Chemical Reactions and Equations Chapter 8

What does a chemical formula represent?  The elements contained in a chemical substance  Indicates the number of atoms of the element just to the right of the element symbol  a whole number that appears in front of a formula in a chemical equation. What is a subscript? What does it indicate? What is a coefficient?

Chemical Formulas 2 C 8 H 18 indicates there are 8 Carbon atoms in the molecule indicates there are 18 hydrogen atoms in the molecule Coefficient means 2 molecules

Visualizing Compounds Example: H 2 O H atoms = ________ O atoms = ________ Model of H 2 O: 2 1

What is a diatomic element?  An element that is made up of two atoms when it is by itself H 2 O 2 F 2 Br 2 I 2 N 2 Cl 2 Hydrogen Molecule Oxygen Molecule

DIATOMIC ELEMENTS You can also mark them off on your periodic table as the SUPER SEVEN!

Chemical Reaction  When bonds are broken and new bonds are formed producing a new substance Chemical Equation Symbolizes or represents a chemical reaction

Reactants Original substances in a chemical reaction Products Substances formed in a chemical reaction REACTANTS yields PRODUCTS

H 2 + O 2  H 2 O Igniting a mixture of hydrogen and oxygen will form water. The symbol equation is: ReactantsProduct

How does a chemical reaction occur?  Atoms must collide for the reaction to occur  Atoms are rearranged in a chemical change - None are lost - None are gained

any reaction that gives off heat Example: 2H 2 (g) + O 2 (g) 2H 2 O (l) + heat Exothermic Reaction

Endothermic Reaction any reaction in which heat is added Example: heat + 2HgO (s) 2Hg (l) + O 2 (g)

Evidence of a Chemical Change 1. Color change (THAT YOU DID NOT CAUSE!!) 2. Temperature change 3. Formation of a gas 4. Change in odor 5. Formation of a precipitate  precipitate: insoluble solid that separates out of solution

Balancing Chemical Equations Notes

A Chemical Reaction Potassium Iodide [KI] Lead Nitrate [Pb(NO 3 ) 2 ] Final Product

Did a chemical change occur? How do you know? Is matter conserved in this reaction?

According to the LAW OF CONSERVATION OF MATTER, the total atoms of reactants must equal the total atoms of products for any given chemical reaction. When you have a chemical reaction, you must add coefficients to balance the reaction!

Atom Inventory __H 2 + __O 2  __H 2 O Reactant SideProduct Side Atom TypeNumber of Atoms Atom TypeNumber of Atoms HH OO UNBALANCED !

To balance an equation, coefficients CAN be added in front of products or reactants. DO NOT CHANGE SUBSCRIPTS!

New Atom Inventory __H 2 + __O 2  __H 2 O Reactant SideProduct Side Atom TypeNumber of Atoms Atom TypeNumber of Atoms HH OO BALANCED!

Balancing Equations ___ Mg + ___ N 2 ---> ___ Mg 3 N 2 ___ H 2 S + ___ Cl 2 ---> ___ S 8 + ___ HCl ___ Al + ___ HCl ---> ___ AlCl 3 + ___ H 2 ___ Fe 2 O 3 + ___ CO ---> ___ Fe + ___ CO 2

Classifying Chemical Reactions

Two or more reactants combine to form ONE product. (marriage) example: 2Mg (s) + O 2 (g) 2MgO (s) A + X AX Synthesis

ONE reactant breaks down into smaller parts. (divorce) AX A + X Decomposition example: 2H 2 O (l)  2H 2 (g) + O 2 (g)

A single element replaces an element that is part of a compound. (affair) example: 2 Al (s) + 3 Fe(NO 3 ) 2(aq) 3 Fe (s) + 2 Al(NO 3 ) 3(aq) A + BX B + AX Y + BX X + BY Single Displacement

Ions from two different compounds exchange places to form two new compounds. (wife swap) AX + BY AY + BX 2KI (aq) + Pb(NO 3 ) 2 (aq) PbI 2 (s) + 2KNO 3 (aq) Double Displacement

A compound (usually a hydrocarbon) reacts with oxygen, releasing carbon dioxide and water. example: combustion of propane C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O (g) AB + O 2 CO 2 + H 2 O Combustion

Writing Chemical Reactions Notes

word equation: equation in which the reactants and products in a chemical reaction are represented by words a word equation is qualitative example: methane + oxygen carbon dioxide + water Word Equations

formula equation: represents the reactants and products of a chemical reaction by their symbols or formulas a formula equation is quantitative example: CH 4(g) + 2O 2(g) CO 2(g) + 2H 2 O (g) Formula Equations

Symbols in a Chemical Reaction