Chapter 7 Chemical Reactions

Introduction Chemical changes always involve the formation of new substances with properties that are different from the original substances. Chemical equations to describe these chemical reactions To conform to the law of conservation of mass, equations must be balanced

Introduction There are five general types of chemical reactions Understanding these types of chemical reactions allow you to predict what will happen when substances undergo a chemical change.

7.1 Chemical Reactions Chemical reactions are constantly occurring; these reactions both occur in the world around you and within you. Examples: digestion, photosynthesis, rust, batteries, cooking. All chemical reactions, both simple and complex, involve changing substances.

7.1 Chemical Reactions In chemical reactions, one or more substances, the reactants, change into one or more new substances, the products. When writing chemical reactions, the reactants are separated from the products with an arrow. The arrow means “yields” or “reacts to product” Reactants → Products

7.1 Chemical Reactions John Dalton explained the way in which substances change in his atomic theory. In a chemical reaction the ways in which the atoms are joined together are changed. As reactants are changed into products, bonds that hold atoms together are broken and new bonds are formed. The atoms themselves are neither created nor destroyed (the law of conservation of mass)

7.1 Chemical Reactions Chemical reactions can be described in different ways: descriptions, word equations, skeleton equations, and balanced equations. In writing chemical equations, reactants are written to the left of the arrow and the products are written to the right. Separate reactants and products are separated by plus signs.

7.1 Chemical Reactions Description: “Iron reacts with oxygen to produce iron (III) oxide (rust)” Word Equation: iron + oxygen → iron (III) oxide Description: “Methane is burned in oxygen to produce carbon dioxide and water” methane + oxygen → carbon dioxide + water

7.2 Writing Chemical Equations Word equations can be difficult to work with, so chemical formulas are often used to communicate more effectively. Word Equation: iron + oxygen → iron (III) oxide Skeleton Equation: Fe(s) + O2(g) → Fe2O3(s)

7.2 Writing Chemical Equations A skeleton equation is a chemical equation that does not indicate the relative amounts of reactants and products A catalyst is a substance that speeds up a reaction without being changed. Because it is not a reactant or a product, the formula for a catalyst is written above the arrow.

7.2 Writing Chemical Equations Symbol Explanation + Used to separate two reactants or two products → “Yields,” separates reactants from products = An alternative to → Used in place of → for reversible reactions (s) Reactant or product in the solid state; placed after the formula (l) Reactant or product in the liquid state; placed after the formula (aq) An aqueous solution; the substance is dissolved in water (g) Reactant or product in the gaseous state; placed after the formula Indicates that heat is supplied to the reaction A formula above the arrow represents a catalyst (in this example, platinum) → ← ∆ heat → → Pt →

7.2 Writing Chemical Equations Description: “Manganese (IV) oxide catalyzes the decomposition of an aqueous solution of hydrogen peroxide into water and molecular oxygen.” Word Equation: hydrogen peroxide → water + oxygen Skeleton Equation: H2O2(aq) → H2O(l) + O2(g) manganese (IV) oxide MnO2