unit 4:Chemical changes

Question 1

What does the pH scale measure?

Answer 1

how acidic or alkaline a solution is

Question 2

If something has a low pH, it is…

Answer 2

acidic

Question 3

if something has a high pH, it is…

Answer 3

alkaline

Question 4

how can you measure the pH of a solution?

Answer 4

using an indicator or pH probe

Question 5

what is a base?

Answer 5

a substance with a ph greater than 7. it is the oxide or hydroxide of a metal

Question 6

What is an acid?

Answer 6

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

Question 7

what is an alkali?

Answer 7

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

Question 8

what is the neutralisation reaction equation?

Answer 8

alkali + acid = water + salt (AAWS)

base + acid = water + salt (BAWS)

Question 9

what is a titration for?

Answer 9

to find out exactly how much acid is needed to neutralise a quantity of alkali or vice versa

Question 10

describe how to carry out a titration

Answer 10

1) using a pipette and pipette filler, add a set volume of some alkali(or acid) to a conical flask. Add a few drops of indicator to the conical flask
2) using a funnel, fill a burette with some acid(or alkali) of known concentration. Record the initial volume of the acid in the burette
3) using the burette, add the acid to the flask a little bit at a time, swirling the flask at regular intervals.
4)The indicator will undergo a colour change when the alkali is neutralised. Once, this happens give the conical flask a few extra swirls to make sure the colour change is permanent
5) record the final volume of acid in the burette and use it and the initial volume to calculate the volume of acid needed to neutralise the alkali
to increase accuracy, repeat the process several times and get a mean result

Question 11

name the single indicators you should use for a titration

Answer 11

litmus: acid to alkali = red to blue
methyl orange: acid to alkali = red to yellow
phenolphthalein: acid to alkali = clear to pink

Question 12

what is the difference between strong and weak acids?

Answer 12

strong acids: ionise completely in water. All particles dissociate to release H+ ions
are more reactive than weak acids of the same concentration
weak acids: don’t fully ionise in water. only a small proportion dissociate to release H+.
the ionisation of a weak acid is reversible

Question 13

what is pH?

Answer 13

the concentration of hydrogen (H+) ions in the solution

Question 14

what is the rule of concentration in the pH scale?

Answer 14

for every decrease of one on the pH scale, the concentration increases by a factor of 10
factor H+ ion concentration changes by 10^(-x)

Question 15

what is the difference between acid strength and acid concentration?

Answer 15

acid strength tells you what proportion of acid molecules ionise in water
acid concentration measures how much acid there is in a certain volume of water

Question 16

what is the equation for the reaction between an acid and metal carbonates?

Answer 16

(metal)carbonate + acid = water + carbon dioxide + salt (CAWCS)

Question 17

describe the practical for making soluble salts using an insoluble base

Answer 17

1) pick the right acid and insoluble base, e.g. an insoluble metal oxide, hydroxide or carbonate. we used hydrochloric acid and copper oxide
2) warm the dilute acid with a bunsen burner
3) add the insoluble base to the acid until it doesn’t react anymore (excess base will sink to the bottom)
4) Filter out the excess solid
5) gently heat the solution using a water bath to evaporate off some of the water. Leave the solution to cool then dry it and take out the crystals that should be formed. This is called crystallisation.

Question 18

what elements are carbon above in the reactivity series?

Answer 18

iron, zinc, copper, gold, silver

Question 19

why are carbon and hydrogen included in the reactivity series?

Answer 19

carbon is often used to extract certain metals in a displacement reaction. If a metal is less reactive than hydrogen it will not react with dilute acids

Question 20

what is the word equation for the reaction between metal an an acid?

Answer 20

metal + acid = salt + hydrogen (MASH)

Question 21

what is the word equation for the reaction between a metal and water?

Answer 21

metal +water = hydrogen + metal hydroxide (MWHM)

Question 22

define oxidation and reduction

Answer 22

oxidation is the loss of electrons
reduction is the gain of electrons
(OIL RIG)

Question 23

what metals can be extracted with reduction

Answer 23

metals less reactive than carbon: zinc, iron, copper

Question 24

what is a redox reaction?

Answer 24

a reaction in which electrons are transferred

Question 25

what do ionic equations show

Answer 25

the particle that react in the reaction and the products they form

Question 26

what is electrolysis?

Answer 26

an electric current is passed through an electrolyte (a molten or dissolved compound). The ions move towards the electrodes, where it reacts and becomes an atom

Question 27

what is produced at the cathode and anode?

Answer 27

at the cathode, positive ions (cations) are produced

at the anode, negative ions(anions) are produced

Question 28

why are some ores extracted using electrolysis?

Answer 28

they are more reactive than carbon so can’t be extracted via a reduction reaction

Question 29

what are the rules of electrolysis of aqueous solutions?

Answer 29

Product at the anode:
non-metal: if a halide is present in the solution
oxygen: if the negative ion is complex (e.g. SO4)
Product at the cathode:
metal: if the metal is less reactive than hydrogen
hydrogen: if the metal is more reactive than hydrogen

Question 30

what are the half equations for each electrode in the reaction of aqueous sodium chloride

Answer 30

cathode:
2H+ + 2e- = H2
anode:
2Cl-=Cl2 + 2e-