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### Topic: difference among coefficient and subscripts in chemical equation  (Read 6809 times)

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#### cube

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##### difference among coefficient and subscripts in chemical equation
« on: November 05, 2008, 10:38:58 AM »
Hey all,

For example: 2Al + 3S --> Al2S3

Where is the difference? I thought, 2Al means that it reacts in a ratio of 2Al atoms. But doesn't mean Al2 the same?
I'd really like to unterstand it, but at the moment it is not clear to me.

Thanks a lot!
cube

#### P

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##### Re: difference among coefficient and subscripts in chemical equation
« Reply #1 on: November 05, 2008, 11:07:21 AM »
Take for example 2H2 + O2 -> 2H2O  right?

the prefix denotes the number of free atoms or molecules of substance needed to balance the equation  -  i.e. it is an amount.  The subscript denotes the number of atoms in that particular molecule.

with the hydrogen + oxygen ->  water  example I wrote above, the 2H2 means there are 2 x Hydrogen molecules which have the configuration of H-H  (the subscript denotes that hydrogen is in molecular form - which has 2 H atoms).

SO: in your example we have 2 Al molecules + 3 Sulphur molecules reacting to give 1 x Al2S3 molecule.   You see? The subscripts are denoting that the Al and S form discrete molecular units which all have 2 Al atoms and 3 S atoms in them. And - you need 2 Al atoms and 3 S atoms to make it.

Hope that is clear and that it helps -

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#### cube

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##### Re: difference among coefficient and subscripts in chemical equation
« Reply #2 on: November 05, 2008, 01:04:55 PM »
Take for example 2H2 + O2 -> 2H2O  right?

I know that it's right, because I kept this fact in mind. But to be honest, I not shure about the way. What is with the sec oxygen on the right side of the equation? It seems to me as one oxygen is disappeared compared with the left side?!?

the prefix denotes the number of free atoms or molecules of substance needed to balance the equation  -  i.e. it is an amount.  The subscript denotes the number of atoms in that particular molecule.

with the hydrogen + oxygen ->  water  example I wrote above, the 2H2 means there are 2 x Hydrogen molecules which have the configuration of H-H  (the subscript denotes that hydrogen is in molecular form - which has 2 H atoms).

Clear.

SO: in your example we have 2 Al molecules + 3 Sulphur molecules reacting to give 1 x Al2S3 molecule. You see? The subscripts are denoting that the Al and S form discrete molecular units which all have 2 Al atoms and 3 S atoms in them. And - you need 2 Al atoms and 3 S atoms to make it.

What means that - discrete molecular units?

Hope that is clear and that it helps -
Yap, better than before! Thank you very much!!
And sorry for my so-so english, I am not from an english speaking country only studying abroad.

Greetings
Jan

#### Borek

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##### Re: difference among coefficient and subscripts in chemical equation
« Reply #3 on: November 05, 2008, 03:34:20 PM »
Take for example 2H2 + O2 -> 2H2O  right?

I know that it's right, because I kept this fact in mind. But to be honest, I not shure about the way. What is with the sec oxygen on the right side of the equation? It seems to me as one oxygen is disappeared compared with the left side?!?

No, coefficient before molecule means - take everything to the right several times. So 2H2O means "two molecules of water", something like 2(H2O).
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#### cube

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##### Re: difference among coefficient and subscripts in chemical equation
« Reply #4 on: November 05, 2008, 05:11:11 PM »
Take for example 2H2 + O2 -> 2H2O  right?

I know that it's right, because I kept this fact in mind. But to be honest, I not shure about the way. What is with the sec oxygen on the right side of the equation? It seems to me as one oxygen is disappeared compared with the left side?!?

No, coefficient before molecule means - take everything to the right several times. So 2H2O means "two molecules of water", something like 2(H2O).

Okay, maybe I didnt understand you. But why is on the left side O2 and on the right side only O ?

#### 7453K

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##### Re: difference among coefficient and subscripts in chemical equation
« Reply #5 on: November 05, 2008, 05:37:07 PM »
because on the right side there are TWO water molecules, which each need one oxygen, so the beginning you need 2 oxygens.

#### JGK

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##### Re: difference among coefficient and subscripts in chemical equation
« Reply #6 on: November 05, 2008, 05:47:38 PM »
Take for example 2H2 + O2 -> 2H2O  right?

I know that it's right, because I kept this fact in mind. But to be honest, I not shure about the way. What is with the sec oxygen on the right side of the equation? It seems to me as one oxygen is disappeared compared with the left side?!?

No, coefficient before molecule means - take everything to the right several times. So 2H2O means "two molecules of water", something like 2(H2O).

Okay, maybe I didnt understand you. But why is on the left side O2 and on the right side only O ?

Because:
H2 is the molecular term for gaseous Hydrogen
O2 is the molecular term for gaseous Oxygen
H2O is the molecular term for water
and
H2O2 is the molecular term for hydrogen peroxide

as water contains only one molecule of oxygen it is written as H2O and not  the more complicated H21/2(O2)

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#### Borek

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##### Re: difference among coefficient and subscripts in chemical equation
« Reply #7 on: November 06, 2008, 03:00:14 AM »
water contains only one molecule of oxygen

MOLECULE of water contains only one ATOM of oxygen.

I think P did similar mistake earlier.
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#### JGK

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##### Re: difference among coefficient and subscripts in chemical equation
« Reply #8 on: November 06, 2008, 08:09:31 AM »
water contains only one molecule of oxygen

MOLECULE of water contains only one ATOM of oxygen.

I think P did similar mistake earlier.

OOPs!  sorry
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