Module 2, Chapter 4: Acids and Redox

What do all acids have in their formula?
What happens when acids are dissolved in water?
an acid releases hydrogen ions as protons, H+ into the solution
Strong Acids
such as HCl release all its hydrogen atoms into solution as H+ ions and completely dissociates in an aqueous solution
Examples of Strong Acids
Weak Acids
such as ethanoic acid, CH3COOH only release a small proportion of its available hydrogen atoms into solution as H+ ions
Examples of Weak Acids
ethanoic acid
Examples of Bases
- metal oxides

- metal hydroxides

- metal carbonates

- ammonia
What do Bases neutralise to form?
form a salt
a base that dissolves in water releasing hydroxide ions (OH-) into the solution
What happens in the neutralisation of acids
- H+ (aq) ions react with a base to form a salt and water

- the H+ ions from the acid are replaced by metal or ammonium ions from the base
How is an acid neutralised?
an acid is neutralised by a metal oxide or metal hydroxide to form a salt and water
Acid + alkali -----)
salt + water
carbonates + acids ---->
salt + water + carbon dioxide
What do Titrations do?
they allow you to find out exactly how much acid is needed to neutralize a quantity of alkali
How to do a Titration
you measure out some alkali using a pipette and put in a flask, along with some indicator (e.g. phenolphthalein)

do a rough titration to get an idea where the end point is (the point where the alkali is exactly neutralised and the indicator changes colour)

Take an initial reading to see how much acid is in the burette to start off with

Add acid to the alkali - giving the flask a regular swirl

Stop when your indicator shows a permanent colour change

Record the final reading from your burette

now do an accurate titration

Run the acid in to within 2cm3 of the end point

Add the dropwise

Work out the amount of acid used to neutralise the alkali
Final reading minus the initial reading = titre

10. Repeat
How to do a titration accurately?
- read from the bottom of the meniscus

- take your reading to the nearest 0.05 cm3
Standard Solution
a solution that has a precisely known concentration is called a standard solution
Steps on How to Make a Standard Solution
1. Using a precise balance, carefully weigh out the required mass of solid onto a watch glass

2. Transfer this solid to a beaker - use some water to wash any bits of solid from the watch glass into the beaker

3. Add water to the beaker to completely dissolve the solid (using a glass rod to stir)

4. Once dissolved, transfer the solution into a volumetric flask and rinse the beaker and glass rod with water, transferring this water into the volumetric flask

5. Use water to fill the volumetric flask to the graduation line and use a pipette to add the final few drops to make sure you don't add too much water and overshoot the graduation line

6. Put the lid back and turn to mix
How to do Titration Calculations
Write balanced equation

2. Decide what you know

3. Work out how many moles of acid

4. Ratio

5. Find concentration of alkali
Polyprotic acid
an acid that can donate more than one proton per molecule
Example of a Diprotic acid
hydrogen sulfie
Example of Triprotic acid
phosphoric acid
Why are indicators that change colour quickly over a very small pH range used?
you know exactly when the reaction has ended
Methyl Orange Colour Change
turns yellow to red when adding acid to alkali
Phenolphthalein colour change
turns pink to colourless when adding acid to alkali
What does the Oxidation Number tell us?
he number of electrons an atom has donated or accepted to form an ion / part of a compound
Oxidation Number of Combined Elements
Oxidation Number of Monatomic Ions
the charge of the ion
Oxidation Number of Molecular Ions
overall charge
Oxidation Number of Oxygen

- EXCEPT when bonded to more electronegative element F

- EXCEPT in Peroxides (-1)

- molecular oxygen (0)
Oxidation Number of Hydrogen

EXCEPT in metal hydride = -1
What do ions ending in -ate contain?
oxygen and another element
How to work out Formulae from Oxidation Numbers
1. Find oxidation number from systematic name

2. Find formula of ion (from oxidation number of oxygen)

3. Overall charge = 0, so find ratio to make charge 0
Working out Systematic Names from Oxidation Numbers
1. Oxygen oxidation number = -2

2. Times Oxidation number by number of atoms of oxygen to find charge

3. Find the overall charge of molecule

4 Work out oxidation number of other element by adding the charges together
loss of electrons
gain of electrons
Redox Reaction
when reduction and oxidation happen simultaneously
Reduction agent
donates electrons and gets oxidised
Oxidation Agent
ccepts electrons and gets reduced
Oxidation Number Increases =
- electrons lost

- oxidation
Oxidation Number Decreases =
- electrons gained

- reduction
Oxidation Numbers when Metals form Compounds
positive oxidation numbers
Oxidation Numbers when Non Metals from Compounds
negative oxidation numbers
What happens to metals when they react with acids?
they are oxidised