Acids

Question 1

What is an acid?

Answer 1

A substance that forms H+ ions when they dissolve in water (aqueous solutions)

Question 2

How do were measure how acidic or alkaline a substance is?

Answer 2

pH scale

Question 3

What is an alkali?

Answer 3

An alkali is a substance that forms OH- ions when they dissolve in water - a soluble base

Question 4

What is a weak acid?

Answer 4

• Only partially ionise in water, meaning not all the acid molecules break up to form H+ ions
• Most acids are weak acids

Question 5

What is an example of a weak acid?

Answer 5

• Ethanoic acid

- CH3COOH ⇌ CH3COO- + H+

Question 6

What is a strong acid?

Answer 6

• Fully ionise in water, meaning that all the acid molecules break up to form H+ ions

Question 7

What is an example of a strong acid?

Answer 7

• Hydrochloric acid

- HCl -> H+ + Cl-

Question 8

What is pH a measure of?

Answer 8

The concentration of H+ ions in a solution

Question 9

How much does the concentration of H+ ions get divided by as the pH goes up by 1?

Answer 9

Divided by 10

Question 10

What is the concentration of H+ ions at pH 0, 1 and 2?

Answer 10

0 - 1 mol/dm3
1 - 0.1 mol/dm3
2 - 0.01 mol/dm3

Question 11

What are the 2 main ways of measuring the pH of a substance?

Answer 11

Universal indicator, pH probe

Question 12

What are 3 ways of determining if a substance is acid or alkali?

Answer 12

Phenolphthalein, Methyl orange, litmus paper

Question 13

Describe a method to test for change in pH when adding a solid base to a fixed volume of acid. (likely to be calcium hydroxide CaO or calcium oxide Ca(OH)2 to HCl)

Answer 13

1) Use a measuring cylinder to add a set volume of acid to the beaker
2) Add the amount of solid base to the beaker (number of spatulas worth)
3) Stir with a glass rod
4) Record the pH of the beakers contents by dipping in glass rod, and tapping some onto universal indicator paper
5) Match the colour to the appropriate pH on a pH colour chart after 30s

Question 14

How can the experiment where you add a soild base to a set volume of acid be improved?

Answer 14

Use a pH probe to measure pH, as this will mean you don’t have to judge the colour, and is quicker

Question 15

What is a concentrated solution?

Answer 15

A solution that contains a large amount of the solute in a given volume of the solvent

Question 16

What is a dilute solution?

Answer 16

Contains a small amount of the solute in a given volume of the solvent

Question 17

What is the result of Acid + Metal

Answer 17

Hydrogen + Salt

Question 18

What is a base?

Answer 18

Any substance that reacts with an acid to form a salt and water.

Question 19

What are examples of substances that are bases?

Answer 19

Metal carbonates, metal oxides, metal hydroxides

Question 20

What is the result of Acid + Carbonate

Answer 20

Salt + water + carbon dioxide

Question 21

What is the result of Acid + Hydroxide

Answer 21

Salt + water

Question 22

What is the result of Acid + Oxide?

Answer 22

Salt + water

Question 23

How are soluble salts made?

Answer 23

Reacting acid, with solid insoluble substances such as pure metals, metal oxides, metal hydroxides or metal carbonates

Question 24

How do you prepare a soluble salt from an acid and a solid?

Answer 24

1) Use a Bunsen burner to gently warm the acid
2) Add the insoluble/soluble solid (with stirring) until the reaction no longer happens

IF USING SOLUBLE SOLIDS, USE TITRATION TO DETERMINE THE EXACT AMOUNT NEEDED

IF USING INSOLUBLE SOLIDS THEN ADD EXCESS AND FILTER

3) Filter the solution to remove excess insoluble solid if its there, leaving a solution of the salt dissolved in water
4) Heat the solution in an evaporating basin above a beaker of water, which ensures gentle heating
5) Let the solution cool and allow more water to evaporate, causing the solution to become more concentrated and the salt will begin to crystallise

Question 25

What is the aim of crystallisation?

Answer 25

Produce a pure, dry sample of a soluble salt from an insoluble oxide or carbonte.

Question 26

What salts are normally soluble in water?

Answer 26

- Sodium, potassium and ammonium salts - all nitrates - most chlorides, except silver and lead chlorides - most sulphates, except lead, barium and calcium sulphates - Sodium, potassium and ammonium carbonates and hydroxides

Question 27

Which salts are normally insoluble in water?

Answer 27

- Silver and lead chlorides - Lead, barium and calcium sulphates - most carbonates and hydroxides, except sodium, potassium and ammonium

Question 28

What is a precipitate?

Answer 28

Insoluble solid in a solution

Question 29

What is a neutralisation reaction?

Answer 29

A reaction between an acid and a base

Question 30

What is titration?

Answer 30

A technique that measures the volumes of acid and alkali solutions that are needed in a neutralisation reaction

Question 31

How do indicators help with titration?

Answer 31

They show the point where a solution turns from acid to alkaline

Question 32

Explain the method for titration

Answer 32

1) Using a pipette, add a set volume of a strong alkaline to a conical flask 2) Add a few drops of a suitable indicator 3) Place a strong acid (HCl, H2SO4, HNO3) of known concentration in a burette 4) Take an initial reading of the volume of acid in the burette 5) Gradually add the acid into the alkali, and ensure you are regularly swirling the conical flask as you add acid. As you get closer to the result slow the swirling. 6) As soon as the indicator changes colour stop adding acid 7) Take a reading of the final volume of acid in the burette. 8) Subtract this reading from the initial volume to find the volume of acid needed to neutralise the alkali

Question 33

What is the equivalence point?

Answer 33

The point where the pH goes changes from acid to alkali or vice versa

Question 34

Why must titration be used when preparing a soluble salt from an acid and a soluble reactant?

Answer 34

Because you need to know the precise amounts of reactants you need to use so that you do not have excess, because as these are soluble, there is no way of separating them from the solution, and this would contaminate the pure salt that you want to crystallise.

Question 35

Why the acid and soluble solid are mixed in correct proportions?

Answer 35

So that there is no excess, as this would be dissolved in solution and very hard to seperate

Question 36

Why after reacting an acid and a soluble solid is the solution left just salt and water?

Answer 36

All reactants were used up and there is no excess