year 1 a level chem: acids + redox

what is the formula for nitric acid?

HNO₃

what is the formula for ethanoic acid?

CH₃COOH

what is the formula for ammonia?

NH₃

what is the formula for ammonium?

NH₄⁺

(ammonia gains a positive hydrogen ion)

what is reduction?

• gain of electrons

• loss of oxygen

• gain of hydrogen

• decrease in oxidation state/number

what is oxidation?

• loss of electrons

• gain of oxygen

• loss of hydrogen

• increase in oxidation state/number

what are oxidation state/numbers?

the theoretical charge an atom would have if it was an ion

what are the rules for oxidation state/numbers?

• Monoatomic ions have the same oxidation state as their charge

• Elements have an oxidation state of 0

• In compounds, the sum of the individual atoms’ oxidation states is 0

• In compound ions (i.e. compounds with a charge), the sum of the oxidation states is the charge of the ion

• In compounds/ions, the oxidation states of these elements are: F = -1, O = -2 (EXCEPT for peroxides like H₂O₂, where O is -1), H = +1 (EXCEPT for metal hydrides, where H = -1), Cl = -1 (EXCEPT when with O or F)

• In a compound/ion, the most electronegative element is given the negative oxidation state

what is neutralisation?

a reaction between an acid and a base to form a salt and water (the OH^- from the base and the H⁺ from the acid join to form H₂O)

examples of bases

• metal carbonates

• metal hydroxides

• metal oxides

• ammonia

how to form an ionic equation

• write out the balanced chemical equation, with state symbols

• split up the aqueous ions into their individual ions

• add the charges

• remove the spectator ions (the ones that don’t change charge)

how do you prepare a standard solution of a known concentration?

1. use a mass balance to measure the required mass of the solid you want to dissolve

2. pour the solid into a beaker, using distilled water to rinse the weighing boat, and add a small volume of distilled water before stirring with a glass rod to dissolve the solute

3. pour the solution into a volumetric flask using a funnel and wash the beaker and stirring rod into it with distilled water

4. add more distilled water until the bottom of the meniscus reaches the scratch mark

5. stopper the flask and invert it to mix

how do you carry out a titration?

1. obtain a known concentration of an acid/alkali

2. wash a volumetric pipette with distilled water then a bit of the standard solution

3. measure 25cm³ of this solution using the volumetric pipette and pipette it into a conical flask

4. add 2-3 drops of indicator

5. wash the burette with distilled water then the acid or alkali of a known concentration you are filling it with (make sure the jet space is still filled afterwards)

6. fill up the burette with the acid/alkali required using a funnel until the bottom of the meniscus reaches the 0cm³ mark

7. remove the funnel to avoid random errors caused by the solution dripping from the funnel into the burette

8. put the conical flask over a white tile under the burette, then open the tap while swirling the flask

9. as soon as the colour changes irreversibly, close the tap and read off the volume on the burette from the bottom of the meniscus while the burette is on a level surface

10. now that you know the approximate volume at which the reaction ends, repeat and add dropwise when you near the volume from the rough titre

11. repeat until you receive concordant results (results which are within 0.1cm³ of each other) and find a mean titre

how do you carry out a back titration to find out % purity of an alkali?

1. make up a standard solution of an alkali and add a known excess of acid to react with it

2. wash a volumetric pipette with distilled water then a bit of the standard solution

3. measure 25cm³ of this solution using the volumetric pipette and pipette it into a conical flask

4. add 2-3 drops of indicator

5. wash the burette with distilled water then the alkali of a known concentration you are filling it with (make sure the jet space is still filled afterwards)

6. fill up the burette with the alkali required using a funnel until the bottom of the meniscus reaches the 0cm³ mark

7. remove the funnel to avoid random errors caused by the solution dripping from the funnel into the burette

8. put the conical flask over a white tile under the burette, then open the tap while swirling the flask

9. as soon as the colour changes irreversibly, close the tap and read off the volume on the burette from the bottom of the meniscus while the burette is on a level surface

10. now that you know the approximate volume at which the reaction ends, repeat and add dropwise when you near the volume from the rough titre

11. repeat until you receive concordant results (results which are within 0.1cm³ of each other) and find a mean titre

how do you find the % purity of the alkaline solution in a back titration?

1. find the volume and so the moles of the solution in the burette that you used in the titration

2. write out a balanced chemical equation for the titration to find out the moles of the solution in the conical flask that reacted with the solution in the burette. this is the number of moles of acid that did not react with alkali

3. subtract the number of moles of unreacted acid from the number of moles of known excess of the acid to find the number of moles of acid that did react with the alkali

4. write out a balanced chemical equation for the reaction at the beginning

5. use this to find the number of moles and so the mass of the alkali

6. % purity = (mass of pure substance) / (mass of impure substance) x 100%