Chemical Reactions Chapter 7

The Nature of Chemical Reactions Section 1

Chemical Reactions A chemical reaction is rearrangement of atoms in the reactants to make new substances in the products. The substance will form new substances after a chemical change Chemical reactions cannot be reversed You can often tell because you can see the changes

Chemical Reactions Some observations that suggest a chemical change has taken place… Heat, light or sound Production of a gas Formation of a precipitate (“stuff forming”) Change in color - This is not conclusive proof though, it needs to be proven that some new compound is present.

Chemical Reactions Reactants: Starting materials in a reaction Products: Ending materials in a reaction The products of a reaction are made up of the same number and type of atoms as were present in the reactants (just in a different arrangement)

Chemical Reactions C6H12O6 + 6O2  6CO2 + 6H2O Glucose and oxygen react to form carbon dioxide and water Same number and kind of atoms on each side, just in different arrangements, it has changed into a different molecule.

Activation Energy All chemical reactions require energy in order to break bonds (covalent, ionic or metallic) Activation Energy: The energy required to break enough bonds for a reaction to start

Exothermic Reactions In this reaction, the energy absorbed by the reactants is less than the amount released from the products The extra chemical energy from the reactants is released as heat when the bonds in the products reform

Endothermic Reactions In this reaction, more energy is required to make the reaction than is released Endothermic reactions take in energy

Chemical Equations section 2

Remember! All MATTER is conserved, but it can change forms All ENERGY is conserved, but it can change forms What goes in must come out, but it can be rearranged! You have to account for all the stuff that goes into the reaction

Chemical Equations Chemical reactions are shown with chemical equations Reactants  Products A chemical equation uses symbols to show the relationship between the reactants and products

Balancing Equations When an equation is balanced, the number of molecules/atoms in the reactants match the number of those in the products – LAW OF CONSERVATION OF MASS! Balancing an equation is changing the ratios of molecules/atoms so that each side of the equation has the lowest possible same whole number and type of atoms

Balancing Equations You CANNOT change the subscript (this would change the formula of the molecules) You CAN change the coefficient in front of any atom/molecule

C6 H12 O6 + 6O2 6C +  O2 6H2 O Carbon - 6 Carbon - 6 Hydrogen - 12 Hydrogen - 12 Oxygen - 18 Oxygen - 18

Let’s Practice! _____H2 + _____ O2  ____ H2O

Let’s Practice! ____ H2 + ____ N2  ____ NH3