Chapter 21 Chemical Reactions (Rxns)

Sec. 1 Chemical Changes Chemical Reaction—a change in which 1 or more substances are converted into new substances. Reactants—substances that react or the starting substances. Products—new substances produced. Ways to tell if a chemical reaction has occurred: Odor, color, bubbles

Conservation of Mass Law of Conservation of Mass— mass cannot be created or destroyed. The mass of reactants is always equal to the mass of products. Ex. 0.7g O g Hg  10.0g HgO 2

Chemical Equations Chemical equation—a way to describe a chemical rxn using chemical formulas and other symbols. Reactants are written on the left, products on the right. Reactants  Products The  is read as “yields” or “produces”

States of Matter & Coefficients The physical state of the reactants and products is written after the formula in parentheses. Solid (s) Liquid (l) Gas (g) Aqueous (aq): a substance dissolved in water. Coefficients (#s to the left of formulas) tell the # of units of each substance in the rxn.

Writing Equations A chemical reaction can be written in words Ex. One unit of solid zinc plus 2 units of hydrochloric acid dissolved in water produce one unit of hydrogen gas plus one unit of zinc chloride dissolved in water. You can use chemical formulas & symbols in the chemical equation. Ex. Zn(s) + 2HCl(aq)  H 2 (g) + ZnCl 2 (aq)

Remember: some elements are diatomic (exist as 2 atoms) Hydrogen: H 2, Nitrogen: N 2, Oxygen: O 2, Fluorine: F 2, Chlorine: Cl 2, Bromine: Br 2, Iodine: I 2 (\ v /) (( )) ^^ ☻ |

Balancing Chemical Equations 1.Determine the formulas for all reactants and products. (this may be given) 2.Write reactants on the left and products on the right of the arrow. 3.Count the # of each element in reactants & products Ex. CH 4 + O 2  CO 2 + H 2 OC = 1 H = 4H = 2 O = 2O = 3

Balancing Chemical Equations 4. Balance each element by using coefficients—number placed in front of the symbol. 5. Make sure all elements are balanced. 6. Make sure all coefficients are in the lowest ratio. Ex. CH 4 + O 2  CO 2 + H 2 OC = 1 H = 4H = 2 O = 2O = 3 ____ CH 4 + ____ O 2  ____ CO 2 + ____ H 2 O 22

Types of Chemical Rxns

Combination/ Synthesis Rxn 2 or more reactants combine to form a single product. Examples: 2K(s) + Cl 2 (g)  2KCl(s) CaO(s) + H 2 O(l)  Ca(OH) 2 (aq)

Decomposition Rxn A single compound is broken down into 2 or more products. Examples: PbO 2 (s)  Pb(l) + O 2 (g) CaCO 3 (s)  CaO(s) + CO 2 (g)

Single-Replacement Rxn One element replaces a second element in a compound. Example: Mg(s) + 2AgNO 3 (aq)  Mg(NO 3 ) 2 (aq) + 2Ag(s)

Double-Replacement Rxn Involve an exchange of positive ions between 2 ionic compounds. Examples: Na 2 S + Cd(NO 3 ) 2  CdS + 2NaNO 3 2NaCN + H 2 SO 4  2HCN + Na 2 SO 4

Combustion Rxn An element or compound reacts with oxygen (O 2 ) often producing CO 2 & H 2 O HeatlightHeat & light are also often given off Example: 2CH 3 OH(l) + 3O 2 (g)  2CO 2 (g) + 4H 2 O(g)

Romance analogy for the four types of reaction CombinationFalling in love DecompositionThe Break-up Single ReplacementThe player Double ReplacementThe swingers

Completing Chem Rxns Some steps for doing reactions 1.Identify the type of reaction 2.Predict the product(s) 3.Balance the equation using COEFFICIENTS Remember: some elements are diatomic (exist as 2 atoms) Hydrogen: H 2, Nitrogen: N 2, Oxygen: O 2, Fluorine: F 2, Chlorine: Cl 2, Bromine: Br 2, Iodine: I 2

When Predicting Products When predicting products, remember to write the formulas correctly—DON’T FORGET TO BALANCE OUT THE CHARGES! Ex. Li + CuSO 4  Type of Rxn: Single Replacement Li replaces Cu & bonds with SO 4 Look at charges of ions: Li +1 SO 4 -2 Products are… Li 2 SO 4 + Cu

State the type of reaction, Predict the products & Balance the equation. Type of RxnReactants Products 1. _____________ __BaCl 2 + __H 2 SO 4  2. _____________ __C 6 H 12 + __O 2  3. _____________ __Zn + __CuSO 4  4. _____________ __Cs + __Br 2  5. _____________ __FeCO 3  __BaSO 4 + __HCl __CO 2 + __H 2 O __ZnSO 4 + __Cu __CsBr __FeO + __CO 2 Double-Replacement Combustion Single-Replacement Combination Decomposition Balanced!!! 22 Balanced!!!

Activity Series for Metals In Single-Replacement, not all metals elements can replace other metals. The single metal element must be more reactive (ha! More attractive) than the one in the compound. Look on P. 643 (figure 13) in textbook for the activity series WRITE THIS DOWN!!!

Google: Metals in Aqueous Solutions Follow Directions as you go and complete all 4 Activities.

Sec. 4 Chem Rxns & Energy Some chemical rxns require heat in order to happen Some rxns give off heat. Thermochemistry—heat changes that occur during chemical reactions.

Exo & Endo Exothermic—a rxn where energy (heat) is given off. Ex: burning wood, exploding dynamite. Exothermic rxns feel warm. Endothermic—a rxn where energy (heat) is absorbed. Ex. Cold packs Endothermic rxns feel cold.

Catalysts & Inhibitors Catalyst—a substance that speeds up a chem rxn without being used in the rxn. Written above the arrow in an equation. Used in making plastics H2O + O2MnO2 H2O2 Inhibitors—prevent certain rxns from occurring. Used in food preservatives to prevent spoiling. Heat as a catalyst