Chapter 21 Chemical Reactions

Sec. 1 Chemical Changes Chemical Reaction—a change in which 1 or more substances are converted into new substances. Reaction can be abbreviated as RXN Ways to tell if a chemical rxn has occurred: Color change, bubbles produced, NEW odor produced or energy change.

Reactants & Products Reactants—substances that combine or change The things you mix together Products—new substances formed. The things made in the end

Chemical Equations Chemical equation—a way to describe a chemical rxn using formulas & other symbols. Write the reactants on the left, & products on the right. Reactants  Products

Other Symbols  : Between reactants & products & is read as “yields” or “produces” + (plus): separates 2 formulas on either the reactant or product side Coefficients: (#s in front of formulas) tell the # of units of each substance in the rxn. Ex. 2Na + Cl2  2NaCl Subscripts tell you how many atoms are in the compound

Aqueous (aq): a substance that is dissolved in water. State of Matter The symbol for the state of matter for each compound is written after formula in parentheses. Solid (s) Liquid (l) Gas (g) Aqueous (aq): a substance that is dissolved in water. 2Na(s) + Cl2(g)  2NaCl(s)

3 ways to write a Chemical Equation Word Equation Skeleton Equation Balanced Equation

Word Equations Describe chemical reactions using words with limited symbols. Hydrogen(g) + Oxygen(g)  Water(l) (word)

Skeleton Equation Equations that show the formulas of the reactants and products NOT the amounts of reactants or products H2 (g) + O2 (g) → H2O(l) skeleton

Balanced Chemical Equations (BCE) Uses COEFFICIENTS to show the amount of reactants and products in the reaction Follows the Law of Conservation of Mass 2H2 (g) + 1O2 (g) → 2H2O(l) balanced

Conservation of Mass Law of Conservation of Mass—mass cannot be created or destroyed; it can just change forms The mass of reactants is always equal to the mass of products. Ex. 0.7g O2 + 9.3g Hg  10.0g HgO2

Closed vs Open System Closed system—the chemical rxn is contained. No product is lost Open system—the chemical rxn is not completely contained. Some product (mostly gases) can escape. BOTH systems follow the LOCO mass—closed systems are easier to measure

Steps for Balancing Equations Determine the formulas for all reactants and products. (this may be given) Reactants on the left & products on the right.

Count the # of each element in reactants & products CH4 + O2  CO2 + H2O C = 1 C = 1 H = 4 H = 2 O = 2 O = 3

3. Balance each element by using coefficients—number placed in front of the symbol. 4. Make sure all elements are balanced. Ex. CH4 + O2  CO2 + H2O C = 1 C = 1 H = 4 H = 2 O = 2 O = 3 2 2 4 4 4

5. Make sure all coefficients are in the lowest ratio. Ex. 2CH4 + 4O2  2CO2 + 4H2O Can be reduced to 1 2  1 2

Chem Balancer Online http://funbasedlearning.com/chemistry/chembalancer/default.htm Complete all 11! Draw out 12 and 13 like shown online!

Types of Chemical Rxns

Combination/ Synthesis Rxn 2 or more reactants combine to form a single product. a + b  ab Examples: 2K + Cl2 2KCl (Can start with 2 elements) CaO + H2O Ca(OH)2 (or 2 compounds)

Decomposition Rxn A single compound is broken down into 2 or more products. ab  a + b Examples: PbO2  Pb + O2 (Can make 2 elements) CaCO3  CaO + CO2 (or 2 compounds)

Single-Replacement Rxn One element replaces a second element in a compound. a + bc ac +b Example: Mg + 2AgNO3Mg(NO3)2 + 2Ag 2NaBr + Cl2  2NaCl + Br2

Double-Replacement Rxn Involve an exchange of cations between 2 ionic compounds. ab + cd ad + cb Examples: Na2S + Cd(NO3)2  CdS + 2NaNO3

Combustion Rxn Heat & light are also often given off A compound containing C & H reacts with oxygen (O2), producing CO2 & H2O Heat & light are also often given off CxHy + O2  CO2 + H2O Example: 2CH3OH + 3O2 2CO2 + 4H2O

Analogies for 4 of the types of reactions Synthesis/Combination Falling in Love Decomposition The Break-up Single Replacement Couple dancing, someone cuts in Double Replacement 2 couples dancing switch partners

Activity Series a + bc ac +b In Single-Replacement rxns, not all metals can replace other metals. a + bc ac +b The single metal element (a) must be more reactive than the one in the compound (b). The activity series is a list of metals in order of reactivity. On the back of your P.T.!!

Tips for balancing equations Identify the type of rxn 1st Start by balancing oxygen or another diatomic element Make odds even by using a coefficient of 2 Balance O2 last in combustion rxns CO2 & H20 are products Go back & forth until all elements balance. Balance in Pencil!

Quiz: Balance the 4 equations and classify the type of rxn. ___1. __Al + __Cl2 __AlCl3 ___2. __Mg + __CuCl  __MgCl2 + __Cu ___3. __FeCl3 + __NaOH Fe(OH)3 + NaCl ___4. __CH4 + __O2  __CO2 + __H2O

Predicting Products Identify the products that should be formed based on type of rxn Write the formulas correctly—DON’T FORGET to look up & balance charges If it is ionic, make sure you have a cation with an anion Remember: some elements are diatomic (exist as 2 atoms): H2, N2, O2, F2, Cl2, Br2, & I2 You MUST have correct formulas BEFORE you balance.

Example __LiCl + __CuSO4 Type of Rxn: Double Replacement Li and Cu are both metals (cations) so they can switch. Look at charges of ions: Li+1 Cl-1 Cu+2 SO4-2 Products are… __Li2SO4 + __CuCl2 Balance

State the type of reaction, Predict the products & Balance the equation. Type of Rxn Reactants Products 1. _____________ __BaCl2 + __H2SO4  2. _____________ __C6H12 + __O2  3. _____________ __Zn + __CuSO4  4. _____________ __Cs + __Br2  5. _____________ __FeCl3  __BaSO4 + __HCl 2 Double-Replacement __CO2 + __H2O Combustion 9 6 6 __ZnSO4 + __Cu Single-Replacement Balanced!!! Synthesis 2 __CsBr 2 2 __Fe + __Cl2 2 3 Decomposition

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 H2O2 MnO2 H2O + O2 Heat as a catalyst Inhibitors—prevent certain rxns from occurring. Used in food preservatives to prevent spoiling.

Some elements are diatomic (exist as 2 atoms) They have the same name as the element (ex. not dihydrogen) Hydrogen: H2 Nitrogen: N2 Oxygen: O2 Fluorine: F2 Chlorine: Cl2 Bromine: Br2 Iodine: I2