1. Aim: How to write chemical equations?
DO NOW:
Define a chemical reaction.
What are some evidence of a chemical reaction occurring?
Give an example of a chemical reaction.
How can scientists represent a chemical reaction?

2. What is a Chemical Reaction?
A reaction in which the substance goes through a chemical change.
Evidence of a chemical reaction:
Release of heat or light
Color change
Production of gas
Formation of a precipitate (solid)

3. What is a Chemical Equation?
A chemical equation represents the formulas and amounts of the reactants and products in a chemical reaction.
(NH4)2Cr2O7 (s)  N2(g) Cr2O3(aq) H2O(g)
REACTANTS PRODUCTS

4. Word Equations An equation written in words.
Names of the reactants are to the left of the arrow separated by the plus sign; names of the products are to the right of the arrow separated by plus signs.

5. Write the word equation
Zinc and lead (II) nitrate react to form zinc nitrate and lead.
2. Potassium metal and chlorine gas combine to form potassium chloride
Zinc lead (II) nitrate  zinc nitrate lead
Potassium + Chlorine  potassium chloride

6. Writing Chemical (Formula) Equations
Represents the reactants and products of a chemical reaction by their symbols or formulas.
Indicate the physical state of a substance; (s) solid, (l) liquid, (g) gas, (aq) aqueous solution (dissolved in water).
Fe(s) + O2(g)  Fe2O3(s)

7. Law of Conservation of Mass
Chemical equations must be BALANCED.
LAW of CONSERVATION of MASS: mass can neither be CREATED nor DESTROYED in a chemical reaction.
# of MOLES of each ELEMENT on the REACTANTS (left) side = # of MOLES of each ELEMENT on the PRODUCTS (right) side

8. Balancing Equations Let’s look at the BALANCED equation below:
C + O2  CO2
*Note that there is 1 mol of carbon and 2 mol of oxygen on each side of the arrow.

9. Balancing Equations
Now, let’s examine the following UNBALANCED equation:
H2 + O2  H2O
Q: How does this unbalanced equation violate the Law of Conservation of Mass?

10. Method for Balancing Equations
Step 1: Draw a line to separate products from reactants
Step 2: List each of the different elements on each side of the line
Step 3: Count up the number of atoms on each side & record next to the element symbol

11. Method for Balancing Equations
Step 4: Find and balance the most complex compound in the equation. Balance the same elements on the opposite side of the equation.
Step 5: Now, continue balancing the elements by changing coefficients until you have the same number of each element on both sides of the equation.

12. NOTE: WE NEVER CHANGE THE SUBSCRIPTS IN A FORMULA
NOTE: WE NEVER CHANGE THE SUBSCRIPTS IN A FORMULA! ONLY CHANGE COEFFICENT.

13. Balancing Chemical Equations
CH4(g) + O2(g)  CO2(g) + H2O(g) C= 1 C= 1 H = 4 H= 2 O = 2 O = 3 1 2 1 2 4 4 4

14. Write and balance the chemical equation.
Zinc and lead (II) nitrate react to form zinc nitrate and lead.
Potassium metal and chlorine gas combine to form potassium chloride
Zn Pb(NO3)2  Zn(NO3) Pb
2K + Cl2  2KCl