1. Chapter 8: Chemical Equations & Reactions

2. Section 8-1: Describing Chemical Reactions
Chemical reaction: a change in which one or more substances are converted into new substances
The substances that react together are called reactants
The new substances produced are called products
The mass of the reactants before the reaction MUST equal the products, according to the law of conservation of mass

3. Describing Chemical Reactions
French chemist Antoine Lavoisier discovered this law in the late 1700s.
He heated 10 grams of mercury (II) oxide in a sealed container
The compound bubbled and changed from a red powder (HgO) to a silver liquid (Hg)
He had 9.3 grams of mercury, and 0.7 grams of oxygen ( = 10.0 g)
Therefore, matter is conserved in a chemical reaction
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4. Writing Equations
Just like we write chemical formulas to save space, we can write chemical equations to show a chemical reaction
2HgO (s)  2Hg (l) + O2 (g)
2 units of solid mercury (II) oxide yield (produce) 2 units of liquid mercury and 1 unit of oxygen gas
The 2’s in front of the HgO and the Hg are called coefficients
The letters in ( ) tell us the state of matter
(aq) means “dissolved in water”

5. Balancing Chemical Equations
Because the law of conservation of mass, chemical equations must be balanced
Steps to balance chemical equations:
Balance the different types of atoms one at a time
First balance the atoms of elements that are combined and that only appear once on each side of the equation
Balance polyatomic ions that appear on both sides of the equation as single units
Balance O and H atoms last

6. Evidence of Chemical Reactions
Production of Heat and/or light
Production of a Gas (bubbles/fizzing)
Formation of a Precipitate (insoluble solid)
Color Change
evidence of chemical reactions

7. Elements That Exist As Diatomic Molecules
p. 243, Table 8-1
BrINClHOF
Br2 I2 N2
Cl2 H2 O2 F2

8. Translating Word Equations
An equation in which the reactants and products in a chemical reaction are represented by words
Methane and oxygen gases react to yield carbon dioxide gas and water vapor
CH4 (g) + O2 (g)  CO2 (g) + H2O (g) (unbalanced)
The letters in ( ) tell us the state of matter; list is found on p. 246 of text

9. Practice Translating Word Equations
Solid sodium oxide is added to water at room temperature to form aqueous sodium hydroxide
Na2O (s) + H2O (l)  2NaOH (aq)
Solid iron reacts with water vapor in a reversible reaction to yield solid iron (III) oxide and hydrogen gas.
2Fe (s) + 3H2O (g)  Fe2O3 (s) + 3H2 (g)

10. Practice Translating Word Equations
Can you translate this?
PbCl2 (aq) + Na2CrO4 (aq)  PbCrO4 (s) + 2NaCl (aq)
Aqueous lead (II) chloride reacts with sodium chromate to yield a precipitate of lead (II) chromate and aqueous sodium chloride

11. Chemical Reactions glycerol and potassium permanganate
dehydration of sugar
sodium acetate
pharaoh's snake
Chemiluminescence
phosphorus in oxygen