Biology; Chemistry; Physics: Post your doubts here!

[Nabeel.Ahmad]

Here is the Question copied from the QP: 5054_s11_qp_22 (May/June 2011 , 5054/22 Question No 2,)

Fig. 2.1 illustrates the journey of a cyclist from point A to point B. Points A and B are at the same
height.

The cyclist starts from rest at A and pedals up and over a hill. Near the bottom of the hill, she
starts to brake and comes to rest at B.
(a) Describe the energy changes that take place as she pedals up the hill at constant speed.

Here is the answer from the Mark Scheme:

2 (a) chemical (potential) energy at start B1

gravitational/potential energy increases B1

thermal energy/heat/internal energy produced B1

So should we say that there is no K.E involved; But Chemical Potential Energy is being converted into G.P.E and Thermal Energy.

Oh, so whenever humans or vehicles running on fuel are concerned, Chemical Potential Energy is converted into G.P.E.
But when we throw an object in the air, then Kinetic Energy in that object gets converted to G.P.E.

 

[irfan.kheerdali]

how the examiner counts the papers of physics for grading.

 

[Sahana :)]

As far as I know there is no such reactivity series. You can only remember the anions in order of increasing ease of their discharge at the anode during electrolysis namely, Sulfate, Nitrate, Chloride, Bromide, Iodide, Hydroxide. If I can find any mnemonic or memory aid I'll let u know later.

Thanks!

 

[hafsa saleem]

mnemonic for learning the reactivity series (its interesting though at first it sounds a bit absurd)for cations:ka na ca mug aal zan fe pb huccu hug ag pt
for anions:sonu kal br aai oh

 

[asadalam]

mnemonic for learning the reactivity series (its interesting though at first it sounds a bit absurd)for cations:ka na ca mug aal zan fe pb huccu hug ag pt
for anions:sonu kal br aai oh

 

[Honey Bee Bentley]

Th

A bistable circuit is a sort of electronic switch: it may use fancy logic, but ultimately it is just a switch that has two 'states', like on and off, or bright and dim. And there is a way that the circuit can be set to one 'state' or the other. So maybe when it is dark in the room, the switch is set make the television screen dimmer, and when it is light, the screen gets brighter. Or a night-light comes on when it is dark in the room. Some condition has to change to make anything happen. Maybe a person throws a switch. A light-switch is a simple bistable circuit.

An astable circuit is one that is going from one state to the other, back and forth, constantly. Lets say it is going on and off, and on and off. it does not need to be turned on and of. That is part of the circuit itself.

That does not mean that the length of the 'on' is the same as the length of the 'off', but it does mean that it never stops switching back and forth. A flashing light is like that. And the 'clock' that makes your computer do 3,000,000,000 logic steps a second (3 gigaherz) is that kind of circuit.

Short: Astable : No stable state...go on oscillating
Bi-stable: has two stable states....need trigger to go from one stable state to another.

thanx u

 

[tdk]

mnemonic for learning the reactivity series (its interesting though at first it sounds a bit absurd)for cations:ka na ca mug aal zan fe pb huccu hug ag pt
for anions:sonu kal br aai oh

Could u plz explain your mnemonic: ka na ca mug aal zan fe pb huccu hug ag pt
& sonu kal br aai oh

 

[Knightmare11]

Could u plz explain your mnemonic: ka na ca mug aal zan fe pb huccu hug ag pt
& sonu kal br aai oh

Cations: Ka (K), Na, Ca, muG(Mg), Aal(Al), Zan(Zn), Fe, Pb , Huccu(H, Cu) , hug(Hg), ag, pt
Potassium (K)
Sodium (Natrium in latin - Na)
Calcium (Ca)
Magnesium (Mg)
Aluminium (Al)
-----------------------------------
Carbon here
-----------------------------------
Zinc (Zn)
Iron (Ferrum in latin - Fe))
Lead (Plumbum- Pb)
Hydrogen (H)
Copper (Cu)
Mercury (Hydragyrum - Hg)
Silver (Argentum-Ag)
Platinum (Pt)




Anions:

Fluoride
Sulphate --------
Carbonate - Never Discharged!
Nitrate ----------

Chloride
Bromide
Iodide
Hydroxyl (OH)

Mnemonic: (Mine ) Foscan clobro -ioxyl

 

[Nabeel.Ahmad]

Cations: Ka (K), Na, Ca, muG(Mg), Aal(Al), Zan(Zn), Fe, Pb , Huccu(H, Cu) , hug(Hg), ag, pt
Potassium (K)
Sodium (Natrium in latin - Na)
Calcium (Ca)
Magnesium (Mg)
Aluminium (Al)
-----------------------------------
Carbon here
-----------------------------------
Zinc (Zn)
Iron (Ferrum in latin - Fe))
Lead (Plumbum- Pb)
Hydrogen (H)
Copper (Cu)
Mercury (Hydragyrum - Hg)
Silver (Argentum-Ag)
Platinum (Pt)




Anions:

Fluoride
Sulphate --------
Carbonate - Never Discharged!
Nitrate ----------

Chloride
Bromide
Iodide
Hydroxyl (OH)

Mnemonic: (Mine ) Foscan clobro -ioxyl

So Fluoride ion is least reactive? Even though Fluorine is the most reactive halogen.

What about Sulphate and Nitrate? Are they discharged?

 

[asadalam]

So Fluoride ion is least reactive? Even though Fluorine is the most reactive halogen.

What about Sulphate and Nitrate? Are they discharged?

Well since more reactive elements form the most stable compounds they may get difficultly discharged,just my 2 cents.But since in aqueous solutions OH ions always get discharged first.i dont see the point of memorizing the anion reactivity series for electrolysis at least,and i havent seen an MCQ or Q relating to it yet...

 

[Nabeel.Ahmad]

Well since more reactive elements form the most stable compounds they may get difficultly discharged,just my 2 cents.But since in aqueous solutions OH ions always get discharged first.i dont see the point of memorizing the anion reactivity series for electrolysis at least,and i havent seen an MCQ or Q relating to it yet...

No, OH ions are not ALWAYS discharged in aqueous solutions.
They are ONLY discharged if the solution is dilute.

 

[asadalam]

No, OH ions are not ALWAYS discharged in aqueous solutions.
They are ONLY discharged if the solution is dilute.

Well MOSTLY anyway,

 

[Knightmare11]

So Fluoride ion is least reactive? Even though Fluorine is the most reactive halogen.

What about Sulphate and Nitrate? Are they discharged?

Fluoride is at the top Reactivity is going down here lol,

As for Sulphate and Nitrate, I tried putting a box saying it's never discharged lol- but no, they are not discharged!

 

[irfan.kheerdali]

help plzzz papers.xtremepapers.com/CIE/Cambridge International O Level/Chemistry (5070)/5070_s12_qp_42.pdf qu 3(b)(iii). explanations

 

[Mubasher96]

Can anyone please tell me the colors of some common metal oxides? Need it urgent. My paper's on Tuesday.

 

[Mubasher96]

help plzzz papers.xtremepapers.com/CIE/Cambridge International O Level/Chemistry (5070)/5070_s12_qp_42.pdf qu 3(b)(iii). explanations

In parts (i) and (ii) you have already calculated the number of moles of the reagents the student used. From the equation you can see that 1 mole of barium nitrate reacts with one mole of sodium sulfate to form 1 mole of barium sulfate and two moles of NaNO3. This means that the reagents the student used had barium nitrate as limiting reactant. So the number of moles of barium sulphate produced would be equal to the number of moles of barium nitrate used. Therefore the answer is 0.075

 

[Knightmare11]

help plzzz papers.xtremepapers.com/CIE/Cambridge International O Level/Chemistry (5070)/5070_s12_qp_42.pdf qu 3(b)(iii). explanations

Calculate the number of moles of sodium sulfate in 100 cm3 of 1.00 mol / dm3
solution.

1 dm3 = 1000 cm3

Therefore, 1000cm3 contains 1.oo mole Na2SO4
1000cm3-->1 mol
100cm3---> (1/1000) * 100 = 0.1 mole

That's all!

 

[Mubasher96]

Calculate the number of moles of sodium sulfate in 100 cm3 of 1.00 mol / dm3
solution.

1 dm3 = 1000 cm3

Therefore, 1000cm3 contains 1.oo mole Na2SO4
1000cm3-->1 mol
100cm3---> (1/1000) * 100 = 0.1 mole

That's all!

You answered for part (ii) but he asked for part (iii)

 

[Knightmare11]

You answered for part (ii) but he asked for part (iii)

Colors of metal oxide?

1st : Group 1-3 are white
2nd: Transition are coloured

CuO - Black
ZnO - yellow whn hot, white when cold
Fe(2)O3 -- Black
Fe(3)O2 --- Red
PbO - Yellow when cold, Orange when hot
CuO2 - Red

 

[Mubasher96]

Colors of metal oxide?

1st : Group 1-3 are white
2nd: Transition are coloured

CuO - Black
ZnO - yellow whn hot, white when cold
Fe(2)O3 -- Black
Fe(3)O2 --- Red
PbO - Yellow when cold, Orange when hot
CuO2 - Red

Thanks alot