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A Study of Chemical Reactions. Equations, Mole Conversions, & Stoichiometry. Types of Reactions. Many chemical reactions have defining characteristics which allow them to be classified as to type. Types of Chemical Reactions. The five types of chemical reactions in this unit are:
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A Study of Chemical Reactions Equations, Mole Conversions, & Stoichiometry
Types of Reactions • Many chemical reactions have defining characteristics which allow them to be classified as to type.
Types of Chemical Reactions • The five types of chemical reactions in this unit are: • Combination/Synthesis • Decomposition/Analysis • Single Replacement/Displacement • Double Replacement/Metathesis • Combustion
Combination Reactions • Two or more substances combine to form one substance. • The general form is A + X AX • Example: • Magnesium + oxygen magnesium oxide • 2Mg + O2 2MgO
Combination Reactions • Combination reactions may also be called composition or synthesis reactions. • Some types of combination reactions: • Combination of elements • K + Cl2 • One product will be formed
Combination Reactions • K + Cl2 • Write the ions: K+ Cl- • Balance the charges: KCl • Balance the equation: 2K + Cl2 2KCl
Combination Reactions • Some types of combination reactions: • Oxide + water • Nonmetal oxide + water acid • SO2 + H2O H2SO3 • Metal oxide + water base • BaO + H2O Ba(OH)2
Combination Reactions • Some types of combination reactions: • Metal oxides + nonmetal oxides • Na2O + CO2 Na2CO3 • CaO + SO2 CaSO3
Decomposition Reactions • One substance reacts to form two or more substances. • The general form is AX A + X • Example: • Water can be decomposed by electrolysis. • 2H2O 2H2 + O2
Decomposition Reactions • Types of Decomposition Reactions: • Decomposition of carbonates • When heated, some carbonates break down to form an oxide and carbon dioxide. • CaCO3 CaO + CO2 • H2CO3 H2O + CO2
Decomposition Reactions • Types of decomposition reactions: • Some metal hydroxides decompose into oxides and water when heated. • Ca(OH)2 CaO + H2O Note that this is the reverse of a similar combination reaction.
Decomposition Reactions • Types of decomposition reactions: • Metal chlorates decompose into chlorides and oxygen when heated. • 2KClO3 2KCl + 3O2 • Zn(ClO3)2 ZnCl2 + 3O2 • Some of these reactions are used in explosives.
Decomposition Reactions • Some substances can easily decompose: • Ammonium hydroxide is actually ammonia gas dissolved in water. • NH4OH NH3 + H2O • Some acids decompose into water and an oxide. • H2SO3 H2O + SO2
Decomposition Reactions • Some decomposition reactions are difficult to predict. • The decomposition of nitrogen triiodide, NI3, is an example of an interesting decomposition reaction.
Single Replacement Reactions • Cationic: A metal will replace a metal ion in a compound. • The general form is A + BX AX + B • Anionic: A nonmetal will replace a nonmetal ion in a compound. • The general form is Y + BX BY + X
Single Replacement Reactions • Examples: • Ni + AgNO3 • Nickel replaces the metallic ion Ag+. • The silver becomes free silver and the nickel becomes the nickel(II) ion. • Ni + AgNO3 Ag + Ni(NO3)2 • Balance the equation: • Ni + 2AgNO3 2Ag + Ni(NO3)
Single Replacement Reactions • Not all single replacement reactions that can be written actually happen. • The metal must be more active than the metal ion. • Aluminum is more active than iron in Al + Fe2O3 in the following reaction:
Thermite Reaction • Al + Fe2O3 • Aluminum will replace iron(III) • Iron(III) becomes Fe and aluminum metal becomes Al3+. • 2Al + Fe2O3 2Fe + Al2O3
Single Replacement Reactions • An active nonmetal can replace a less active nonmetal. • The halogen (F2, Cl2, Br2, I2) reactions are good examples. • F2 is the most active and I2 is the least. • Cl2 +2 NaI 2 NaCl + I2
Double Replacement Reactions • Ions of two compounds exchange places with each other. • The general form is AX + BY AY + BX • Metathesis is an alternate name for double replacement reactions.
Metathesis (sink or float?) • NaOH + CuSO4 • The Na+ and Cu2+ switch places. • Na+ combines with SO42- to form Na2SO4. • Cu2+ combines with OH- to form Cu(OH)2 • NaOH + CuSO4 Na2SO4 + Cu(OH)2 • 2NaOH + CuSO4 Na2SO4 + Cu(OH)2
Double Replacement • CuSO4 + Na2CO3 • Cu2+ combines with CO32- to form CuCO3. • Na+ combines with SO42- to form Na2SO4. • CuSO4 + Na2CO3 CuCO3 + Na2SO4
Double Replacement • Na2CO3 + HCl • Notice that gas bubbles were produced rather than a precipitate. • What was the gas? • Write the double replacement reaction first.
Double Replacement • Na2CO3 + HCl • Na+ combines with Cl- to form NaCl. • H+ combines with CO32- to form H2CO3. • Na2CO3 + 2HCl 2NaCl + H2CO3 • H2CO3 breaks up into H2O and CO2.
Double Replacement • The gas formed was carbon dioxide. • The final balanced reaction is: Na2CO3 + HCl NaCl + H2O + CO2. • Balance the equation. • Na2CO3 + 2HCl 2NaCl + H2O + CO2
Combustion Reaction • When a substance combines with oxygen, a combustion reaction results. • The combustion reaction may also be an example of an earlier type such as 2Mg + O2 2MgO. • The combustion reaction may be burning of a fuel.
Combustion Reaction • Methane, CH4, is natural gas. • When hydrocarbon compounds are burned in oxygen, the products are water and carbon dioxide. • CH4 + O2 CO2 + H2O • CH4 + 2O2 CO2 + 2H2O
Combustion Reactions • Combustion reactions involve light and heat energy released. • Natural gas, propane, gasoline, etc. are burned to produce heat energy. • Most of these organic reactions produce water and carbon dioxide.
Practice • Classify each of the following as to type: • H2 + Cl2 2HCl • Combination • Ca + 2H2O Ca(OH)2 + H2 • Single replacement
Practice • 2CO + O2 2CO2 • Combination and combustion • 2KClO3 2KCl + 3O2 • Decomposition
Practice • FeS + 2HCl FeCl2 + H2S • Double replacement • Zn + HCl ? • Single replacement • Zn + 2HCl ZnCl2 + H2
Balancing Equations: Chemical
Ca O O H H How molecules are symbolized Cl2 2Cl 2Cl2 • Molecules may also have brackets to indicate numbers of atoms. E.g. Ca(OH)2 • Notice that the OH is a group • The 2 refers to both H and O • How many of each atom are in the following? • a) NaOH • b) Ca(OH)2 • c) 3Ca(OH)2 Na = 1, O = 1, H = 1 Ca = 1, O = 2, H = 2 Ca = 3, O = 6, H = 6
Mg Mg O O O + Balancing equations: MgO • The law of conservation of mass states that matter can neither be created or destroyed • Thus, atoms are neither created or destroyed, only rearranged in a chemical reaction • Thus, the number of a particular atom is the same on both sides of a chemical equation • Example: Magnesium + Oxygen (from lab) • Mg + O2 MgO • However, this is not balanced • Left: Mg = 1, O = 2 • Right: Mg = 1, O = 1
Balance equations by “inspection” From Mg + O2 MgO 2Mg + O2 2MgO is correct Mg + ½O2 MgO is incorrect Mg2 + O2 2MgO is incorrect 4Mg + 2 O2 4MgO is incorrect Hints: start with elements that occur in one compound on each side. Treat polyatomic ions that repeat as if they were a single entity. 5 a) P4 + O2 P4O10 b) Li + H2O H2+ LiOH c) Bi(NO3)3 + K2S Bi2S3 + KNO3 d) C2H6 + O2 CO2 + H2O 2 2 2 2 3 6 3.5 2 3 C2H6 + O2 CO2 + H2O 2 7 4 6
Balance these skeleton equations: a) Mg + 2HCl MgCl2 + H2 b) 3Ca + N2 Ca3N2 c) NH4NO3 N2O + 2H2O d) 2BiCl3 + 3H2S Bi2S3 + 6HCl e) 2C4H10 + 13O28CO2 + 10H2O f) 6O2 + C6H12O66CO2 + 6H2O g) 3NO2 + H2O 2HNO3 + NO h) Cr2(SO4)3+ 6NaOH 2Cr(OH)3+ 3Na2SO4 i) Al4C3 + 12H2O 3CH4 + 4Al(OH)3
The Mole Q: how long would it take to spend a mole of $1 coins if they were being spent at a rate of 1 billion per second?
Background: atomic masses • Look at the “atomic masses” on the periodic table. What do these represent? • E.g. the atomic mass of C is 12 (atomic # is 6) • We know there are 6 protons and 6 neutrons • Protons and neutrons have roughly the same mass. So, C weighs 12 u (atomic mass units). • What is the actual mass of a C atom? • Answer: approx. 2 x 10-23 grams (protons and neutrons each weigh about 1.7 x10-24 grams) Two problems • Atomic masses do not convert easily to grams • They can’t be weighed (they are too small)
The Mole Withtheseproblems,whyuseatomicmassatall? • Masses give information about # of p+, n0, e– • It is useful to know relative mass E.g. Q - What ratio is needed to make H2O? A - 2:1 by atoms, but 2:16 by mass • It is useful to associate atomic mass with a mass in grams. It has been found that 1gH,12gC,or 23gNahave6.02x1023atoms • 6.02 x 1023 is a “mole” or “Avogadro’s number” • “mol” is used in equations, “mole” is used in writing; one gram = 1 g, one mole = 1 mol.
Mollionaire Q: how long would it take to spend a mole of $1 coins if they were being spent at a rate of 1 billion per second? A: $ 6.02 x 1023 / $1 000 000 000 = 6.02 x 1014 payments = 6.02 x 1014 seconds 6.02 x 1014 seconds / 60 = 1.003 x 1013 minutes 1.003 x 1013 minutes / 60 = 1.672 x 1011 hours 1.672 x 1011 hours / 24 = 6.968 x 109 days 6.968 x 109 days / 365.25 = 1.908 x 107 years A: It would take 19 million years
Comparing sugar (C12H22O11) & H2O Same 1 gram each 1 mol each volume? No, they have dif. densities. No, molecules have dif. sizes. mass? Yes, that’s what grams are. No, molecules have dif. masses # of moles? No, they have dif. molar masses Yes. # of molecules? No, they have dif. molar masses Yes (6.02x1023 in each) # of atoms? No, sugar has more (45:3 ratio) No
Molar mass • The mass of one mole is called “molar mass” • E.g. 1 mol Li = 6.94 g Li • This is expressed as 6.94 g/mol • What are the following molar masses? S SO2 Cu3(BO3)2 32.06 g/mol 64.06 g/mol 308.27 g/mol Calculate molar masses (to 2 decimal places) CaCl2 (NH4)2CO3 O2 Pb3(PO4)2 C6H12O6 Cu x 3 = 63.55 x 3 = 190.65 B x 2 = 10.81 x 2 = 21.62 O x 6 = 16.00 x 6 = 96.00 308.27