Why can't we change subscripts when we balance chemical equations? I don't understand why we can change coefficients but not subscripts. Thanks.
There is a VERY good reason. BECAUSE NaCl is sodium chloride. Not NaCl2, Na2Cl, NaCl3 or any other arrangement. NaCl is the ONLY one that is a 1:1 ratio. Balancing equations would be SO MUCH easier if we could change subscripts BUT then you have changed the reactant or the product and only NaCl is NaCl.
NaCl + Pb(NO3)2 ==> PbCl2 + NaNO3.
So what you want to do is to rewrite this as
NaCl + PbNO3 ==> PbCl + NaNO3.
Easy, huh? BUT PbNO3 is NOT lead nitrate and PbCl is NOT lead chloride. In fact, PbNO3 and PbCl do not exist.
Regarding changing coefficients, we can change coefficients because we are simply saying we have twice as many or three times as many or any times as many; for example, 2NaCl instead of 1NaCl or we have 3AgCl instead of 1AgCl. Look at it this way. We go into a grocery store to buy jelly + bread + banana. Now we can buy 2 jars of jelly, 3 loaves of bread and 5 bananas and we haven't changed what we went to the store to purchase. BUT, if instead of this I purchase 1 lb of hamburger, a loaf of bread and a banana, I have changed the shopping list and my family will be mad at me, especially if they wanted a jelly/bread/banana sandwich. They aren't likely to like a hamburger, bread, and banana sandwich. In the first case (changing coefficients), my family can make more jelly/bread/banana sandwiches but if I change the shopping list they can't make any sandwiches.
When balancing chemical equations, we balance the number of atoms on both sides of the equation by adjusting the coefficients (the numbers in front of the chemical formulas). The key concept here is that the subscripts in a chemical formula represent the number of atoms of each element in a molecule or compound.
Let's consider an example to illustrate why changing subscripts is not permitted when balancing chemical equations. Suppose we have the unbalanced equation:
H2 + O2 -> H2O
On the left side, we have two hydrogen atoms (H2) and two oxygen atoms (O2). On the right side, we have two hydrogen atoms (H2) and only one oxygen atom (O). To balance the equation, we need to make sure that the number of atoms of each element is the same on both sides.
If we were allowed to change subscripts, we might be tempted to balance this equation by changing the subscript on the right side from 1 to 2, like this:
H2 + O2 -> H2O2
Now, both sides have two oxygen atoms, but we have introduced a new compound, hydrogen peroxide (H2O2), which was not part of the original equation.
The reason we cannot change subscripts is because they represent the chemical makeup of a substance. Changing the subscripts would alter the identity of the compound, leading to a different chemical reaction altogether.
To balance the equation H2 + O2 -> H2O, we must only adjust the coefficients. In this case, the balanced equation is:
2H2 + O2 -> 2H2O
By doubling the coefficients, we ensure that there are now four hydrogen atoms and two oxygen atoms on both sides, maintaining the same compounds involved in the reaction.