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 2

Describe the pH scales

Answer 2

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

Question 2

How do acids and alkalis react to phenolphthalein?

Answer 2

Alkalis turn pink
Acids are colourless

Question 3

How do acids and alkalis react to methyl orange?

Answer 3

Alkalis turn yellow
Acids are red

Question 3

How do acids and alkalis react to litmus?

Answer 3

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 4

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

Answer 4

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 5

How does hydrogen ion concentration effect pH?

Answer 5

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

Question 6

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

Answer 6

​ ​​​​​​​​​​​Concentrated​ ​=​ ​larger​ ​amount​ ​of​ ​substance​ ​in​ ​a​ ​given​ ​volume​ ​of​ ​a​ ​solution
Dilute​ ​=​ ​lesser​ ​amount​ ​of​ ​substance​ ​in​ ​a​ ​given​ ​volume​ ​of​ ​a​ ​solution

Question 6

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 6

-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 7

Explain the terms weak and strong acids

Answer 7

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

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

Question 7

What is a base?

Answer 7

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

Question 8

What is an alkali?

Answer 8

A soluble base

Question 9

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

Answer 9

A salt + hydrogen gas (H2)

Question 10

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

Answer 10

A salt + water

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

Question 10

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

Answer 10

A salt + water

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

Question 11

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

Answer 11

A salt + water + carbon dioxide

Question 12

Describe the chemical test for hydrogen

Answer 12

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

Question 13

Describe the chemical test for carbon dioxide (using limewater)

Answer 13

Bubble​ ​the​ ​gas​ ​through​ ​the​ ​limewater​ ​(calcium​ ​hydroxide​ ​solution)​ ​and​ ​it
will​ ​turn​ ​milky​ ​(cloudy)

Question 14

Describe a neutralisation reaction

Answer 14

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

Question 15

​ ​​ Explain an acid-alkali neutralisation

Answer 15

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 16

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

Answer 16

-Excess​ ​of​ ​the​ ​reactant​ ​is​ ​added
* this​ ​is​ ​to​ ​ensure​ ​your​ ​volume​ ​of​ ​acid​ ​reacts​ ​completely

-excess​ ​reactant​ ​is​ ​removed
* 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,​ ​and​ ​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 17

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

Answer 17

-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 17

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 17

-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 18

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

Answer 18

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

Question 19

Which common substances are always soluble in water?

Answer 19

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

Question 20

What is the general solubility rule for chlorides?

Answer 20

Common chlorides are soluble except those of silver and lead

Question 21

What is the general solubility rule for sulfates?

Answer 21

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

Question 22

What is the general solubility rule for carbonates and hydroxides?

Answer 22

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

Question 23

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

Answer 23

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

Soluble salts remain in solution

Question 24

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

Answer 24

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)