Nature of Chemical Reactions

Objectives:1. Describe the Characteristics of a chemical

reaction.2. Distinguish between reactants and

products.Key Terms:Chemical reaction, Reactants, Products,

subscripts, coefficients, yield

Chemical Reactions

A chemical reaction occurs when substances (reactants) are converted into a new substances (products) with new physical and chemical (intensive & extensive) properties. Consider the chemical reaction of photo synthesis:

H2O + CO2 g C6H12O6 + O2

In this reaction water and carbon dioxide are changed into sugar and oxygen. H2O and CO2 are the reactants of the reaction and C6H12O6 and O2 are the products. The arrow means yields (becomes).

The change is permanent. That is your test for a chemical reaction. If the reaction cannot be reversed physically and get the original reactants, a

chemical change has taken place. A chemical reaction must follow the Laws of Conservation of

Matter, Mass, and Energy. Matterreactants = Matterproducts, Massreactants = Massproducts, Energyreactants = Energyproducts

Evidence of Chemical Reactions

change in physical appearance (color, texture, luster)

transfer on energy (exothermic vs endothermic)

production of gas (effervescence)formation of a solid in solution (precipitate) pH change

Chemical Equations

A chemical equation lists all the elements in the reactants and in the products they form Must obey Conservation of Matter, Mass, & Energy

6H2O + 6CO2 g C6H12O6 + 6O2

The 6 listed before water, carbon dioxide, and oxygen is a coefficient Multiplies the entire compound

The subscripts tell how many atoms there are of the element directly preceding it

The g is the yield. It is the action word in a word problem (yields, forms, becomes, produces, …)

If there is a D above the g, the reaction requires an input of energy.