Unit 4 Questions MEGATHREAD!

[homosapiens]

More calorimetry questions:
1. What would be the effect on the calibration factor obtained from an experiment if you use 50 mL of water instead of 100 mL of water to calibrate the calorimeter?
Please correct if I'm wrong, but since the temperature will increase by double, the CF will be half of what it would be if 100mL was used?

2. What energy transformations occur when the calorimeter and its contents are being heated?
Is this just electrical energy being converted into heat energy?

3. If a calorimeter is calibrated using 100mL water, does it matter if I'm actually experimenting with another solution of the same volume?

[Graphite]

1. Sounds good
2. Should be a chemical transformation
3. I'm not sure what you mean, are you asking wheher it is required to experiment with the same volume as the calibration volume? If so, yes you do to be accurate

[Mao]

2. Yes, if you are using the heating coil to calibrate it. Otherwise in a chemical reaction it's typically chemical --> thermal.

3. Theoretically, it shouldn't matter. If both are aqueous solutions with the same volume, you should have the same CF.
In reality though, dissolved species do change the specific heat capacity of the solution by a small amount, so the CF do change from solution to solution. And this is also why calibration should be done AFTER the reaction has taken place, without changing the solution.

[nacho]

in a redox reaction between SHE and a nickel
over time, the reading of the voltmeter will seem to decrease,
why is this so?
 

[Mr. Study]

Can someone explain why this is the case:

Q9.
Use the electrochemical series to determine which one of the following would not be
expected to occur to an appreciable extent.
A 2H+(aq) + Fe(s) ? Fe2+(aq) + H2(g)
B 2Ag+(aq) + Ni(s) ? 2Ag(s) + Ni2+(aq)
C Br2(aq) + 2Fe2+(aq) ? 2Br–(aq) + 2Fe3+(aq)
D 2I–(aq) + Pb2+(aq) ? I2(s) + Pb(s)
A9.
D. A reaction will only occur if the oxidant of the conjugate redox pair with the higher
E° is added to the reductant of the conjugate redox pair having the lower E°. In D, a
reaction will not occur because the E° of the half reaction involving the reductant I– is
higher than the E° of the half reaction involving the oxidant, Pb2+.

Thank you very much.

[Vincezor]

Can someone explain why this is the case:

Q9.
Use the electrochemical series to determine which one of the following would not be
expected to occur to an appreciable extent.
A 2H+(aq) + Fe(s) ? Fe2+(aq) + H2(g)
B 2Ag+(aq) + Ni(s) ? 2Ag(s) + Ni2+(aq)
C Br2(aq) + 2Fe2+(aq) ? 2Br–(aq) + 2Fe3+(aq)
D 2I–(aq) + Pb2+(aq) ? I2(s) + Pb(s)

If you look at the electrochemical series, the reaction of D appears in this order (from top to bottom)




As is a stronger oxidant than and is a stronger reducdant that , the reaction mention above is unlikely to occur as you are reacting a weaker oxidant with a weaker reductant and that is less likely to happen.

[meli001]

Pg 409 Q5 of the Heinemann textbook

Q5.
A calorimeter containing 100 mL of water is calibrated by passing a 3.00 A current through the instrument for 36.0 s at a potential difference of 3.50 V. The temperature rises by 0.82°C.
Potassium hydroxide weighing 0.654 g is added to the calorimeter and dissolved rapidly by stirring. The temperature rises from 20.82°C to 22.23°C.
b   Determine the enthalpy change for the equation:

KOH(s) -> KOH(aq)

Ans:

Step 1   Calculate the energy released by the reaction.
      E   = CF ´ DT
         = 461 J °C–1 ´ (22.23 – 20.82)°C
         = (461 ´ 1.41) J
         = 650 J
   Step 2   Calculate the amount of KOH, using n =  .
      n(KOH)   = 
         = 0.01166 mol
   Step 3   Calculate DH for the equation by using stoichiometry.
      0.01166 mol released 650 kJ
      1 mol will release 
      So delta H= –5575 kJ mol–1
            = –55.8 MJ mol–1

Why did it jump from 650 J to 650 kJ?

[Mr. Study]

Can someone explain why this is the case:

Q9.
Use the electrochemical series to determine which one of the following would not be
expected to occur to an appreciable extent.
A 2H+(aq) + Fe(s) ? Fe2+(aq) + H2(g)
B 2Ag+(aq) + Ni(s) ? 2Ag(s) + Ni2+(aq)
C Br2(aq) + 2Fe2+(aq) ? 2Br–(aq) + 2Fe3+(aq)
D 2I–(aq) + Pb2+(aq) ? I2(s) + Pb(s)

If you look at the electrochemical series, the reaction of D appears in this order (from top to bottom)




As is a stronger oxidant than and is a stronger reducdant that , the reaction mention above is unlikely to occur as you are reacting a weaker oxidant with a weaker reductant and that is less likely to happen.

I love you! That makes so much sense now!

Thank you so much!

[Vincezor]

Pg 409 Q5 of the Heinemann textbook

Q5.
A calorimeter containing 100 mL of water is calibrated by passing a 3.00 A current through the instrument for 36.0 s at a potential difference of 3.50 V. The temperature rises by 0.82°C.
Potassium hydroxide weighing 0.654 g is added to the calorimeter and dissolved rapidly by stirring. The temperature rises from 20.82°C to 22.23°C.
b   Determine the enthalpy change for the equation:

KOH(s) -> KOH(aq)

}











(-ve as temp increased)

Yeah I know all the above is not needed, but I guess maybe it might help someone. In short, the answer obviously made a mistake and didn't convert to kJ instantly (I like to do this specifically for this  reason - to prevent mistakes like the answer in the worked solutions)

[generalkorn12]

Would anyone be able to explain what solutions are requireed in Galvanic and Electrolytic Cells? I'm having trouble understanding, if you're mean't to have a solution of the elements that are being oxidised and reduced.

And, for Electrolytic Cells, how do you label the anode and cathode ends such as being a Tin Cathode, would that be part of the question? Or is it always going to be Tin?

Sorry for the trouble ;S

[Vincezor]

I have found a question (found in TSFX 2010) that I would like an explanation to:

An increase in pressure (by decreasing the volume) does not increase the reaction rate for the following reaction type:



Explain why this is the case.

Yeah I know it sounds somewhat silly, but I would love to know!

[Mao]

[Assuming that there is no back reaction]

The forward reaction is a decomposition reaction. It does not depend on collision rate, as no collision is required for the reaction to proceed. Instead, it depends on fluctuations of internal energy of AB, i.e. when AB (somehow) gains too much internal energy, it will shake itself apart. Thus, the reaction rates do not follow the usual collision theory.

Increasing the pressure increases the rate of collision, but this doesn't affect the rate of decomposition. Increasing the pressure doesn't necessarily raise internal energy (and so does not necessarily increasethe rate of decomposition).

However, raising the temperature will have a profound impact.

Again, this is all assuming we are only assessing the forward case. The backwards reaction obviously is affected by pressure changes.

[Mr. Study]

I apologize if this is a noob question but I would love clarification.

1.Is a monoprotic acid, an acid that contains 1 Hydrogen. Example HCl.
   Also, a diprotic acid is an acid that contains 2 Hydrogen. Example H2SO4

2. Also, What's CH3COOH? (I know its Ethanoic Acid but How much protons does it donate?)

Thank you very much!

[Greatness]

A monoprotic acid is one that can donate 1 proton, a diprotic is one that can donate 2 protons.
CH3COOH is monoprotic, it donates 1 proton and becomes CH3COO-

[Mr. Study]

Haha! Thank god I was right!! Thanks!

Just this one for today.

What do Ka and Kw actually do? (Does it indicate the strength of an acid or the mole? :S) I know that it's the Acidity Constant and Self Ionisation of Water.