Unit 6 Chemical Reactions and Equations What They Represent & How to Balance Them

Chemical Reactions Reactions are chemical changes in which new substances form. How do you know???? If the material looks different, smells different, behaves in a new way, guess what??? It IS different!!! A chemical reaction is simply a REARRANGEMENT of atoms.

ACTIVITY IN THIS ACTIVITY, YOU WILL EXAMINE A VARIETY OF CHEMICAL AND PHYSICAL CHANGES IN A ZIPLOCK BAG.

POST ACTIVITY DISCUSSION What happens at various points in the minilab??? When water is added to calcium chloride? What kind of change is this????? When baking soda is added to calcium chloride?

5 Indications of Chemical Rxn Color Change Heat / Light Released or Absorbed Formation of a Solid (precipitate) Formation of a Gas Formation of an Odor

5 Types of Chemical Reactions Synthesis (Combination) A + B  AB Decomposition AB  A + B Single Displacement A + BC  B + AC Double Displacement AB + CD  AD + CB Combustion CnHm + O2  CO2 + H2O A, B, C, D are elements. AB, BC, AC, CD, CB, and AD are all compounds of elements A, B, C, and D. CnHm is a specific hydrocarbon.

Classify the following reactions: Fe2(SO4)3 + Ba(OH)2  BaSO4 + Fe(OH)3 Double displacement Al + CuSO4  Al2(SO4)3 + Cu Single displacement KClO3  KCl + O2 Decomposition Mg + N2  Mg3N2 Synthesis C6H14 + O2  CO2 + H2O Combustion

The Chemical Equation 2 Fe(s) + 3 Cl2(g)  2 FeCl3(s) The chemical equation is a shorthand representation of the substances involved in a chemical reaction. Reactants are on the left. Products are on the right. The arrow means “produces” or “changes into”. The letters in parentheses are the physical states of the substances: (s) -- solid (l) -- liquid (g) -- gas (aq) -- aqueous

The Chemical Equation 2 Fe(s) + 3 Cl2(g)  2 FeCl3(s) The smaller numbers within a formula are called subscripts, indicating the number of each atom in the formula for a substance. The larger numbers are called coefficients, indicating the number of atoms or molecules of each substance reacting or being produced in the chemical change.

The Balanced Chemical Equation What is it? A balanced chemical equation has the same number of each type of atom on BOTH sides of the equation. Why? All (non-nuclear) chemical reactions must obey the law of conservation of mass. The mass of the reactants equals the mass of the products in ordinary chemical reactions.

The Balanced Chemical Equation How? Examine the chemical equation to see if there are the same number of each type of atom on both sides of the arrow. If not, place coefficients in front of each formula to make them equal. Re-inventory each time a coefficient is added. When counting the atoms of each element, the coefficient gets distributed onto each atom in the formula it precedes.

Balancing Equations Two Helpful Hints 1. Balance H & O last. (usually) 2. If there is an even # on one side and an odd # on the other side, always change odd to even by making a common multiple. Fe + O2  Fe2O3 4 Fe + 3 O2  2 Fe2O3

Balance The Following BF3 + H2O  HF + H3BO3 1,3,3,1 SO2+ CaCO3+ O2  CaSO4 + CO2 2,2,1,2,2 C3H8 + O2  H2O + CO2 1,5,4,3

ACTIVITY YOU WILL CLOSELY EXAMINE THE CHEMICAL REACTION BETWEEN SILVER NITRATE AND COPPER WIRE.

PREDICTING PRODUCTS IN CHEMICAL REACTIONS How is it done? One must know the type of reaction. Then, elements must be combined, separated, or switched and recombined according to the type of reaction that occurs.

The formulas for the products are determined by: Formulas for Products The formulas for the products are determined by: Switching charges for compounds Determining if a subscript is needed for the stand alone elements. (Diatomic elements are H2, N2, O2, F2, Cl2, Br2, and I2) Most others have no subscript like Fe, Al, Mn, Si, and Xe.

SYNTHESIS A + B  AB This is the reaction where elements combine to form a compound: H2 + Cl2  what is the product? When hydrogen and chlorine combine, what are the subscripts? Look up hydrogen H+1 Look up chlorine Cl-1 The resulting formula is HCl, the result of the criss-cross and drop the sign method from the formula chapter.

SYNTHESIS The resulting unbalanced equation is: H2 + Cl2  HCl The balanced equation is: H2 + Cl2  2 HCl

SYNTHESIS This is the reaction where elements combine to form a compound: Ca + N2  what is the product? When calcium and nitrogen combine, what are the subscripts? Look up calcium Ca+2 Look up nitrogen N-3 The resulting formula is Ca3N2.

SYNTHESIS The resulting unbalanced equation is: Ca + N2  Ca3N2 The balanced equation is: 3 Ca + N2  Ca3N2

Must go back and balance all of these!! 311, 232, and 212 SYNTHESIS Na + P  Na+1 P-3 = Na3P Na + P  Na3P Al + F2  Al+3 F-1 = AlF3 Al + F2  AlF3 Zn + O2  Zn+2O-2 = ZnO Zn + O2  ZnO Must go back and balance all of these!! 311, 232, and 212 21

DECOMPOSITION AB  A + B This is when a compound breaks up into simpler substances. When H2O decomposes, what are the products? The products are hydrogen and oxygen in their elemental forms. Check to see if either or both are diatomic elements, written with a 2 as the subscript. H2O  H2 + O2 unbalanced 2 H2O  2 H2 + O2 balanced

DECOMPOSITION NH3  The products are nitrogen and hydrogen Are they diatomic or not? Yes, therefore products are N2 and H2 NH3  N2 + H2 unbalanced 2 NH3  N2 + 3 H2 balanced

DECOMPOSITION CuBr  Copper is not diatomic, bromine is. CuBr  Cu + Br2 GaF3  Gallium is not diatomic, fluorine is. GaF3  Ga + F2 N2O  Nitrogen and oxygen are both diatomic. N2O  N2 + O2 Must go back and balance these: 221, 223, 221

SINGLE DISPLACEMENT A + BC  B + AC If A is a metal, it will replace B, the metal or H, in BC, releasing B as an element. For the reaction to occur, A must be higher on the activity series than B. Otherwise, the reaction will not occur.

SINGLE DISPLACEMENT Mg + HCl  Mg is a metal that will replace H in HCl. They switch places, releasing H as an element. Check to see if H is diatomic. Yes, so H2. Check the charges of Mg and Cl. Mg+2 and Cl-1 , so MgCl2 is the formula. Then balance the final equation. Mg + HCl  H2 + MgCl2 1,2,1,1

SINGLE DISPLACEMENT A + BC  C + BA If A is a non-metal, it will replace C, the nonmetal, in BC, releasing C as an element. BA will form as a compound in a ratio that depends on the charges. For the reaction to occur, A must be higher on the activity series than C. Otherwise, the reaction will not occur.

SINGLE DISPLACEMENT F2 + KBr  F is a non-metal that will replace Br in KBr. They switch places, releasing Br as an element. Check to see if Br is diatomic. Yes, so Br2. Check the charges of K and F. K+1 and F-1 , so KF is the formula. Then balance the final equation. F2 + KBr  Br2 + KF 1,2,1,2

SINGLE DISPLACEMENT Zn + Cu(NO3)2  Na + MgCl2  Cl2 + AgI  I2 + AgCl 

DOUBLE DISPLACEMENT AB + CD  CB + AD A and C switch places. C combines with B. A combines with D. The ratio of the atoms depends on the charge. Look up charges of each ion, criss-cross and drop the sign in the formula. Balance the equation.

DOUBLE DISPLACEMENT KI + Pb(NO3)2  K and Pb switch places. Pb combines with I. K combines with NO3. Pb+2 I-1 = PbI2 and K+1 NO3-1 = KNO3 So, KNO3 and PbI2 are the products. Balance the equation. KI + Pb(NO3)2  KNO3 + PbI2 2,1,2,1

DOUBLE DISPLACEMENT MgCl2 + AgNO3  Na2CO3 + CaCl2  NaOH + H3PO4  K2CrO4 + PbCl2 

COMBUSTION CnHm + O2  CO2 + H2O Simple combustion reactions involve the reaction of hydrocarbons with oxygen to form carbon dioxide and water. Balance the C, H and O atoms in that order. If, in the end, there is an odd number of O atoms on the right, you may need to double the hydrocarbon by simply placing a 2 coefficient in front of the CnHm compound.

COMBUSTION C5H12 + O2  Complete the reaction with CO2 and H2O. Balance C, then H then O. C5H12 + O2  CO2 + H2O 1,8,5,6

COMBUSTION C4H10 + O2  Complete the reaction with CO2 and H2O. Balance C, then H then O. C4H10 + O2  CO2 + H2O 2,13,8,10