Physics, Chemistry and Biology: Post your doubts here!

[EpicLol]

Please help me with these questions.. In an easy way

1. Describe bond breaking as endothermic and bond making as exothermic.
2.describe the production of heat energy by burning fuels
3.describe production of electrical energy from simple cells.

This image might help for part 3
http://images.google.com/imgres?q=e...w=114&start=0&ndsp=49&ved=1t:429,r:4,s:0,i:79

 

[Maz]

Please help me with these questions.. In an easy way

1. Describe bond breaking as endothermic and bond making as exothermic.
2.describe the production of heat energy by burning fuels
3.describe production of electrical energy from simple cells.

1. Endothermic reactions involve breaking of bonds eg.melting of ice.Exothermic reactions involve bonding eg.freezing of water to ice.
2.Heat energy is produced by burning fuels.eg burning methane.
3.A simple cell consists of 2 different metals in a solution of an electrloyte. The more reactive metal(anode) goes into the solution as ions and electrons flow through the wire to the positive electrode.

 

[thats_me]

on the same paper could u help me with 7)a and b plz? I sort of understand but Im still confused....

sure!
U knw abt the reactivity series right? K,Na,Ca,Mg,Al,Zn..... here we are dealing with Mg , Al & Zn
so 7a: metal a will be magnesium as its the fastest reaction and its the most reactive one
b: metal b will be aluminium as the graph is slow at first and thn goes fast because aluminium forms an oxide layer so the reaction goes fast after it's removed
c: is abviously Zinc least reactive among them n the only left n it reacts slowly
7b: here we'll write the equations
Zinc :2Hcl+ Zn -->2Zncl2 + H2 ( notice the ratio of hydrogen in each of these)
Magnesium: 2Hcl +Mg --> Mgcl2 +H2
Aluminim : 6HcL +Al --> Alcl3 +3H2
Now, for zinc n magnesium the ratio of H2 in the eqn is 1 where as for aluminium its 3H2
the reason is that zinc n magnesium have the same valency 2 but aluminium has a diff 3

 

[thats_me]

so if the question said pick a redox reaction, then a or c would be correct?
Thx

Yes! but that is if they donot mention abt 2 acid/base reactions

 

[gelato]

http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_s05_qp_6.pdf
3e, please

 

[Maz]

http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_s05_qp_6.pdf
3e, please

Resistance is directly proportional to length of the wire.
So, you find the resistance using R1/l1=R2/l2.

 

[thats_me]

http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_s05_qp_6.pdf
3e, please

u need to cross multiply using any value frm the table (Fig.3.1)
eg: if we take AD it's lenght is 100 and resistance is 5.20
so cross multiply it i.e if 100is 5.20 ,thn 150 is how much? (150*5.20)/100=7.8 ohms

 

[Yousif Mukkhtar]

Guys when we add potassium manganate (vii) to a solution, they sometimes ask what is observed?
Can we just say the color changes from purple to colorless?

 

[Maz]

Guys when we add potassium manganate (vii) to a solution, they sometimes ask what is observed?
Can we just say the color changes from purple to colorless?

Depends on the marks, i suppose.
I don't know any other observation.

 

[Yousif Mukkhtar]

Depends on the marks, i suppose.
I don't know any other observation.

Sometimes in the mark scheme it says the solution changes color. So should we include the word solution?

 

[sma786]

by showing that they shared 2 pair of electrons

thanks al0t

 

[sma786]

Here i drew this up for u .
Notice that Chlorine still has its 7 electrons, oxygen has its 6 electrons, carbon still has its 4 electrons. that you must show.
Also that each Atoms has a total of 8 electrons (stable) = as some of its own + the ones shared.
Double bond : Pair of electrons shared (oxygen shares 2 electrons with carbon, carbon shares 2 electrons with oxygen.)

great diagram, thanks

 

[Yousif Mukkhtar]

I got it now The question said aqueous.

 

[tribianni]

http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Chemistry (0620)/0620_w10_qp_31.pdf Q8b ASAP

 

[Stormcry72]

http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_s05_qp_6.pdf
3e, please

you either add the resistance of the 1m and 0.5m length wires or u multiply the resistance of the 0.5m wire by 3

 

[za Shadow]

http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_qp_33.pdf
http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_ms_33.pdf
Q 5 b ii) plzzz help why all 0.2

http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_qp_32.pdf
http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_ms_32.pdf
1 d) why F why not C ????????

HELLLLLLPPPPPPPPPPP MMMMMEEEE PPLZZZZ

 

[Stormcry72]

http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_qp_33.pdf
http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_ms_33.pdf
Q 5 b ii) plzzz help why all 0.2

http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_qp_32.pdf
http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_ms_32.pdf
1 d) why F why not C ????????

HELLLLLLPPPPPPPPPPP MMMMMEEEE PPLZZZZ

y u want help?? isn't paper 3 already over??

 

[Manobilly]

http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_qp_33.pdf
http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_ms_33.pdf
Q 5 b ii) plzzz help why all 0.2

http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_qp_32.pdf
http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_ms_32.pdf
1 d) why F why not C ????????

HELLLLLLPPPPPPPPPPP MMMMMEEEE PPLZZZZ

Paper 3 is over.

 

[thats_me]

Paper 3 is over.

o/n 10 61 physics q3bii how is it done? :S
http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_qp_61.pdf

 

[Manobilly]

o/n 10 61 physics q3bii how is it done? :S
http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_qp_61.pdf

Which question?