AQA A-level Chemistry 7405 - Paper 3 - 19th June 2019 unofficial mark scheme

Idk why but I remember my answer being x10-6?????

That's odd. It was definitely x10^-3.

Do you remember any of the numbers? I know pH was 1.72

isn't the % uncertainty ＝0.005/0.685 something

There was 6xx mg of NaHSO4, Could be 638 but I can't remember exactly. That was dissolved in 100cm3 of water.

ah i think i know my mistake now

Did you think it said grams? Easy mistake to make and only drops 1 mark.

Ok i just worked it out again using 685mg i think
I got a completely wrong answer - 6.36 x10-6?????
I worked out [H+] then worked out moles of nahso4 and substituted into the equation - not sure what i did tbh

S2O3 2- + 2H+  S + So2 + h20
so2 is acidic
SO2 + H2O --> H2SO3
Sodium thiosulfate and hcl, water bath, range of temperatures, conical flask on top of a black cross stopwatch, time taken for cross to disappear, rate is 1/time. Plot 1/time against temperature, exponential shape due to Arhenius equation

H2SO4  H+ + HSO4-
HSO4- <-> H+ +so4 2-
Ka is 7.21x10^-3
Increased conc of so4 2-, so equilibrium shifts left, H+ decreases and ph Increases

Salt bridge
Allows movement of ions
Platinum
C; HCL/1 moldm3
D: hydrogen gas at 100kPa
E: solution of fe2+ and fe 3+ at 1 moldm3
2Fe3+ + H2 --> 2Fe2+ + 2H+

-0.013V (gradient)
303K
-0.80V

Nucleophilic addition
Racemic mixture
Planar C=O, Equal probability of nucleophilic attack from above and below the plane, racemate formed
Draw optical isomers of 2hydroxypropanenitrile (mirror images in 3D)
C.h2s04/ 600K
3 repeating units with trailing bonds

Fe + h2so4  feso4 +h2
MnO4-(aq) + 8H+(aq) + 5Fe2+ (aq) Mn2+ (aq) + 4H2O (l) + 5Fe3+ (aq)
Colourless to pale pink/purple
22.65 cm3
1.47% as two readings

Some multiple choice
-1139
+0.78V
Benzoic acid

Don't worry about it now, chemistry is done. Was that your last exam or do you do futher maths?

Final!

also lone pairs in water don’t conduct

Bonded electrons in metals also don’t conduct electricity.

Maybe it meant the sea of delocalised electrons in the metal structure (which do conduct) as they are part of the metallic bonding of the metal

Yes it did

Ah, I see the AQA examiner is present to give a definitive answer 😂

S2O3 2- + 2H+  S + So2 + h20
so2 is acidic
SO2 + H2O --> H2SO3
Sodium thiosulfate and hcl, water bath, range of temperatures, conical flask on top of a black cross stopwatch, time taken for cross to disappear, rate is 1/time. Plot 1/time against temperature, exponential shape due to Arhenius equation

H2SO4  H+ + HSO4-
HSO4- <-> H+ +so4 2-
Ka is 7.21x10^-3
Increased conc of so4 2-, so equilibrium shifts left, H+ decreases and ph Increases

Salt bridge
Allows movement of ions
Platinum
C; HCL/1 moldm3
D: hydrogen gas at 100kPa
E: solution of fe2+ and fe 3+ at 1 moldm3
2Fe3+ + H2 --> 2Fe2+ + 2H+

-0.013V (gradient)
303K
-0.80V

Nucleophilic addition
Racemic mixture
Planar C=O, Equal probability of nucleophilic attack from above and below the plane, racemate formed
Draw optical isomers of 2hydroxypropanenitrile (mirror images in 3D)
C.h2s04/ 600K
3 repeating units with trailing bonds

Fe + h2so4  feso4 +h2
MnO4-(aq) + 8H+(aq) + 5Fe2+ (aq) Mn2+ (aq) + 4H2O (l) + 5Fe3+ (aq)
Colourless to pale pink/purple
22.65 cm3
1.47% as two readings

Some multiple choice
-1139
+0.78V
Benzoic acid