Chemical Changes

Question 1

What does the pH scale measure?

Answer 1

How acidic or alkaline a solution is:
-lower pH -> more acidic
-higher pH -> more alkaline

Question 2

What is the pH of a neutral substance?

Answer 2

7

Question 3

What are the two ways of measuring pH?

Answer 3

-indicator dyes ->change colour.
-pH probe -> attached to a pH meter to measure it electronically, gives a number

Question 4

What are acids, bases and alkalis?

Answer 4

-acids -> aqueous solution with pH less than 7. Form H+ ions in water
-base -> substance with pH greater than 7
-alkali -> base that dissolves in water. Form OH- ions

Question 5

What is the equation for the reaction between acids and bases?

Answer 5

acid + base -> salt + water

Question 6

What do titrations allow you to find out?

Answer 6

How much acid is needed to neutralise a certain quantity of an alkali and vice versa

Question 7

What is the equation for the neutralisation reaction between acids and alkalis?

Answer 7

H+ + OH- -> H2O

Question 8

How do use titration to find out the concentration of an alkali (and vice versa)?

Answer 8

-add set volume of alkali to conical flask
-add two or three drops of indicator
-funnel acid of a known concentration into a burette
-record initial volume
-add acid one bit at a time and swirl
-indicator changes colour when all the acid is neutralised
-record final volume of acid4

Question 9

Why do strong acids ionise completely in water but not weak ones?

Answer 9

Because all of the acid particles dissociate to release H+ ions but with weak acids only a small proportion of the particles ionise

Question 10

How does the change of pH affect the concentration of H+ ions?

Answer 10

H+ ion concentration changes by 10^-x where x is the difference in pH.

Question 11

What is the difference between the strength and concentration of an acid?

Answer 11

-strength tells you what proportion of the acid molecules ionise in water
-concentration tells you how much acid there is in a certain volume of water (how dilute it is)

Question 12

What is the word equation for the reaction between an acid and a metal oxide? Give an example

Answer 12

Acid + Metal Oxide –> Salt + Water
E.g. Hydrochloric acid (2HCl) + Copper Oxide (CuO) –> Copper Chloride (CuCl2) + Water (H2O)

Question 13

What is the word equation for the reaction between an acid and a metal hydroxide? Give an example

Answer 13

Acid + Metal Hydroxide –> Salt and Water
E.g. Sulfuric Acid (H2SO4) + Potassium Hydroxide (2KOH) –> Potassium Sulfate (K2SO4) + Water (2H2O)

Question 14

What is the word equation for the reaction between an acid and a metal carbonate? Give an example

Answer 14

Acid + Metal Carbonate –> Salt + Water + Carbon Dioxide
E.g. Hydrochloric Acid (2HCl) + Sodium Carbonate (Na2CO3) –> Sodium Chloride (2NaCl) + Water (H2O) + Carbon Dioxide (CO2)

Question 15

Describe the process of making a soluble salt using an insoluble base.

Answer 15

-Gently warm the dilute acid with a bunsen burner
-add insoluble base to acid a bit at a time until it stops reacting
-filter out the excess solid
-gently heat the solution with water bath or electric heater to evaporate some of the water
-leave solution to cool
-crystals of the salt should be formed
-filter them out

Question 16

What is the order of the elements in the reactivity series?

Answer 16

Potassium (K)
Sodium (Na)
Lithium (Li)
Calcium (Ca)
Magnesium (Mg)
Carbon (C)
Zinc (Zn)
Iron (Fe)
Hydrogen (H)
Copper (Cu)

Question 17

What is the word equation for the reaction between an acid and some metals?

Answer 17

Acid + Metal –> Salt + Hydrogen gas

Question 18

How does the reaction between acids and (some) metals work?

Answer 18

-speed of reaction indicated by the rate of hydrogen bubble production
-the more reactive the metal, the faster the reaction
-the more reactive the metal, the larger the temperature change over a set period of time.

Question 18

What is the word equation for the reaction between water and metals? Which metals will and won’t react?

Answer 18

Metal + Water –> Metal Hydroxide + Hydrogen
-potassium, sodium, lithium, calcium will
-zinc, iron, copper won’t

Question 19

What is a reduction reaction?

Answer 19

A reaction that separates a metal from its oxide.

Question 20

What is the difference between oxidation and reduction?

Answer 20

Oxidation is the gain of oxygen. E.g magnesium is oxidised to give magnesium oxide.
Reduction is the loss of oxygen E.g. copper oxide is reduced to give copper

Question 21

How can metals be extracted from their ores using carbon? Which metals does this work for?

Answer 21

-By reacting them with carbon. The metal is reduced and the carbon is oxidised.
-only works for metals lower than carbon in the reactivity series
-metals high than carbon have to be extracted through electrolysis

Question 22

What is the difference between oxidation and reduction in REDOX reactions (in terms of electrons?

Answer 22

Oxidation
Is
Loss of electrons
Reduction
Is
Gain of electrons
These happen at the same time during a reaction

Question 23

How do displacement reactions work?

Answer 23

A more reactive metal will displace a less reactive metal from its compound. The metal ion is always reduced and the metal atom is always oxidised
E.g if you put iron into copper sulfate solution, the iron will react to give you iron sulfate solution and copper metal

Question 24

What are spectator ions?

Answer 24

Ions that don't change during a displacement reaction and therefore don't need to be represented in the ionic equation

Question 25

How does electrolysis work?

Answer 25

-electric current passed through electrolyte (molten or dissolved compound) -positive ions move to the cathode and are reduced -negative ions move towards the anode and are oxidised -ions become uncharged -creates a flow of charge through the electrolyte as the ions travel

Question 26

Why must ionic compounds be molten to be electrolysed?

Answer 26

Because this is the only way that they're ions can move freely.

Question 27

Describe the process of the electrolysis of aluminium oxide.

Answer 27

-aluminium is mixed with cryolite to lower its melting point -melted down -positive ions (Al^3+) electrons are attracted to the cathode and reduced and sink -negative ions (O^2-) are attracted to the positive electrode where they are oxidised and combine to form O2 molecules -anode is made of carbon and reacts with oxygen to make CO2 so should be replaced regularly

Question 28

How does electrolysis work in aqueous solutions?

Answer 28

-hydrogen ions (H+) and hydroxide ions (OH-) -hydrogen gas produced at the cathode if hydrogen and metal ions that are more reactive than hydrogen are present -if not a layer of pure metal is formed -chlorine, bromine or iodine is produced at the anode if OH- and halide ions are present -if not, the ions are discharged and oxygen is formed

Question 29

What are half equations used to show?

Answer 29

The reactions at the electrodes, concerning one compound and its electrons