Topic 3.1-3.21 Acids

Question 1

What are acids in solution sources of?

Answer 1

Hydrogen ions

Question 2

What are alkalis in solution sources of?

Answer 2

Hydroxide ions

Question 3

Describe the pH scales

Answer 3

A neutral solution has a pH of 7 and that acidic solutions have lower pH values and alkaline solutions higher pH values

Question 4

How do acids and alkalis react to phenolphthalein?

Answer 4

Alkalis turn pink
Acids are colourless

Question 5

How do acids and alkalis react to methyl orange?

Answer 5

Alkalis turn yellow
Acids are red

Question 6

How do acids and alkalis react to litmus?

Answer 6

Alkalis turn blue
Acids turn red

(Same for blue/red litmus paper.
Red litmus paper stays red in acid, turns blue in alkali.
Blue litmus paper turns red in acid, stays blue in alkali)

Question 7

How does the concentration of hydrogen and hydroxide ions effect the pH of an acidic/alkaline solution?

Answer 7

The higher the concentration of hydrogen ions in an acidic solution, the lower the pH.
The higher the concentration of hydroxide ions in an alkaline solution, the higher the pH.

Question 8

How does hydrogen ion concentration effect pH?

Answer 8

As hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1 unit.

Question 9

​ ​​​​​​Explain the terms dilute and concentrated

Answer 9

​ ​​​​​​​​​​​Concentrated​ ​=​ ​larger​ ​quantity​ ​of​ ​solute​ ​in​ ​a​ ​given​ ​volume​ ​of​ ​a​ ​solution
Dilute​ ​=​ ​smaller​ ​quantity​ ​of​ ​solute​ ​in​ ​a​ ​given​ ​volume​ ​of​ ​a​ ​solution

Question 10

Describe the core practical for ​​​​ ​​​​ ​​​​​​​​​​​​​​​​ ​​​​​​​​ ​​
investigating the change in pH on adding powdered calcium hydroxide or calcium oxide to a fixed volume of dilute hydrochloric acid

Answer 10

-Add​ ​dilute​ ​HCl​ ​to​ ​the​ ​beaker​ ​and​ ​measure​ ​pH
-Add​ ​weighed​ ​mass​ ​of​ ​calcium​ ​hydroxide​ ​and​ ​stir​ ​then​ ​record​ ​pH
-Keep​ ​adding​ ​weighed​ ​masses​ ​of​ ​calcium​ ​hydroxide​ ​until​ ​there​ ​is​ ​no​ ​more change​ ​to​ ​the​ ​pH
To analyse it:
-draw​ ​a​ ​line​ ​graph​ ​with​ ​mass​ ​added​ ​on​ ​the​ ​horizontal​ ​axis​ ​and​ ​with​ ​pH​ ​on the​ ​vertical​ ​axis
-draw​ ​a​ ​line​ ​of​ ​best​ ​fit​ ​(remember​ ​to​ ​ignore​ ​any​ ​anomalies)

Question 11

Explain the terms weak and strong acids

Answer 11

Strong​ ​acid​ ​=​ ​fully​ ​dissociates​ ​in​ ​aqueous​ ​solution​ ​(dissociation​ ​is​ ​where​ ​an​ ​acid breaks​ ​down​ ​to​ ​release​ ​H​+​​ ​ions​ ​in​ ​solution)

Weak​ ​acid​ ​=​ ​partially​ ​dissociates​ ​in​ ​aqueous​ ​solution as it has fewer H+ ions than expected

Stronger​ ​an​ ​acid,​ ​greater​ ​the​ ​dissociation,​ ​the​ ​more​ ​H​+​​ ​ions​ ​released,​ ​the​ ​lower the​ ​pH​ ​(for​ ​a​ ​given​ ​conc.​ ​of​ ​aq.​ ​solutions)

Question 12

What is a base?

Answer 12

A base is any substance that reacts with an acid to form a salt and water only

Question 13

What is an alkali?

Answer 13

A soluble base

Question 14

What results from the reaction of an aqueous solution of acid with a metal?

Answer 14

A salt + hydrogen gas (H2)

Question 15

What results from the reaction of an aqueous solution of acid with a metal hydroxide?

Answer 15

A salt + water

(Metal​ ​hydroxides​ ​are​ ​bases/alkalis​ ​if​ ​insoluble/soluble)

Question 16

What results from the reaction of an aqueous solution of acid with a metal oxide?

Answer 16

A salt + water

(Metal​ ​oxides​ ​are​ ​normally​ ​bases​ ​(because​ ​insoluble))

Question 17

What results from the reaction of an aqueous solution of acid with a metal carbonate?

Answer 17

A salt + water + carbon dioxide

Question 18

Describe the chemical test for hydrogen

Answer 18

Use a burning split held at the open end of a test tube of the gas. If it contains hydrogen it creates a ‘squeaky pop’ sound

Question 19

Describe the chemical test for carbon dioxide (using limewater)

Answer 19

Bubble​ ​the​ ​gas​ ​through​ ​the​ ​limewater​ ​(calcium​ ​hydroxide​ ​solution)​ ​and​ ​it will​ ​turn cloudy/output a white precipitate if it contains carbon dioxide

Question 20

Describe a neutralisation reaction

Answer 20

​A reaction​ ​between​ ​an​ ​acid​ ​and​ ​a
base

Question 21

​ ​​ Explain an acid-alkali neutralisation

Answer 21

A reaction in which hydrogen ions (H+) from the acid react with hydroxide ions (OH–) from the alkali to form water

H​+(​aq)​​+​​OH​-(​aq)​​->​​H2​O​(l)

Question 22

Explain what you should do to prepare soluble salts from an acid and an insoluble reactant and why

Answer 22

-Add excess​ ​of​ ​the​ ​reactant
* this​ ​is​ ​to​ ​ensure​ ​your​ ​volume​ ​of​ ​acid​ ​reacts​ ​completely

-Remove excess​ ​reactant
* this​ ​is​ ​done​ ​by​ ​filtration​ ​of​ ​the​ ​insoluble​ ​reactant​ ​and​ ​is​ ​done​ ​so​ ​that​ ​you are​ ​left​ ​with​ ​just​ ​a​ ​salt​ ​and​ ​water

-The​ ​remaining​ ​solution​ ​is​ ​only​ ​salt​ ​and​ ​water
* this​ ​is​ ​because​ ​all​ ​of​ ​your​ ​acid​ ​has​ ​fully​ ​reacted​ ​and​ ​you​ ​have​ ​filtered​ ​off your​ ​other​ ​reactant,​ ​so​ ​that​ ​the​ ​only​ ​products​ ​of​ ​your​ ​reaction​ ​are​ ​a​ ​salt and​ ​water
* if​ ​you​ ​have​ ​used​ ​a​ ​carbonate​ ​you​ ​would​ ​still​ ​only​ ​have​ ​a​ ​salt​ ​and​ ​water remaining​ ​as​ ​carbon​ ​dioxide​ ​gas​ ​would​ ​have​ ​been​ ​given​ ​off​ ​into​ ​the atmosphere

Question 23

What should you do if soluble salts are prepared from an acid and an soluble reactant and why?

Answer 23

-Titration​ ​must​ ​be​ ​used:
* both​ ​reactants​ ​are​ ​liquids/soluble,​ ​so​ ​if​ ​you​ ​have​ ​an​ ​excess​ ​of​ ​one​ ​you would​ ​not​ ​be​ ​able​ ​to​ ​easily​ ​remove​ ​it​ ​from​ ​your​ ​mixture​ ​of​ ​products,​ ​this means​ ​you​ ​need​ ​to​ ​measure​ ​the​ ​exact​ ​amount​ ​of​ ​volumes​ ​that​ ​react, which​ ​is​ ​easily​ ​done​ ​using​ ​a​ ​titration.

-You​ ​can​ ​then​ ​mix​ ​the​ ​exact​ ​proportions​ ​of​ ​the​ ​two​ ​reactants

-The​ ​exact​ ​amount​ ​of​ ​acid​ ​has​ ​thus​ ​been​ ​added​ ​to​ ​the​ ​soluble​ ​reactant,​ ​meaning that​ ​the​ ​leftover​ ​solution​ ​is​ ​only​ ​salt​ ​and​ ​water,​ ​no​ ​acid​ ​or​ ​alkali,​ ​because​ ​they have​ ​been​ ​completely​ ​neutralised

Question 24

Describe the core practical for investigating the preparation of pure, dry hydrated copper sulfate crystals starting from copper oxide including the use of a water bath

Answer 24

-Add​ ​an​ ​excess​ ​of​ ​copper​ ​oxide​ ​(insoluble)​ ​to​ ​your​ ​acid​ ​(sulfuric​ ​acid — H​2S​O​4 — as​ ​you​​ are​​ making​​ copper​​ SULFATE)
-use​ ​a​ ​filter​ ​and​ ​filter​ ​paper​ ​to​ ​filter​ ​off​ ​any​ ​copper​ ​oxide​ ​that​ ​hasn’t reacted​ ​(your​ ​solution​ ​should​ ​be​ ​blue​ ​as​ ​copper​ ​sulfate​ ​solution​ ​has​ ​been formed)
-evaporate​ ​off​ ​the​ ​water​ ​by​ ​placing​ ​your​ ​final​ ​solution​ ​in​ ​a​ ​water​ ​bath

Question 25

Describe how to carry out an acid-alkali titration, using burette, pipette and a suitable indicator, to prepare a pure, dry salt

Answer 25

1. Wash​ ​burette​ ​using​ ​the​ ​acid​ ​and​ ​then​ ​water
2. Fill​​ burette​ ​to​​ 100cm3​​​ w​ith​​ acid​​ with​​ the​​ meniscus’​ ​base​​ on​​ the​​ 100cm3​​​​ line
3. Use 25cm3​ pipette to add 25cm3​ of alkali into a conical flask, drawing alkali into​ ​the​ ​pipette​ ​using​ ​a​ ​pipette​ ​filler
4. Add a few drops of a suitable indicator to the conical flask (eg: phenolphthalein​ ​which​ ​is​ ​pink​ ​when​ ​alkaline​ ​and​ ​colourless​ ​when​ ​acidic)
5. Add acid from burette to alkali until end-point is reached (as shown by the indicator).
6. The titre (volume of alkali needed to exactly neutralise the acid) is the difference​ ​between​ ​the​ ​first​ ​(100cm3)​ ​and​ ​second​ ​readings​ ​on​ ​the​ burette
7. Repeat​ ​the​ experiment​ ​to​ ​gain​ ​more​ ​precise​ ​results
8. To prepare a pure, dry salt you warm the salt solution to evaporate the water
9. Crystals​ ​form

Question 26

Describe the affect of traces of water in the pipette

Answer 26

The traces of water in the pipette reduce the titration result as it lowers the concentration of the barium hydroxide solution

Question 27

Describe the affect of traces of water in the conical flask

Answer 27

The water has no effect on the titration result as any water present does not affect the amount of barium hydroxide present

Question 28

Which common substances are always soluble in water?

Answer 28

All common sodium, potassium and ammonium salts and all nitrates are soluble

Question 29

What is the general solubility rule for chlorides?

Answer 29

Common chlorides are soluble except those of silver and lead

Question 30

What is the general solubility rule for sulfates?

Answer 30

Common sulfates are soluble except those of lead, barium and calcium

Question 31

What is the general solubility rule for carbonates and hydroxides?

Answer 31

Common carbonates and hydroxides are insoluble except those of sodium, potassium and ammonium

Question 32

When solutions are mixed together what will cause a precipitate to be formed?

Answer 32

If a salt in the reaction is insoluble it will form as precipitate.

Soluble salts remain in solution

Question 33

Describe the method used to prepare a pure, dry sample of an insoluble salt

Answer 33

1. mix​ ​the​ ​two​ ​solutions​ ​needed​ ​to​ ​form​ ​the​ ​salt
2. filter​ ​the​ ​mixture​ ​using​ ​filter​ ​paper,​ ​which​ ​the​ ​insoluble​ ​salt​ ​will​ ​be​ ​left​ ​on
3. wash​ ​the​ ​salt​ ​using​ ​distilled​ ​water
4. leave​ ​the​ ​salt​ ​to​ ​dry​ ​on​ ​filter​ ​paper​ ​(water​ ​will​ ​evaporate,​ ​speed​ ​this​ ​process​ ​up by​ ​drying​ ​it​ ​in​ ​an​ ​oven)