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Chapter 6 Chemical Reactions and Change

Chapter 6 Chemical Reactions and Change. In a chemical change , reacting substances form new substances with different compositions and properties; a chemical reaction takes place. In a chemical reaction ,old bonds are broken and new bonds formed;

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Chapter 6 Chemical Reactions and Change

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  1. Chapter 6 Chemical Reactions and Change

  2. In a chemical change, reacting substances form new substances with different compositions and properties; a chemical reaction takes place In a chemical reaction,old bonds are broken and new bonds formed; atoms in the reactants are rearranged to form one or more different substances

  3. A chemical equationgives the chemical formulas of the reactants on the left of the arrow and the products on the right. Since matter in a chemical reaction is conserved, the number of atoms you begin with must equal the number oand type you end up with.

  4. In a balanced chemical equation, there must be the same number of each type of atom on the reactant side and on the product side numbers called coefficientsare used in front of one or more formulas. Al + S Al2S3Not balanced 2Al + 3S Al2S3Balanced 2Al = 2Al 3S = 3S Coefficients that can be changed 2Al + 3S 1Al2S3 Coefficients that can’t be changed 2Al + 3S Al2S3 Balance the following reaction: P4(s) + Br2(l) = PBr3(g) P4(s) 4PBr3(g) P4(s) + 6Br2(l) = 4PBr3(g) Al(s) + Fe2O3(s) = Fe(s) + Al2O3(s) 2Al(s) Al2O3(s) 2Al(s) + Fe2O3(s) = 2Fe(s) + Al2O3(s) Thermite reaction

  5. Burning of natural gas, CH4 CH4 + O2 = CO2 + H2O CH4 + O2 = CO2 + 2H2O Burning of butane, C4H10 C4H10 + O2 = CO2 + H2O C4H10 + O2 = 4CO2 + 5H2O C4H10 + 13/2O2 = 4CO2 + 5H2O 2C4H10 + 13O2 = 8CO2 + 10H2O

  6. Magnesium phoshate, Mg3(PO4)2, is insoluble while sodium phosphate, Na3PO4, and magnesium chloride, MgCl2, are water soluble Na3PO4(aq) + MgCl2(aq)= Mg3(PO4)2(s) + NaCl(aq) 6 NaCl (aq)

  7. Reactions of some metals with water Na(s) + H2O (l) = NaOH(aq) + H2 (g) Na + 3/2H2O = NaOH + H2 2Na + 3H2O = 2NaOH + 2H2 K(s) + H2O (l) = KOH(aq) + H2 (g) K + 3/2H2O = KOH + H2 2K + 3H2O = 2KOH + 2H2 Reactions of some elements with oxygen Mg(s) + O2 (g) = MgO (s) 2Mg + O2 = 2MgO Fe(s) + O2(g) = Fe2O3 (s) 2Fe + 3/2O2 = Fe2O3 4Fe + 3O2 = 2Fe2O3 P4 (s, white phosphorus) + O2 (g) = P4O10(s) P4 + 5O2 = P4O10 (s) Note that when Mg and Fe reacted with oxygen, they lost electrons and becamepositively charged in the process; the loss of electrons is called oxidation. The oxygen gained electrons and this is called reduction. All elements in their elemental state are neutral and are assigned an oxidation state of zero. Similarly, when Na and K reacted with H2Othey became positively charged. In this case the hydrogen in water, H+1 was reduced to its elemental state H20

  8. Quantities Definition of terms. 1 ream of paper = 500 sheets 1 case = 24 cans 1 dozen donuts = 12 donuts Definition of a new quantity, the mol 1 mol = 6.03*1023 Why choose this number? Several lectures ago we discussed the mass of various particles mass of a proton (p) 1.672*10-24 g mass of a neutron (n) 1.674*10-24 g mass of an electron (e) 9.1*10-28 g Consider the element H; it consists of a proton and electron; the mass of the elctrom is small in comparison so lets ignore it. What is the mass of 1 mol of H atoms? The mass of 1 mol of H atoms: (6.03*1023)*(1.67*10-24g) = 10.0*10-1 = 1g

  9. What is the mass of 1 mol of 12C6 atoms? mass of a proton (p) 1.672*10-24 g mass of a neutron (n) 1.674*10-24 g mass of a proton ≈ mass of a neutron ≈ 1.67*10-24 g What is the mass of one 12C6 atom? C = 6 p + 6 n = 12(1.67*10-24) g The mass of 1 mol of 12C6 atoms? 12 (6.03*1023)* (1.67*10-24g) = 12*10.0*10-1 = 12g The mass of 1 mol of yMx atoms? y (6.03*1023)* (1.67*10-24g) = y*10.0*10-1 = yg where y = atomic mass of M The atomic mass of any element is also numerically equal to the mass in grams of 6.03*1023 atoms of that element

  10. How many mols of O are in 1 mole of aspirin, C9H8O4? 4 moles How many O atoms are in 1 mole of aspirin, C9H8O4? 4 oxygen atoms/aspirin* 6.03*1023 = 24*1023

  11. What is the mass in grams of a mol of aspirin, C9H8O4? 9*12+8*1+4*16= 180 g/mol 180 is also referred to as the molar mass

  12. 6*12+12*1+6*16 = 180 To calculate the molar mass simply add up the (atomic masses of each element times the number of times they appear) in the molecule

  13. The Law of Conservation of Mass indicates that in an ordinary chemical reaction, matter cannot be created or destroyed no change in total mass occurs in a reaction mass of products is equal to mass of reactants ReactantsProducts 2 moles of Ag + 1 mole of S = 1 mole Ag2S 2 (107.9 g) + 1(32.1 g) = 1 (247.9 g) Ag = 215.8g S = 32.1g Total = 247.9

  14. Consider the following equation: 4Fe(s) + 3O2(g) = 2Fe2O3(s) 4 atoms of iron react with 3 molecules of oxygen to produce 2 molecules of iron(III) oxide This equation can be read in “moles” by placing the words “moles of” between each coefficient and formula. 4 moles of Fe + 3 moles of O2 equals 2 moles of Fe2O3

  15. Energy Changes in Chemical Reactions Ever seen water go up hill on it own?

  16. Why do chemical reactions occur? Consider the reaction of paper which is mainly cellulose, (C6H10O5)n with oxygen Does a reaction occur? What are the products? Why doesn’t paper react spontaneously with oxygen? or does it? The driving force for most chemical reaction is the conversion of potential energy to kinetic energy (ie the release of heat) similar to water going over a dam

  17. Far less common is the situation illustrated in the photo and described by the diagram on the right Identify each of the following reactions as exothermic or endothermic A. N2(g) + 3H2(g) = 2NH3(g) + 22 kcal B. CaCO3(s) + 133 kcal = CaO(s) + CO2(g) C. 2SO2(g) + O2(g) = 2SO3(g) + heat A and C are exothermic; B is endothermic and it proceeds because the CO2 is driven off by heating and escapes into the environment

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