

79
Q

What occurs during a displacement reaction?

A

During a displacement reaction oxidation and reduction occurs.

80
Q

Explain which elements are oxidised an reduced in this equation:

Fe + CuSO4 ͢ FeSO4 + Cu

A
  • Fe has been oxidised as it has gained oxygen and lost electrons.
  • Cu has been reduced because it has gained electrons to become stable and have a full outer shell. It has also lost oxygen.
81
Q

Write the ionic half equations for Fe and Cu from this:

Fe + CuSO4 ͢ FeSO4 + Cu

Write the overall ionic equation..

A

-Ionic equations for displacement reactions only concentrate on the substances which are oxidised or reduced, so you don’t

Oxidation half equation (loss of electrons):
Fe ͢ Fe2+ + 2e-

Reduction half equation (gain of electrons):
Cu2+ + 2e- ͢ Cu

Overall ionic equation: Fe + Cu2+ ͢ Fe2+ + Cu

Remember- if anything is repeated (e.g. in this case 2e-) cross it out!!!

82
Q

What happens to the structure of an ionic compound when is melted or dissolves?

What are the liquids/solutions called?

A

When an ionic compound is melted or dissolved in water, the ions are free to move about within the liquid or solution.

These liquids and solutions are able to conduct electricity and are called electrolytes.

83
Q

Define electrolysis…

A

Electrolysis is the process by which ionic (metal and non-metal) substances are decomposed (broken down) into simpler substances when an electric current is passed through them.

84
Q

What occurs during electrolysis?

A

1) During electrolysis, an electric current is passed through an electrolyte (a MOLTEN or DISSOLVED ionic compound). The ions move towards the electrodes, where they react, and the compound DECOMPOSES.
2) The POSITIVE IONS in the electrolyte will move towards the cathode (negatively charged electrode) and GAIN electrons (they are REDUCED).
3) The NEGATIVE IONS in the electrolyte will move towards the ANODE (positively charged electrode) and LOSE electrons (they are OXIDISED).
4) This creates a FLOW OF CHARGE through the ELECTROLYTE as ions travel to the electrodes.
5) As ions gain or lose electrons, they form the uncharged element (they become atoms or molecules) and are RELEASED from the electrolyte. These atoms or molecules are the products of electrolysis.

85
Q

Why are dissolved ionic substances only used in electrolysis and not solid ionic substances?

A

-Electrolysis requires a liquid to conduct electricity, called the ELECTROLYTE .

-An IONIC SOLID CAN’T be electrolysed because the ions are in fixed positions and can’t move to carry current.
Whereas…
-Electrolytes contain free ions-they’re usually a molten or dissolved ionic substance. The ions can MOVE FREELY and conduct electricity. It’s the FREE IONS which conducts the electricity and allow the whole process to work.

86
Q

What types of reactions are involved in electrolysis?

A

Electrolysis always involves an OXIDATION and a REDUCTION reaction.

87
Q

Where does reduction and oxidation occur in the process of electrolysis?

A
  • Reduction is happening at the negative electrode as the positive ions are gaining electrons.
  • Oxidation is happening at the positive electrode as the negative ions are losing electrons.
88
Q

What is produced at the positive and negative cathode when a molten ionic compounds is electrolysed?

A

Molten ionic liquids, e.g. lead bromide, are always broken up into their ELEMENTS.

  • Positive METAL ions are REDUCED to the element at the CATHODE, e.g. Pb2+ + 2e- ͢ Pb
  • Negative NON-METAL ions are OXIDISED to the element at the ANODE, e.g. 2Br- ͢ Br2 + 2e-
89
Q

What is an electrolyte?

A

An electrolyte is just a liquid or solution that can conduct electricity.

90
Q

What is an electrode?

A

An electrode is a solid that conducts electricity and is submerged in a electrolyte.

91
Q

What are electrodes made from?

A

Electrons should be made out of an inert material, such as graphite or platinum, so they don’t react.

92
Q

What are binary compounds?

A

Binary compounds are ionic compounds containing two elements which are ions- a positive metal ion and a negative non-metal ion.

93
Q

What is the product of electrolysis of binary compounds?

A

Electrolysis of the molten binary compounds gives the neutral metal and non-metal elements. During the electrolysis of these substances, the metal ions move to the cathode and gain electrons to become neutral, while the non-metal ions move to the anode and lose electrons to become neutral.

94
Q

What is the product of electrolysis of lead bromide?

Tip- lead bromide only contains two elements so can be described as a binary compound.

A

When a simple ionic compound (eg lead bromide) is electrolysed in the molten state using inert electrodes, the metal (lead) is produced at the cathode and the non-metal (bromine) is produced at the anode.

95
Q

Other than reducing metals with carbon, how else can metals be extracted?

A

Metals can be extracted from molten compounds using electrolysis.

96
Q

Why is electrolysis used to extract metal?

A

Electrolysis is used if the metal is too reactive to be extracted by reduction with carbon or if the metal reacts with carbon.

97
Q

What are the disadvantages of using electrolysis?

A

Large amounts of energy are used in the extraction process to melt the compounds and to produce the electrical current.

98
Q

How is aluminium extracted?

A

*Aluminium is manufactured/extracted by the electrolysis of a MOLTEN MIXTURE of aluminium OXIDE (called bauxite ore) and CRYOLITE using CARBON as the positive electrode (ANODE).

99
Q

Why is aluminium oxide dissolved in cryolite before it undergoes electrolysis?/*explain why a mixture is used as the electrolyte.

A

Aluminium oxide has a VERY HIGH melting temperature so it’s mixed with CRYOLITE to lower the melting point.

100
Q

Explain the process of aluminium electrolysis…

A
  1. The main ore of aluminium is BAUXITE, which can be mined and purified to give ALUMINIUM OXIDE. Aluminium is then extracted by ELECTROLYSIS.
  2. Aluminium has a VERY HIGH melting temperature of 2070℃ so melting it would be very EXPENSIVE. Instead, the aluminium oxide is DISSOLVED in molten cryolite (a less common ore of aluminium). This brings the melting point DOWN to about 900℃, which SAVES ENERGY, making the process CHEAPER and EASIER.
  3. The electrodes are made of CARBON (graphite), a good conductor of electricity. And the MOLTEN MIXTURE contains FREE IONS-so it’ll CONDUCT ELECTRICITY.
  4. The POSITIVE Al3+ IONS are attracted to the NEGATIVE ELECTRODE where they EACH PICK UP THREE ELECTRONS and turn into neutral ALUMINIUM ATOMS. These then SINK to the bottom of the electrolysis tank.
  5. The NEGATIVE O2- IONS are attracted to the POSITIVE ELECTRODE where they EACH LOSE TWO ELECTRONS.
    The neutral oxygen atoms will then combine to form O2 molecules.
  6. Because of the heat, the oxygen produced at the anode reacts with the graphite (carbon) anode causing it to react (corrode) and produce carbon dioxide:

C (s) + O2 (g) ͢ CO2 (g)

101
Q

*Explain why the positive electrode must be continually replaced?

A

Because of the heat, the oxygen produced at the anode reacts with the graphite (carbon) anode causing it to react (corrode) and produce carbon dioxide:

C (s) + O2 (g) ͢ CO2 (g)

Therefore, the graphite anodes need replacing on a frequent basis.

102
Q

What are the half equations from the overall reaction of aluminium electrolysis?

The overall equation is:

2Al2O3 ͢ 4Al + 3O2

A

The half equation for the reaction taking place at the electrodes when aluminium oxide is electrolysed are:

Negative electrode: Al3+ + 3e- ͢ Al

Positive electrode: 2O2- ͢ O2 + 4e-

103
Q

What determines the products of electrolysis of an aqueous solution using inert electrodes?

A

Sometimes there are more than two types of free ions in the electrolyte. For example, if a salt is dissolved in water there will be some H+ and OH- ions as well as the ions from the salt in the solution. In this situation, the products of electrolysis depend on how REACTIVE the ELEMENTS involved are.

104
Q

What will be produced at the negative electrode (cathode) when electrolysing an aqueous solution?

A

(Remember-in AQUEOUS SOLUTIONS, as well as the IONS from the ionic compound there will be HYDROGEN IONS from the ionic compound, there will be HYDROGEN IONS H+ and HYDROXIDE IONS OH- from the water.)

At the cathode, if H+ INS AND METAL IONS are present, HYDROGEN GAS will be produced if the metal is MORE REACTIVE than hydrogen (e.g. sodium ions).

If the metal ions are from an element LESS REACTIVE than hydrogen (e.g. copper ions), a solid layer of the PURE METAL will be produced instead.

105
Q

Why is hydrogen formed at the cathode when electrolysing an aqueous solution where the metal is more reactive than hydrogen?

A

This is because IT IS HIGH IN THE REACTIVITY SERIES, and, as covered previously, the most REACTIVE METALS ARE THE ONES BEST ABLE TO RID THEMSELVES OF ELECTRONS. As a result H+ ions present in small amounts in the water are forced to accept electrons at the cathode.

106
Q

What will be produced at the positive electrode (anode) when electrolysing an aqueous solution?

A

If OH- and HALIDE IONS (F-, Cl-, Br-, etc) are present, the corresponding HALOGEN (F2, Cl2, Br2, etc) is produced.

But, if NITRATE, NO3-, or SULPHATE, are present, OYGEN gas is produced instead.

If NO halide is present, then OH- ions are discharged and OXYGEN and WATER will be formed.

GCSE Chemistry (5 decks)

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