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The Arithmetic of Chemistry

The Arithmetic of Chemistry. Stoichiometry and chemical reactions. 3 ways of representing the reaction of H 2 with O 2 to form H 2 O. reactants. products. A process in which one or more substances is changed into one or more new substances is a chemical reaction.

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The Arithmetic of Chemistry

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  1. The Arithmetic of Chemistry Stoichiometry and chemical reactions

  2. 3 ways of representing the reaction of H2 with O2 to form H2O reactants products A process in which one or more substances is changed into one or more new substances is a chemical reaction A chemical equation uses chemical symbols to show what happens during a chemical reaction 3.7

  3. Remember this? • Law of mass conservation • Law of definite proportions

  4. Balancing equations • Number of atoms conserved: atoms in reactants equal to atoms in products • Change coefficients, not subscripts • Balance atoms that occur in only one reactant and one product • Balance the rest • Check number of atoms on each side

  5. Test yourself • Test yourself: ethane gas reacts with oxygen gas to produce carbon dioxide and water • Heating potassium chlorate yields potassium chloride and oxygen gas • Demo reaction: Pb(NO3)2 and KI

  6. Atomic mass is the mass of an atom in atomic mass units (amu) • Atoms: small scale • Macroscopic scale: the mole By definition: 1 atom 12C “weighs” 12 amu On this scale 1H = 1.008 amu 16O = 16.00 amu

  7. Average atomic masses: weighted averages Natural lithium is: 7.42% 6Li (6.015 amu) 92.58% 7Li (7.016 amu)

  8. Dozen = 12 Pair = 2 The mole (mol) is the amount of a substance that contains as many entities as there are atoms in exactly 12.00 grams of 12C 3.2

  9. How Big is Avogadro’s Number? • an Avogadro’s number of softdrink cans would cover the surface of the earth to a depth of over 200 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 an Avogadro’s number of atoms

  10. eggs shoes Molar mass is the mass of 1 mole of in grams marbles atoms molecules 1 mole 12C atoms = 6.022 x 1023 atoms = 12.00 g 1 12C atom = 12.00 amu 1 mole 12C atoms = 12.00 g 12C 1 mole lithium atoms = 6.941 g of Li For any element atomic mass (amu) = molar mass (grams) 3.2

  11. One Mole of: S C Hg Cu Fe 3.2

  12. Converting small scale to large (and back) • Dimensional analysis: method of calculation utilizing a knowledge of units. • Conversion factors are used to manipulate units:

  13. Test yourself • How many atoms are in 0.551 g of K? • How many molecules of water are in a 15g sample? How many individual H atoms? O atoms?

  14. Back to chemical equations • 2H2 + O2 2H2O can be read as: • 2 molecules H2 of react with 1 molecule of O2 to produce 2 molecules of water, OR • 2 moles of H2 react with 1 mole O2 to form 1 mole of water • Convert to grams.

  15. 6 green used up 6 red left over Limiting reagent 3.9

  16. Do You Understand Limiting Reagents? 2Al + Fe2O3 Al2O3 + 2Fe g Al mol Al mol Fe2O3 needed g Fe2O3 needed OR g Fe2O3 mol Fe2O3 mol Al needed g Al needed 1 mol Fe2O3 160. g Fe2O3 1 mol Al = x x x 27.0 g Al 2 mol Al 1 mol Fe2O3 Start with 124 g Al need 367 g Fe2O3 In one process, 124 g of Al are reacted with 601 g of Fe2O3 Calculate the mass of Al2O3 formed. 367 g Fe2O3 124 g Al Have more Fe2O3 (601 g) so Al is limiting reagent 3.9

  17. General types of simple chemical reactions • Combination : A + Z  AZ • Decomposition: AZ  A + Z • Single replacement (substitution) A + BZ  AZ +B • Double replacement/displacement • Neutralization (acids and bases) • Combustion: reaction with O2, energy produced

  18. 1. Combination Reactions • Two or more substances (elements or compounds) react to produce one compound • Also known as “synthesis” or “addition” • A + Z  AZ • Metal oxygen metal oxide • Nonmetal oxygen nonmetal oxide • Metal nonmetal salt • Water metal oxide base • Metal oxide nonmetal oxide salt

  19. 2. Decomposition • ONE substance reacts to form two or more substances (elements or compounds). • AZ  A + Z • Compound compound/element oxygen • Metal carbonate metal oxide carbon dioxide • Hydrated salt salt water • Compound compound/element water

  20. 3. Single Replacement • Also known as single displacement or substitution. • A metal (A) replaces a metal ion (B=metal) in its salt or a hydrogen ion (B=H) in an acid • A + BZ  AZ +B • Ex. Fe(s) + CuSO4(aq)  FeSO4(aq) + Cu(s) • Ex. Sn(s) + HCl(aq)  SnCl2(aq) + H2(g) • A nonmetal (X) replacing another nonmetal (Z) in its salt (B=metal) or acid (B=H) • Cl2(g) + NaBr (aq)  _____________________________ • Bromine + Potassium Iodide ______________________

  21. 4. Double Replacement • Also known as “Double displacement”, “Metathesis”, or “Double decomposition.” • Two compounds are involved with the cation of one compound EXCHANGING with the cation of another compound. • AX + BZ  AZ + BX • These reactions proceed if one of the ff. is satisfied: • An insoluble/slightly soluble product is formed (PRECIPITATE formation) • A weakly ionized species is produced. The most common species of this type is water. • A gas is produced as a product.

  22. 5. Neutralization • Reaction of an acid and a base that usually produces a salt and water. • HX + MOH  MX + HOH • Hydrochloric acid and sodium hydroxide. • Hydrochloric acid and magnesium hydroxide. • Sulfuric acid and Barium hydroxide • To be discussed in a separate lesson

  23. 6. Combustion • Reactions involving oxygen (metal +oxygen), (nonmetal + oxygen), (organic compounds + oxygen) are sometimes called combustion reactions. • Energy is given off in combustion reactions. • For organic compounds, water and carbon dioxide are usual byproducts. • Ex. Metabolism of food, fuel combustion: • Sucrose (C22H12O11) combusts • Combustion of propane • Combustion of hydrogen

  24. Quiz • Insoluble silver chloride is produced in the reaction between silver nitrate and hydrochloric acid. • What type of reaction is this? • Balance the equation.

  25. Quiz • Sodium azide produces nitrogen gas and sodium metal. • Type of reaction? • Balanced equation? • How many grams of nitrogen gas is produced from 60.0g sodium azide?

  26. Quiz • Working at 273.15K and 1atm (STP), I have 10.0g of carbon and 56L of oxygen. Under these conditions, I know 1mol of any gas has a volume of 22.4L (Molar volume at STP). What is the limiting reagent? If carbon monoxide made sure to be formed, what will be its volume?

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