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Chemical changes - Chemistry Flashcards

1
Q

The lower the pH of a solution …

A

The more acidic it is

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2
Q

The higher the pH of a solution …

A

The more alkaline it is

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3
Q

What are wide range indicators useful for

A

They contain a mixture of dyes so gradually change colour over a broad range of pH. Useful for estimating the pH of a solution

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4
Q

How can we measure pH electronically

A

A pH probe attached to a pH meter. Probe is placed in the solution your measuring and the pH is given on a digital display as a numerical value.

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5
Q

Why is a pH meter better than universal indicator

A

It gives a numerical value so it’s more accurate

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6
Q

Acid

A

A substance that forms aqueous solutions with a pH of less than 7. Acids form H+ ions in water.

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7
Q

Base

A

A substance with a pH greater than 7

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8
Q

Alkali

A

Base that dissolves in water to form a solution with a pH greater than 7. Alkalis form OH- ions

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9
Q

What is the reaction between acid and base called

A

Neutralisation

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10
Q

When an acid neutralises the base what pH are the products

A

7 - they are neutral

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11
Q

Titrations practical

A
  1. Use the pipette and pipette filler to add 25 cm3 of alkali to a clean conical flask.
  2. Add a few drops of indicator and put the conical flask on a white tile (so you can see
    the colour of the indicator more easily).
  3. Fill the burette with acid and note the starting volume.
  4. Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix.
  5. Stop adding the acid when the end-point is reached (the appropriate colour change
    in the indicator happens). Note the final volume reading.
  6. Repeat steps 1 to 5 until you get concordant results
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12
Q

Why is a pipette not as useful as a burette

A

A pipette only measures one volume of solution, but Burettes measure different volumes and let you add the solution drop by drop

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13
Q

Safety precautions for titrations

A

Do below eye level to prevent acid accidentally getting in your eye

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14
Q

Appropriate colour change for phenolphthalein

A

Pink in alkaline conditions, colourless in acidic conditions

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15
Q

Universal indicator colour change

A

Red in acidic solutions to violet in alkali solutions

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16
Q

Why do we use single indicators in titration practicals and not universal

A

We want to see a sudden colour change at the end point

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17
Q

Strong acids

A

Ionise completely in water. All particles dissociate to release H+ ions

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18
Q

Weak acids

A

Do not fully ionise in solution. Only a small proportion of acid particles dissociate to release H+ ions

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19
Q

What type of reaction is the ionisation of weak acids

A

Reversible reaction - sets up an equilibrium between the undissociated and dissociated acid.

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20
Q

Why are stronger acids more reactive than weaker ones

A

The the concentration of H+ ions is higher in a stronger acid so the rate of reactions will be faster

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21
Q

What is the pH a measurement of

A

The concentration of H+ ions in the solution

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22
Q

H+ ion concentration changes by

A

10 to the power of negative x - x being the difference in Ph

23
Q

Difference between strong acids and concentrated acids

A

Acid strength tells you what proportion of the acid molecule ionise with water whereas the concentration of an acid measures how much acid there is in a certain volume of water. pH will decrease with increasing acid concentration regardless of whether it’s strong or weak.

24
Q

Metal oxides and metal hydroxides are

A

Bases or alkalis so react with acid to form a salt and water

25
Q

Metal carbonates are

A

Bases

26
Q

Crystallization practical

A

use a measuring cylinder to add 25cm cubed of dilute sulphuric acid in a beaker
Measure into a weighing boat, 5g of copper oxide
gently heat the beaker on a tripod and gauze with a bunsen burner - DO NOT BOIL IT
Carefully add a spatula of copper oxide powder to the beaker and stir the solution with a glass rod,
keeping adding more copper oxide powder until it no longer disappears (add in excess)
filter the mixture to remove the excess copper oxide using filter paper and a funnel , then pour the filtrate (the copper sulphate solution) into an evaporating basin
place the evaporating basin above a bunsen burner again, and heat the copper sulphate solution to evaporate off half of the water
Leave the solution to cool for 24 hours
Remove the crystals with a spatula on to a filter paper and gently pat the crystals dry

27
Q

Reactivity series

A

Potassium
Sodium
Lithium
Calcium
Magnesium
Carbon
Zinc
Iron
Hydrogen
Copper

28
Q

How can you investigate the reactivity of metals

A
  • React metals with dilute acid and note the amount of hydrogen bubbles produced
  • Could also measure the temperature change of the acid, the more reactive the metal, the greater the temperature change is
29
Q

Reduction reaction

A

A reaction that separates a metal from it’s oxide

30
Q

Reduction (oxygen)

A

Loss of oxygen

31
Q

Oxidation (oxygen)

A

Gain of oxygen

32
Q

Ways in which metals can be extracted from their ores

A

Using carbon

33
Q

What depends on whether the metal can be extracted by reduction with carbon

A

The position of the metal in the reactivity series. IF the metal is higher than carbon in the reactivity series have to be extracted using electrolysis. Metals below carbon can be extracted by reduction using carbon because carbon can only take the oxygen away from the metals which are less reactive than carbon itself

34
Q

Oxidation (electrons)

A

A loss of electrons

35
Q

Reduction (electrons)

A

A gain of electrons

36
Q

REDOX reactions

A

Reduction and oxidation happen that happens at the same time

37
Q

What type of reaction is a displacement reaction

A

Redox

38
Q

In a displacement reaction what reactant always gains electrons and is reduced

A

Metal ion

39
Q

What happens during electrolysis

A

An electrical current passes through an electrolyte. The ions move towards the electrodes, where they react, and the compound decomposes.

40
Q

Positive ions move to the

A

Cathode

41
Q

Negative ions move to

A

Anode

42
Q

Why can an ionic solid not be electrolysed

A

Because the ions are in a fixed position and are not free to move

43
Q

Why can molten ionic compounds be electrolysed

A

Because the ions can move freely and conduct electricity. Positive metal ions are reduced, negative non-metal ions are oxidised

44
Q

Problem with electrolysing metal ores

A

Very expensive as lots of energy is required to melt the ore and produce the required current

45
Q

Why is aluminium oxide mixed with cryolite

A

Has a very high melting point so cryolite lowers the melting point

46
Q

Where does the aluminium go in its extraction of aluminium oxide

A

The al3+ ions are attracted to the cathode where they pick up 3 electrons and turn into neutral aluminium atoms. These then sink to the bottom of the electrolysis tank

47
Q

Where do the oxygen ions go in the extraction of aluminium oxide

A

The o2- ions are attracted to the anode where they each lose two electrons. The neutral oxygen atoms will then combine to form 0z molecules

48
Q

What electrode are metals attracted to

A

Metals form positive ions so they’re attracted to the negative electrode

49
Q

What electrode are non-metals attracted to

A

Non-metals form negative ions and therefore ar attracted to the positive electrode

50
Q

What other ions will be in aqueous solutions

A

As well as the ionic compound, there will be hydrogen ions and hydroxide ions

51
Q

At the cathode what is discharged if hydrogen ions and metal ions are present

A

Hydrogen gas will be produced if the metal ions form an elemental metal that is more reactive than hydrogen. If the metal ion forms an elemental metal that is less reactive than hydrogen a solid layer of pure metal will be produced instead.

52
Q

At the anode what happens if any halide ions are present

A

Molecules of the chlorine, bromine or iodine will formed. If no halide ions are present then the OH- ions are discharged and oxygen will be formed. The oxygen can be seen as bubbles

53
Q

When a halide isn’t present in the aqueous solution, the half equation for the anode is

A

4OH= ——-> O2 + 2H20 + 4e-

54
Q

Colour change of phenophaylene

A

Pink to colourless

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