Chemical Reactions Chapter 10

How Many Atoms in a Molecule? Part I: Counting Atoms How Many Atoms in a Molecule?

Counting Atoms Most 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.

Counting Atoms A molecular formula gives the actual number of atoms of each element in a molecule of a compound. Hydrogen peroxide H2O2 Water H2O Glucose C6H12O6 A structural formula uses lines to represent covalent bonds, and shows how the atoms in a molecule are joined together: H—O—O—H H—O—H O=C=O

Counting Atoms Example: How Many Atoms? C6H12O6 6 C’s + 12 H’s + 6 O’s = 24 atoms K3PO4 3 K’s+1 P’s + 4 O’s = 8 atoms C2H5OH 2 C’s+ 5 H’s + 1 O’s + 1 H = 9 atoms H2O2 2 H’s and 2 O’s = 4 atoms

Counting Atoms with Polyatomic Ions - Al2(SO4)3 = S O

Counting Atoms with Polyatomic Ions When counting atoms with polyatomic ions; Count number of atoms in one polyatomic ion Ions inside the parentheses Multiply 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 atoms Mg(NO3)2 – 1 Mg + 2(1 N + 3 O’s) = 1 + 2(4) = 1+8 = 9 atoms

Hydrates Hydrates 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 #H2O Example: MgSO4•7H2O. They are named as ionic compounds, followed by a counting prefix and the word “hydrate” CuSO4•5H2O copper(II) sulfate pentahydrate BaCl2•6H2O barium chloride hexahydrate MgSO4•7H2O magnesium sulfate heptahydrate (Epsom salts)

How Many Atoms in a Hydrate? When counting atoms in the hydrate, count the water atoms also. Example: CuSO4•5H2O 1 Cu + 1 S + 4 O’s + 5(2 H’s + 1 O’s) =1+1+4+5(3) = 6+15 = 21 atoms BaCl2•6H2O 1 Ba + 2 Cl + 6(2 H’s + 1 O’s) = 1+2+6(3) =3 + 18 = 21 Atoms

Part II: Conservation of Mass

Conservation of Mass In 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 reaction Number of atoms for EACH ELEMENT STAYS THE SAME in a chemical reaction The elements just rearrange their organization The beginning MASS of the reaction EQUALS the ending MASS of the reaction

Conservation of Mass Total Mass stays the same in a chemical reaction 2g H2 + 16g O2 yields 18g H2O Number and Identity of Atoms stays the same in a chemical reaction 2 H2 + 1 O2 yields 2 H2O O H H H O H O

Part III: Writing Reactions How Do You Write a Chemical Reaction?

III. Chemical Reactions Definition – process by which the atoms of one or more substances are rearranged KEY: new substances are formed KEY: No Atoms are Gained or Lost A chemical reaction is the process by which atoms of one or more substances are rearranged into new substances Chemical change occurs How do you know?

III. Evidences of a Chemical Reaction 1) gas production 2) light production 3) temperature change (endo/exothermic) 4) precipitate formed (solid from 2 liquids) 5) permanent color change

III. Energy Changes Energy is stored in compounds as chemical potential energy due to specific arrangements of atoms. A chemical reaction changes the potential energy present.

Energy Changes When energy is lost as heat, it is called an __________________. exothermic reaction These reactions get hotter. When energy is gained; heat is added for a reaction to occur. These are called ______________________, endothermic reactions These reactions get colder. Energy in a reaction is shown with: ΔH (heat) kJ Joules Heat energy

III. Chemical Reactions Representing Chemical Reactions: Reactants – the ‘stuff’ you start with An ‘arrow’ which means ‘yields’, or ‘becomes’ Products – the ‘stuff’ you end up with Principle of “Conservation of Mass” applies to chemical reactions. Why?

III. Chemical Reactions Word Equations: Reactant-A + Reactant-B yields Product-AB Example: 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

Part IV: Balancing Equations Applying Conservation of Mass to Equations

VI. Chemical Equations Step 1: Write a Skeleton Equation Skeleton Equation uses chemical formulas and symbols instead of words: Words: Sodium + Chlorine gas yields Sodium Chloride Symbols: Na(s) + Cl2(g) → NaCl Skeleton Equations are not complete equations, but are the first step in writing a complete equation

IV. Chemical Equations Chemical Equation is BALANCED Balanced means that “conservation of mass” is upheld All atoms in reactants are also in products No more, no less Just rearranged

IV. Chemical Equations Balancing Equations Use a number before the compound/element symbol to indicate how many of them are needed Called a COEFFICIENT Written in front of the atom/compound KEY: Coefficient is a MULTIPLIER Number of atoms per molecule is SUBSCRIPT Change ONLY the COEFFICIENTS to balance the equation

IV. Chemical Reactions Steps to Balance Equations Write the skeleton equation Count the atoms of EACH element in the reactants Count the atoms of EACH element in the products Change the coefficients to make the number of atoms of each element equal on both sides of the equation Write the coefficients in the lowest possible ratio Check your work NEVER CHANGE A SUBSCRIPT

IV. Chemical Equations Write the skeleton equation: Al + O2 → Al2O3 2. Count Number of atoms for each element on both sides This is not balanced because the numbers don’t match 3. Multiply coefficients until they match – multiply the entire units 2 Al + O2 → Al2O3 Go to 6 Oxygens

IV. Balancing Equations Al + O2 2 Al2O3 Multiply each atom by 2 4 Al + 2 Al2O3 O2 3 Balanced

IV. 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? N2 + H2  NH3

IV. 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 nitrogen 2NH3 H2 + N2

IV. 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 side only 2 of them on the reactant side. The next step requires multiplying the number of reactant hydrogen molecules by three to give: Balanced                          2NH3 H2 + N2 3H2

IV. 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 Cl2 Ex: 2 HCl + 2K  2 KCl + H2

IV. Balancing Equations 3 1. Start with an unbalanced equation 2. Draw boxes around the compounds so you don’t mess with the groups Don’t be threatened by how complex it looks!

IV. Balancing Equations 2 3. Make an element inventory – count number of atoms for each element on each side of the equation

IV. 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 reaction Save Oxygen and Hydrogen for last, Treat Polyatomic like an atom. Let’s start with Sodium We have 2 in products, so I need 2 in reactants Multiply reactant with sodium by 2 and recount atoms 2 3 1 H SO4 O Na Product Reactant Element Balanced? 1 2 Y N 1 2 N Y 3 4 N Y

IV. Balancing Equations 3 Inventory Shows: Reactant side has FOUR hydrogen atoms Product side has TWO hydrogen atoms Using your amazing powers of mathematics two hydrogen multiplied two becomes four hydrogen Balanced 2 4 1 H SO4 O Na Product Reactant Element Balanced? Y 1 2 Y N 2 4 Y N Y

Helpful Hints Balance hydrogen and oxygen last Balance polyatomic ions as a group if present on both reactants and products You can consider a polyatomic ion as a single element If the balancing starts to get very complex: Stop Start over Select a different atom to balance first.

Example Using PolyAtomics Before MgCl2 + NaOH  Mg(OH)2 + NaCl 1 Mg 1 Na 1 Mg 1 Na 2 Cl 1 OH 1 Cl 2 OH After MgCl2 + 2 NaOH  Mg(OH)2 + 2 NaCl 1 Mg 2 Na 1 Mg 2 Na 2 Cl 2 (OH) 2 Cl 2 (OH)

Types of Chemical Reactions Part V

Classifying Chemical Reactions Synthesis Decomposition Single replacement Double Replacement Combustion

Synthesis Definition – two or more substances react to form ONE product Product is usually bigger or more complex than either reactant A + B  AB

Hey baby let’s get jiggy.

Al3+ Cl1- Synthesis 2 3 2 ___Al + ___Cl2  ___AlCl3 reaction of two elements 2 3 2 ___Al + ___Cl2  ___AlCl3 Al3+ Cl1-

CaO + H2O  Ca(OH)2 SO2 + O2  SO3 Synthesis 2 2 reaction of two compounds reaction of element and compound CaO + H2O  Ca(OH)2 2 SO2 + O2  2 SO3

Decomposition definition – one substance breaks down into two or more simpler products AB  A + B

Break yoself fool!

Decomposition __ NaN3 (s)  ___ Na (s) + ___ N2 (g) __ CaO (s)  Example reaction: 2 2 3 __ NaN3 (s)  ___ Na (s) + ___ N2 (g) 2 2 1 __ CaO (s)  ___ Ca (s) + ___ O2 (g)

Single Replacement Reactions Definition – one element replaces another element in a compound to form new compound A + BX  AX + B

I’m gon’ dance with yo’ lady

A B + C D  AD + CB Double Replacement Defn – exchange of cations between two ionic compounds A B + C D  AD + CB switch

3 possible products of double replacement reactions Precipitate Gas Water

Reactivity Series (or Activity Series) More active will replace less active Less active will NOT replace more active metals Li K Ca Na Mg Al Mn Zn Fe Ni Sn Pb Cu Ag Au most active least active halogens F Cl Br I most active least active

+ Al3+ O2- examples + Al Fe2O3 Fe Al2O3 Fe3+ O2- 2 1 2 1 aluminum + iron (III) oxide Fe3+ O2- Stronger? 2 1 2 + 1 Al + Fe2O3 Fe Al2O3 Al3+ O2-

+ examples Ag CuNO3 Cu1+ NO31- silver + copper (I) nitrate NO RXN Stronger? Ag + CuNO3 NO RXN

+ examples F2 NaBr NaF + Br2 1 2 2 1 Na1+ F1- fluorine gas + sodium bromide Stronger? 1 2 2 1 F2 + NaBr NaF + Br2 Na1+ F1-

examples chlorine gas + hydrogen fluoride Stronger? Cl2 + HF NO RXN

Example Problem Li I Li1+ I1- Ag1+ NO31- Ag I (s) + AgNO3 LiNO3 + lithium iodide and aqueous silver nitrate react Li1+ I1- Ag1+ NO31- Li I Ag I (s) + AgNO3 LiNO3 +

Combustion definition – compound reacts with O2 Hydrocarbon – compound w/ only carbon and hydrogen

Combustion CxHy + O2 CO2 + H2O Combustion of hydrocarbons ALWAYS produces CO2 and H2O CxHy + O2 CO2 + H2O

I sell propane and propane accessories! Ex problem show combustion of propane (C3H8) gas 1 C3H8 + 5 O2 3 CO2 + 4 H2O I sell propane and propane accessories!