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Z Chemistry GCSE > Acids > Flashcards

Acids Flashcards

1
Q

What is an indicator?

A
  • Substances which indicate the presence of an acid or soluble base
  • Turn different colours in acidic and basic (alkali)
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2
Q

What colours does litmus show?

A
  • Acid: Red
  • Neutral: Purple
  • Base: Blue
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3
Q

What colours does Methyl Orange show?

A
  • Changes at pH4
  • Acid: Red
  • Neutral and Base: Yellow
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4
Q

What colours does phenolphalein show?

A
  • Changes at pH9
  • Acid and Neutral: Colourless
  • Base: Pink
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5
Q

What is universal indicator?

A
  • A mixture of several indicators, a spectrum (solution or paper): red, orange, yellow, green, blue, violet
  • 0-14
  • Can be used to classify solutions as strongly acidic, weakly acidic, neutral, weakly alkaline or strongly alkaline
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6
Q

What does the pH scale show?

A
  • How acidic or alkaline a substance is
  • pH 7: neutral and there is an equal concentration of H+ ions and OH- ions
  • pH7: alkaline and there is a higher concentration of OH- ions than pure water
  • Substances that do not dissolve in water have no pH value
  • Use universal indicator or pH meter (more accurate)
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7
Q

What are acids?

A
  • They are a source of H+, hydrogen ions
  • Strong acid: pH 0-3 and is fully ionised E.G. HCL, H2SO4 High concentration of H+ ions
  • Weak acid: 3-6 and is partially ionised E.G. Carbonic acid, Ethanoic acid Low concentration of H+ ions
  • All reactions in acids are due to the H+ ions
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8
Q

What are bases?

A

-They are substances that neutralise acids
-Most metal oxides and metal hydroxides are bases
E.G. NaOH, Ammonia

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9
Q

What are alkalis?

A

-Source of OH-, hydroxide ions
-Soluble bases which dissolve in water, (all alkalis are bases but not all bases are alkalis)
-Strong alkali: 11-14 high concentration of OH- ions
-Weak alkali: 8-11 low concentration of OH- ions
E.G. NaOH, Ammonia

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10
Q

What does metal and acid give?

A

Salt and hydrogen

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11
Q

What does alkali and acid give?

A

Salt and water

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12
Q

What does metal oxide and acid give?

A

Salt and water

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13
Q

What does metal carbonate and acid give?

A

Salt and water and carbon dioxide

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14
Q

What is neutralisation?

A

An acid is neutralised when reacting with a base or carbonate, the four reactions all form a salt

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15
Q

What is a salt?

A

A salt is a substance derived from an acid where the hydrogen has been replaced by a metal

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16
Q

What salt does H2SO4 produce?

A

Sulphate

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17
Q

What salt does HCL produce?

A

Chloride

18
Q

What salt does HNO3 produce?

A

Nitrate

19
Q

How can the acidity in lakes be changed?

A

Acidity in soil and lakes are affected by acid rain, can be neutralised by adding slaked lime (calcium hydroxide)

20
Q

How can neutralisation help crops?

A

Neutralisation of sulphuric acid by ammonia is used to form ammonium sulphate with is an important fertiliser

21
Q

How can neutralisation cause erosion?

A

Neutralisation of a carbonate (e.g. marble or limestone) by acid rain can cause erosion

22
Q

What are neutralisation reactions?

A

Exothermic

23
Q

What happens in the neutralisation between acids and alkalis?

A

Neutralisation between acids and alkali involves hydrogen ions and hydroxide ions reacting to form neutral water molecules
H+ and OH- goes to H2O

24
Q

Are all common sodium, potassium and ammonium salts soluble?

A

Yes

25
Q

Are nitrates insoluble?

A

No

26
Q

Are common chlorides soluble?

A

Yes except silver chloride

27
Q

Are common sulphates soluble?

A

Yes except those of barium and calcium

28
Q

Are common carbonates soluble?

A

No, except those of sodium, potassium and ammonium

29
Q

How do you make salts using acids and insoluble bases?

A
  1. You need to pick the right acid and insoluble base (most metal oxides, metal carbonates, and metal hydroxides are insoluble) E.G. to make copper nitrate, mix nitric acid and copper carbonate
  2. You add the metal oxide, carbonate, or hydroxide to the acid, the solid will dissolve in the acid as it reacts, you will know when all the acid has been neutralised because the excess solid will sink to the bottom of the flask and remain there
  3. You can then filter out the excess base to get the salt solution. To get pure, solid crystals of the salt, evaporate off the water
    - CuCO3 (s) + 2HNO3 (aq) –> Cu(NO3)2 (aq) + CO2 (g) + H2O (l)
30
Q

How do you make soluble salts using an alkali?

A
  1. You cannot use the method above if you want to neutralise an acid with an alkali. Alkalis are soluble bases so you cannot filter them out if you add to much, and you cannot tell when you have added too much
  2. You have to add exactly the right amount of alkali to just neutralise the acid, you need to use an indicator to show when the reaction is finished. The best way of doing this is to do a titration
  3. Then repeat using exactly the same volumes of alkali and acid but without an indicator so the salt is not contaminated
    - To make a soluble salt which is sodium, potassium or ammonium salt then you use a titration method when you react an acid with a solution of sodium or potassium hydroxide or carbonate (or use ammonia solution)
31
Q

How do you make insoluble salts? (precipitation reactions)

A
  1. If the salt you want to make is insoluble, you can use a precipitation reaction
  2. You just need to pick two solutions that contain the ions you need, one containing the correct positive ion and the other the correct negative ion
    E.G. to make barium sulphate (which is insoluble) you need a solution which contains barium ions and one which contains sulphate ions
  3. So you can mix barium chloride (most chlorides are soluble) with sulphuric acid
    -BaCl2 (aq) + H2SO4 (aq) –> BaSO4 (s) + 2HCL (aq)
  4. The salt is obtained by FILTER the mixture to get the precipitate. WASH the salt with PURE/Distilled water to remove any traces of other solutions. Finally the salt is LEFT TO DRY
  5. Dissolve in water, add together and mix, filter, wash with distilled water, leave to dry
32
Q

What are titrations used to find out?

A
  • Concentrations

- Titrations allow you to find out exactly how much acid is needed to neutralise a quantity of alkali (or vice versa)

33
Q

Explain how you do a titration

A
  1. Using a pipette and pipette filler, add known volume of alkali (usually about 25cm cubed) to a conical flask, along with two or three drops of indicator
  2. Fill a burette with the acid. Make sure you do this below eye level, because you do not want to be looking up if some acid spills over
  3. Using the burette, add the acid to the alkali a bit at a time, giving the conical flask a regular swirl . Go especially slowly when you think the end pint (colour change) is about to be reached
  4. The indicator changes colour when all the alkali has been neutralised, e.g. phenolphthalein is pink in alkalis, but colourless in acids
  5. Record the volume of acid used to neutralise the alkali
  6. It is best to repeat this process a few times, making sure you get (pretty much) the same answer each time, this makes for more reliable results
    - The solution is transferred to an evaporating dish and heated to partially evaporate the water causing crystallisation or can be left to slowly evaporate, which tends to give bigger crystals
34
Q

What does the pipette filler do in titration?

A

Stops you from getting a mouthful of alkali

35
Q

What does a pipette do in titration?

A

Pipettes measure only one volume of solution, fill the pipette to about 3cm above the line, then drop the level down carefully to the line

36
Q

What does a burette do in titration?

A

Burettes measure different volumes and let you add the solution drop by drop

37
Q

How do you make lead (II) chloride? (How to prepare soluble salts from acids / using precipitation reactions)

A
  1. Place dilute HNO3 in a beaker
  2. Warm the beaker gently on a tripod and gauze, do not boil the acid
  3. Remove the beaker from the gauze and add lead (II) oxide one spatula at a time, stirring the acid until no more dissolves
  4. Allow the solution to cool until room temperature
  5. Filter off any excess lead (II) oxide and collect the filtrate in the boiling tube
  6. Add dilute HCL to the filtrate
  7. Filter the precipitate with 20cm cubed of distilled water to remove excess acid
  8. Dry the lead (II) chloride between pieces of filter paper and leave in a warm place to dry
38
Q

Why did we not directly react the lead oxide with HCL?

A
  • Because then you would have two soluble products at the end which you could not separate easily (lead chloride and water so would not be able to separate)
  • Instead: lead oxide and nitric acid = lead nitrate (soluble) and water (soluble): but this is ok as you filter the solution and there is excess lead oxide so you are left with lead nitrate solution
  • Add dilute HCL to the lead nitrate solution (creates a mixture)
  • Then you filter this, and lead (II) chloride stays in filter whilst the rest goes through
39
Q

How do you make a soluble salt which is not sodium, potassium or an ammonium salt?

A
  • By reacting an acid with a metal or an insoluble base (e.g. metal oxide or an insoluble metal carbonate)
    1. Add insoluble salt of copper oxide to (a measured volume of acid using a measuring cylinder) sulphuric acid and stir, the copper oxide is added in small proportions to the acid in the beaker with stirring
    2. Warm gently on a bunsen burner (to speed up reaction), stir the reaction mixture and add more copper oxide using the spatula until it is in excess, until no more copper oxide dissolves (do not boil), all the acid is neutralised and there should be a little left over solid
    3. The solution turns blue as the reaction occurs, showing that copper sulphate has been formed
    4. Filter the solution once it has cooled to remove excess copper oxide, using a filter funnel and filter paper
    5. Collect filtrate and put it in an evaporating dish/basin
    6. Solution left in warm place for a few days to allow the crystals to from
40
Q

Why do you add excess copper oxide to sulphuric acid?

A
  1. As excess H2SO4 would have been hard to remove from the solution and so contaminated the solution at the end
  2. To make sure the reaction is completed and all the sulphuric acid is used up in the reaction
41
Q

What colour changes would you observe when preparing CuSO4 crystals?

A
  • The solution went from colourless to blue (blue solution formed) and the black colour of copper oxide disappeared
  • Initially the copper oxides appears to be dissolved but when copper oxide added in excess the black colour remained
  • Not all the water was evaporated by heating the bunsen briner because you would not have got the crystals

Z Chemistry GCSE (16 decks)

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