Balancing Equations: High School Chemistry | C2H6 + O2 = 2CO2 + 3H2O

[Barclay]

This is a question, not a home work.

The book I'm reading explains how to balance equations step-by step.

UNBALANCED EQUATION IS THIS: C₂H₆ + O₂ = CO₂ + H₂O

The book gets to this: C₂H₆ + O₂ = 2CO₂ + 3H₂O

The book says "There are 7 oxygen on the right-hand-side but only 2 on the left". All fine so far.

Then the book says "Oxygen's HAVE to go around in PAIRS so how can you get an odd number (7) of them on the left side".

Then the book goes on to balance the equation: 2C₂H₆ + 7O₂ = 4CO₂ + 6H₂OMy questions are this:

1. Do Oxygen's HAVE to go around in PAIRS? What if I balanced an equation (any equation) and there were say 7 oxygen on left side and 7 on the right side - would that be incorrect? Would I have to get 14 on both sides (or an even number on both sides)?

2. If the book is correct and "Oxygen's HAVE to go around in PAIRS" then what other elements HAVE to go around in pairs or triplets etc (for the purpose of balancing equations)?

[This is HIGH SCHOOL CHEMISTRY]

Thank you.

 

[mfb]

Oxygen has to be paired as gas - it is perfectly fine to have 7 oxygen atoms at both sides, as long as some odd number is bound in molecules on both sides.
Your left side just has oxygen as O₂, that cannot give an odd number.
Ozone is O₃, but you will rarely see that in those questions.

then what other elements HAVE to go around in pairs or triplets etc (for the purpose of balancing equations)?

The typical gases apart from noble gases: hydrogen, nitrogen, oxygen, all halogens, and some more exotic gases.

 

[Borek]

To make balancing easier you can use something like 3\frac 1 2 O_2 to get 7 atoms of oxygen, but after the equation is balanced you should get rid of all fractional coefficients.

In other words: during balancing feel free to use fractions if it makes balancing easier, just remove them as the final step. Properly balanced equation should use a set of the lowest integer coefficients. Doesn't matter how you get there.

 

[Barclay]

Oxygen has to be paired as gas - it is perfectly fine to have 7 oxygen atoms at both sides, as long as some odd number is bound in molecules on both sides.
Your left side just has oxygen as O₂, that cannot give an odd number.

I'm not sure what the book means by "oxygen must be paired".

Does it mean there must be an EVEN number of oxygen atoms (in total) on each side of the equation?

PAIRED means an EVEN number yet MFB you say that 7 oxygen is okay on both sides.

Edited bit later:
I've just been looking at a few balanced equations and it seems that each compound that contains oxygen (in a balanced equation) must have an EVEN number of oxygen atoms on each individual molecule. Is that right?Why must oxygen be paired?

 

[Barclay]

To make balancing easier you can use something like 3\frac 1 2 O_2 to get 7 atoms of oxygen, but after the equation is balanced you should get rid of all fractional coefficients.

Yes the next step in the book does mention 3 1/2 fraction

 

[mfb]

Does it mean there must be an EVEN number of oxygen atoms (in total) on each side of the equation?

No.
It just means you cannot have 1/2, 3/2 or similar fractions of O₂ in the final equation.

 

[Barclay]

No.
It just means you cannot have 1/2, 3/2 or similar fractions of O₂ in the final equation.

Thanks for that BUT can there be single oxygens in the balanced equation?
E.g in the balanced equation:
Can there exist say H₂O or does it have to be 2H₂O ?
Can there exist say MgO or does it have to be 2MgO ?

 

[Borek]

Can there exist say H₂O or does it have to be 2H₂O ?
Can there exist say MgO or does it have to be 2MgO ?

H₂O and MgO are perfectly OK. Pure, gaseous oxygen is made of diatomic molecules, so it is O₂. In other compounds it is no longer pure, gaseous oxygen, so it doesn't have to be in pairs.

 

[mfb]

Thanks for that BUT can there be single oxygens in the balanced equation?

Yes.
Just forget the book's comment if it causes so much confusion. There is nothing special about oxygen.

 

[Borek]

Just forget the book's comment if it causes so much confusion. There is nothing special about oxygen.

As I wrote earlier - the only way book can be confusing is when the comment about "single oxygens" is applied to every oxygen in every compound, and not only to gaseous, elemental oxygen O₂.