Chemical Reactions CHAPTER 7 1

The Nature of Chemical Reactions SECTION 1 2

LAST CHAPTER BEFORE EXAMS!!! 3

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  You can often tell because you can see the changes 4

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. 5

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) 6

Chemical Reactions C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O 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. 7

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 8

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 9

Endothermic Reactions  In this reaction, more energy is required to make the reaction than is released  Endothermic reactions take in energy 10

Chemical Equations SECTION 2 11

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 12

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 13

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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!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 15

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 16

C 6 H 12 O 6 6C6H 2  Carbon - 6 6O 2 + O O 2 + Hydrogen - 12 Oxygen - 18 Carbon - 6 Hydrogen - 12 Oxygen

Balancing Equations  Write down your given equation 18

Balancing Equations  Write down the number of atoms per each element that you have on each side of the equation.  Look at the subscripts next to each atom to find the number of atoms in the equation. 19

Balancing Equations  Choose the element that appears in only a single molecule of reactants and in only a single molecule of products. 20

Balancing Equations  Add coefficients to the single atom to make it balanced on both sides 21

Balancing Equations  Then balance hydrogens 22

Balancing Equations  Then balance the oxygens 23

Balancing Equations  Check your work!  This process works mostly by trial and error 24

Let’s Practice!  _____H 2 + _____ O 2  ____ H 2 O 25

Let’s Practice!  ____ H 2 + ____ N 2  ____ NH 3 26