Chapter 8 Chemical Reactions and Equations

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?

Coefficient means 2 molecules Chemical Formulas 2 C8H18 Coefficient means 2 molecules indicates there are 8 Carbon atoms in the molecule indicates there are 18 hydrogen atoms in the molecule

Visualizing Compounds Example: H2O H atoms = ________ O atoms = ________ Model of H2O: 1

What is a diatomic element? An element with two atoms when it is by itself H2 O2 F2 Br2 I2 N2 Cl2 Hydrogen Molecule Oxygen Molecule

DIATOMIC ELEMENTS You can also mark them off on your periodic table as the SUPER SEVEN! 7

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

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

H2 + O2  H2O Reactants Product Igniting a mixture of hydrogen and oxygen will form water. The symbol equation is: H2 + O2  H2O Reactants Product

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 Exothermic Reaction any reaction that gives off heat Example: 2H2 (g) + O2 (g) 2H2O (l) + heat

heat + 2HgO (s) 2Hg (l) + O2 (g) Endothermic Reaction any reaction in which heat is added Example: heat + 2HgO (s) 2Hg (l) + O2 (g)

Evidence of a Chemical Change Color change (THAT YOU DID NOT CAUSE!!) Temperature change Odor change Formation of a gas 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(NO3)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 [matter can’t be created or destroyed!] When you have a chemical reaction, you must add coefficients to balance the reaction!

UNBALANCED! __H2 + __O2  __H2O Reactant Side Product Side 2 2 2 1 Atom Inventory __H2 + __O2  __H2O Reactant Side Product Side Atom Type Number of Atoms H O UNBALANCED! 2 2 2 1

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

UNBALANCED! New Atom Inventory 2 __H2 + __O2  __H2O Reactant Side Product Side Atom Type Number of Atoms H O UNBALANCED! 2 4 2 2

BALANCED! New Atom Inventory 2 2 __H2 + __O2  __H2O Reactant Side Product Side Atom Type Number of Atoms H O BALANCED! 4 4 2 2

Balancing Equations ___ Mg + ___ N2 ---> ___ Mg3N2 ___ H2S + ___ Cl2 ---> ___ S8 + ___ HCl ___ Al + ___ HCl ---> ___ AlCl3 + ___ H2 ___ Fe2O3 + ___ CO ---> ___ Fe + ___ CO2

Classifying Chemical Reactions

A + X AX Synthesis Two or more reactants combine to form ONE product. (marriage) A + X AX example: 2Mg (s) + O2 (g) 2MgO (s)

AX A + X Decomposition ONE reactant breaks down into smaller parts. (divorce) AX A + X example: 2H2O (l)  2H2 (g) + O2 (g)

A + BX B + AX Y + BX X + BY Single Displacement A single element replaces an element that is part of a compound. A + BX B + AX Y + BX X + BY example: 2 Al(s) + 3 Fe(NO3)2(aq) 3 Fe(s) + 2 Al(NO3)3(aq)

AX + BY AY + BX Double Displacement Ions from two different compounds exchange places to form two new compounds. (wife swap) AX + BY AY + BX 2KI(aq) + Pb(NO3)2 (aq) PbI2 (s) + 2KNO3 (aq)

Combustion A compound (usually a hydrocarbon) reacts with oxygen, releasing carbon dioxide and water. AB + O2 CO2 + H2O example: combustion of propane C3H8 (g) + 5O2 (g) 3CO2 (g) + 4H2O (g)

What is the activity series? When elements are ranked in order of their relative reactivities

Activity Series of the Elements

1.Why is sodium not used for silverware? It is highly reactive and reacts with water.

2. Why has so much gold jewelry survived from ancient civilizations? It is unreactive with oxygen or water and therefore does not readily break down.

3. Why is food canned in containers made from steel coated with a thin layer of tin? Steel makes a strong container and the tin lining does not react with the contents.  

To prevent the iron from rusting by being exposed to oxygen in air. 4. Why are cars painted? To prevent the iron from rusting by being exposed to oxygen in air.

Writing Chemical Reactions Notes

a word equation is qualitative Word Equations 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

a formula equation is quantitative Formula Equations formula equation: represents the reactants and products of a chemical reaction by their symbols or formulas a formula equation is quantitative example: CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

Symbols in a Chemical Reaction

Writing Chemical Reactions Steps to writing a reaction: Balance the equation. Fill in the phases of matter. Identify the reaction type.

Example 1 Aluminum metal reacts with a solution of hydrochloric acid to produce hydrogen gas and aluminum chloride solution.

Example 2 Magnesium metal burns in oxygen gas to produce solid magnesium oxide.

Example 3 Hydrogen peroxide (H2O2) solution breaks down to produce oxygen gas and liquid water in the presence of the catalyst “catalase”.