Chapter 4.3 Chemical Reactions

Chemical Reactions Chemical Change – the transformation of one or more substances into different substances with different properties Chemical Reactions -include a change in energy energy is absorbed or releases (heat, light , sound) -occur at different rates: some are fast others are slow

Chemical Equations Represent chemical reactions by using words or symbols and formulas Starting materials are called “reactants” New substances are called “products” An arrrow () is read as “produces” States of matter appear in brackets (s)=solid (l)=liquid (g)=gas (aq) dissolved in water

Word Equations Use words to represent reactions Use all of your naming rules to do this! Ex. Magnesium + Hydrochloric acid  Magnesium chloride + Hydrogen

Formula Equations Use formulas to represent reactions Use your formula rules to do this! Notice states of matter are always included Coefficients are used to show the ratios of the different substances present in the reaction Ex. Mg (s) + 2HCl (aq)  MgCl2 (aq) + H2 (g)

Law of Conservation of Mass States that in a chemical reaction, the mass of the products ALWAYS EQUALS the mass of the reactants since elements cannot be created or destroyed in a chemical reaction, they can only be rearranged!

Counting Atoms Rules: Just a symbol = 1 atom Ex. N is one atom of nitrogen Subscripts tell you the number of atoms Ex. H2 is two atoms of hydrogen Coefficients mean to multiply everything after it by that number Ex. 2 MgCl2 means 2x1 = 2 atoms of Mg 2x2 = 4 atoms of Cl

Counting Atoms 4. Brackets mean to multiply everything inside by the subscript number outside the brackets Ex. Al2(SO4)3 2= atoms Al 3x1 =3 atoms S 4X3 = 12 atoms O

Counting Atoms Examples Count the atoms in the following examples BaCl2 3 BeF2 Al (NO3)3 4 Mg3PO3

Homework Counting Atoms Worksheet

Skeleton Equation An unbalanced equation that shows all reactants and products in a chemical reaction written as formulas Rules: -write the symbol of a metal element that is NOT in a compound ex. “Iron Nail” is Fe

Skeleton Equations -If the non-metal is part of Hockey stick and puck, write it as a diatomic molecule H2, N2, O2, F2, Cl2, Br2, I2 Ex. Oxygen gas reacts with fluorine gas O2 + F2 - Write the formula of any other compound using the rules you have learned

Example of Skeleton Equations Ex. Write the skeleton equation for the reaction of sodium chloride with Magnesium phosphate to produce magnesium chloride and sodium phosphate

Balanced Equations Include coefficients infront of chemical formulas to ensure that the number of each element is the same on both sides of the equation WHY?????? You must follow the Law of Conservation of mass!

Balancing Rules Balance polyatomic ions first if the same ion appears on both sides of the equation Balance all other elements except H an O Balance H Balance O

Examples