Anatomy of a Chemical Reaction

1. Anatomy of a Chemical Reaction • A chemical reaction is a recipe for making a chemical PRODUCT(S) from ingredient REACTANT(S) in the right proportion from the COEFFICIENTS: 4Fe(s) + 3 O2(g)  2 Fe2O3(s) coefficients States (s, l, g, aq) subscripts reactants product(s)‏

2. Bonds are broken and/or made in every chemical reaction 4 Fe + 3 O2 2 Fe2O3 2 C6H2O6 + 7 O2  12 CO2 + 2 H2O NaOH + HCl  NaCl + H2O

3. Just because you can write a chemical reaction, doesn’t mean it occurs • Reactions must conserve matter (can’t create or destroy atoms so have to have the same number before and after) • Reactions must respect bond rules (can’t create CaCl because Ca2+ needs 2 Cl- to be a compound) • Some reactions just don’t happen (when AgCl & O2 mixed, chemists never find any Ag2O made, just AgClO3) 4 AgCl + O2 2 Ag2O + 2 Cl2not observed even though all rules followed 2 AgCl + 3 O2 2 AgClO3

4. Balancing Reactions • Adjust COEFFICIENTS until the total number of each type of atom is the same on both sides of the chemical equation • Do not adjust subscripts – they are determined by compound rules and what is observed in nature

5. Chemists like to classify reactions by how the atoms are rearranging*: Combination (aka synthesis)‏ Two substances (element, molecule or ionic) combine to form ONE PRODUCT A + B  AB or AB + C  ABC or AB + CD  ABCD Decomposition The reverse of synthesis; ONE REACTANT is broken up into 2 or more products AB  A + B Single Replacement An IONIC COMPOUND reacts with an ELEMENT (element can be a diatomic) AB + E  A + EB Double Displacement TWO IONIC COMPOUNDS switch anion partners AB + CD  AD + CB *there are other ways to classify reactions

6. Chemists also classify reactions by certain types of reactant or product*: *there are other ways to classify reactions Precipitation:‏ an insoluble ionic compound is made ? + ?  AB (s) + ? Often double replacement reactions form insoluble ionic compounds You can check if an ionic compound is insoluble using a solubility chart Gas Evolution: a gas is made ? + ?  X (g) + ? Often double replacement reactions form one of 3 intermediates that are known to always break down into gases H2CO3CO2 (g) + H2O (l) H2SO3  SO2 (g) + H2O (l) NH4OH  NH3 (g) + H2O (l) Single replacement reactions can form H2S (g) or a diatomic element gas O2, N2, etc. Neutralization: acid and base react to form water and a salt HB + MOH  MB + H2O (l) (for now) Acids contain an H+ ion, Bases contain an OH- ion Reduction-Oxidation: electrons are being transferred (ions becoming elements or elements becoming ions) Many single replacement reactions are reduction-oxidation Combustion is a sub-category of reduction-oxidation (it’s also a gas evolution) Hydrocarbon + Oxygen  CO2 + H2O CxHy + O2 CO2 + H2O

7. Predicting Products • First, determine the type of reaction (we will only predict products for double displacement, single displacement, and a select few combination and decomposition reactions) • Then, use the reaction ‘type’ to guide you in forming products • D.D. : switch ions ; make sure valid subscripts ; don’t break up polyatomic ions • S.D. : switch ions ; make sure valid subscripts ; don’t break up polyatomic ions; watch for diatomics • Comb: use specific sub-type of reaction to guide you in products ; make sure valid subscripts • Decomp: use specific sub-type of reaction to guide you in products ; make sure valid subscripts

8. Determining if Reactions are Spontaneous(do they occur naturally) • Single displacement: • The element must be more active than the metal ion is replaces; Activity is found on a special Activity Series table • Double displacement: • The products must not all be aqueous; need at least one solid, liquid or gas state product ; use solubility table to determine if product is insoluble solid (s) or soluble (aq)

9. Helpful Tables (on reference) Solubility table of ionic compounds in water

10. Helpful definitions • “salt” = an ionic compound • (“binary salt” = an ionic compound of 2 elements) • “acid” = an ionic compound with H+ ion (for now; in Ch 14 we’ll update that definition)

11. Combination ReactionsTwo substances (element, molecule or ionic) combine to form ONE PRODUCT We will work with 6 sub-types of combination reactions: • metal element + oxygen  metal oxide • nonmetal + oxygen  nonmetal oxide* • metal element + nonmetal element  salt • metal oxide + H2O  metal hydroxide • nonmetal oxide + H2O  oxy-acid* • metal chloride + O2 metal chlorate *you will not have to predict products for these subtypes

12. Combination Reaction tips • Make sure it’s combination and not single or double replacment; single displacement is an element (except O2) reacting with ionic compound (including H2O); double displacement is 2 ionic compounds reacting unless 1 of them is H2O)‏ • Use list to identify WHICH SUBTYPE of combination reaction it is • The product formula will be determined by the CHARGES of the two ions: • Find out what the metal is in the reactants and write it with charge* on the right • (If there is no metal in the reactants, the products are hard to predict)‏ • Find out the anion from the KIND of combination reaction (oxide = O-2, chlorate = ClO3-, hydroxide = OH-)‏ • Use the charges to determine the subscripts of the ionic compound • Balance the reaction * For a metal than has more than one charge, look for a clue as to which charge to use (sometimes you try both and 1 won’t be able to balance)‏

13. Decomposition Reactions ONE REACTANT is broken up into 2 or more products We will work with 4 sub-types of decomposition reactions: • metal oxide  metal element + O2 OR  another metal oxide + O2 * • Binary Salts  metal element + nonmetal element • Carbonates  metal oxide + CO2 * • Chlorates  O2 + metal chloride • Many polyatomic salts  another salt + O2 * *you will not have to predict products for these subtypes

14. Decomposition Reaction tips • Once you’ve identified that it is a decomposition reaction (only 1 reactant), use the list to identify WHICH SUBTYPE of decomposition reaction it is • Write down the “given” product for that kind of reaction (O2 or CO2 or the metal* or nonmetal elements) – don’t forget the diatomic 7 will appear with a subscript of 2 as an element. • The other product formula will be determined by the CHARGES of the two ions: • Find out what the metal is in the reactants and write it with charge* on the right • (If there is no metal in the reactants, the products are hard to predict)‏ • Find out the anion from the KIND of decomp. reaction (oxide = O-2, chloride = C-)‏ • Use the charges to determine the subscripts of the ionic compound • Balance the reaction * For a metal than has more than one charge, look for a clue as to which charge to use (sometimes you try both and 1 won’t be able to balance); in the metal oxide reaction, if using the metal element as a product will not balance, try making a metal oxide instead

15. Single displacement ReactionsAn IONIC COMPOUND reacts with an ELEMENT (element can be a diatomic)‏ Some specific examples (many of these are also reduction-oxidation reactions): • metal element + salt  metal element + salt (metal element switches with metal ion)‏ • halogen element + halide salt  halogen + halide salt (halogen element switches with halogen ion)‏

16. Single displacement tips • Find the element (watch for O2 - that’s a combination reaction)‏ • Determine which half of the ionic compound the element will replace IF it reacts (metals replace metals, nonmetals replace nonmetals…)‏ • Look at the activity series to see if the reaction will occur spontaneously. • If it does, write the half of the ionic compound being replaced as an element on the product side (don’t forget the diatomic 7 will have subscripts of 2 as an element!)‏ • Write the reactant element as an ION (lookup charge*) and the other half of the ionic compound as an ION, and use the charges to determine the subscripts of the ionic compound product • Balance the reaction * in the case of a metal element that has more than one possible charge, you should look for a clue in the question as to which charge ion is to be used

17. Double displacement ReactionsTWO IONIC COMPOUNDS switch anion partners Notice that these often can be classified as other types of reactions: • Neutralization: Acid + base  salt + water • (no special name, also can be considered a combination reaction sometimes): Metal oxide + acid  salt + water • Precipitation: Salt + salt  salt (s) + salt‏ • Gas Formation: Salt + salt  salt (g) + salt

18. Double Displacement tips • Find the two halves of each ionic compound (watch out for water reacting with a metal oxide – that’s a combination reaction)‏ • Write each half with charge* on the right, but SWITCH the anions • Use the charges* to determine the subscripts of the ionic compound for EACH of the two products • To determine if there IS a reaction, must make at least a (s),(l) or(g): • Lookup on solubility table to see if either of the two products is solid (I)‏ • If not, try to determine if one of the products is H2 gas • If not, see if one of the products is H2O (liquid, never aqueous)‏ • If you do not have at least one gas or solid or liquid (water) state product, it is considered “NO REACTION” • If H2CO3, H2SO3, or NH4OH is a product, break it down (H2CO3 = H2O + CO2; H2SO3= H2O + SO2 ; NH4OH = H2O + NH3)‏ • Balance the reaction • if a polyatomic ion stays together, count it as a unit when balancing • If water is made, consider it H(OH) and count the H separate from OH units * For a metal than has more than one charge, use the same charge that it is on the reactant side

19. What STATE should I pick? • For elements: • metals are solids (s) (except for Hg (l))‏ • Group 18 and the diatomic 7 are gases (g) (except for Br2 (l))* • All other nonmetals are solids (s)‏ • For ionic compounds • If one of the reactants is (aq), then you need to determine if it is soluble or not (Appendix V)‏ • Soluble ionic compounds in (aq) reactions are (aq)‏ • Insoluble ionic compounds in (aq) reactions are (s)‏ • If it isn’t an aqueous reaction, assume its solid • Covalent compounds • Many are gases (g), but it is hard to say • H2S, CO2, SO2, NH3 are all gases