Evidence & Types of Chemical Reactions 9/30/10

total mass of reactants = total mass of products Part I: Describing Chemical Reactions chemical reaction = the process by which one or more substances are changed into one or more different substances. in any chemical reaction, the original substances are known as the reactants and the resulting substances are known as the products. according to the law of conservation of mass: total mass of reactants = total mass of products chemical reactions are described by chemical equations. chemical equation = represents, with symbols and formulas, the identities and molar amounts of the reactants and products in a chemical reaction. For example: “The reactant ammonium dichromate yields the products nitrogen, chromium (III) oxide, and water.” is the same as: the equation above is UNBALANCED. We will balance it using coefficients later. total mass of reactants = total mass of products (NH4)2Cr2O7(s)  N2(g) + Cr2O3(s) + H2O(g)

(NH4)2Cr2O7(s)  N2(g) + Cr2O3(s) + H2O(g)

CO2(g) + NaC2H3O2(aq) + H2O(l) Part II: Evidence of Chemical Reactions to know for certain that a reaction has taken place requires evidence that one or more substances have undergone a change in identity. Evolution of heat and light. A change in matter that releases energy as both heat and light is strong evidence that a chemical reaction has taken place. Production of a gas. The evolution of gas bubbles when two substances are mixed is often evidence of a chemical reaction. Ex: bubbles of CO2 gas form immediately when baking soda is mixed with vinegar. Formation of a precipitate. Many reactions take place between substances that are dissolved in liquids. If a solid appears after two solutions are mixed, a reaction has likely occurred. precipitate = a solid that is produced as a result of a chemical reaction in solution that separates from the solution. NaHCO3(s) + CH3COOH(l)  CO2(g) + NaC2H3O2(aq) + H2O(l)

precipitate = a solid that is produced as a result of a chemical reaction in solution that separates from the solution. Color change. A change in color is often an indication of a chemical reaction. Ex: Bleach breaks down dye molecules attached to fabric, altering the wavelength of light they reflect, thus changing the color. Evolution of an odor. Odors that suddenly appear indicate new, aromatic chemicals have been produced. Part III: Types of Chemical Reactions there are 5 distinct types of chemical reactions. Each has a general formula that represents the types of substances that react within each type of reaction. synthesis decomposition single replacement double replacement combustion The first 4 types of reactions often involve ionic compounds. It’s helpful to think of the cations (A and B) as boys and the anions (X and Y) as girls.

A + X  AX AX  A + X AX + B  BX + A AX + Y  AY + X synthesis = combines 2 small compounds or atoms into one larger compound the “marriage” reaction Ex: Na + Cl2  NaCl K + CO3  K2CO3 Al + O2  Al2O3 decomposition = splits 1 compound into 2 or more smaller compounds or atoms the “divorce” reaction Ex: NaCl  Na + Cl2 Cr2O3  Cr + O2 N3S5  N2 + S single replacement = Replaces cation or anion of a compound with another free cation or anion the “one-cheater” reaction Ex: NaCl + K  KCl + Na Al2(CO3)3 + Li  Li2CO3 + Al NaCl + Br2  NaBr + Cl2 Mn3P2 + O2  MnO + P A + X  AX AX  A + X AX + B  BX + A AX + Y  AY + X

AX + BY  AY + BX CnHn + O2  CO2 + H2O double replacement = the cation of one compound is combined with anion of the other compound, creating 2 new compounds the “switching partners” reaction Ex: NaCl + KBr  NaBr + KCl Li2S + TiF2  LiF + TiS Sn3P4 + Zn(NO3)2  Zn3P2 + Sn(NO3)4 combustion = Hydrocarbon + O2 produces CO2/CO, H2O plus energy the “involves hydrocarbons” reaction (not ionic compounds) sometimes results in CO2 as a product, other times CO Ex: C2H6 + O2  CO2 + H2O C7H14 + O2  CO2 + H2O C10H22 + O2  CO2 + H2O C7H14 + O2  CO + H2O now let’s take a closer look at the single-replacement reaction again... AX + BY  AY + BX CnHn + O2  CO2 + H2O

NaHCO3 (s) + CH3COOH(l)  CO2 + NaC2H3O2(aq) + H2O(l) Li Rb K Ba Sr Ca Na Mg Al Mn Zn Cr Fe Cd let’s take a closer look at the single-replacement reaction... the activity series helps determine whether or not one element can replace another in a single-replacement rxtn. if an element is above another, it can replace it. so, can these reactions happen or not? Ex: NaCl + K  KCl + Na Al2(CO3)3 + Li  Li2CO3 + Al NaCl + Br2  NaBr + Cl2 Part IV: Symbols Found in Chemical Equations Co Ni Sn Pb H2 Sb Bi Cu Hg Ag Pt Au F2 Cl2 Br2 I2 NaHCO3 (s) + CH3COOH(l)  CO2 + NaC2H3O2(aq) + H2O(l)  (s)   (g)  (l) 2 atm (aq) 0°C  MnO2

NaHCO3 (s) + CH3COOH(l)  CO2 + NaC2H3O2(aq) + H2O(l) Part IV: Symbols Found in Chemical Equations Part V: Other Info About Chemical Reactions some elements occur only in diatomic form, meaning they are bonded to another atom of that element. H2 N2 O2 F2 Cl2 Br2 I2 these gases are never found by themselves in any chemical formula or equation!!! NaHCO3 (s) + CH3COOH(l)  CO2 + NaC2H3O2(aq) + H2O(l)  (s)   (g)  (l) 2 atm (aq) 0°C  MnO2 solid ( for precipitate) gas ( for gaseous product) liquid in aqueous solution yields or produces reversible reaction heat applied to reactants specific pressure for rxtn specific temp for reaction catalyst needed for rxtn

5Al2(SO4)3 4Na2S 2MgCl2 Part V: Other Info About Chemical Reactions some elements occur only in diatomic form, meaning they are bonded to another atom of that element. H2 N2 O2 F2 Cl2 Br2 I2 these gases are never found by themselves in any chemical formula or equation!!! coefficients = large numbers in front of a chemical formula subscripts = little numbers within a chemical formula 5Al2(SO4)3 4Na2S 2MgCl2 we will learn about balancing equations (using coefficients) tomorrow!