1. Bellringer/6.3 Notes
You have already used scientific shorthand by writing symbols for elements and formulas for compounds. You can use these formulas to write chemical equations that summarize what happens during a chemical reaction and how much of each substance is involved. Examine the reaction model for the water synthesis reaction shown below, and answer the items that follow.
What is the difference between reaction models A and B?
Why is reaction model A not fully complete as written? (Hint: Consider how many atoms of each element exist before and after the reaction.)
A friend tells you that an easier way to make sure the same number of atoms are on both sides of the equation is to change the subscript on the product so that it is H2O2 instead of H2O. What’s wrong with this reasoning? (Hint: If you did this, would it still be a synthesis reaction for water?)

2. Ag(cr) + H2S Ag2S(cr) + H2(g)
I. Checking for Balance
Balanced chemical equations-same number of atoms of each element on both sides of the equation.
Example: 2
Ag(cr) + H2S Ag2S(cr) + H2(g)
We use coefficients to balance equations.
If no coefficient is present, it is assumed to be one.

3. Solid magnesium plus gaseous oxygen produces solid magnesium oxide
II. Writing balanced equations
Follow these steps to write a balanced equation.
Solid magnesium plus gaseous oxygen produces solid magnesium oxide
Write a chemical equation for the reaction using formulas and symbols.
Mg(cr) + O2(g) MgO(cr)

4. 2 2 NOT BALANCED! BALANCED! Mg(cr) + O2(g) MgO(cr)
Check the equation for atom balance.
Mg(cr) + O2(g) MgO(cr)
NOT BALANCED!
Balance the equation 2 2
Mg(cr) + O2(g) MgO(cr)
Mg(cr) + O2(g) MgO(cr)
BALANCED!