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Edexcel Chemsitry > Topic 3: Chemical Changes > Flashcards

Topic 3: Chemical Changes Flashcards

1
Q

What are acids in a solution a source of?

A

Hydrogen ions, H+

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2
Q

what is a ph scale

A

a measure of how acidic or alkaline a solution is

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3
Q

what is the pH of a neutral substance

A

7

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4
Q

what is an acid

A

a substance with a pH of less than 7

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5
Q

relationship between concentration of hydrogen ions and pH

A

as concentration of hydrogen ions increases, pH decreases

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6
Q

what is a base

A

a substance that reascts iwth an acid to produce a salt and water

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7
Q

What are alkalis in a solution a sourece of?

A

Hydroxide ions, OH-

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8
Q

What is the pH of an alkaline solution?

A

over 7

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9
Q

what is an alkali

A

a base that is soluble in water

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10
Q

what is the relationship between concentration of hydroxide ions and pH

A

the higher the concentration of hydroxide ions, the higher the pH

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11
Q

what is an indicator

A

a dye that chnages colour depending on whether it’s above or below a certain pH

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12
Q

What colour does litmus turn in acidic and alkaline environments?

A

red in acid, blue in alkali

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13
Q

What colour does methyl orange turn in acidic and alkaline environments?

A

red in acid, yellow in alkali

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14
Q

What colour does phenolphthalein turn in acidic and alkaline environments?

A

colourless in acid, pink in alkali

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15
Q

What happens as the concentration of H+ ions in an acidic solution increases?

A

pH decreases

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16
Q

What happens as the concentration of OH- ions in an alkaline solution increases?

A

What happens as the concentration of OH- ions in an alkaline solution increases?

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17
Q

What happens as pH increases by 1?

A

H+ ion concentration increases by a power of 10

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18
Q

what is the reaction between an acid and a base called

A

neutralisation

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19
Q

what products are produced in a neutralisation reaction

A

salt and water

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20
Q

show how neutralisation recations happen in aqueous solution

A

use ionic equations

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21
Q

describe the practical: Invesitgating the neutralisation reaction between calcium oxide (a base) and dilute hydrochloric acid

A
  1. Start by measuring out a set volume of dilute hydrochloric acid into a conical fask. Use a pipette or a measuring cylinder for this
    2.Measure out a fixed mass of calcium oxide using a mass balance.
  2. Add the calcium oxide to the hydrochloric acid.
  3. Wait for the base to completely react, then record the pH of the solution, using either a pH probe or Universal indicator paper. (You can use a glass rod to spot samples of the solution onto the paper).
  4. Repeat steps 2 to 4 until all the acid has reacted.
    You’ll know you’ve reached this point when you get unreacted calcium oxide sitting at the bottom of the flask.
  5. You can then plot a graph to see how pH changes with the
    mass of base added. You should find it looks a bit like this.
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22
Q

How does the pH change when powdered calcium hydroxide is added to a fixed volume of dilute hydrochloric acid?

A

pH is initially low but as calcium hydroxide is added it increases to the point of neutralization and then become alkaline as it is in excess

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23
Q

Describe how the pH change when calcium hydroxide is added to a fixed volume of dilute hydrochloric acid can be measured

A
  1. 50cm3 of acid added to a beaker with its pH recorded using universal indicator
  2. 0.3g of calcium hydroxide powder measured out and added 7 times with pH recorded after each addition
  3. Graph plotted showing change in pH and point of neutralisation
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24
Q

Why does pH increase during the experiment?

A

hydrogen ions neutralised as concentration of them falls because hydroxide ion concentration increases

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25
Q

What is formed in the reaction of calcium hydroxide and hydrochloric acid?

A

Calcium chloride (salt) and water

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26
Q

What is the reaction of calcium hydroxide and hydrochloric acid an example of?

A

Neutralisation, where a base + acid -> salt + water

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27
Q

Explain the term dilute

A

A solution containing small amounts of solute compared to the volume of solvent

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28
Q

Explain the term concentrated

A

A solution containing large amounts of solute compared to the volume of solvent

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29
Q

Explain the term weak acid

A

Partially disassociates into ions in solutions

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30
Q

Explain the term strong acid

A

Completely disassociates into ions in solutions

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31
Q

Define a base

A

A substance that reacts with an acid to form salt and water only

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32
Q

Define an alkali

A

A soluble base- metal hydroxides/oxides usually

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33
Q

Explain the general reactions of aqueous solutions of acids with metals

A

metal + acid -> salt + hydrogen

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34
Q

Explain the general reactions of aqueous solutions of acids with metal oxides (alkalis)

A

metal oxide + acid -> salt + water
Neutralisation reaction

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35
Q

Explain the general reactions of aqueous solutions of acids with metal hydroxides (alkalis)

A

metal hydroxide + acid -> salt + water
Neutralisation reaction

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36
Q

Explain the general reactions of aqueous solutions of acids with metal carbonates

A

metal carbonate+ acid -> salt + water + carbon dioxide

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37
Q

Describe the chemical test for hydrogen

A

Light a splint and put it at the mouth of a test tube. If hydrogen is in it there will be a squeaky pop as it ignites.

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38
Q

Describe the chemical test for carbon dioxide using limewater

A

Limewater turns cloudy if carbon dioxide bubbled through it

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39
Q

Describe a neutralisation reaction

A

Reaction between an acid and a base

40
Q

Explain an acid-alkali neutralisation

A

Reaction in which H+ ions from the acid react with OH- ions from the alkali to form water

41
Q

Describe the process for preparing a soluble salt from an acid and an insoluble base

A
  1. react an acid that contains one of the ions you wnat in the salt with an insoluble base that contains the other ion you need (often a metla oxide or a metal hydroxide)
  2. start by heating the acid in a water bath - this speeds up the recation between te acid and the insoluble base. do this in a fume cupboard to avoid releasing acid fumes into the room
  3. then add the base tp 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 acud has been neutralised because the excess solid will just sink tp the bottom of the flask.
  4. filter off the excess solid to get a solution contain 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 to crystallise. Filter off the solid salt and leave it to dry.
42
Q

Explain why, if soluble salts are prepared from an acid and an insoluble reactant excess of the reactant is added

A

To make sure all the acid is used up

43
Q

Describe what you would see from adding excess reactant to an acid

A

some solid remains, and change in colour of solution as reactant reacts with acid

44
Q

Explain why, if soluble salts are prepared from an acid and an insoluble reactant the excess reactant is removed

A

To make sure the salt prepared from it is pure

45
Q

Explain why, if soluble salts are prepared from an acid and an insoluble reactant the solution remaining is only salt and water

A

An acid and a metal oxide reacting will always only form salt and water

46
Q

what is the process for preparing a soluble salt from an acid and a soluble reactant (alkali)

A

titration

47
Q

Explain why for the process of preparing a soluble salt from an acid and a soluble reactant (alkali) titration must be used

A

So the correct proportions of acid and alkali can be determined for a neutral solution

48
Q

Explain why for the process of preparing a soluble salt from an acid and a soluble reactant (alkali) the acid and soluble reactant are mixed in the correct proportions

A

So they react to produce a salt (and water). If it wasn’t neutral the salt would be contaminated with an excess of one reactant

49
Q

Explain why for the process of preparing a soluble salt from an acid and a soluble reactant (alkali) the solution remaining is only salt and water

A

If the correct proportions of acid and alkali are mixed a neutralisation reaction will occur which will only produce salt and water

50
Q

describe the protocol for a titration practical

A
  • measure out a set amount of acid into a conical flask using a pippette. ass a few drops of indicator
  • Slowly add alkali to the acid, using a burette, until you reach the end point — this is when the acid’s been exactly neutralised and the indicator changes colour
  • Then, carry out the reaction using exactly the same volumes of alkali and acid but with no indicator, so the salt won’t be contaminated with indicator
  • The solution that remains when the reaction is complete contains only the salt and water.
  • Slowly evaporate off some of the water and then leave the solution to crystallise (see page 38 for more on crystallisation). Filter off the solid and dry it — you’ll be left with a pure, dry salt.
51
Q

define solubale

A

able to dissolve in water

52
Q

define insoluble

A

unable to dissolve in water

53
Q

are common salts of sodium, potassium and ammonium solube or insoluble

A

soluble

54
Q

are nitrates soluble or insoluble

A

soluble

55
Q

are common chlorides soluble or insoluble

A

soluble (except silver chloride and lead chloride)

56
Q

are common sulfates soluble or insoluble

A

soluble (except lead, barium and calcium sulfate)

57
Q

are common carbonates and hydroxides soluble or insoluble

A

insoluble (except for sodium, potassium and ammonium ones)

58
Q

what type of reaction makes insoluble salts

A

precipitation reaction

59
Q

method of precipitate reaction

A
  • Add 1 spatula of lead nitrate to a test tube. Add water to dissolve it. You should use deionised water to make sure there are no other ions about.
    Shake it thoroughly to ensure that all the lead nitrate has dissolved. Then, in a separate test tube, do the same with 1 spatula of sodium chloride.
  • Tip the two solutions into a small beaker, and give it a good stir to make sure it’s all mixed together. The lead chloride should precipitate out.
  • Put a folded piece of filter paper into a filter funnel,
    and stick the funnel into a conical flask.
  • Pour the contents of the beaker into the middle of the filter paper. Make sure that the solution doesn’t go above the filter paper otherwise some of the solid could dribble down the side.
  • Swill out the beaker with more deionised water, and tip this into the filter paper — to make sure you get all the precipitate from the beaker.
  • Rinse the contents of the filter paper with deionised water to make sure that all the soluble sodium nitrate has been washed away
  • Then just scrape the lead chloride onto fresh filter paper and leave it to dry in an oven or a desiccator
60
Q

Describe how to prepare pure, dry hydrated copper sulfate crystals from copper oxide and sulfuric acid

A
  1. 20cm3 dilute sulfuric acid in conical flask heated in 50°C water bath
  2. Copper oxide powder added bit by bit until it stops disappearing when added (in excess)
  3. Copper sulfate solution filtered and filtrate heated in evaporating basin over bunsen burner until crystals form. Residue would be unreacted copper oxide
  4. Solution poured into watch glass and left for a few days to let water evaporate
61
Q

Describe how to carry out an acid-alkali titration to prepare a pure, dry salt

A
  1. BURETTE filled with acid and jet filled by running some out.
  2. Initial reading recorded on burette
  3. Conical flask filled with set volume of alkali using PIPETTE and placed in a conical flask below
  4. Indicator added to alkali and acid slowly added to alkali
  5. END-POINT occurs when indicator changes to indicate neutralisation
  6. Repeat titration adding correct volume of acid to burette without indicator
  7. Evaporate water from solution formed
62
Q

Why is distilled water added in preparing an insoluble salt

A

Precipitate insoluble in water whilst solution is, so remaining solution can be removed from it

63
Q

What is an electrolyte

A

Ionic compounds in the molten state or dissolved in water

64
Q

What is electrolysis

A

A process in which electrical energy, from a direct current supply decomposes electrolytes

65
Q

Why does the ionic solid have to be dissolved in water as an electrolyte

A

To break its ionic bonds so that the ion are free to move and conduct electricity

66
Q

How does electrolysis work

A

Two inert electrodes placed in the electrolyte are connected to DC and opposite charged ions will migrate to opposite electrodes and gain or lose electrons to produce a metal and a non-metal at either electrode

67
Q

Why are electrodes inert

A

To not affect the products formed

68
Q

Explain the movement of ions during electrolysis

A
  • Positively charged cations migrate to negatively charged cathode and produce a metal
  • Negatively charged anions migrate to positively charged anode and produce a non-metal
69
Q

Predict the products of electrolysis of other binary, ionic compounds in the molten state

A

Compound is decomposed regularly. Water doesn’t ionise since it isnt present- eg ZnCl2

70
Q

What happens when aqueous salt solutions are used in electrolysis

A

Water ionises to a small extent into H+ and OH- ions so these are present alongside the ions of the dissolved salt. The least reactive of the ions is discharged at each electrode so can vary with salt - eg molten NaCl(aq)

71
Q

Describe the order that determines what will be discharged at the cathode and anode during the electrolysis of an aqueous salt solution

A

Cathode: Least reactive series discharged- dependent on if metal cation is more/less reactive than H+
Anode: Halide > Hydroxide > everything else

72
Q

What does H+ form if discharged in electrolysis

A

hydrogen gas and effervescence will be observed

73
Q

What does OH- form if discharged in electrolysis

A

oxygen gas and water

74
Q

Explain the formation of the products in the electrolysis, using inert electrodes, of some electrolytes, including: a copper chloride solution

A
  • CuCl2 splits into Cu2+(aq) and Cl-(aq)
  • Water splits into H+ (aq) and OH-(aq)
  • Cu2+ and H+ attracted to cathode but copper ions discharged more readily than hydrogen ions, so copper is formed
  • Cl- and OH- attracted to anode but chloride ions discharged more readily than hydroxide ions so chlorine formed
    Sodium and hydroxide ions remain in solution to form 2NaOH
75
Q

Write the formula of the reaction that occurs between sodium and hydroxide ions in the electrolysis of copper chloride solution

A
76
Q

Write the half equation that occurs at the anode during the electrolysis of copper chloride solution

A

Cl- and OH- attracted to anode but chloride ions discharged more readily than hydroxide ions so chlorine formed

77
Q

Write the half equation that occurs at the cathode during the electrolysis of copper chloride solution

A

Cu2+ and H+ attracted to cathode but copper ions discharged more readily than hydrogen ions, so copper is formed

78
Q

Explain the formation of the products in the electrolysis, using inert electrodes, of some electrolytes, including sodium chloride solution

A
  • NaCl splits into Na+(aq) and Cl-(aq)
  • Water splits into H+ (aq) and OH-(aq)
  • Na+ and H+ attracted to cathode but hydrogen ions discharged more readily than sodium ions, so hydrogen formed
  • Cl- and OH- attracted to anode but chloride ions discharged more readily than hydroxide ions so chlorine formed
  • Sodium and hydroxide ions remain in solution to form 2NaOH(aq)
79
Q

Explain the formation of the products in the electrolysis, using inert electrodes, of some electrolytes, including sodium sulfate solution

A
  • Na2SO4 splits into 2Na+(aq) and SO2- 4(aq)
  • Water splits into H+ (aq) and OH-(aq)
  • 2Na+ and H+ attracted to cathode but hydrogen ions discharged more readily than sodium ions, so hydrogen gas formed
  • OH- and SO2- 4 attracted to anode but hydroxide ions discharged more readily than sulfate ions so 02 gas formed
    Sodium and sulfate ions remain in solution to form 2NaSO4(aq)
80
Q

Explain the formation of the products in the electrolysis, using inert electrodes, of some electrolytes, including water acidified with sulfuric acid

A
  • H2SO4 splits into 2H+(aq) and SO2- 4(aq)
  • Water splits into H+ (aq) and OH-(aq)
  • H+ attracted to cathode so hydrogen gas formed
  • OH- and SO2- 4 attracted to anode but hydroxide ions discharged more readily than sulfate ions so 02 gas formed
    sulfate ions remain in solution
81
Q

Explain the formation of the products in the electrolysis, using inert electrodes, of some electrolytes, including molten lead bromide

A
  • PbBr2 splits into 2Br(l) and Pb2+ (l)
  • Pb2+ attracted to cathode so molten lead formed
  • 2Br attracted to anode so bromine gas formed
82
Q

Write the half equation that occurs at the cathode during the electrolysis of molten lead bromide

A
83
Q

Write the half equation that occurs at the anode during the electrolysis of molten lead bromide

A
84
Q

Write half equations for reactions occurring at the anode and cathode in electrolysis

A
85
Q

Explain oxidation and reduction in terms of loss or gain of electrons

A
  • Oxidation is loss of electrons
  • Reduction is gain of electrons
86
Q

Recall where oxidation and reduction occur during electrolysis

A
  • Oxidation at anode
  • Reduction at cathode
87
Q

Explain the formation of the products in the electrolysis of copper sulfate solution, using copper electrodes, and how this electrolysis can be used to purify copper

A
  1. Impure copper used as anode and pure copper used as cathode in copper sulfate solution
  2. Copper atoms in anode lose 2 electrons to form Cu2+ ions that dissolve in the solution and migrate to cathode to be deposited as pure copper
  3. Impurities from anode don’t form ions and collect as sludge that can be collected for other metallic elements
  4. Impure anode loses mass and pure copper gains mass
88
Q

Write the half equation that occurs at the cathode during the purification of copper in copper sulfate solution by electrolysis

A
89
Q

Write the half equation that occurs at the anode during the purification of copper in copper sulfate solution by electrolysis

A
90
Q

Core Practical: Investigate the electrolysis of copper sulfate solution with copper electrodes

A
  1. Fill beaker with copper sulfate solution
  2. Record masses of 2 copper electrodes and connect them to opposite terminals of a dc supply to make an anode and cathode at 0.2A for 20 minutes
  3. Remove copper electrodes from beaker, wash with distilled water.
  4. Dip electrodes into propanone and shake it off. Wait for it to evaporate before measuring change in masses of dry electrodes
  5. Repeat with currents of 0.3A, 0.4A and 0.5A
91
Q

Why is propanone used to clean the electrodes?

A

It evaporates faster than water as it has a lower boiling point so will dry electrodes faster

92
Q

What will happen in the electrolysis of copper sulfate solution?

A

Cathode increases in mass, anode decreases
- Copper atoms oxidised at anode and form Cu2+ ions
- Cu2+ reduced at the cathode, forming copper atoms
- Gain in mass by negative electrode same as loss by positive electrode, so concentration of the Cu2+ ions in solution remains constant

93
Q

How does the change in mass of the electrodes differ with current with the electrolysis of copper sulfate solution?

A

The higher the current, the greater the change in mass because charge and thus rate of flow of electrons is higher, boosting the rate of reaction so more ions will undergo electrolysis in the same amount of time.

94
Q

Core Practical: Investigate the electrolysis of copper sulfate solution with inert electrodes

A
  1. Fill beaker with copper sulfate solution
  2. Record masses of 2 graphite electrodes and connect them to opposite terminals of a dc supply to make an anode and cathode at 0.2A for 20 minutes
  3. Remove graphite electrodes from beaker, wash with distilled water.
  4. Dip electrodes into propanone and shake it off. Wait for it to evaporate before measuring change in masses of dry electrodes
  5. Repeat with currents of 0.3A, 0.4A and 0.5A
95
Q

What products are formed during the electrolysis of copper sulfate solution with inert electrodes

A
  • Cu2SO4 splits into 2Cu+(aq) and SO2- 4(aq)
  • Water splits into H+ (aq) and OH-(aq)
  • 2Cu+ and H+ attracted to cathode but copper ions discharged more readily than hydrogen ions, so copper formed
  • OH- and SO2- 4 attracted to anode but hydroxide ions discharged more readily than sulfate ions so 0₂ gas formed
    hydrogen and sulfate ions remain in solution to form H₂SO₄
96
Q

Safety precautions

A
  • Copper(II) sulfate corrosive and a skin irritant and can cause serious eye damage- wear goggles and gloves
  • Propanone is flammable so keep away from naked flames

Edexcel Chemsitry (9 decks)

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