Start with the unbalanced equation:
#"Zn + H"_2"O" → "ZnO" + "H"_2#
A method that often works is to balance everything other than #"O"# and #"H"# first, then balance #"O"# and finally balance #"H"#.
Let's start with #"ZnO"#. It has one #"Zn"# atom.
Put a #1# in front of #"ZnO"#. That number will stay fixed unless we have to change it later to remove fractions.
#"Zn + H"_2"O" → color(red)(1)"ZnO" + "H"_2#
Balance #"Zn"#:
We have fixed #"1 Zn"# on the right, so we need #"1 Zn"# on the left.
Put a #1# in front of #"Zn"#.
#color(red)(1)"Zn + H"_2"O" → color(red)(1)"ZnO" + "H"_2#
Balance #"O"#:
We have fixed #"1 O"# on the right, so we need #"1 O"# on the left.
Put a #1# in front of #"H"_2"O"#.
#color(red)(1)"Zn" + color(blue)(1)"H"_2"O" → color(red)(1)"ZnO" + "H"_2#
Balance #"H"#:
We have fixed #"2 H"# on the left, so we need #"2 H"# on the right.
Put a #1# in front of #"H"_2#.
#color(red)(1)"Zn" + color(blue)(1)"H"_2"O" → color(red)(1)"ZnO" + color(orange)(1)"H"_2#
Every formula now has a fixed coefficient. We should have a balanced equation.
Let’s check:
#ulbb("Atom"color(white)(m)"Left hand side"color(white)(m)"Right hand side")#
#color(white)(m)"Zn"color(white)(mmmmml)1color(white)(mmmmmmmm)1#
#color(white)(m)"H"color(white)(mmmmmm)2color(white)(mmmmmmmm)2#
#color(white)(m)"O"color(white)(mmmmmm)1color(white)(mmmmmmmm)1#
All atoms balance. We do not usually write coefficients of #1#, so the balanced equation is
#"Zn + H"_2"O" → "ZnO" + "H"_2#
