Chapter Seven: Chemical Reactions Understanding Chemical Reactions Energy and Chemical Reactions

Understanding chemical reactions Ice melting into water is an example of a physical change. During a physical change, a substance changes its form but remains the same substance. The water can be refrozen into ice again.

Understanding chemical reactions A chemical change turns one or more substances into different substances that usually have different properties.

Chemical changes rearrange chemical bonds A chemical reaction is a system of chemical changes that involves the breaking and reforming of chemical bonds to create new substances.

Evidence of chemical change Four indicators of chemical change are: 1.Formation of new gas 2.Formation of new solid 3.Release of energy (heat or light) 4.Color change

Products and reactants In chemical reactions, you start with reactants that are combined to make products. –The reactants are the starting substances. –The products are the new substances which result from the chemical reaction.

Reactants The first step in understanding a reaction is to see what atoms are in the reactants. Counting atoms is one way to understand a reaction. How many hydrogen atoms? How many sodium ions? How many oxygen atoms? How many carbon atoms?

Products The chemical reaction rearranges the same atoms in the reactants to become new compounds in the products. How many hydrogen atoms? How many sodium ions? How many oxygen atoms? How many carbon atoms?

Chemical equations A chemical equation is an abbreviated way to show the exact numbers of atoms and compounds in a chemical reaction. The arrow shows the direction the reaction goes, from reactants to products. ReactantsProduct

Chemical equations The reaction combines hydrogen and oxygen molecules.

Chemical equations Count the atoms to see if there are the same number of each type of atom on the reactant and product sides of the equation. Since there is one more oxygen atom on the reactant side than there is on the product side, the reaction equation is not balanced.

Balancing chemical equations A balanced chemical equation has the same number of each type of atom on the product side and the reactant side. To balance the equation, we add another water molecule to the product side and add another hydrogen molecule to the reactant side. We can practice balancing equations using periodic table tiles and pencil and paper.

Numbers in equations

1. Synthesis Reactions A + B AB Example: 2Na + Cl 2 2NaCl 2. Decomposition Reactions AB A + B Example: 2H2O 2H 2 + O 2 TYPES OF REACTIONS

3. Displacement reactions In single-displacement reactions, one element replaces a similar element in a compound. A + BC B + AC Example:

4. Double Replacement reactions A precipitate is a new solid product that comes out of solution in a chemical reaction. The formation of a cloudy precipitate is evidence that a double-displacement reaction has occurred. AB + CD AD + CB Example: Pb(NO 3 ) 2 + 2KI PbI 2 + 2KNO 3