Equations Section 1

8 Representing Changes: Equations Notation using chemical formulas showing… Net changes that occur in a reaction Can be used for physical changes: Ex: Ice melting (water changes from solid to liquid) Also used for chemical changes (reactions) Ex: gaseous hydrogen burns by combining with oxygen from the air to form water vapor H2O(s) H2O (l) H2(g) + O2(g) H2O(g)

Equations have two parts: Reactants: Products: “Hydrogen and oxygen gas combine to form of water vapor” Reactants: hydrogen gas & oxygen gas Product: water vapor chemicals that will change (what you start with) what they change in to (the final form) Word equation: Hydrogen + oxygen  water Molecular equation: H2 + O2  H2O This is sometimes called a skeleton equation (since its not balanced) Reactants product on left on right Read as “ hydrogen gas… reacts with… oxygen gas… …to form … water vapor “

↑ gas bubbles from liquids escapes ↓ a solid that precipitates out Equations can include notation: H2 (g) + O2 (g)  H2O (g) (g) = gas or vapor Others (aq) dissolved in a water solution (l) pure liquid ↑ gas bubbles from liquids escapes (s) A pure solid ↓ a solid that precipitates out Δ heat applied MnO2 a Catalyst (ex: manganese dioxide) (write above the arrow)

Examples: Manganese dioxide catalyzes the decomposition of Hydrogen peroxide H2O2 H2O + O2 Heating a hydrate releases water vapor: CuSO4 5H2O CuSO4 + 5H2O↑ MnO2 ∆

We must next balance the equation – by inspection with coefficients (not subscripts – changes the chemicals involved) Atoms present as reactants are rearranged into atoms of products H2 + O2  H2O balanced equation: 2 H2 + O2  2 H2O 2( ) = 4 H’s 2 H’s Vs. 2 H’s 4 H’s 2 O’s 2 O’s Vs. Vs. 2( ) = 2 O’s 1 O 2 O’s

Try these balancing problems: 1. Ba + O2  BaO Mg + N2  Mg3N2 3. Zn + HCl  ZnCl2 + H2 2 2 1 Ba 2 O’s 1 Ba 2( ) = 2 O’s, + 1 O 2( ) = 2 Ba’s 1 Ba 2 Ba 3 Try this one on your own! This should be an easy one.

Try these balancing problems: 4. Cu + Ag NO3  Ag + Cu (NO3)2 5. ZnCl2 + (NH4)2 S  ZnS + NH4 Cl 6. Na OH + H2 SO4  H2O + Na2SO4 NO3 (NO3)2 Hint: treat NO3 as a unit (NH4)2 NH4 Water is also hydrogen hydroxide! OH H2 HOH

2 H2 + O2  2 H2O H2 (g) + O2 (g)  H2O (g) Draw structures or Particle diagrams help to visualize: H2 (g) + O2 (g)  H2O (g) H - H + O = O  H – O - H H - H H – O - H 2 H2 + O2  2 H2O

Chlorine gas reacts with sodium metal to produce solid sodium chloride   Word eq: Skeleton eq: Particle diagram: Balanced eq: Chlorine + Sodium  sodium chloride Cl2 + Na  NaCl Cl - Cl Na Na Cl Na Na Cl Cl2 (g) + 2 Na (s)  2 NaCl (s)

Carbon dioxide and water vapor are produced when methane gas is burned in the presence of oxygen from the air:   Word eq: Skeleton eq: Particle diagram: Balanced eq:

Test your understanding Iron(III)oxide – rust, forms when iron objects are exposed to the air. What are the reactants? The products? Write a formula (skeleton) equation for the reaction, then balance. What evidence would you see indicating that a chemical change has occurred? (which of the three indicators?)

Reaction Types Section 2

4+1 Types of Reactions A B 2 A2 + B2 2 A2B 2 H2 + O2  2H2O CaO + H2O  Ca(OH)2 2 Mg + O2 2 MgO 1) These are Synthesis Reactions 2 combine  to form 1 General form: A + B  AB Element + element  New compound or or Compound compound Notice in each how two substances combine to form a new compound A B 2 A2 + B2 2 A2B

∆ 2 Ag2O  4 Ag + O2(g) 2NaHCO3 Na2O + H2O + 2CO2(g) 2 H2O 2 H2 + O2(g) 2 Ag2O  4 Ag + O2(g) 2NaHCO3 Na2O + H2O + 2CO2(g) MnO2 2H2O2  2H2O + O2(g) 2) These are Decomposition Reactions 1 breaks  in 2 general form: AB  A + B Compound  Element + element or or new new compound compound Notice in each how a substance breaks apart into two or more simpler chemicals ∆ Many decompositions involve heating Δ MnO2 is catalyst ( a helper)

Decomposition Questions: A B 1. Write the balanced equation shown in the two diagrams 2. Why is this classified as decomposition?

Look at the oxidation states 3) Single Replacement Reactions Mg + CuCl2  Cu + MgCl2 F2 + 2NaCl  Cl2 + 2NaF element + compound  new element + new compound Or A + BC  B + AC Zn (s) + HCl (aq)  H2 (g) + ZnCl2 (aq) (zinc is losing electrons to hydrogen ions) Notice loser replaces loser Or gainer replaces gainer? +1 +2 Look at the oxidation states

Why does the first reaction happen, but not the second? If a piece of zinc metal is placed in a solution of copper sulfate…reddish copper collects on the gray zinc. Zn(s) + CuSO4(aq)  Cu(s) + ZnSO4(aq) If a piece of copper metal is placed in a solution of zinc sulfate…nothing happens. Cu (s) + ZnSO4(aq)  Zn(s) + CuSO4(aq) Why does the first reaction happen, but not the second? Zinc is a more active metal than copper…but how would you know?

Activity series: table J 1) Zn(s) + CuSO4(aq)  Cu(s) + ZnSO4(aq) Vs. 2) Cu (s) + ZnSO4(aq)  Zn(s) + CuSO4(aq) Higher (more active) Always replaces Lower (less active) Reaction 1 will occur!

Any above Hydrogen on table J Predict the products when magnesium metal is placed into hydrochloric acid: Mg + HCl  ? Complete and balance the equation: Which metals will bubble when placed in acid? Mg + HCl  H + MgCl 2 2 2 Any above Hydrogen on table J

Gases: H2CO3 = H2O (l) + CO2 (g) Double Replacement Reaction (exchange of ions) K2CrO4 + Pb(NO3)2  PbCrO4 + KNO3 HCl + NaOH  HOH + NaCl AX(aq) + BY(aq)  BX+ (aq) + AY (s) loser 1 replaces loser 2, gainer 1 replaces gainer 2 Occurs when aqueous solutions of ions are mixed How does one tell if a double replacement reaction will occur? Product must form a gas, liquid or solid (precipitate) (aq) (aq) (s) (aq) (aq) (aq) (l) (aq) Gases: H2CO3 = H2O (l) + CO2 (g)

Soluble will be (aq) aqueous insoluble will be precipitate (s) Solid For precipitates See reference table F – solubility guidelines Soluble Ions side Insoluble Ions side Soluble will be (aq) aqueous insoluble will be precipitate (s) Solid

Solubility Problems: 1) Na HCO3 2) PbCl2 3) NH4OH Soluble (aq) Insoluble (s) precipitate Soluble (aq)

Will a reaction occur? 1) Na3PO4 (aq) + Zn(NO3)2 (aq)  ? Na3PO4 (aq) + Zn(NO3)2 (aq)  Zn3 (PO4)2 + NaNO3 Table F: Zn3 (PO4)2 insoluble – precipitates NaNO3 soluble – stays aqueous Na3PO4 (aq) + Zn(NO3)2 (aq)  Zn3 (PO4)2 (s) + NaNO3 (aq) Reaction occurs since a precipitate is produced.

Will a reaction occur? 2) NaC2H3O2 (aq) + Zn(NO3)2 (aq)  ? NaC2H3O2 (aq) + Zn(NO3)2 (aq)  Zn(C2H3O2)2 + NaNO3 Table F: Zn(C2H3O2)2 soluble – stays aqueous NaNO3 soluble – stays aqueous NaC2H3O2 (aq) + Zn(NO3)2 (aq)  no products formed No reaction since nothing precipitates.

+1 Combustion reactions Burning compounds: usually organics with CH Produces CO2 and H2O Ex: CH4 + O2  CO2 + H2O CH4 + 2O2  CO2 + 2H2O Glucose: C6H12O6 + 6O2  6CO2 + 6H2O

3 1/2 Use of fractions in balancing: C2H6 + O2  2 CO2 + 3 H2O ____ O2’s 7 O’s 3 1/2 C2H6 + 3 ½ O2  2 CO2 + 3 H2O 2 C2H6 + 7 O2  4 CO2 + 6 H2O

Incomplete combustion In limited oxygen – forms carbon monoxide CH4 + O2  CO + H2O CH4 + O2  CO + 2 H2O CH4 + 1½ O2  CO + 2 H2O 2 CH4 + 3 O2  2 CO + 4 H2O

Practice problems 1. K + O2  2. Hg2O  3. F2 + NaCl  Predict the products of these reactions 1. K + O2  2. Hg2O  3. F2 + NaCl  4. KNO3 + NaCl  5. C3H6 + O2  K+