1. Thermite reaction 2. Enthalpy of reaction, H 3. Heating/cooling curves and changes in state 4. More thermite thermodynamics

Transcription

1 Chem 105 Fri Thermite reaction 2. Enthalpy of reaction, H 3. Heating/cooling curves and changes in state 4. More thermite thermodynamics 10/23/2009 1

2 Please PICK UP your graded EXAM in front. ALSO an EXAM KEY. Graded Hour Exam 2 Your numeric grade out of 100 is circled in red on the last page 10/23/2009 2

3 10/23/2009 3

4 Thermite is a mixture of about 2:1 molar ratio of aluminum powder (Al) and iron (III) oxide powder. Ignited by glycerol-potassium permanganate reaction (KMnO 4 ). 4C 3 H 3 (OH) KMnO 4 --> 5CO 2 (g) + 16H 2 O(g) + 7Mn 2 O 3 + 7K 2 CO 3 (For this reaction... which element is oxidized? which element is reduced? which reactant is the oxidizing agent? which reactant is the reducing agent?) 10/23/2009 4

5 A chemical reaction (the system ) takes place inside the surroundings. Define E = total energy of the system. surroundings system E is the energy change in a chemical reaction E = heat gained or lost + work done by system on surroundings H H is change in enthalpy = change in energy content of atoms, including bond strength, ionic attraction, & solvation forces. P V P-V work = work done or by caused by change in volume of gases = P * V If no gases are produced or used up, or the system is kept at constant volume (such as in a bomb calorimeter), then V = 0. And the whole term drops out of the E expression. 10/23/2009 5

6 In exothermic reactions, heat flows out of the system. In endothermic reactions, heat flows into the system. surroundings system 10/23/2009 6

7 H is the enthalpy(or heat content) of a chemical. It is the amount of energy stored in Chemical bonds within the molecule or ion, Water-ion interactions (if the chemical is dissolved in water), or Ion-Ion or molecule-molecule interactions inside a crystal (solids) 10/23/2009 7

8 H = H final H initial It is the change in energy content of atoms, including bond strength, ionic attraction, & solvation forces. Endothermic products less stable than reactants (contain more chemical energy) H Initial (reactants) Final (products) Exothermic products more stable than reactants (contain less chemical energy) 10/23/2009 8

9 The enthalpy of reaction H is proportional to the # of moles of products or reactants. H = kj H is enthalpy of reaction Units of J or kj Written to right of equation - for exothermic reaction + for endothermic reaction Applies to molar quantities given in the balanced chemical equation 10/23/2009 9

10 Why is no /mole shown in units of H rxn? H = kj Because H is a stoiochiometric quantity. It applies to the whole reaction as written to the left. You can write thermochemical conversion factors for all reactants and products. THESE DO include moles in the units. For example, 851 kj 2mol Al 851 kj 1mol Fe 2 O 5 Coefficient in the balanced chemical equation as given. 10/23/

11 If we used 125 g of iron oxide with excess Al powder, how much heat (in kj) is produced by this exothermic reaction? 125g Fe2 O3 = kj 125g Fe2 O3 = g Fe2O 3 mol Fe O kj g Fe2 O3 = g Fe2O 3 mol Fe2O 3??? kj kj kj 1mol Fe O 851kJ g Fe2 O3 = g Fe2O 3 1mol Fe2O 3 666kJ 10/23/

12 If we used 125 g of iron oxide with excess Al powder, how much heat (in kj) is produced by this exothermic reaction? 1mol Fe O 851kJ g Fe2 O3 = g Fe2O 3 1mol Fe2O 3 666kJ Notice that here we do not use -851 kj in the numerator of this conversion factor, because the question asks how much heat is produced? That quantity is always positive. One does not say for instance that -851 kj of heat was produced because that would be redundant. In contrast, if the question had asked What is H for reaction of 125 g of iron (III) oxide with excess Al..?, then the correct answer would be -666 kj because the question did not imply an exothermic or endothermic reaction. 10/23/

13 Heating/Cooling Curves (eventually we ll compare the amount of heat produced in the thermite reaction to the amount of heat required to make molten iron.) 10/23/

14 Melting or or boiling (s( (s-> > l l or or l l -> > g) gg) ) requires added heat. Changes of state occur at constant temperature. 10/23/

15 H 2 O (s) -----> > H 2 O (l) H H = kj H fus fus = kj/mol ( molar( heat of fusion) = +333 J/g ( mass( heat of fusion ) q = (mass)(heat( of fusion) = m Hm fus 10/23/

16 Heating/Cooling Curve for Water 10/23/

17 What quantity of heat is required to melt 500. g of ice and heat the water to steam at 100 o C? Given data: Heat of fusion of ice = 333 J/g Specific heat of water = 4.2 J/g K Heat of vaporization = 2260 J/g +333 J/g J/g 10/23/

18 What quantity of heat is required to melt 500. g of ice (at 0 o C) and convert the water to steam at 100 o C? 1. To melt ice q = (500. g)(333 J/g) = 1.67x 10 5 J 2.To warm water from 0 o C to100 o C q = (500. g)(4.184 J/g K)(100 K)(100-0)K = x 10 5 J Temperature To evaporate water at 100 o C q = (500. g)(2260 J/g) = x 10 6 J Heat added 4. Total heat = sum = x 10 6 J = 1.51 x 10 3 kj 10/23/