SC4 - Acids and Alkalis

Question 1

What is concentration?

Answer 1

The amount of solute per unit volume. Concentrated acids have lots of H+ ions per unit volume, concentrated alkalis have lots of OH- ions per unit volume.

Question 2

What is the pH scale?

Answer 2

A measure of the acidity or alkalinity of a substance.

1 2 3 4 5 6 7 8 9 10 11 12 13 14
Acidic Neutral Alkaline

Question 3

What are indicators?

Answer 3

Substances that change colour depending on the pH.

Question 4

What colour is universal indicator in acids, alkalis and neutral?

Answer 4

Acids - red
Neutral - green
Alkalis - blue/purple

Question 5

What colour is litmus in acids and alkalis?

Answer 5

Acids - red

Alkalis - blue

Question 6

What colour is methyl orange in acids and alkalis?

Answer 6

Acids - red

Alkalis - yellow

Question 7

What colour is phenolphthalein in acids and alkalis?

Answer 7

Acids - colourless

Alkalis - pink

Question 8

What is the difference in concentration between an acid of pH1 and pH3?

Answer 8

Each time the pH of an acid increases by 1 it has been diluted by a factor of 10. So, pH1 is 10x10 more concentrated than pH3 (x100 difference).

Question 9

How can you measure pH?

Answer 9

Universal indicator can be used then matched to a colour chart.
A pH probe can be used to measure the pH to to 2d.p. . This is better as it removes human error and gives the value to 2d.p. whereas universal indicator will only give a whole number.

Question 10

What is a strong acid?

Answer 10

Strong acids fully dissociate (split up) when in solution. For example, all molecules of hydrochloric acid (HCl) are spilt into H+ and Cl- ions. This means that strong acids generally have lower pHs as all the molecules produce H+ ions, however this may not always be the case if it is very dilute.

Question 11

What are weak acids?

Answer 11

Weak acids only partially dissociate (split up into ions) in solution. For example, only a few ethanoic acid molecules (CH3COOH) split up into CH3COO- and H+, the rest stay the same. Weak acids generally have higher pHs (closer to neutral) as only some molecules produce H+ ions, however this is not always the case as it may be very concentrated.

Question 12

What is the equation for the reaction between an acid and a base?

Answer 12

Base + acid -> salt + water (BASH2O)

Question 13

What is the equation for the reaction between an acid and a metal?

Answer 13

Metal + acid -> salt + hydrogen (MASH2)

Question 14

What is the equation for the reaction between an acid and a metal carbonate?

Answer 14

Metal carbonate + acid -> salt + water + carbon dioxide (CASH2OCO2)

Question 15

How would you prepare a soluble salt from an insoluble base?

Answer 15

Add an excess of the insoluble base to the acid (you know it’s in excess when it stops dissolving). Filter off the excess then crystallise the salt solution to form crystals of the soluble salt.

Question 16

Core practical - preparing copper sulphate

Answer 16

• measure 20cm3 of dilute sulphuric acid using a measuring cylinder and pour it into a small conical flask.
• warm the acid in a water bath of 50 degrees
• add a little copper oxide to the flask and stir
• if it all reacts, add a little more until it no longer reacts
• filter and transfer to an evaporating basin
• heat the solution with a Bunsen burner until around half has evaporated, then take off the heat and leave to cool
• dry the crystals in an oven.

Question 17

What are acids?

Answer 17

Acids are substances which release H+ ions when dissolved in solution.

Question 18

What are alkalis?

Answer 18

Alkalis are soluble bases. They release OH- ions when dissolved in solution.

Question 19

Core practical - investigating neutralisation

Answer 19

• Use a measuring cylinder to add 50cm3 of dilute hydrochloric acid to a beaker
• put a piece of universal indicator paper on a white tile and dip a glass rod into the liquid and tap it on the U.I. paper, wait 30secs and match the colour to a pH colour chart.
• measure out 0.3g calcium hydroxide power onto a weighing boat
• add the calcium hydroxide to the beaker and stir then estimate and record the pH
• repeat 7 times so you have added a total of 2.4g of calcium hydroxide powder.
• plot a graph with pH on the y axis and mass of calcium hydroxide added on the x axis.

Question 20

Why do acids and alkalis neutralise each other (ionic equation)?

Answer 20

H+ + OH- -> H2O

Question 21

How would you carry out a titration?

Answer 21

• use a glass pipette and pipette filler to measure out a fixed volume of alkali and add to a conical flask.
• fill the burette to the 0.0cm3 mark
• add a few drops of a suitable indicator (e.g. phenolphthalein) to the alkali
• open the tap on the burette and swirl the flask as you go, adding the acid more slowly as you near the end point (you know when it’s the end point as the indicator changes colour).

Question 22

How would you obtain a pure, dry salt using titration?

Answer 22

• carry out a titration
• note the exact volume of acid needed to neutralise the alkali
• use the burette to add the correct volume of acid without the indicator
• use crystallisation to obtain the salt.

Question 23

What would you observe when metals react with acids?

Answer 23

Effervescence (due to hydrogen gas being released - note you do not actually see the hydrogen), fizzing, bubbles, metal disappears

Question 24

What are the different gas tests?

Answer 24

Hydrogen - apply lit split, squeaky pop sound
Oxygen - apply glowing splint, which relights
Carbon dioxide - bubble through limewater, which goes cloudy

Question 25

How do you write ionic equations?

Answer 25

• Write the balanced symbol equation with state symbols
• split all ionic compounds into their ions (e.g. HCl would be written as H+ + Cl- but H20 would stay H20 as it is covalent).
• cancel out spectator ions (any ions which appear on both sides of the equation)
• write out what is left - this is the ionic equation.

Question 26

How do you write half equations?

Answer 26

Choose one reactant and show what product it forms. Then, add electrons to one side of the equation to balance the charges.

2H+ + 2e-> H2

Question 27

What are oxidation and reduction reactions?

Answer 27

Oxidation is the loss of electrons, reduction is the gain of electrons (OIL RIG). If electrons appear on the right hand side of a half equation, it is oxidation, if they appear on the left hand side, it is reduction.

Question 28

What would you observe in a reaction between a metal carbonate and an acid?

Answer 28

Effervescence (due to carbon dioxide being given off), bubbles, metal carbonate disappears

Question 29

What are the solubility rules?

Answer 29

Soluble in water:

• all group 1 and ammonium salts
• all nitrates
• most chlorides
• most sulphates
• group 1 and ammonium carbonates
• group 1 and ammonium hydroxides

Insoluble in water:

• silver, lead chlorides
• lead, barium, calcium sulphates
• most carbonates
• most hydroxides

Question 30

What are precipitation reactions?

Answer 30

Reactions in which soluble substances in solution cause an insoluble precipitate to form. In precipitation reactions, the metals ‘swap’ e.g. silver nitrate (aq) + sodium chloride (aq) -> silver chloride (s) + sodium nitrate (aq)

Question 31

How would you prepare an insoluble salt?

Answer 31

• measure out the correct amounts of each solution and mix in a beaker
• filter the mixture and rinse the beaker with a little distilled water
• pour a little distilled water over the precipitate in the funnel
• remove the filter paper containing the precipitate and dry it in a warm oven.