OCR A Level Chemistry- Acids and Redox

What is an acid?

substances that release H+ ions when dissolved in water. Proton donors

What is a strong acid?

an acid that fully dissociates when dissolved in water

Give 3 examples of strong acids

HCl, H2SO4, HNO3

Define weak acid

acids that partially dissociate when dissolved in water

Give an example of a weak acid

CH3COOH

What is the difference between a base and an alkali?

an alkali is a base that dissolves in water, releasing OH- ions

Give 3 examples of common alkalis

NaOH, KOH, NH3

How can you identify a salt in a reaction?

an ionic compound formed when the H+ ion of an acid is replaced by a metal ion or ammonium ion

What are the products of a reaction between an acid and a metal carbonate?

salt, water and carbon dioxide

What are the common bases?

metal oxides, metal hydroxides, metal carbonates and alkalis

What are the products of a reaction between an acid and a metal oxide?

salt and water

What is a titre?

the volume of acid needed to neutralise the alkali
(the vol of solution added from the burette)

why do you invert the volumetric flask containing the standard solution?

to mix it thoroughly
if not, conc is not consistent throughout the mixture

What are the products of a reaction between an acid and an alkali?

salt and water

Give the ionic equation of a neutralisation reaction

H+ + OH- = H2O

What is formed from the reaction of an acid with ammonia?

ammonium salts

What are titrations used for?

Finding out exact conc. or vol. of acid needed to neutralise a quantity of alkali

finding conc./vol of a solution
identifying unknown chemicals
finding purity of a substance

What precise apparatus is used in a titration?

burette and pipette

methyl orange in acid v alkali

red in acid
yellow in alkali

phenolphthalein in acid v alkali

colourless in acid
pink in alkali
(pale pink when neutralised from acid to alkali)

What is a standard solution?

a solution of known concentration

Describe the preparation of a standard solution

1) Work out mass of substance needed to be weighed by multiplying the concentration by the volume to get the mole. Then, multiply the mole by the molar mass to get the mass.
2) Dissolve substance in a beaker with distilled water. Then, pour solution into a volumetric flask, including the rinsings from the beaker. Add distilled water until bottom of meniscus is on the graduation line

Describe how to carry out an acid-base titration

1) Add 25cm^3 of the solution of known concentration into a conical flask
2) Place flask on white tile and add indicator (e.g. methyl orange)
3) Add solution of unknown concentration and volume into a burette
4) Turn on tap, swirling the flask until there is a colour change signalling the end-point
5) Calculate the volume of solution added. Repeat titration until there are at least 2 concordant results

What is the process to calculate unknown concentration?

1) Write a balanced equation for the reaction
2) Calculate the amount in moles of the known solution that reacted
3) Use stoichiometry to calculate the amount in moles of the unknown solution that reacted
4) Calculate the unknown concentration by dividing the moles by the volume

monoprotic, diprotic, polyprotic

1 mole of HCl produces 1 mole of H+ ions
1 mole of H2SO4 produces 2 moles of H+ ions
1 mole of H3SO4 produces 3 moles of H+ ions

How can the percentage uncertainty be calculated?

((uncertainty * number of readings) / measured value ) * 100

Define oxidation number

a number assigned to an element in chemical combination which represents the number of electrons lost or gained by an atom of that element in the compound

What is the rule for assigning an oxidation number to a pure element?

the oxidation number is zero

What is the rule for assigning an oxidation number to monatomic ions?

the oxidation number is equal to ionic charge

What are the rules for assigning oxidation numbers for compounds?

fluorine= -1
hydrogen= +1
oxygen= -2

Describe the common exceptions for assigning oxidation numbers to hydrogen and oxygen

hydrogen- in a metal hydride, it has an oxidation number of -1
oxygen- when it is bonded to fluorine, it has an oxidation number of +2; in a peroxide, it has an oxidation number of -1

What does the sum of the oxidation numbers equal?

the overall charge of the compound (e.g. in a polyatomic ion, it equals the overall charge on the ion)

What are Roman numerals used for?

show the oxidation number of elements that can have different oxidation numbers in different compounds or ions

What is a redox reaction?

a chemical reaction involving both oxidation and reduction

What is oxidation?

the loss of electrons and increase in oxidation number

What is reduction?

the gain of electrons and decrease in oxidation number

What do half equations show?

the electron transfer in a redox reaction by splitting the overall equation into two halves