CHEMICAL REACTIONS Chapter 7

Chemical Equations Reactants  Products In a chemical reaction, the substances that undergo change are called reactants. The new substances formed as a result of that change are called products.

Chemical Equations A chemical equation is a representation of a chemical reaction in which the reactants and products are expressed as formulas. Carbon + Oxygen  Carbon dioxide C + O 2  CO 2

Chemical Equations The law of conservation of mass states that mass is neither created or destroyed in a chemical reaction. When carbon burns, the mass of carbon dioxide produced is equal to the mass of the carbon and oxygen that reacted. If there are 15 g of reactant in a chemical reaction, there must be 15 g of product when the reaction is finished.

Balancing Chemical Equations Chemical equations must be balanced to show that mass is conserved during a reaction. Chemical equations are balanced by adding coefficients in front of the formulas in the equation.

Balancing Chemical Equations H 2 + O 2  H 2 O How many of each atom are in the equation? _2_ H _2_ _2_ O _1_ There needs to be the same number of each atom on both sides of the equation. We need 2 oxygen on the right hand side, so we add a “2” in front of the H 2 O. H 2 + O 2  2H 2 O

Balancing Chemical Equations H 2 + O 2  2H 2 O How many of each atom are in the equation? _2_ H _4_ _2_ O _2_ Now there are 2 oxygen on each side, however there are not enough hydrogen on the left hand side. We will put a “2” in front of the H 2 on the left. 2H 2 + O 2  2H 2 O

Balancing Chemical Equations 2H 2 + O 2  2H 2 O How many of each atom are in the equation? _4_ H _4_ _2_ O _2_ Now the equation is balanced!

Exothermic and Endothermic Reactions During a chemical reaction, energy is either released or absorbed. A chemical reaction that releases energy is called an exothermic reaction. A chemical reaction that absorbs energy is called an endothermic reaction.

Classifying Reactions Synthesis A synthesis reaction is a reaction in which two or more substances react to form a single substance.  The general equation for a synthesis reaction is: A + B  AB Synthesis reactions may also be classified as combustion reactions if oxygen gas (O 2 ) is one of the reactants.

Classifying Reactions Decomposition A decomposition reaction is a reaction in which one compound breaks down into two or more simpler substances.  The general equation for a decomposition reaction is: AB  A + B

Classifying Reactions Single Replacement A single replacement reaction is a reaction in which one element takes the place of another element in a compound.  The general equation for a single replacement reaction is: A + BC  B + AC

Classifying Reactions Double Replacement A double replacement reaction is a reaction in which two different compounds exchange positive ions and form two new compounds.  The general equation for a double replacement reaction is: AB + CD  AD + CB

Classifying Reactions Combustion A combustion reaction is a reaction in which a substance reacts rapidly with oxygen, often producing heat and light. If a carbon compound is combusted, the products of the combustion are always carbon dioxide (CO 2 ) and water (H 2 O).  An example of a carbon-compound combustion reaction is: CH 4 + 2O 2  CO 2 + 2H 2 O

Reactions as Electron Transfer Redox Reactions A reaction in which electrons are transferred from one reactant to another is called an oxidation-reduction reaction, or redox reaction. “LEO the lion says GER” Losing Electrons Oxidation Gaining Electrons Reduction

Reactions as Electron Transfers LEO: When an atom is oxidized, it loses electrons and becomes a cation. GER: When an atom is reduced, it gains electrons and becomes an anion.