Chapter 8: Chemical Equations & Reactions

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

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 (9.3 + 0.7 = 10.0 g) Therefore, matter is conserved in a chemical reaction video

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”

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

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

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

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

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)

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

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