Describing Chemical Reactions The process by which one or more substances change into one or more new substances.

Reactant – the original substance/s substance/s that undergo a change found on the left side of chemical equation Product – the substance/s created found on the right side of chemical equations

Evidence of a Chemical Change Change in energy Release of energy as heat or light Production of sound Reduction or increase of temperature Absorption or release of electrical energy Formation of gas change in odor bubbling or fizzing Change in color Formation of a precipitate An insoluble solid

Exothermic and Endothermic Reactions Exothermic reactions – energy is released Energy can be thought of as a product Endothermic reactions – energy is absorbed Energy can be thought of as a reactant

Constructing a Chemical Equation A correctly written chemical equation shows the chemical formulas and relative amounts of all reactants and products. The first step in writing a chemical equation is to write a word equation. A word equation consists of the names of reactants and products separated by the yield sign, an arrow, which means produces or forms.

Formula Equations The next step is to change a word equation into a formula equation. Replace the names of substances with their formulas. This kind of formula does not give any quantitative information.

Equations and Reactions Information Equations give information about the amounts of each substance in the reaction Information about specific conditions and that are needed for the reaction to take place can also be included in the equation. This information is written over or under the yield sign. 350 C, 25000 kPa N2(g) + 3H2(g) ------------- 2NH3(g) catalyst

Symbols Represent States of Matter (g) gas (s) solid (l) liquid (aq) aqueous (dissolved in water)  yields or produces  reversible reaction in which product can reform into reactants, results in a mixture of products and reactants heat conditions can be placed over or under the arrow to show conditions necessary for the reaction to take place