1. Chemical Equations
8.P.1.4

2. Balancing Equations
Chemical reactions occur when bonds (between the electrons of atoms) are formed or broken
Chemical reactions involve
changes in the chemical composition of matter
the making of new materials with new properties
energy changes:
Bond breaking absorbs Energy (endothermic process)
Bond making releases Energy (exothermic process)
Symbols represent elements
Formulas describe compounds
Chemical equations describe a chemical reaction

3. Chemical Equations
A chemical equation is written as an expression similar to a mathematic equation that can be compared to a recipe that a chemist follows in order to produce desired results.

4. Chemical Equations 4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)
Chemical Equations
Depict the kind of reactants and products and their relative amounts in a reaction.
4 Al (s) O2 (g) ---> 2 Al2O3 (s)
The numbers in the front of chemical symbols are called coefficients
The numbers behind chemical symbols are called subscripts
The letters (s), (g), and (l) are the physical states of compounds.

5. Chemical Equations
Because of the Law of Conservation of Matter (matter can not be created or destroyed) an equation must be balanced.
It must have the same number of atoms of the same kind on both sides.
Law of Conservation of Energy MUST ALSO BE FOLLOWED!
Energy changes are written in (endo-/ exothermic reactions)
Lavoisier, 1788

6. Law of Conservation of Matter
During a chemical reaction, matter cannot be created or destroyed.
Even though the matter may change from one form to another, the same number of atoms exists before and after the change takes place!

7. Since the overall mass of the matter does not change…
The mass of the reactants – the substances there before the reaction occurs – must equal the mass of the product – the substances there after the reaction takes place.
Mass of
Reactants
Mass of
Products

8. An Example of the L.C.M.
Test tube containing lead nitrate is placed upright in a flask that contains potassium iodide.
Open system – an environment where matter can enter or escape
What are these?
REACTANTS!
They don’t mix!

9. A stopper is put in place to create a closed system – an environment where matter cannot enter or escape.
The mass of the system is found.

10. After the mass is found, the system is turned upside down so the lead nitrate can mix with the potassium iodide.
They create lead iodide and potassium nitrate.

11. Although the matter changed in form, the total mass of the system remains unchanged.

12. Chemical Equations Reactants  Products
All chemical equations have reactants and products.
We express a chemical equation as follows:
Reactants  Products
The arrow is equivalent to an “=“ math. When we describe the equation we use the word “yields” or “produces” instead of equals
Example
C + O2  CO2
This reads “carbon plus oxygen react to yield carbon dioxide”

13. Balancing a Chemical Equation
A chemical equation is balanced when the ions or atoms found on the reactant side of the equation equals that found on the product side.
The arrow can be considered the balance point (kind of like an equal sign).

14. Symbols Used in Equations
Solid (s)
Liquid (l)
Gas (g)
Aqueous solution (aq)
Catalyst H2SO or Pt
Escaping gas ()

15. Chemical Reactions and Chemical Equations
Reactants Products
Driving forces:
Color change
Formation of a solid/precipitate
Evolution (release) of a gas
Evolution (release) or absorption of heat
Precipitate is a solid that is produced from a liquid solution during a chemical change/reaction

16. Balancing 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 the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)
Think back to naming compounds/ determining formulas. NaCl exists, because Na is + and Cl is -, but NaCl2 does NOT exist since you would not have a neutral compound! You can’t just add a number to a formula to balance an equation.

17. Subscripts vs. Coefficients
The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

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

20. Steps to Balancing Equations
There are four basic steps to balancing a chemical equation.
Write the correct formula for the reactants and the products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first.
**And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS!
Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side.

21. Steps to Balancing Equations continued
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:
The numbers of atoms on both sides of the equation are now balanced.
The coefficients are in the lowest possible whole number ratios. (reduced)

22. Some Suggestions to Help You
Some helpful hints for balancing equations:
Take one element at a time, working left to right except for H and O. Metals, then nonmetals are a good way, too. 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)

23. Balancing Equations 2 2 ___ H2(g) + ___ O2(g) ---> ___ H2O(l)2 2
___ H2(g) + ___ O2(g) ---> ___ H2O(l)
What Happened to the Other Oxygen Atom?????
This equation is not balanced!
Two hydrogen atoms from a hydrogen molecule (H2) combine with one of the oxygen atoms from an oxygen molecule (O2) to form H2O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H2 molecule) to make a second H2O molecule.

24. Balance this equation! Na + Cl2 NaCl Na- 1 Na-1 Cl- 2 Cl-1
Balance this equation!
Na + Cl NaCl
Na- 1 Na-1
Cl- 2 Cl-1
**note that the number of sodiums balance but the chlorine does not. We will have to use coefficients in order to balance this equation.

25. Inserting subscripts Na + Cl2 2 NaCl Na- 1 Na- 1 2 Cl- 2 Cl- 1 2
Inserting subscripts
Na + Cl NaCl
Na- 1 Na- 1 2
Cl- 2 Cl- 1 2
** Now the chlorine balances but the sodium does not! So we go back and balance the sodium.

26. Finally balanced! 2Na + Cl2 2 NaCl Na- 1 2 Na-1 2 Cl- 2 Cl-1 2
Finally balanced!
2Na + Cl NaCl
Na Na-1 2
Cl- 2 Cl-1 2
**Since the number of each element on the reactant side and the product side of the equation are equal, the equation is balanced.

27. Balancing Equations 2 3
___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s)

28. Balancing Equations Na3PO4 + Fe2O3 ----> Na2O + FePO4 2 2 3
Sodium phosphate + iron (III) oxide  sodium oxide + iron (III) phosphate
Na3PO Fe2O > Na2O FePO4 2 2 3

29. Video
Balancing Chemical Equations for Beginners