Topic 4 - Chemical Changes

Question 1

What are wide range indicators and give an example of one

Answer 1

-A dye that changes colour depending on whether it’s above or below a certain PH
-They can contain a mixture of dyes that mean they gradually change over a broad range of pH
-Universal indicator gives the colours on a scale of 1-14

Question 2

What is a pH probe and how is it used

Answer 2

-Attatched to a pH meter it measures pH electronically
-The problem is placed in the solution and the pH is given on a digital display as a numerical value, so it is more accurate than an indicator

Question 3

Definition of an acid

Answer 3

-A substance that forms aqueous solutions and H+ ions in water
-Has a pH less than 7

Question 4

Definition of a base/alkali

Answer 4

-A base is a substance with pH over 7
-An alkali is a base that dissolves in water to form a solution with a pH great than 7
-They form OH- ions in water

Question 5

what is a neutralisation reaction

Answer 5

-A reaction between acids and bases
acid + base > salt + water

Question 6

What happens to the pH of a substance during a neutralisation reaction

Answer 6

The products are equal

Question 7

Describe how to carry out a titration experiment (e.g. to find out concentration of an alkali)

Answer 7

-Using a pipette and conical flask add a set volume of the alkali to a conical flask and add two or three drops of indicator
-Use a funnel to fill a burette with an acid of known concentration and record initial volume of it in the burette
-Using the burette add the acid to the alkali a bit at a time giving the conical flask a swirl and go slowly when you think the end point is about to be reached
-When the indicator changes colour all the alkali has been neutralised
-Record final volume of acid in the burette and use it along with the initial reading to calculate the volume of the acid used to neutralise the alkali

Question 8

What should you do after you finish one titration experiment

Answer 8

-Repeat the whole thing a few times, as the first one you do should be a rough one to get an approximate idea of where the solution changes
-You should get the same answer each time after this (within 0.1cm)
-Calculate a mean ignoring any anomalies

Question 9

Why should you repeat the titration experiment

Answer 9

-To increase the accuracy of the titration and spot any anomalous results

Question 10

Why should you use a single indicator and not a universal one for calculating titrations

Answer 10

• Because universal indicators gradually change colour, but during a titration you want to see a sudden colour change at the end point

Question 11

Name the three examples of single indicators and their colour changes

Answer 11

-Litmus : red as an acid, purple as an alkali
-Methyl orange : red as an acid, yellow as an alkali
-Phenolphthalein : Clear as an acid, pink as an alkali

Question 12

What do strong acids do in water and what then happens to the particles

Answer 12

They ionise completely in water, so the particles all dissociate to release H+ ions

Question 13

What do weak acids do in water and what then happens to the particles

Answer 13

-Do not fully ionise in solution and only a small proportion of acid particles dissociate to release H+ ions

Question 14

what does the Ionisation of weak acids set up equilibrium of? And what side does it lie.

Answer 14

-Undissociated and dissociated acid
-Since only a few of the acid particles release H+ ions the position of equilibrium lies well to the left

Question 15

Why are stronger acids more reactive than weaker ones or the same concentration

Answer 15

-Reactions involve H+ ions reacting with other substances, so is the concentration of H+ ions is higher, the rate of reaction will be faster, so strong acids are more reactive

Question 16

What factor do H+ ions increase for every 1 decrease on the pH scale and give an example

Answer 16

-For every decrease of 1 on the pH scale the concentration of ions increase by a factor of 10
-E.g. for a decrease of 2 on the pH scale the concentration increase by a factor of 100

Question 17

rule for factor of ions concentration in comparison to the pH factor

Answer 17

Factor H+ ion concentration changes by = 10 ^-x

Question 18

Difference between strong and concentrated acids

Answer 18

-Acid strength is what proportion of the acid molecules ionise in water
-Concentration measures how much acid is in a certain volume of water

Question 19

What happens to the pH of an acid when the concentration increases

Answer 19

The pH decreases regardless of the acids strength

Question 20

Give three examples of strong acids

Answer 20

Sulfuric, hydrochloric and nitric acid

Question 21

give three examples of weak acids

Answer 21

carbonic, citric and ethanoic acids

Question 22

What kind of metals take part in neutralisation reactions

Answer 22

-Hydroxide and oxide

Question 23

What do metal carbonates produce in a neutralisation reactions

Answer 23

They react with acids to produce salt + water + carbon dioxide

Question 24

How to make soluble salts using an insoluble base (e.g Magnesium sulphate from sulphuric acid + magnesium oxide)

Answer 24

-Gently heat the dilute acid and add the insoluble base to the acid a bit at a time until no more reacts
-Add it in excess to guarantee we have the maximum amount of product
-It will be neutralised and all excess will sink to the bottom
-Filter out the excess to get just the salt solution
-Gently heat the solution using a water bath to evaporate the water and then stop heating it and leave the solution to cool, and it will form crystals of the salt which can be filtered out of the solution and then dried

Question 25

What is the reactivity series

Answer 25

A list of metal in an order of their reactivity towards other substances

Question 26

How are metal’s reactivity affected by electrons

Answer 26

-It depends on how easily they lose electrons/making positive ions
-The higher up the reactivity series the more easily they form positive ions

Question 27

How does the reactivity series affect reactions with water and acids

Answer 27

-When metals react with these they lose electrons and form positive ions
-So the higher a metal is in the reactivity series, the more easily it reacts with water and acid

Question 28

What do some metals and acids produce when reacted together

Answer 28

-Salt and hydrogen gas

Question 29

What is the speed of reaction indicated by when a metal and acid react (reactivity series)

Answer 29

-The rate at which the bubbles of hydrogen are given off

Question 30

What happens when very reactive metals (e,g magnesium) react with acid

Answer 30

They act vigorously even when the acid is cold and dilute and produce a lot of hydrogen bubbles

Question 31

What happens when very fairly metals (e,g iron) react with acid

Answer 31

-Slowly with dilute acid but more strongly if you heat them up

Question 32

How does measuring temperature change of a reaction investigate the reactivity of a metal

Answer 32

-If you measure the temperature change during a reaction with acid or water over a set period of time
-you can use the same mass and surface area of metal each time then the more reactive the metal the greater the temperature change should be

Question 33

What is an oxidation reaction

Answer 33

-When common metals react with oxygen to form oxides
(Gain of oxygen)

Question 34

What is a reduction reaction

Answer 34

A reaction that separates s metal from its oxide
(Loss of oxygen)

Question 35

What do carbons do in a reduction reactions

Answer 35

-The ore is reduced as oxygen is removed from it and the carbon gains oxygen so it is oxidised.
This can only be done if the metal is lower than carbon in the reactivity series, otherwise it cannot be displaced

Question 36

How are metals that can’t be reduced with carbon able to lose their oxygen

Answer 36

Through electrolysis which is an expensive process

Question 37

How is iron reduced by carbon (e.g)

Answer 37

Using a blast furnace iron oxide is reduced to make iron