Topic 4 - chemical changes

Question 1

What is the PH scale?

Answer 1

A measure of how acidic or alkaline a solution is.

Question 2

What are the two ways that we can measure PH and compare them.

Answer 2

An indicator- a dye that changes colour depending on the PH
A PH probe and PH meter- The probe is placed into the solution and the PH is given as a numerical value
The ph probe and meter is more accurate than an indicator.

Question 3

What is an acid and what ions do they form?

Answer 3

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

Question 4

What is an alkali and what ions do they form?

Answer 4

An alkali is a base that dissolves in water to form a solution with a PH greater than 7.
Alkalis form OH- ions .

Question 5

What is the reaction between acids and bases called?

Answer 5

neutralisation

Question 6

What is the word and formula equations for a neutralisation reaction between acids and bases?

Answer 6

acid + base -> salt + water
H+ (aq) + OH- (aq) -> H20 (l)

Question 7

Describe how you would carry out a titration to work out the concentration of an alkali.

Answer 7

1) using a pipette and pipette filler, measure 25cm^3 of your alkali and add it into a conical flask.
2) place the conical flask on a white tile.
3) Add a few drops of suitable indicator to the conical flask.
4) Use a funnel to fill the burette with acid of known concentration (wearing safety glasses) and record the initial volume of the acid in the burette.
5) Complete a rough titration by opening the tap and adding acid to the alkali whilst swirling the conical flask untill end point has been reached (colour change).
6) Then record the final volume of acid in the burette and use it to calculate the volume of acid used to neutralise the alkali.
7) repeat accurately untill you get concordant results.

Question 8

What is the colour change with litmus indicator?

Answer 8

blue in alkali
red in acid

Question 9

What is the colour change with phenolphthalein indicator?

Answer 9

pink in alkali
colourless in acid

Question 10

What is the colour change with methyl orange indicator?

Answer 10

yellow in alkali
red in acid

Question 11

What is a strong acid ? And give examples.

Answer 11

One that completely ionises or dissociates in water.
e.g. sulfuric, hydrochloric and nitric acids

Question 12

What is a weak acid ? And give examples.

Answer 12

One that does not fully ionise or dissociate in solution
e.g. ethanoic, citric and carbonic acids

Question 13

The ionisation of a weak acid is a r_________ reaction which set up an equilibrium which lies well to the _____.

Answer 13

reversible
left

Question 14

Acid + metal oxide -> _______ + _________

Answer 14

salt + water

Question 15

Acid + metal Hydroxide -> _____ + _______

Answer 15

salt + water

Question 16

Acid + metal carbonate -> ____ + ______ + ________ ________

Answer 16

salt + water + carbon dioxide

Question 17

Describe a method to obtain pure salt crystals from a reaction between an acid and an insoluble base.

Answer 17

1) Gently heat your acid in a beaker using a bunsen burner.
2) A bit at a time, add in your insoluble base untill it is in excess (excess solid will float to the bottom.)
3) using filter paper and a funell , filter the soluion to get rid of excess solid.
4) Gently heat the solution in a water bath untill it reaches the point of crystalisation.
5) leave to cool , wash and then dry to obtain pure salt crystals.

Question 18

What is the order of the reactivity series?
including carbon and hydrogen

Answer 18

potassium Please stop looking crazilly magnetic always, cause zebras in halls complain
sodium
lithium
calcium
magnesium
aluminium
carbon
zinc
iron
hydrogen
copper

Question 19

Acid + metal -> ____ + ___________

Answer 19

salt + hydrogen

Question 20

Compare the ractivity of metals in the reactivity series when they react with acid.

Answer 20

potassium zinc copper
sodium iron usually no reaction
lithium react less vigorously
calcium

react explosively

Question 21

metal + water -> ______ __________ + _________

Answer 21

metal hydroxide + hydrogen

Question 22

What is oxidation in terms of oxygen?

Answer 22

oxidation = gain of oxygen
formation of a metal ore
e.g. magnesium is oxidised to make magnesium oxide

Question 23

What is reduction in terms of oxygen ?
and what is it used for ?

Answer 23

reduction = loss of oxygen
a reaction that seperates a metal from its oxide
e.g. copper oxide is reduces to copper

Question 24

What metals can be extracted by reduction with carbon?
and why ?

Answer 24

Metals below carbon in the reactivity series
carbon can only displace oxygen from metals which are less reactive

Question 25

Describe how metals are extraced by reduction with carbon .

Answer 25

The metal ore is reduced as oxygen is removed from it , the carbon gains the oxygen and becomes oxidised leaving metal and carbon dioxide as the products.

Question 26

What is a redox reaction?

Answer 26

When reduction and oxidation happen at the same time (electrons)

Question 27

What is oxidation in terms of electrons?

Answer 27

Loss of electrons

Question 28

What is reduction in terms of electrons?

Answer 28

Gain of electrons

Question 29

OIL RIG

Answer 29

Oxidation is loss
reduction is gain

Question 30

When writing ionic equations , remember that only the aqueous substances form ions

Question 31

What is the ionic equation for Mg(s) + ZnCl2(aq) -> MgCl2(aq) + Zn (s)

Answer 31

Mg (s + Zn2+ (aq) -> Mg 2+ (aq) + Zn (s)

Question 32

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

Answer 32

by electrolysis

Question 33

What is electrolyte?

Answer 33

A liquid / solution that can conduct electricity

Question 34

Explain how electrolysis works.

Answer 34

An electric current is passed through an electrolyte.
The positive ions in the elextrolyte will move towards the cathode (negative electrode) and gain electrons ( they are reduced)
The negative ions in the elctrolyte will move towards the anode ( positive electrode) and lose electrons ( they are oxidised)
This creates a flow of charge through the electrolyte as ions travel to the electrodes
as ions gain or lose electrons , they form uncharged elements and are discharged from the electrolyte

Question 35

What are the disadvantages of electrolysis ?

Answer 35

Very expensive
requires a lot of energy

Question 36

explain how aluminium is extracted from its ore by elextrolysis.

Answer 36

Aluminium is extracted from the ore bauxite
Aluminium has a very high melting point so it is mixed with cryolite to lower its melting point
The molten mixture contains free ions - so it will conduct electricity
The positive Al3+ ions are attracted to the negative electrode where they pick up three electrons and turn into neutral aluminium atoms.
These then sink to the bottom of the electrolysis tank.
The negative 02- ions are attracted to the positive electrode where they each lose two electrons .
The neutral oxygen atoms will then combine to form o2 molecules.

Question 37

explain how aluminium is extracted from its ore by elextrolysis.

Answer 37

Aluminium is extracted from the ore bauxite
Aluminium has a very high melting point so it is mixed with cryolite to lower its melting point
The molten mixture contains free ions - so it will conduct electricity
The positive Al3+ ions are attracted to the negative electrode where they pick up three electrons and turn into neutral aluminium atoms.
These then sink to the bottom of the electrolysis tank.
The negative 02- ions are attracted to the positive electrode where they each lose two electrons .
The neutral oxygen atoms will then combine to form o2 molecules.

Question 38

During elextrolyisis of aqueous solutions, what happens at the cathode?

Answer 38

If H+ and metal ions are present , hydrogen gas will be produces if the metal ions form a metal that is more ractive than hydrogen.
If the metal ions form a metal that is less reactive than hydrogen , a solid layer of the pure metal will be produced instead.

Question 39

During electrolysis of aqueous solutions , what happens at the anode?

Answer 39

If OH- and halide ions are present, molecules of chlorine bromine or iodine will be formed. If no halide ions were present then OH- ions are discharged and oxygen will be formed.

Question 40

How do you test if chlorine is present?

Answer 40

Litmus paper turns white in its presence

Question 41

how do you test if hydrogen is present?

Answer 41

place a lighted splint near the gas
if hydrogen is present there will be a squeaky pop.

Question 42

How do you test if oxygen is present?

Answer 42

oxygen will relight a glowing splint.