A quicker way to balance an equation? Watch

coconut64
Badges: 16
Rep:
?
#1
Report Thread starter 3 years ago
#1
Can anyone tell me a quick way to balance this equation. Some people say that you should start with the non mental first and leave H and O at last but any suggestions?? What is the quickest way to balance an equation like this: P4+H2O ---> PH3 +H3PO4 than you.
0
reply
Pronged Lily
Badges: 3
Rep:
?
#2
Report 3 years ago
#2
Leave the elements til last if there are any (like the P4 in your equation above).
Do the O's first because they only appear in one compound on each side so you can just adjust the ratios. Then fix the H ratios by changing the PH3. Since you've left the element til last it'll now be easy to change the P4 to whatever it needs to be. I also find it's often easier also to just leave things in fractions and then multiply out the fractions at the end:

P4+H2O ---> PH3 +H3PO4 ----------------- Unbalanced equation
P4+4H2O ---> PH3 +H3PO4 --------------- change the O ratios (1 to 4)
P4+4H2O ---> (5/3)PH3 +H3PO4 -------- 8 H on LHS so there should be 8 Hs on the RHS
(2/3)P4+4H2O ---> (5/3)PH3 +H3PO4 --- 8/3 Ps on RHS, so should be 8/3 Ps on LHS

Both sides are now equal, so multiply both sides through by 3 to remove fractions:

2P4+12H2O ---> 5PH3 +3H3PO4

and done
0
reply
charco
  • Community Assistant
Badges: 17
Rep:
?
#3
Report 3 years ago
#3
(Original post by coconut64)
Can anyone tell me a quick way to balance this equation. Some people say that you should start with the non mental first and leave H and O at last but any suggestions?? What is the quickest way to balance an equation like this: P4+H2O ---> PH3 +H3PO4 than you.
This is a redox reaction (actually disproportionation)

Write down each half equation using oxidation numbers to decide how many electrons are transferred.

equation 1 ===> P + 3H+ +3e --> PH3
equation 2 ===> P + 4H2O --> H3PO4 + 5H+ + 5e

now equalise electrons

from equation 1 ===> 5P + 15H+ + 15e --> 5PH3
from equation 2 ===> 3P + 12H2O --> 3H3PO4 + 15H+ + 15e
-------------------------------------------------------------------- add together
8P + 12H2O --> 5PH3 + 3H3PO4

put the phosphorus into molecules

2P4 + 12H2O --> 5PH3 + 3H3PO4
0
reply
coconut64
Badges: 16
Rep:
?
#4
Report Thread starter 3 years ago
#4
(Original post by charco)
This is a redox reaction (actually disproportionation)

Write down each half equation using oxidation numbers to decide how many electrons are transferred.

equation 1 ===> P + 3H+ +3e --> PH3
equation 2 ===> P + 4H2O --> H3PO4 + 5H+ + 5e

now equalise electrons

from equation 1 ===> 5P + 15H+ + 15e --> 5PH3
from equation 2 ===> 3P + 12H2O --> 3H3PO4 + 15H+ + 15e
-------------------------------------------------------------------- add together
8P + 12H2O --> 5PH3 + 3H3PO4

put the phosphorus into molecules

2P4 + 12H2O --> 5PH3 + 3H3PO4
I haven't learnt how to balance the equation using oxidation number yet but is there a simple way to work this out? Thanks
0
reply
coconut64
Badges: 16
Rep:
?
#5
Report Thread starter 3 years ago
#5
(Original post by Pronged Lily)
Leave the elements til last if there are any (like the P4 in your equation above).
Do the O's first because they only appear in one compound on each side so you can just adjust the ratios. Then fix the H ratios by changing the PH3. Since you've left the element til last it'll now be easy to change the P4 to whatever it needs to be. I also find it's often easier also to just leave things in fractions and then multiply out the fractions at the end:

P4+H2O ---> PH3 +H3PO4 ----------------- Unbalanced equation
P4+4H2O ---> PH3 +H3PO4 --------------- change the O ratios (1 to 4)
P4+4H2O ---> (5/3)PH3 +H3PO4 -------- 8 H on LHS so there should be 8 Hs on the RHS
(2/3)P4+4H2O ---> (5/3)PH3 +H3PO4 --- 8/3 Ps on RHS, so should be 8/3 Ps on LHS

Both sides are now equal, so multiply both sides through by 3 to remove fractions:

2P4+12H2O ---> 5PH3 +3H3PO4

and done
So I should always start with element that only comes up once on both sides? Also, i never tried using fractions to balance an equation, could you give a summary of how to balance an equation (any equation, not just this one). Thanks for the help, this is actually really helpful!
0
reply
charco
  • Community Assistant
Badges: 17
Rep:
?
#6
Report 3 years ago
#6
(Original post by coconut64)
I haven't learnt how to balance the equation using oxidation number yet but is there a simple way to work this out? Thanks
no, this is the correct method and can always be used for reactions involving redox regardless of complication.
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • Birkbeck, University of London
    Undergraduate Open Day - Startford Undergraduate
    Thu, 21 Mar '19
  • University of Wolverhampton
    Postgraduate Open Evening Postgraduate
    Thu, 21 Mar '19
  • Edge Hill University
    Undergraduate and Postgraduate - Campus Tour Undergraduate
    Fri, 22 Mar '19

Where do you need more help?

Which Uni should I go to? (70)
15.95%
How successful will I become if I take my planned subjects? (44)
10.02%
How happy will I be if I take this career? (82)
18.68%
How do I achieve my dream Uni placement? (61)
13.9%
What should I study to achieve my dream career? (47)
10.71%
How can I be the best version of myself? (135)
30.75%

Watched Threads

View All