Section 2.6—Chemical Reactions If we’re going to do chemical reactions with antacids, we’d better know how to write them!

Watch as 2 H2 and 1 O2 undergo a chemical reaction Chemical Reactions Bonds are broken. New bonds are made to form new compounds. 2 H2 + O2  2 H2O O H H Watch as 2 H2 and 1 O2 undergo a chemical reaction

Chemical Reactions Bonds are broken. New bonds are made to form new compounds. 2 H2 + O2  2 H2O H O O H O H The compounds in the end are different from those in the beginning H Bonds are broken and formed between different atoms

Chemical Equations What is a Chemical Equation?? A shorthand method of showing how reactants convert to products

Examples of Chemical Equations Word equation: magnesium metal is reacted with aqueous hydrochloric acid to produce aqueous magnesium chloride and hydrogen gas  Formula equation: Mg (s) + 2 HCl (aq) MgCl2 (aq) + H2 (g) Visualization

Parts of a Chemical Equation The starting materials for the reaction—each compound is separated by a “+” Formed in the reaction Reactants Products NaCl (aq) + AgNO3 (aq)  AgCl(s) + NaNO3 (aq) States of matter Arrow s = solid l = liquid g = gas aq = aqueous (dissolved in water) Read as: Yields Produces Forms Makes etc.

Special Note about Diatomics Lets break the word down Di= two atomic=atoms Diatomics are elements that exist naturally as two atoms bonded together There are seven of them: H2 N2 O2 F2 Cl2 Br2 I2

Point to Hydrogen then go to N (#7) and draw a “7” I Never Bring Classwork Home On Fridays  I Have No Bright Or Clever Friends HOBrFINCl - Its pronounced “hober-finkle”

Double Replacement Reactions The cations from two compounds replace each other. NaCl + AgNO3  AgCl + NaNO3 Cl Ag Cl Na Ag O N Na O N Two ionic compounds switch ions

Double Replacement Reactions General format of a double replacement reaction:

Write the word equation into symbol form Let’s Practice #1 Aqueous solutions of Copper (II) nitrate and sodium hydroxide form solid copper (II) hydroxide and aqueous sodium nitrate Example: Write the word equation into symbol form

Write the word equation into symbol form Let’s Practice #1 Aqueous solutions of Copper (II) nitrate and sodium hydroxide form solid copper (II) hydroxide and aqueous sodium nitrate Example: Write the word equation into symbol form Cu(NO3)2(aq) + NaOH(aq)  Cu(OH)2(aq) + NaNO3(aq)

Products of a Double Replacement 1 Combine the cation of the first reactant with the anion of the second reactant Ca Cl2 + Ag NO3

Products of a Double Replacement 2 Combine the cation of the second reactant with the anion of the first reactant Ca Cl2 + Ag NO3

Products of a Double Replacement 3 Remember to write cations first … & balance charges with subscripts when writing formulas Only leave subscripts that are in the original compound there if they are a part of a polyatomic ion! Ca Cl2 + Ag NO3 Ca Cl2 + Ag NO3 Ca(NO3)2 + AgCl

Neutralization Reactions General format of a neutralization reaction: salt Water Neutralization reactions are double replacement reactions where one cation is “H” and one anion is “OH” and water is formed. The other compound formed will be a salt.

Products of a Neutralization Reaction 1 Combine the cation of the first reactant with the anion of the second reactant H2 SO4 + Na OH

Products of a Neutralization Reaction 2 Combine the cation of the second reactant with the anion of the first reactant H2 SO4 + Na OH

Products of a Neutralization Reaction 3 Remember to write cations first … & balance charges with subscripts when writing formulas Only leave subscripts that are in the original compound there if they are a part of a polyatomic ion! H2 SO4 + Na OH H2 SO4 + Na OH Na2SO4 + HOH You can write the water as “HOH” when you combine the cation & anion or you can change it to “H2O”

Write the products for this reaction Let’s Practice #2 Example: Write the products for this reaction Sr(OH)2 + HBr 

Write the products for this reaction Let’s Practice #2 Example: Write the products for this reaction Sr(OH)2 + HBr  SrBr2 + HOH

Write the products for this reaction Let’s Practice #3 Example: Write the products for this reaction H2SO4 + Ca(OH)2 

Write the products for this reaction Let’s Practice #3 Example: Write the products for this reaction H2SO4 + Ca(OH)2 HOH + CaSO4

Section 2.7—Balancing Equations We need to finish writing those equations we started!

Watch as 2 H2 and 1 O2 undergo a chemical reaction Chemical Reactions Bonds and atoms are rearranged to form new compounds. 2 H2 + O2  2 H2O O H H Watch as 2 H2 and 1 O2 undergo a chemical reaction

Chemical Reactions Bonds and atoms are rearranged to form new compounds. 2 H2 + O2  2 H2O H O O H O H The compounds in the end are different from those in the beginning H Bonds are broken and formed between different atoms

Law of Conservation of Matter/Mass Law of Conservation of Matter – Matter cannot be created nor destroyed during chemical or physical changes Also called the Law of Conservation of Mass (since all matter has mass)

How Does the Law Lead to Balancing? We have to have the same number and types of atoms on each side of the reaction! We CAN’T lose atoms as we move from reactant to product!!

How do we Balance Equations How do we Balance Equations? Let’s start with discussing what the numbers mean!! Number of compounds in the reaction Coefficients 2 H2 + O2  2 H2O Subscripts # of atoms in a compound Subscripts balance charges within a compound. Coefficients balance atoms in an equation

What do Coefficients Really Mean? CH4 + 2 O2  CO2 + 2 H2O Total: 1 C 4 H 4 O Total: 1 C 4 H 4 O The equation is balanced.

How to Balance Chemical Equations

How to Balance By Inspection: 1 Make a table of elements _____ C H4 + _____ O2  _____ H2 O + _____ C O2 Reactants Products C H O

How to Balance By Inspection: 2 Count the number of each element or ion on the reactants and products side. Don’t forget to add all the atoms of the same element together—even if it appears in more than one compound! _____ C H4 + _____ O2  _____ H2 O + _____ C O2 Reactants Products C 1 1 H 4 2 O 2 3

How to Balance By Inspection: 3 Add coefficients to balance the numbers Each time you add a coefficient, update your table with the new quantities of each atom. _____ C H4 + _____ 2 O2  _____ 2 H2 O + _____ C O2 Reactants Products C 1 1 H 4 2 4 O 2 4 3 4

Tips to Balancing How do you know what order to balance in? Start With: Elements that appear only 1 time per side Save for later Elements that are uncombined Pb + PbO2 + H+  Pb2+ + H2O Start With: Elements in most complicated molecules Save for later Elements that appear more than 1 time per side To balance this equation, use the order: O, H, Pb

Polyatomic Ions and the balanced equation Polyatomic ion – Group of atoms that together has a net charge e.g. Nitrate NO31- Carbonate CO32- A polyatomic ion is a group of atoms that together have a charge.

Balancing with Polyatomic Ions: 1 Make a table of elements You may leave polyatomic ions together—IF they appear intact on both sides of the reaction. OH-1 is a polyatomic ion that is sometimes “hidden” in H2O. Re-write H2O as HOH to “see” the OH polyatomic ion. H OH _____ H3 PO4 + _____ Ca (OH)2  _____ Ca3 (PO4)2 + _____ H2 O Reactants Products H PO4 Ca OH

Balancing with Polyatomic Ions: Count the number of each element or ion on the reactants and products side. 2 H OH _____ H3 PO4 + _____ Ca (OH)2  _____ Ca3 (PO4)2 + _____ H2 O Reactants Products H 3 1 PO4 1 2 Ca 1 3 OH 2 1

Balancing with Polyatomic Ions: 3 Add coefficients to balance the numbers H OH _____ 2 H3 PO4 + _____ 3 Ca (OH)2  _____ Ca3 (PO4)2 + _____ 6 H2 O Reactants Products H 3 6 1 6 PO4 1 2 2 Ca 1 3 3 OH 2 6 1 6

Balance the following equation Let’s Practice #1 Example: Balance the following equation __ HCl + __ Ca(OH)2  __ CaCl2 + __ H2O

Balance the following equation Let’s Practice #1 Example: Balance the following equation Did you see the “OH” polyatomic ion? Should we change H2O to HOH? 2 1 1 2 __ HCl + __ Ca(OH)2  __ CaCl2 + __ H2O HOH

Balance the following equation Let’s Practice #2 Example: Balance the following equation __ H2 + __ O2  __ H2O

Balance the following equation Let’s Practice #2 Example: Balance the following equation 2 1 2 __ H2 + __ O2  __ H2O

Balance the following equation Let’s Practice #3 Example: Balance the following equation __ Fe + __ O2  ___ Fe2O3

Balance the following equation Let’s Practice #3 Example: Balance the following equation 4 3 2 __ Fe + __ O2  ___ Fe2O3