posted by Mary Lame on .
To balance an equation
if the equation was
CaOH2 + HCl ---> CaCl2 + H20
the question is , if i want to balance , should i count the OH ( hydroxide ) as a whole thing or separately so the O goes with the other oxygens and the H goes with the other hydrogens
can someone explain how can i balance the whole equation
The proper way to write calcium hydroxide is Ca(OH)2, not CaOH2. The O and H atoms in the OH- ionic part of the molecule remain close to one another and transfer as a pair in chemical reactions or when dissolved in solutions. The reaction you are trying to balance is a neutralization with the acid HCl. Each HCl atom gives up one H+ ion in the reaction, and it combines with one OH- from the base to form an H2O (or HOH) molecule. Ca(OH)2 contains two OH- ions, so it takes two HCl molecules to neutralize each Ca(OH)2 molecule.
Therefore the balanced reaction is in this case
Ca(OH)2 + 2 HCl = CaCl2 + 2 H2O
don't understand !
The point is that you need the parentheses to show that it is two OH- ions in Ca(OH)2
OH Ca OH
H Ca OH
Maybe it would be easier if you wrote it as two separate reactions
Ca++ + 2 Cl- ---> CaCl2
2 H+ + 2 (OH)- ---> 2 H2O
------------------------- add those
Ca++ + 2 (OH)- + 2 H+ + 2 Cl- ---> CaCl2 + 2 H2O
You would not really write it this way but perhaps it makes it easier to see how the ions combine. The calcium ions grab the chlorine ions to make the salt CaCl2 and the hydrogen and hydroxide ions are left in solution to form water.
Yet another way to look at it.
Three points here.
First, Ca has a valence of +2.
OH, as a unit, has a valnece of -1.
Therefore, it takes two OH units at -1 to balance the charge on Ca at +2. We writre the symbol for calcium hydroxide as Ca(OH)2.
Second point. If you write CaOH2, it means, by definition, that we have 1 Ca, 1 O and 2H atoms. That isn't what we want for the OH is -1 (not O and H2 a -1). If we want to show that the ENTIRE OH part is used and we want two of them, we write (OH)2. When parentheses are used, everything inside the parentheses are multiplied by 2. Thus OH2 means 1 O and 2 H atoms, but (OH)2 means 2O and 2 H atoms. By writing the symbol for calcium hydroxide as Ca(OH)2, we see that Ca at +2 and OH as -1 each gives us a total of +2 charge and that equals the -2 charge.
Third point. Writing a cryptic note that you don't understand tells us almost nothing about what your problem is. The three of us have TRIED to understand, each taking a different tack to explaining what we think you don't understand. Perhaps one of us hit the mark, perhaps none of us did. If you still don't understand, please explain in detail what your problem is and we can take it from there. Otherwise, we are stumbling in the dark as we try to guess.