1. Aim: How to write chemical equations? DO NOW: If done with yesterday’s classwork answer the following questions. If not done, continue working on classwork. 1. Define a chemical reaction. 2. What are some evidence of a chemical reaction occurring? 3. Give an example of a chemical reaction. 4. 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.  The coefficients (number in front of chemical formula) tell us the number of moles, molecules, or formula units of a species (NH 4 ) 2 Cr 2 O 7 (s)  N 2 (g) + Cr 2 O 3 (aq) + 4H 2 O(g) REACTANTS PRODUCTS

4. Writing Chemical 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) + O 2 (g)  Fe 2 O 3 (s)

5. 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.  Mass of the reactants is equal to the mass of the products.  # of ATOMS of each ELEMENT on the REACTANTS (left) side = # of ATOMS of each ELEMENT on the PRODUCTS (right) side.

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

7. Balancing Equations  Now, let’s examine the following UNBALANCED equation: H 2 + O 2  H 2 O  Q: How does this unbalanced equation violate the Law of Conservation of Mass?

8. 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

9. Method for Balancing Equations  Step 4: Balance one atom at a time, using coefficients. Start with atoms that appear only once in the reactants and only once in the products. Usually leave Hydrogen atoms followed by Oxygen atoms until last.  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.

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

11. Balancing Chemical Equations CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O(g)C= 1 H = 4H= 2 O = 2O = 3 1212 4 4 4

12. Write and balance the chemical equation. 1. Zinc and lead (II) nitrate react to form zinc nitrate and lead. 1. Potassium metal and chlorine gas combine to form potassium chloride Zn + Pb(NO 3 ) 2  Zn(NO 3 ) 2 + Pb 2K + Cl 2  2KCl