Chemical changes - Chemistry

Question 1

The lower the pH of a solution …

Answer 1

The more acidic it is

Question 2

The higher the pH of a solution …

Answer 2

The more alkaline it is

Question 3

What are wide range indicators useful for

Answer 3

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

Question 4

How can we measure pH electronically

Answer 4

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.

Question 5

Why is a pH meter better than universal indicator

Answer 5

It gives a numerical value so it’s more accurate

Question 6

Acid

Answer 6

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

Question 7

Base

Answer 7

A substance with a pH greater than 7

Question 8

Alkali

Answer 8

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

Question 9

What is the reaction between acid and base called

Answer 9

Neutralisation

Question 10

When an acid neutralises the base what pH are the products

Answer 10

7 - they are neutral

Question 11

Titrations practical

Answer 11

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

Question 12

Why is a pipette not as useful as a burette

Answer 12

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

Question 13

Safety precautions for titrations

Answer 13

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

Question 14

Appropriate colour change for phenolphthalein

Answer 14

Pink in alkaline conditions, colourless in acidic conditions

Question 15

Universal indicator colour change

Answer 15

Red in acidic solutions to violet in alkali solutions

Question 16

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

Answer 16

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

Question 17

Strong acids

Answer 17

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

Question 18

Weak acids

Answer 18

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

Question 19

What type of reaction is the ionisation of weak acids

Answer 19

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

Question 20

Why are stronger acids more reactive than weaker ones

Answer 20

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

Question 21

What is the pH a measurement of

Answer 21

The concentration of H+ ions in the solution

Question 22

H+ ion concentration changes by

Answer 22

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

Question 23

Difference between strong acids and concentrated acids

Answer 23

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.

Question 24

Metal oxides and metal hydroxides are

Answer 24

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

Question 25

Metal carbonates are

Answer 25

Bases

Question 26

Crystallization practical

Answer 26

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

Question 27

Reactivity series

Answer 27

Potassium Sodium Lithium Calcium Magnesium Carbon Zinc Iron Hydrogen Copper

Question 28

How can you investigate the reactivity of metals

Answer 28

- 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

Question 29

Reduction reaction

Answer 29

A reaction that separates a metal from it's oxide

Question 30

Reduction (oxygen)

Answer 30

Loss of oxygen

Question 31

Oxidation (oxygen)

Answer 31

Gain of oxygen

Question 32

Ways in which metals can be extracted from their ores

Answer 32

Using carbon

Question 33

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

Answer 33

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

Question 34

Oxidation (electrons)

Answer 34

A loss of electrons

Question 35

Reduction (electrons)

Answer 35

A gain of electrons

Question 36

REDOX reactions

Answer 36

Reduction and oxidation happen that happens at the same time

Question 37

What type of reaction is a displacement reaction

Answer 37

Redox

Question 38

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

Answer 38

Metal ion

Question 39

What happens during electrolysis

Answer 39

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

Question 40

Positive ions move to the

Answer 40

Cathode

Question 41

Negative ions move to

Answer 41

Anode

Question 42

Why can an ionic solid not be electrolysed

Answer 42

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

Question 43

Why can molten ionic compounds be electrolysed

Answer 43

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

Question 44

Problem with electrolysing metal ores

Answer 44

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

Question 45

Why is aluminium oxide mixed with cryolite

Answer 45

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

Question 46

Where does the aluminium go in its extraction of aluminium oxide

Answer 46

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

Question 47

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

Answer 47

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

Question 48

What electrode are metals attracted to

Answer 48

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

Question 49

What electrode are non-metals attracted to

Answer 49

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

Question 50

What other ions will be in aqueous solutions

Answer 50

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

Question 51

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

Answer 51

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.

Question 52

At the anode what happens if any halide ions are present

Answer 52

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

Question 53

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

Answer 53

4OH= -------> O2 + 2H20 + 4e-

Question 54

Colour change of phenophaylene

Answer 54

Pink to colourless