Chemical Reactions

In a chemical reaction: there is a change in the way atoms are joined together there is a change in the way atoms are joined together the Law of Conservation of Mass applies: atoms cannot be created or destroyed, only rearranged the Law of Conservation of Mass applies: atoms cannot be created or destroyed, only rearranged can be described several ways: can be described several ways: 1. In a sentence Copper reacts with chlorine to form copper (II) chloride. 2. In a word equation Copper + chlorine copper (II) chloride 3. Using symbols Cu + Cl 2 CuCl 2 Cu + Cl 2 CuCl 2

Symbols in Chemical Reactions yield sign - separate the reactants (on the left) from the products (on the right) yield sign - separate the reactants (on the left) from the products (on the right) (s) = solid (s) = solid (g) = gas (g) = gas (l) = liquid (l) = liquid (aq) = aqueous solution (dissolved in water) (aq) = aqueous solution (dissolved in water)

Symbols in Chemical Reactions heat or Δ indicates heat is required for the reaction to occur heat or Δ indicates heat is required for the reaction to occur catalyst - substance that speeds up a reaction without being changed or used up by the reaction catalyst - substance that speeds up a reaction without being changed or used up by the reactionExample: Cu(s) + AgNO 3 (aq) Ag(s) + Cu(NO 3 ) 2 (aq) Cu(s) + AgNO 3 (aq) Ag(s) + Cu(NO 3 ) 2 (aq) reactantsproducts reactantsproducts

Balancing Chemical Equations A balanced equation must have the same number of atoms on both sides of the equation in order to observe the Law of Conservation of Mass. A balanced equation must have the same number of atoms on both sides of the equation in order to observe the Law of Conservation of Mass.Example: Fe + Cl 2 FeCl 3 coefficient – whole number written in front of a reactant or product; not usually written if it is one coefficient – whole number written in front of a reactant or product; not usually written if it is one

Rules for balancing equations 1. Write the correct formulas for the reactants and products. 2. Count the number of atoms of each type appearing on both sides. 3. Balance the elements one at a time by adding coefficients in lowest possible ratio. 4. Check to make sure it is balanced.

Things you should NEVER do Never change a subscript to balance an equation. Never change a subscript to balance an equation. If the formula changes, then it ’ s a different compound. If the formula changes, then it ’ s a different compound. H 2 O is different from H 2 O 2 H 2 O is different from H 2 O 2 Never put a coefficient in the middle of a formula. Never put a coefficient in the middle of a formula. 2 NaCl, not Na2Cl 2 NaCl, not Na2Cl

Types of Chemical Reactions 1. Combination (Synthesis) Reaction – two substances combine to form one product Examples: 2 Ca (s) +O 2 (g)  2 CaO (s)

Types of Chemical Reactions 2. Decomposition Reaction – one reactant splits apart into two or more elements or compounds; energy is usually required Examples: 2 NaCl (aq)  2Na (s) + Cl 2 (g)

Types of Chemical Reactions 3. Combustion - hydrocarbon compound composed of C and H reacts with oxygen hydrocarbon compound composed of C and H reacts with oxygen for complete combustion, products are CO 2 & H 2 O for complete combustion, products are CO 2 & H 2 OExample: C 3 H O 2 → 3 CO H 2 0

Types of Chemical Reactions 4. Single Replacement Reaction – reactants will be an element and a compound; one element replaces another metals replace other metals (and hydrogen) metals replace other metals (and hydrogen) Example: K (s) + NaCl (aq) Na (s) + KCl (aq) can predict these reactions using an activity series of metals – higher (more active) metals on the list replace lower (less reactive) metals can predict these reactions using an activity series of metals – higher (more active) metals on the list replace lower (less reactive) metals nonmetals replace other nonmetals (F 2, Cl 2, Br 2, I 2 ) nonmetals replace other nonmetals (F 2, Cl 2, Br 2, I 2 )

Types of Chemical Reactions 5. Double Replacement Reaction two elements or ions “ switch ” partners two elements or ions “ switch ” partners reactants are always two ionic compounds, usually in (aq) solution reactants are always two ionic compounds, usually in (aq) solution Will only occur if one of products is: a precipitate (solid that doesn ’ t dissolve in water) a precipitate (solid that doesn ’ t dissolve in water) a gas that bubbles out a gas that bubbles out H 2 O or another molecular compound (nonmetals) H 2 O or another molecular compound (nonmetals)Example: 3 NaOH (aq) + FeCl 3 (aq) Fe(OH) 3 (s) + 3 NaCl (aq) 3 NaOH (aq) + FeCl 3 (aq) Fe(OH) 3 (s) + 3 NaCl (aq)