Module 2 (chapter 4) - acid and redox

Question 1

acid

Answer 1

a species that donates a proton (H+) when dissolved in water as it dissociates into its components
-they give PHs of less than 7

Question 2

three common inorganic acids

Answer 2

• sulphuric acid (H2SO4)
• hydrochloric acid (HCl)
• Nitric acid (HNO3)
• hydrogen is always at the start of inorganic acids

Question 3

three common organic acids

Answer 3

• ethanoic acid (CH3COOH) vinegar
• methanoic acid (HCOOH) in insert bites
• citric acid (C6H8O4) in citrus fruits

Question 4

what is a strong acid?

Answer 4

-completely dissociates when dissolved in water or aqueous solution
-this meanest releases all its H+ atoms
(HCL –> H+ + Cl -)

Question 5

what is a weak acid?

Answer 5

-only partially dissociates in water or aqueous solution
-only releases a small proportion of its available hydrogen atoms
(CH3COOH H+ + CH3COO-) the equilibrium sign indicates that the forward reaction is incomplete

Question 6

what is important to note about compounds that contain hydrogen?

Answer 6

• they’re not all acidic
• e.g. each molecule of ethnic acid contains four hydrogen atoms but only the hydrogen atom on the COOH group is released as H+

Question 7

bases

Answer 7

a species that is a H+ (proton) accepter

• a base neutralises an acid to form a salt
• common bases are metal oxides and hydroxides
• ammonia is also classified as a base as it has a lone pair
• alkalis are a type of base

Question 8

alkalis

Answer 8

a type of base that dissolves in water to form hydroxide (OH-) ions

• a chemical which gives a solution above 7 when dissolved in water
• 3 common alkalis are ammonia, sodium and potassium hydroxide
• can sometimes be more corrosive and ore dangerous than acids

Question 9

ammonia

Answer 9

• a gas that dissolves in water to form a weak alkaline solution (NH3 + H2O NH4+ + OH-)
• ammonia is a weak base because only a small proportion of the ammonia dissolves

Question 10

what is a neutralisation reaction?

Answer 10

H+ ions react with a base to form a salt and neutral water

-the H+ ions from the acid are replaced by metal or ammonium ions from the base

Question 11

what is the neutralisation of acids like?

Answer 11

-when neutralised by a metal oxide or hydroxide it forms salt and water only
acid + alkali – salt and water
-when neutralised with carbonate the products are salt, water and carbon dioxide

Question 12

what is a titration?

Answer 12

a technique used to accurately measure the volume of one solution that reacts exactly with another
-They’re often used when finding the concentrating of a solution, identification of unknown chemicals and finding the purity of a substance

Question 13

how do you prepare a standard solution?

Answer 13

use a volumetric flask
-weight the solid accurately first
-solid is dissolved in a beaker of distilled water
-transfer solution to a volumetric flask
-carefully fill to the graduation line by adding distilled water drop-wise. try to view the meniscus at eye level
-slowly invert the flask several times to mix the solution thoroughly
to increase accuracy:
- 2 or more decimal place balance, rinse beaker and transfer washings to flask, invert with stopper

Question 14

what are the uncertainties in pipettes and burettes

Answer 14

• a 10cm3 pipette (+/- 0.04cm3)
• a 25cm3 pipette (+/- 0.06cm3)
• a 50cm3 pipette (+/- 0.1cm3)
• a burette reading is recoded to the nearest half division with the bottom of the meniscus on the mark of between two marks (each reading is measured to the nearest +/- 0.05cm3)

Question 15

working out the mean titre

Answer 15

-repeat titre until two are concordant of each other and rejecting other inaccurate readings

Question 16

what is oxidation?

Answer 16

• loss of electrons
• gain of oxygen
• loss of hydrogen
• rise in oxidation number/increase in oxidation state

Question 17

what is reduction?

Answer 17

• gain of electrons
• loss of oxygen
• gain of hydrogen
• decrease in oxidation number/decrease in oxidation state

Question 18

oxidation number

Answer 18

a measure of the number of electrons that an atom uses to bond with atoms of another element

• sum of oxidation numbers equals charge
• used to see what has been oxidised and what reduced

Question 19

oxidation number rules of elements

Answer 19

-always zero for elements or things in tier elemental form (P4, S8)

Question 20

oxidation number rules for compounds and ions

Answer 20

• each atom in a compound has an oxidation number

- an oxidation number is a sign placed before the number

Question 21

combined oxygen

Answer 21

-2

Question 22

combined hydrogen

Answer 22

+1

Question 23

simple ion

Answer 23

charge on ion

Question 24

combined halogens and fluorine

Answer 24

-1

Question 25

peroxides

Answer 25

-1

Question 26

key exceptions and points for oxidation numbers

Answer 26

- when bonded to metal hydrides, hydrogen has an oxidation number of -1 - the more electronegative an atom the more negative the oxidation state - O bonded to F is +2

Question 27

oxide, ate and ide endings

Answer 27

- ate with oxygen - ide without oxygen - oxide- oxygen attached to a non oxygen

Question 28

NO2- and NO3-

Answer 28

- nitrite, +3 | - nitrate, +5

Question 29

rules for half equations

Answer 29

if electrons on the - RHS its oxidation - LHS its reduction - balance elements and then the charges using electrons in half equations - to form an ionic equation combine without electron notations and spectator ions

Question 30

redox reaction

Answer 30

are when both oxidation and reduction both happen simultaneous

Question 31

reducing agent

Answer 31

a species being oxidised so forcing another species to be reduced

Question 32

oxidising agent

Answer 32

a species itself which is reduced so forcing the other species to be oxidised

Question 33

disproportionation reaction

Answer 33

a type of redox where one species one species is both oxidised and reduced at the same time