Chemical Reactions Chemical Reaction One or more substances undergoing a change in identity. – Indications of a chemical reaction: Temperature change Production of a gas Formation of a precipitate Color change Odor

Chemical Reactions Chemical Equation – Can summarize any chemical change – Characteristics of a chemical equation Must represent known facts. Must contain the correct formulas for products and reactants. Law of conservation of mass Must be satisfied.

Chemical Reactions Law of Conservation of Mass – In any chemical reaction, the total mass of the reactants equals the total mass of the products. – Atoms are rearranged but are never lost or gained.

Chemical Reactions Writing a chemical equation 1.Write a Word Equation – Use words to represent products and reactants. E.g.Hydrogen gas reacts with oxygen gas to produce liquid water

Chemical Reactions 2. Write a Formula Equation – Use chemical formulas to represent reactants and products E. g. H 2 (g) + O 2 (g)  H 2 O(l) Once formulas are correctly written, they are NEVER changed.

Chemical Reactions Some symbols used in chemical equations –  yields (makes, forms, produces) – (s)in the solid form – (aq)in aqueous solution (dissolved in water) – (l) in a liquid state – (g)gaseous state – Δusing heat (written above the arrow) –  Gas evolves –  Precipitate forms

Chemical Reactions 3.Balance the formula equation in order to satisfy the Law of Conservation of Mass. Need to have the same number of atoms of each type in the reactants and the products. – Use Coefficients to balance – Coefficient » small whole number that appears in front of a chemical formula

Chemical Reactions Balancing chemical equations Step 1 – Identify the names of the reactants and products and write a word equation hydrogen gas plus oxygen gas produces liquid water

Chemical Reactions Step 2 – Write a formula equation H 2 (g) + O 2 (g)  H 2 O(l) This is called a “skeleton equation.”

Chemical Reactions Step 3 – Count the number of each atom present H 2 (g) + O 2 (g)  H 2 O(l)H – 2 O – 2O - 1

Chemical Reactions Step 4 – Add coefficients to balance the formula equation according to the Law of Conservation of Mass 2H 2 (g) + O 2 (g)  2H 2 O(l) – Count atoms to be sure the equation is balanced H – 4 atomsH- 4 atomsO – 2 atoms TA-DA BALANCED

Chemical Reactions Types of Chemical Reactions (Rxn) 1.Synthesis Reaction (sometimes called combination) – Two or more materials combine to produce one product A + B = AB

Chemical Reactions Types of Chemical Reactions 2.Decomposition – One material breaks down into two or more simpler materials. AB = A + B

Chemical Reactions Types of Chemical Reactions 3.Single Replacement – – A free element in solution replaces an element in a compound, producing a new compound and a different free element. A + BX = AX+ B

Chemical Reactions Types of Chemical Reactions 4. Double Replacement – the ions of two compounds interchange to produce two new compounds. AX +BY = AY + BX

Chemical Reactions A special kind of double replacement reaction is a “Neutralization “ reaction. Acids and bases react to form a salt (ionic compound) and water. HX + YOH  YX + HOH (acid) (base) (salt) (water) HCl + NaOH  NaCl +H 2 O

Chemical Reactions Types of Chemical Reactions 5.Combustion – a hydrocarbon combines with oxygen to form carbon dioxide and water. C x H y + O 2  CO 2 + H 2 O