2How Many Atoms in a Molecule? Part I: Counting AtomsHow Many Atoms in a Molecule?
3Counting AtomsMost substances that we encounter are compounds, not elements.• A chemical compound is a pure substance formed from the combination of two or more different elements.The properties of the compound may be completely unlike those of the elements that form it.• The formula for a compound lists the symbols of the individual elements followed by subscripts which indicate the number of atoms of that element.(If no subscript is given, it is understood to be “1.”) E.g., NaCl, H2O, C12H22O11.
4Counting AtomsA molecular formula gives the actual number of atoms of each element in a molecule of a compound.Hydrogen peroxide H2O2Water H2OGlucose C6H12O6A structural formula uses lines to represent covalent bonds, and shows how the atoms in a molecule are joined together:H—O—O—HH—O—HO=C=O
6Counting Atoms with Polyatomic Ions - Al2(SO4)3 =SO
7Counting Atoms with Polyatomic Ions When counting atoms with polyatomic ions;Count number of atoms in one polyatomic ionIons inside the parenthesesMultiply by number of polyatomic groups in the molecule (number outside the parenthesis)Examples:Al2(SO4)3 - 2 Al’s + 3(1 S + 4 O’s) = 2 + 3(5) = 2+15 = 17 atomsMg(NO3)2 – 1 Mg + 2(1 N + 3 O’s) = 1 + 2(4) = 1+8 = 9 atoms
8HydratesHydrates are ionic compounds which also contain a specific number of water molecules associated with each formula unit. The water molecules are called waters of hydration.The formula for the ionic compound is followed by a raised dot and #H2OExample: MgSO4•7H2O.They are named as ionic compounds, followed by a counting prefix and the word “hydrate”CuSO4•5H2O copper(II) sulfate pentahydrateBaCl2•6H2O barium chloride hexahydrateMgSO4•7H2O magnesium sulfate heptahydrate (Epsom salts)
9How Many Atoms in a Hydrate? When counting atoms in the hydrate, count the water atoms also.Example:CuSO4•5H2O1 Cu + 1 S + 4 O’s + 5(2 H’s + 1 O’s)= (3)= 6+15 = 21 atomsBaCl2•6H2O1 Ba + 2 Cl + 6(2 H’s + 1 O’s)= 1+2+6(3)= = 21 Atoms
11Conservation of MassIn a normal chemical reaction, the mass of substances in a closed system will remain constant, no matter what processes are acting inside the system.How ever many atoms a reaction starts with, ends with the same number.Atoms don’t change their identity in a chemical reactionNumber of atoms for EACH ELEMENT STAYS THE SAME in a chemical reactionThe elements just rearrange their organizationThe beginning MASS of the reaction EQUALS the ending MASS of the reaction
12Conservation of Mass Total Mass stays the same in a chemical reaction 2g H2 + 16g O2 yields 18g H2ONumber and Identity of Atoms stays the same in a chemical reaction2 H2 + 1 O2 yields 2 H2OOHHHOHO
13Part III: Writing Reactions How Do You Write a Chemical Reaction?
14III. Chemical Reactions Definition – process by which the atoms of one or more substances are rearrangedKEY: new substances are formedKEY: No Atoms are Gained or LostA chemical reaction is the process by which atoms of one or more substances are rearranged into new substancesChemical change occursHow do you know?
15III. Evidences of a Chemical Reaction 1) gas production2) light production3) temperature change (endo/exothermic)4) precipitate formed (solid from 2 liquids)5) permanent color change
16III. Energy ChangesEnergy is stored in compounds as chemical potential energydue to specific arrangements of atoms.A chemical reaction changes the potential energy present.
17Energy ChangesWhen energy is lost as heat, it is called an __________________.exothermic reactionThese reactions get hotter.When energy is gained; heat is added for a reaction to occur. These are called ______________________,endothermic reactionsThese reactions get colder.Energy in a reaction is shown with:ΔH (heat)kJJoulesHeatenergy
18III. Chemical Reactions Representing Chemical Reactions:Reactants – the ‘stuff’ you start withAn ‘arrow’ which means ‘yields’, or ‘becomes’Products – the ‘stuff’ you end up withPrinciple of “Conservation of Mass” applies to chemical reactions.Why?
19III. Chemical Reactions Word Equations:Reactant-A + Reactant-B yields Product-ABExample:Sodium(s) + Chlorine(g) → Sodium Chloride(s)The small letters in paretheses () indicate the state of the reactant or product (solid, liquid, gas, or aqueous solution)(s) = solid(l) = liquid(g) = gas(aq) = aqueous = dissolved in water
20Part IV: Balancing Equations Applying Conservation of Mass to Equations
21VI. Chemical Equations Step 1: Write a Skeleton Equation Skeleton Equation uses chemical formulas and symbols instead of words:Words: Sodium + Chlorine gas yields Sodium ChlorideSymbols: Na(s) + Cl2(g) → NaClSkeleton Equations are not complete equations, but are the first step in writing a complete equation
22IV. Chemical Equations Chemical Equation is BALANCED Balanced means that “conservation of mass” is upheldAll atoms in reactants are also in productsNo more, no lessJust rearranged
23IV. Chemical Equations Balancing Equations Use a number before the compound/element symbol to indicate how many of them are neededCalled a COEFFICIENTWritten in front of the atom/compoundKEY: Coefficient is a MULTIPLIERNumber of atoms per molecule is SUBSCRIPTChange ONLY the COEFFICIENTS to balance the equation
24IV. Chemical Reactions Steps to Balance Equations Write the skeleton equationCount the atoms of EACH element in the reactantsCount the atoms of EACH element in the productsChange the coefficients to make the number of atoms of each element equal on both sides of the equationWrite the coefficients in the lowest possible ratioCheck your workNEVER CHANGE A SUBSCRIPT
25IV. Chemical Equations Write the skeleton equation: Al + O2 → Al2O3 2. Count Number of atoms for each element on both sidesThis is not balanced because the numbers don’t match3. Multiply coefficients until they match – multiply the entire units2Al + O2 → Al2O3Go to 6 Oxygens
26IV. Balancing Equations Al + O22Al2O3Multiply each atom by 24Al +2Al2O3O23Balanced
27IV. Balancing Equations 2 The work of balancing a chemical equation is in many ways a series of trials and errors.Consider the equation given below.Does this represent a balanced chemical equation?N H NH3
28IV. Balancing Equations 3 To balance this reaction, it is best to choose one kind of atom to balance initially. Let's choose nitrogen in this case.2 Nitrogen Atoms in Reactants requires 2 Ammonia molecules in Product to balance the nitrogen2NH3H2+N2
29IV. Balancing Equations 2 Once we know what the molecules are (N2, H2, and NH3 in this case) we cannot change them (only how many of them there are).The nitrogen atoms are now balanced, but there are 6 atoms of hydrogen on the product sideonly 2 of them on the reactant side.The next step requires multiplying the number of reactant hydrogen molecules by three to give:Balanced2NH3H2+N23H2
30IV. Don’t Forget: Diatomic Elements Definition – 7 elements that NEVER occur as singular atoms (always paired with an the same or different element)H2 O2 F2 Br2 I2 N2 Cl2Ex: HCl K 2 KCl H2
31IV. Balancing Equations 3 1. Start with an unbalanced equation2. Draw boxes around the compounds so you don’t mess with the groupsDon’t be threatened by how complex it looks!
32IV. Balancing Equations 2 3. Make an element inventory – count number of atoms for each element on each side of the equation
33IV. Balancing Equations 3 4. Write coefficients in front of each of the boxes until the inventory for each element is the same both before and after the reactionSave Oxygen and Hydrogen for last, Treat Polyatomic like an atom.Let’s start with SodiumWe have 2 in products, so I need 2 in reactantsMultiply reactant with sodium by 2 and recount atoms231HSO4ONaProductReactantElementBalanced?1 2YN1 2NY3 4NY
34IV. Balancing Equations 3 Inventory Shows:Reactant side has FOUR hydrogen atomsProduct side has TWO hydrogen atomsUsing your amazing powers of mathematicstwo hydrogen multiplied two becomes four hydrogenBalanced241HSO4ONaProductReactantElementBalanced?Y1 2YN2 4YNY
35Helpful Hints Balance hydrogen and oxygen last Balance polyatomic ions as a group if present on both reactants and productsYou can consider a polyatomic ion as a single elementIf the balancing starts to get very complex:StopStart overSelect a different atom to balance first.
503 possible products of double replacement reactions PrecipitateGasWater
51Reactivity Series (or Activity Series) More active will replace less activeLess active will NOT replace more activemetalsLi K Ca Na Mg Al Mn Zn Fe Ni Sn Pb Cu Ag Aumost activeleast activehalogensF Cl Br Imost activeleast active
52+ Al3+ O2- examples + Al Fe2O3 Fe Al2O3 Fe3+ O2- 2 1 2 1 aluminum + iron (III) oxideFe3+O2-Stronger?212+1Al+Fe2O3FeAl2O3Al3+O2-
53+ examples Ag CuNO3 Cu1+ NO31- silver + copper (I) nitrate NO RXN Stronger?Ag+CuNO3NO RXN