⭐️SC8 - Acids and Alkalis

Question 1

For litmus:

• what colour is it In alkaline solutions?
• what colour is it in acidic solutions?

Answer 1

Blue in alkaline and red in acid

Question 2

For methyl orange:

• what colour is it In alkaline solutions?
• what colour is it in acidic solutions?

Answer 2

Yellow in alkaline and red in acid

Question 3

For phenolphthalein,

• what colour is it In alkaline solutions
• what colour is it in acidic solutions

Answer 3

Pink and colourless

Question 4

When are polyatomic ions formed?

Answer 4

When small groups of atoms held together by covenant bonds loose or gain electrons e.g. OH-

Question 5

What does a concentration depend on?

Answer 5

The number of hydrogen ions for an acid or hydroxide ions for an alkaline in a certain volume, so the higher the concentration of H+ ions, the more acidic the solution and the higher the concentration of OH- ions the more alkaline the solution

Question 6

How is the concentration of H+ ions linked to the pH?

Answer 6

As if concentration increased by a factor of 10, the pH decreases by 1

Question 7

What makes an acid strong?

Answer 7

The molecules of the acid dissociate completely into ions when they dissolve in water and produce high concentrations of hydrogen ions, they also have a low pH value

Question 8

What are bases?

Answer 8

Substances that neutralise acids to form salts and water only

Question 9

What is the general word equation for neutralisation reactions?

Answer 9

Metal oxide + acid -> salt + water

Question 10

What happens to ions in general during neutralisation reactions?

Answer 10

Hydrogen ions in the acid combine with oxide ions to form water. This removes the hydrogen ions causing the pH to increase

Question 11

In terms of ions how are salts produced in neutralisation reactions?

Answer 11

By replacing hydrogen ions with metal ions

Question 12

When are bases alkali? What are these?

Answer 12

When they are soluble bases like sodium hydroxide and other group 1 hydroxides and calcium hydroxide and other group 2 hydroxides

Question 13

What bases are not alkali? What are these?

Answer 13

Insoluble ones like copper hydroxide and most other metal hydroxides

Question 14

What happens during a neutralisation reactions?

Answer 14

The reaction mixture becomes warmer when alkali is added to the acid and pH increases giving products of salt and water but pH may go higher than 7 if enough alkali is added

Question 15

What happens to ions In a neutralisation reaction

Answer 15

The hydrogen ions from the acid react with hydroxide ions from the alkali and water being a simple molecular covalently bonded substance with molecular bonds is formed. The ions from the acid and alkali stay in solution as ions of the dissolves salt

Question 16

How can you make a pure dry salt?

Answer 16

• Carry out a titration
• this gives the exact volume of acid needed to neutralise an alkali
• use the Burnett to add the correct volume of acid without any indicator
• evaporate the water from the solution formed

Question 17

In a titration why do you use a Burette and not a measuring cylinder?

Answer 17

Because you can add one drop of acid at a time providing more accurate and repeatable measurements

Question 18

What indicators are used in titration and why?

Answer 18

Phenolphthalein and methyl orange as they give you a sharp end point due to the obvious colour change

Question 19

Give the general word equation for when you react metal and acids

Answer 19

Metal + acid -> salt + water

Question 20

How do you test for hydrogen?

Answer 20

Put a light splint in a hydrogen and air mixture and you’ll hear a squeaky pop

Question 21

What are oxidation and reduction?

Answer 21

Oxidation is loss of electrons

Reduction is the gain of electrons

Question 22

What’s the general word equation when an acid reacts with a metal carbonate?

Answer 22

Metal carbonate + acid -> salt + water + carbon dioxide

Question 23

What do you see in a reaction between a metal carbonate and an acid?

Answer 23

You can see bubbles (of the carbon dioxide) and the solid metal carbonate disappears (providing there’s enough acid )

Question 24

What is a precipitate reaction “

Answer 24

Where soluble substances in solution cause an insoluble precipitate to form

Question 25

In general what substances are soluble and what are insoluble

Answer 25

• All common sodium, potassium and ammonium salts
• all nitrates
• most chlorides except silver and lead chloride
• most sulphates except Lead, barium and calcium sulphate
• only sodium, potassium and ammonium carbonates, all others are insoluble
• only sodium, potassium and ammonium hydroxides, all others are insoluble

Question 26

How can you test for carbon dioxide?

Answer 26

Bubble the gas through limewater and will turn milky if lime water is present

Question 27

How can ionic equations show the formation of precipitates?

Answer 27

The formula of the ions the react together to form the precipitate is on the left and the formula of the precipitate is on the right

Question 28

How can you predict wether a precipitate will form?

Answer 28

By checking the solubility of the product as if both products are insoluble then no precipitate forms

Question 29

Give a method to prepare a pure dry sample of insoluble salt from two soluble ones

Answer 29

1. Wear eye protection
2. Mix the two solutions in a beaker and then filter the mixture
3. Mix the beaker with a little distilled water and then pour it through the funnel
4. Pour a little distilled water over the precipitate in the funnel. This is so little salt can be wasted from being stuck on the sides
5. Carefully remove filter paper containing the precipitate and dry it in a warm oven