Module 2 - Acids and Redox Flashcards
formula for sulphuric acid
H2SO4
formula for nitric acid
HNO3
formula for ethanoic acid
CH3COOH
formula for sodium carbonate
Na2CO3
formula for sodium hydroxide
NaOH
formula for potassium hydroxide
KOH
formula for phosphoric acid
H3PO4
define a hydrogen ion
has no electrons in its outer shell so it is also known as a proton
define an acid
in water an acid releases hydrogen ions (H+) into solution (proton donor)
define a strong acid
releases all of its hydrogen ions in solution and completely disassociates/ ionises
define a weak acid
only releases a small number of its hydrogen ions in solution and partially disassociates/ ionises
define a base
a compound that neutralises an acid, by accepting a hydrogen ion, to form a salt (proton acceptor)
what are bases generally
metal oxides
ammonia
describe what a base does
-donates 2 electrons to the hydrogen ion to form a dative covalent bond
-bases can either have a lone pair or a negative charge
define an alkali
a type of soluble base that releases hydroxide ions
what are alkalis typically
hydroxides
how is ammonia classified as an alkali
it takes a proton from water to form ammonium and hydroxide
define a salt
the product of a reaction in which the H+ ions from the acid are replaced by the metal or ammonium ions
product for metal + acid
salt + hydrogen
product for metal oxide + acid
salt + water
product for metal hydroxide + acid
salt + water
product for metal carbonate + acid
salt + carbon dioxide + water
product for ammonia + acid
ammonium salt
what do neutralisation reactions form
salts
observations in carbonate reactions
-there will be effervescence due to the CO2 gas evolved
-the solid carbonate will dissolve
describe the method for making a standard solution
- weigh out the mass of the solid using weigh by difference, to 2 decimal places
- place the solid into a beaker
- add 100cm3 of distilled water to the beaker
- stir the solution to dissolve the solid
- transfer the solution to a 250cm3 volumetric flask
- rinse the equipment into the volumetric flask
- make up the solution to the 250cm3 line with distilled water using a pipette, with the bottom of the meniscus on the line
- put on the lid and invert several times to mix
what is used to calculate the mean titre
- only concordant results
describe concordant results
-2 or 3 values within 0.10cm3
-so results are accurate and repeatable
-the titration technique is consistent
rules for recording titrations
-results should be clearly recorded in a table
-result should be recorded in full (i.e. both initial and final readings)
-record titre volumes to 2dp (0.05 cm3)
describe the method for titrations
1.rinse all glassware with distilled water and the solution that will be used
2.measure 25cm3 of standard solution with a pipette and filler and put in the conical flask
3.add 3-4 drops of indicator
4. place on a white tile to see the colour change better
5. using a funnel, fill the burette with acid to the zero mark with the bottom of the meniscus on this line
6. complete rough titration looking for the endpoint while swirling the conical flask
7. repeat the experiment adding the acid drop by drop to reach the endpoint
8. rinse the conical flask between titrations with distilled water
9. record out titres into a table
10. calculate the mean titre using concordant results
safety precautions of a titration
-acids and alkalis are corrosive
(at low concentrations acids are irritants)
-wear eye protection and gloves
-if spilled immediately wash affected parts after spillage
-if substance is unknown treat it as potentially toxic and wear gloves
why must the jet space in the burette be filled with acid
-it will lead to errors if it then fills during the titration
-leading to a larger than expected titre reading
why must a conical flask be used in a titration
-it is easier to swirl the mixture in a conical flask than a beaker without spilling the contents
why must only a few drops of indicator be used in a titration
-they are generally weak acid
-if too much is added the result will be affected
why can distilled water be used to wash the flask during a titration
-so all the acid on the side is washed into the reaction mixture to react with the alkali
-does not react with reagents or change the number of moles of acid added
steps for titration calculation
-moles
-ratio
-question
levels of precision of different equipment
-different equipment can measure to different levels of precision
uncertainty in measurements
-there is uncertainty when you make a measurement
-the uncertainty is half the smallest possible unit you can make
equation for percentage uncertainty
uncertainty of equipment/ size of measurement
x100
how to calculate maximum percentage uncertainty in a final result
-add all the individual equipment uncertainties together
what needs to be taken into account when calculating uncertainty
-how many times you read off a piece of equipment to get an answer
how do you decrease apparatus uncertainties
-increase the size of the measurement made
what is a volumetric flask
-a type of laboratory flask calibrated to contain a precise volume at a particular temperature
what are volumetric flasks used for
-precise dilution and preparation of standard solution
equation for calculating new concentration
original volume/ new volume
x original concentration
what is an oxidation number/state
-a measure of the number of electrons that an atom uses to bond with an atom of another element
-oxidation numbers are derived from a set of rules
rule 1 for oxidation numbers
1- the oxidation number of a neutral element is zero
rule 2 for oxidation numbers
2- the oxidation number of a monatomic ion is the same as the charge on the ion
rule 3 for oxidation numbers
3- the sum of all oxidation numbers in a neutral compound is zero
rule 4 for oxidation numbers
4- the sum of all oxidation numbers in a polyatomic ion is equal to the charge on the ion
rule 5 for oxidation numbers
5- in compounds the elements of :
group 1 - have an oxidation number of +1
group 2 - have an oxidation number of +2
group 3 - have an oxidation number of +3
rule 6 for oxidation numbers
6- the oxidation number of hydrogen in a compound is usually +1
-if the hydrogen part of a binary metal hydride (compound of hydrogen and some metal), then the oxidation state of hydrogen is -1
rule 7 for oxidation numbers
7- fluorine always has an oxidation number of -1 in compounds
rule 8 for oxidation numbers
8- the oxidation number of oxygen in a compound is usually -2
-if however the oxygen is in a class of compounds called peroxides (e.g hydrogen peroxide), then the oxygen has an oxidation number of -1
-if the oxygen is bonded to fluorine, the number is +1
rule 9 for oxidation numbers
9- in transition metals the oxidation number can vary
rule 10 for oxidation numbers
10- chlorine, bromine and iodine usually have oxidation numbers of -1, except when it is in a compound with oxygen
define a redox reaction
-a reaction involving reduction and oxidation
what is oxidation
-oxidation is the process of electron loss
-it involves an increase in oxidation number
what is reduction
-reduction is the process of electron gain
-it involves a decrease in oxidation number
when do we use roman numerals as oxidation numbers
-when we write the oxidation state into the name of the compound for the element in the compound where the oxidation state may be different
how do metals generally form ions
-losing electrons
-with an increase in oxidation number
-to form positive ions
how do non metals generally form ions
-gaining electrons
-with a decrease in oxidation number
-to form negative ions
how to reduce uncertainty in a titration
-replacing measuring cylinders with pipettes or burettes which have lower apparatus uncertainty will lower the % uncertainty
how to reduce uncertainty in a titration in a burette
-make the titre a larger volume
-this could be done by:
-increasing the volume and concentration of the substance in the conical flask
-or by decreasing the concentration of the substance in the burette
how to reduce uncertainty in measuring mass
-using a balance that measures to more decimal places
-use a larger mass
-weigh by difference
