Chemistry B11 Chapter 5 Chemical Reactions

Chemical Reactions Chemical transformation or change = Chemical reaction Substance(s) is used up (disappear / re-arrange) New substance(s) is formed. Different physical and chemical properties.

Chemical Reactions

A + B  AB or C + D Chemical Reactions Reactants Product or Products Elements are not created nor destroyed Chemical Equation

Chemical Reactions 1. A + B  AB 2. AB  A + B 3. A + BC  AC + B 4. Synthesis reaction (combination) 2H2 + O2  2H2O AB  A + B 2. Decomposition 2NaCl  2Na + Cl2 A + BC  AC + B 3. Single replacement reaction Fe + CuSO4  FeSO4 + Cu AB + CD  AD + CB 4. Double replacement reaction NaCl + AgNO3  NaNO3 + AgCl

Physical state irrelevant Chemical Reactions 5. AB + xO2  yCO2 + zH2O Combustion “Burning” C3H8 + 5O2  3CO2 + 4H2O Physical state irrelevant Solid (s) Liquid (l) Gas (g) Aqueous (aq) Ca(OH)2(s) + 2HCl(g)  CaCl2(s) + H2O(l)

Chemical reactions and heat Chemical reactions speed up With increased heat This is due to several reasons: Activation energy Increased energy Quicker motion more interactions

Balance a chemical equation Why balancing?

Balance a chemical equation Law of conservation of mass Atoms are neither destroyed nor created. They shift from one substance to another. “Fruit Salad”

Balance a chemical equation Begin with atoms that appear in only one compound on the left and right. If an atom occurs as a free element, balance it last. Change only coefficients (not formulas). C3H8(g) + O2(g)  CO2(g) + H2O(g) begin last

Formula and Molecule Formula Weight of NaCl: Ionic & covalent compounds  Formula formula of NaCl Covalent compounds  Molecule molecule of H2O Formula Weight of NaCl: 23 amu Na + 35.5 amu Cl = 58.5 amu NaCl Molecular Weight of H2O: 2 (1 amu H) + 16 amu O = 18 amu H2O

Mole Mole (mol): formula weight of a substance (in gram). 12g of C = 1 mol C 23g of Na = 1 mol Na 58.5 g of NaCl = 1 mol NaCl 18 g of H2O = 1 mol of H2O

Avogadro’s number (6.02×1023): number of formula units in one mole. 1 mole of apples = 6.02×1023 apples 1 mole of A atoms = 6.02×1023 atoms of A 1 mole of A molecules = 6.02×1023 molecules of A 1 mole of A ions = 6.02×1023 ions of A Molar mass (g/mol): mass of 1 mole of substance (in gram) (Formula weight) molar mass of Na = 23 g/mol molar mass of H2O = 18 g/mol

2H2O(l)  2H2(g) + O2(g) Stoichiometry 2 2 1 2 moles 2 moles 1 mole Relationships between amounts of substances in a chemical reaction. Look at the Coefficients! 2H2O(l)  2H2(g) + O2(g) 2 2 1 2 moles 2 moles 1 mole 2 liters 2 liters 1 liter 2 particles 2 particles 1 particle 2 grams 1 gram

1 step: use coefficient in the balanced equation. mole B mass volume Particle (atom) (molecule) (ion) 1 step: use coefficient in the balanced equation. CH4 + 2O2  CO2 + 2H2O

STP: 1 mole of substance (gas) = 22.4 L = 22400 cc (cm3 or mL) CH4 + 2O2  CO2 + 2H2O 2 moles H2O 23 mole CH4 = ? moles H2O 23 mole CH4 ( ) = 46 moles H2O 1 mole CH4 10 cc O2 = ? cc CO2 10 cc O2 ( 1 cc CO2 2 cc O2 ) = 5 cc CO2 32 g CH4 = ? moles CO2 32 g CH4 ( 1 mole CH4 16 g CH4 ) = 2 mole CO2 1 mole CO2 )( 40 g CH4 = ? L CH4 40 g CH4 ( 1 mole CH4 16 g CH4 ) = 56 L CH4 22.4 L CH4 )( STP: 1 mole of substance (gas) = 22.4 L = 22400 cc (cm3 or mL)

Limiting Reagents N2(g) + O2(g)  2NO(g) Stoichiometry: 1 mole 1 mole 2 moles Before reaction: 1 mole 4 moles After reaction: 0 mole 3 moles 2 moles

Limiting Reagents N2(g) + O2(g)  2NO(g) Stoichiometry: 1 mole 1 mole 2 moles Before reaction: 1 mole 4 moles Left over After reaction: 0 mole 3 moles 2 moles Used up first

Limiting Reagents N2(g) + O2(g)  2NO(g) Limiting reagent Stoichiometry: 1 mole 1 mole 2 moles Before reaction: 1 mole 4 moles Left over After reaction: 0 mole 3 moles 2 moles Used up first

Limiting Reagents N2(g) + O2(g)  2NO(g) Limiting reagent: is the reactant that is used up first. Limiting reagents can control a reaction: N2(g) + O2(g)  2NO(g)

Limiting Reagents C(s) + O2(g)  CO2(g) Example: 12g of C 64g of O2 ? Limiting reagent ? g of CO2 will be formed Make sure that the chemical equation is balanced. C(s) + O2(g)  CO2(g) 1 mol 12g C ( 1 mole C 12g C ) = 1 mole C 64g O2 ( 1 mole O2 32g O2 ) = 2 mole O2 C is the limiting reagent.

Limiting Reagents A B C(s) + O2(g)  CO2(g) ? g of CO2 will be formed: We should use the mass of the limiting reagent. (because it controls our reaction). 12g C ( 1 mole C 12g C )( 1 mole CO2 44g CO2 ) = 44g CO2

Percent Yield actual yield Percent yield = × 100 theoretical yield actual yield: mass of product formed (experimental) theoretical yield: mass of product that should form (according to stoichiometry) N2(g) + O2(g)  2NO(g) theoretical yield = 40g NO actual yield = 37g NO Percent yield = 37g NO 40g NO × 100 = 92.5% 7.5% error or lost

Aqueous Solution (ionic compounds) H2O NaCl(s) Na+(aq) + Cl-(aq) Dissociation (Ionization) aqueous solution: solvent is water AgNO3(s) Ag+(aq) + NO3-(aq) H2O NaCl AgNO3 NaCl(aq) + AgNO3(aq)  AgCl(s) + NaNO3(aq)

total charge on left side = total charge on right side Molecular equation: NaCl(aq) + AgNO3(aq)  AgCl(s) + NaNO3(aq) Ionic equation: Na+(aq) + Ag+(aq) + Cl-(aq) + NO3-(aq)  AgCl(s) + Na+(aq) + NO3-(aq) Net ionic equation: Na+(aq) + Ag+(aq) + Cl-(aq) + NO3-(aq)  AgCl(s) + Na+(aq) + NO3-(aq) Spectator ions Ag+(aq) + Cl-(aq)  AgCl(s) 2As3+(aq) + 3s2-(aq)  As2S3(s) total charge on left side = total charge on right side

oxidation: is the loss of electrons. Oxidation and Reduction reactions (redox) oxidation: is the loss of electrons. reduction: is the gain of electrons. Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s) redox reaction Zn(s)  Zn2+(aq) + 2e- Zn is oxidized (reducing agent) Cu2+(aq) + 2e-  Cu(s) Cu2+ is reduced (oxidizing agent)

double replacement reactions  non redox Oxidation and Reduction reactions (redox) oxidation: is the gain of oxygen / loss of hydrogen. reduction: is the loss of oxygen / gain of hydrogen. CH4(s) + 2O2(g)  CO2(g) + 2H2O(g) redox reaction C gains O and loses H is oxidized (reducing agent) O gains H Is reduced (oxidizing agent) single replacement reactions and combustion reactions  redox reactions double replacement reactions  non redox

2HgO(s) + heat (energy)  2Hg(l) + O2(g) Heat of reaction 2HgO(s) + heat (energy)  2Hg(l) + O2(g) Endothermic reaction C3H8(s) + 5O2(g)  3CO2 + 4H2O + heat (energy) Exothermic reaction All combustion reactions are exothermic.