1. Balancing and writing equations
Balancing Equations
Balancing and writing equations

2. Balancing Chemical Equations
When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts.
Changing subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)

3. Symbols Used in Equations
solid (s)
liquid (l)
gas (g)
aqueous solution (aq)
catalyst H2SO4

4. Subscripts vs. Coefficients
Subscripts tell how many atoms of a particular element are within a compound.
Coefficient tells you the number of molecules of the compound.

5. Chemical Equations 4 Al(s) + 3 O2(g) ---> 2 Al2O3(s)
This equation means
4 Al atoms + 3 O2 molecules
produces
2 molecules of Al2O3

6. 4 Steps to Balancing Equations
Write the correct formula for each of the reactants and the products. Once you have written them correctly do not change formulas.
Find the number of atoms for each element on the left hand side of the equation. Compare them with the number of the atoms of the same element on the right hand side.

7. 4 Steps to Balancing Equations
Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation.
Check your answer to see if:
(a) The numbers of atoms on both sides of the equation are now balanced.
(b) The coefficients are in the lowest possible whole number ratios.

8. Some Suggestions to Help You
Take one element at a time, working left to right except for H and O. Save H for next to last, and O until last.
IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again (because O is diatomic as an element).
Shortcut: Polyatomic ions that appear on both sides of the equation should be balanced as independent units (ie if you have a sulfate on the left you probably have one on the right).

10. Law of Conservation of Matter
Matter can neither be created nor destroyed, but is only rearranged.
In a chemical reaction (and equation) the reactant elements are the same as the product elements – no new elements are made!
In a chemical reaction, the number of atoms in reactants must equal the number of atoms in the products.
Atoms get rearranged, through bond breaking and new bonds forming, to make different compounds

11. Balancing Chemical Equations
To paraphrase, the
Law of Conservation of Atoms states that the number of atoms of each type of element must be the same on each side of the chemical equation.

12. Balancing a chemical equation:
Hydrogen + oxygen water
H2 + O H2O
Hydrogen and oxygen are diatomic elements.
Their subscripts cannot be changed.
The subscript on water cannot be changed.

13. H2 + O H2O
Strategy: Count the atoms of each element, on each side of the equation:
Reactant side: 2 atoms H and 2 atoms O
Product side: 2 atoms H and 1 atom O

14. H2 + O H2O
If the subscripts cannot be altered, how can the number of atoms of each type be made equal on each side?
Adjust the number of molecules by changing the coefficients on the molecules.

15. H2 + O2 2H2O Now you have: Reactants: 2 atoms of H, 2 atoms of O
Products: 4 atoms of H, 2 atoms of O
But number of H is no longer balanced!

16. 2H2 + O2 2H2O Now check what you have:
Reactant side: 4 atoms H, 2 atoms O
Product side: 4 atoms H, 2 atoms O
It’s balanced!

17. Balancing a chemical equation
N H NH3
nitrogen + hydrogen ammonia
Strategy: Count atoms of N and of H:
Reactants: 2 atoms N and 2 atoms H
Products: 1 atom N and 3 atoms of H

18. N2 + H2 2NH3 Neither the N nor the H are balanced.
Balance the nitrogen first by placing a coefficient of 2 in front of the NH3 (since nitrogen is diatomic, needs an even number)
You get
N2 + H NH3

19. N2 + 3H2 2NH3 Hydrogen is still not balanced.
Since you have 6 atoms of H on the right hand side, place coefficient 3 in front of H2 on the left hand side.
Now you have:
Reactant side: 2 atoms N, 6 atoms H
Product side: 2 atoms N, 6 atoms H
N H NH3

20. A trickier equation to balance
Cu + H2SO4 CuSO4 + H2O + SO2
Copper is balanced.
There is one sulfate ion on each side, but the element sulfur is not balanced.

21. To double the number of sulfur atoms on the left hand side, place coefficient 2 in front of sulfuric acid.
Count atoms in 2 H2SO4 : H = 4, S = 2, O = 8
On the left hand side there are S = 2, H = 2 and O = 6
So now the H are not balanced. Try doubling the number of water molecules …..

22. Cu + 2H2SO CuSO4 + 2H2O + SO2
Count the number of atoms on each side again to check:
Cu = 1, S = 2, H = 4, O = 8 on each side
It’s balanced!

23. Writing and naming
“nickel and hydrochloric acid react together to form mickel (II) chloride and hydrogen”
Strategy: Write the corresponding formula equation and then balance the equation.

24. Ni + HCl NiCl2 + H2 Ni + 2HCl NiCl2 + H2
Write the formula of each compound independently.
Change the “and” to “+” and “produces” to “”
Balance the equation after writing the formulas.
Ni + HCl NiCl2 + H2
Ni + 2HCl NiCl2 + H2