Chapter 8 (8.1-8.3) Chemical Reactions

Chemical Reactions and Equations What is a chemical reaction? A chemical equation uses symbols to represent what happens at the atomic level. What happens when you boil water? What happens during the electrolysis of water? Chemical equations follow the Law of Conservation of Mass Conservation of atoms, not conservation of molecules (molecular structures can change) Reaction Equation 2

Balancing Equations Adjust coefficients, not subscripts, to balance an equation Coefficients are whole-number integers in the lowest ratio Reactants  Products H2 + O2  H2O 3

Balancing Equations Coefficients vs. Subscripts

Balancing Equations Draw structures for the following substances: H2, Br2, HBr, CO, CO2, Cl2, AlCl3, N2, H2, NH3 Use your drawings to balance the equations below. Add more substances as needed by adding coefficients. H2 (g) + Br2 (l)  HBr (g) CO (g) + O2 (g)  CO2 (g) Al (s) + Cl2 (g)  AlCl3 (g) N2 (g) + H2 (g)  NH3 (g) 5

Guidelines to Balancing Equations Write correct formulas for reactants and products. Begin balancing with the most complex formula. Balance polyatomic ions as a single unit. Check each reactant and product to verify the coefficients. Mg(NO3)2 + Cr2(SO4)3  MgSO4 + Cr(NO3)3 6

Practice Balancing Equations Gummy Bear Balance the following equations: Mg + O2  MgO H2O2  H2O + O2 NaN3  Na + N2 CaCO3  CaO + CO2 Mg3(PO4)2 + NaOH  Mg(OH)2 + Na3PO4 KClO3  KCl + O2 Na + H2O  NaOH + H2 AgNO3 + MgCl2  AgCl + Mg(NO3)2 CH3OH + O2  CO2 + H2O Na + H2O Alkali metals in H2O Burn methanol 7

Practice Balancing Equations Balanced equations: 2 Mg + O2  2 MgO 2 H2O2  2 H2O + O2 2 NaN3  2 Na + 3 N2 CaCO3  CaO + CO2 Mg3(PO4)2 + 6 NaOH  3 Mg(OH)2 + 2 Na3PO4 2 KClO3  2 KCl + 3 O2 2 Na + 2 H2O  2 NaOH + H2 2 AgNO3 + MgCl2  2 AgCl + Mg(NO3)2 2 CH3OH + 3 O2  2 CO2 + 4 H2O 8

Group Quiz #17 Balance the following equations: __Fe2(SO4)3 + __K3PO4  __K2SO4 + __FePO4 __Hg(NO3)2 + __ KI  __HgI2 + __ KNO3 __Li2O(s) + __ H2O  __LiOH __HBr + __Ba(OH)2  __BaBr2 + __H2O __K2PtCl4 + __NH3  __Pt(NH3)2Cl2 + __KCl

Chapter 2 Section 2.6

Amounts of Reactants and Products In lab, we want to predict amount of product we can expect from a reaction Have to use stoichiometry: quantitative study of reactants and products in a reaction (mole to mole ratio) Use the mole to relate one substance in a reaction to another (equation coefficients, see Table 3.1) 2H2 (g) + O2 (g)  2H2O (g)

Mole Ratios We use coefficients in a balanced equation to determine the mole ratio in which substances need to react. MgCl2(aq) +2AgNO3(aq) 2AgCl(s) + Mg(NO3)2(aq) 1 molecule of MgCl2 reacts with 2 molecules of AgNO3 to produce 2 molecules of AgCl and 1 molecule of Mg(NO3)2 Molecules are not practical! Scale up to moles!

2 ways to calculate moles of substance Use molar mass: Mass ÷ molar mass = moles Concentration: amount of solute present in a given amount of solution Molarity: moles of solute in 1 L of solution moles solute / liters solution; M = mol / L If I have 1.50 mL of a 0.100 M solution of NaCl, how many moles are in the sample? M * V = moles 13

Mole Diagram molar mass 1 mole = 6.022 x 1023 particles Use coefficients from equation molar mass Mole 1 mole = 6.022 x 1023 particles (atoms or molecules) volume Molarity Particles Courtesy: www.unit5.org/christjs/Stoichiometry/Mole%20Island%20Diagram.ppt

Stoichiometry Calculations Mole to mole conversions MgCl2(aq) +2AgNO3(aq) 2AgCl(s) + Mg(NO3)2(aq) 1.00 mol xs ? How many moles of solid product do you predict to form? Cannot compare grams of one to grams of another!! 15

Mole Ratios Predict products (with phases) and balance each equation. Calculate the moles of precipitate (ppt) that can be made in the following reactions. NaCl (aq) + Hg(NO3)2 (aq)  0.500 mol NaCl FeCl3 (aq) + NaOH (aq)  2.35 mol NaOH Li2SO4 (aq) + AgCH3COO (aq)  0.1098 mol AgCH3COO

Avogadro’s Number and the Mole We need to be able to determine amounts of chemicals to use so they react in the exact ratio needed. For example, we need 2 molecules of H2 and 1 molecule of O2 to make 2 molecules of H2O. Do you want to count individual atoms and molecules? Size of atoms

Avogadro’s Number Who wants to count: Peppers? Water Molecules? Grains of rice?

The Mole We can measure the mass of a pure sample but how do we know how many atoms or molecules are in that sample? Mole: amount of a substance that contains a specific number of particles. A mole (mol) is a quantity that tells us two things: the number of particles in a substance the mass of that substance (containing a specific number of particles)

Avogadro’s Number and the Mole 1 12C atom weighs 12 amu, 1 mole of 12C atoms weighs 12 grams. 1 dozen = 12 objects; 1 mole = 6.022 x 1023 objects. This is Avogadro’s number. The moral: 1 mole of carbon-12 weighs 12 grams and contains 6.022 x 1023 atoms.

Avogadro’s Number and the Mole How big IS Avogadro’s number? An Avogadro's number of standard soft drink cans would cover the surface of the earth to a depth of over 200 miles. If you had Avogadro's number of unpopped popcorn kernels, and spread them across the US, the country would be covered in popcorn to a depth of over 9 miles. If we were able to count atoms at the rate of 10 million per second, it would take about 2 billion years to count the atoms in one mole.

Avogadro’s Number and the Mole What do these substances have in common? Atoms measured in amu, moles measured in grams amu and g are the same value, different units 1 atom 12C = 12 amu; 1 mol 12C atoms = 12 g 1 atom 4He = 4 amu; 1 mol 4He atoms = 4 g 22

Avogadro’s Number and the Mole 1 mole contains 6.0221415 x 1023 particles Particles can be atoms, ions, molecules, or formula units Determine what type of particle Avogadro’s number will describe for the following: Kr C6H12O6 MgCO3 SF6 CaBr2 P

Molar Masses Atomic, molecular, and formula masses are not practical in lab. We can scale up to calculate the mass of a mole of substances. Molar mass is the same value as atomic mass, just with different units. Molar mass (g/mol: (mass per 1 mole of substance) What is molar mass of C6H6? What is molar mass of Mg(OH)2? Molar masses have 6.022 x 1023 particles in the substance.

Calculate Molar Masses of the following: CO2 Cl2 SO2 CaCl2 H2SO4 AlCl3 Mg3(PO4)2 Na2CO3

Avogadro’s Number and the Mole Conversions: If we know the mass of a substance, how can we calculate number of moles? mass  moles (use molar mass) If we know how many moles of a substance, how can we calculate number of particles? moles  number of particles (use Avogadro’s number)

Mole Diagram molar mass 1 mole = 6.022 x 1023 particles (atoms or molecules) Particles Courtesy: www.unit5.org/christjs/Stoichiometry/Mole%20Island%20Diagram.ppt 27

Practice – Group Work How many moles are in a 2.67 g sample of Ag? How many grams are in a 0.45 mol sample of Mg3(PO4)2? How many atoms are in a 16.3 g sample of S? How many formula units are in 0.81 g of MgCl2? How many ions are in 0.81 g of MgCl2? (Hint: How many ions are in 1 formula unit of MgCl2?) How many moles of ions are in 0.81 g of MgCl2? How many C atoms are in 3.64 g of C6H12O6? How many O atoms are in 0.0765 g of CO2 Answers: 0.0248 mol Ag, 1.2 x 102 g Mg3(PO4)2, 3.06 x 1023 S atoms, 5.12 x 1021 f. un., 1.54 x 1022 ions, 0.026 mol ions, 7.30 x 1022 C atoms, 2.09 x 1021 O atoms

Practice – Group Work Calculate the number of atoms that are in a 6.00 g sample of carbon. Calculate the number of moles in 12.04 x 1023 atoms of uranium. Calculate the mass of 12.04 x 1023 atoms of uranium. If you have 1 g samples of NaCl, CO2, and H2O, which sample will contain the fewest particles? Answers: 4.34 x 1025 C atoms, 2 mole Uranium, 476.1 g Uranium, NaCl (fewest moles because largest MM)

Group Quiz #18 How many atoms are in 12.987 g of iron? How many H atoms are in a 3.4215 gram sample of H2O?

The End Atom/molecule/mole ratios in C6H12O6