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Thermochemistry Unit 7. thermochemistry the study of heat changes in chemical reactions and physical changes.

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Presentation on theme: "Thermochemistry Unit 7. thermochemistry the study of heat changes in chemical reactions and physical changes."— Presentation transcript:

1 Thermochemistry Unit 7

2 thermochemistry the study of heat changes in chemical reactions and physical changes

3 open mass & energy**Exchange: closed energy isolated nothing SYSTEMSURROUNDINGS systemspecific part being analyzed surroundingseverything outside the system (usually the immediate area) universe = system + surroundings

4 Energy is the capacity to do work Thermal energy is the energy associated with the random motion of atoms and molecules Chemical energy is the energy stored within the bonds of chemical substances Nuclear energy is the energy stored within the collection of neutrons and protons in the atom Electrical energy is the energy associated with the flow of electrons Potential energy is the energy available by virtue of an objects position

5 Heat is the transfer of thermal energy between two bodies that are at different temperatures. Energy Changes in Chemical Reactions Temperature is a measure of the thermal energy. Temperature = Thermal Energy 90 0 C 40 0 C greater thermal energy

6 Units for Measuring Heat Flow calorie - the amount of heat needed to raise the temperature of 1 g of H 2 O by 1 o C. -Used except when referring to food Calorie (capital C) refers to the energy in food

7 Units for Measuring Heat Flow 1 Calorie = 1 kilocalorie = 1000 cal SI unit of heat and energy = Joule J = 1 cal 1 J = cal

8 Energy Transformations Heat – (variable is q) – energy that transfers from one object to another –only changes can be detected! –flows from warmer to cooler object

9 Exothermic - process that gives off heat- energy goes from system to surroundings. 2H 2 (g) + O 2 (g) 2H 2 O (l) + energy Energy is written on the product (right) side of the equation q and ΔH are negative (-) Δ is a Greek letter – means change in

10 The products are lower in energy than the reactants Thus, energy is released. ΔH = -395 kJ The negative sign does not mean negative energy, but instead that energy is lost. exothermic

11 Endothermic - process in which heat has to be supplied – energy goes from surroundings to system energy is written on the reactant (left) side of the equation q and ΔH are positive (+) energy + 2HgO (s) 2Hg (l) + O 2 (g)

12 The products are higher in energy than the reactants Thus, energy is absorbed. ΔH = +176 kJ The positive sign means energy is absorbed endothermic

13 When a substance changes state the temperature remains constant during the actual phase change.- this is because the heat energy is being used up by the phase change process. Changes of state

14 The specific heat (C) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius. q = heat m = mass ( in grams) t = t final – t initial C = specific heat q = H Specific heat formula q = m X t X C

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16 How much heat is given off when an 869 g iron bar cools from 94 0 C to 5 0 C? C of Fe = J/g 0 C m = 869 g t = t final – t initial = 5 0 C – 94 0 C = C q = mC t = 869 g x J/g 0 C x –89 0 C= -34,000 J 6.4

17 The specific heat of water is high due to hydrogen bonding

18 Calorimetry the measurement of the heat into or out of a system heat released = the heat absorbed enthalpy changes are measured with a calorimeter No heat enters or leaves!

19 The Law of Conservation of Energy in any chemical or physical process, energy is neither created nor destroyed. - All the energy is accounted for as work, stored energy, or heat.

20 thermo chemical equations thermo chemical equations equations that show heat changes enthalpy (H)heat content of a substance Cannot measure directly but we CAN measure change in enthalpy = ΔH; –heat change for a process; usually measured in kJ (kilojoules)

21 H 2 O (s) H 2 O (l) H = 6.01 kJ The physical states of all reactants and products must be specified Heat of reaction H 2 O (l) H 2 O (g) H = 44.0 kJ Equation – = given mol x H 1 # mol

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23 Thermo chemical Equations How much heat is evolved when 266 g of white phosphorus (P 4 ) burn in air? P 4 (s) + 5O 2 (g) P 4 O 10 (s) H = kJ 266 g P 4 1 mol P g P 4 x 3013 kJ 1 mol P 4 x = 6470 kJ

24 ΔH f = HEAT OF FORMATION = heat absorbed or released to make 1 mol of a compound from its elements ΔH soln = Heat of Solution = heat change caused by dissolving of one mole of substance

25 H f for the formation of rust (Fe 2 O 3 ) is –826 kJ/mol. How much energy is involved in the formation of 5 grams of rust 5.0 g Fe 2 O mol Fe 2 O g Fe 2 O 3 x 826 kJ 1 mol Fe 2 O 3 x = 25.9 kJ

26 When 1.0 g of solid NaOH ( H soln = – kJ/mol) dissolves in 10 L of water, how much heat is released? 1.0 g NaOH 1 1 mol NaOH 40.0 g NaOH x kJ 1 mol NaOH x = 11.1 kJ

27 Molar Heat of Combustion = ΔH comb = heat released in combustion of 1 mol of substance Molar Heat of Fusion ( H fus.) = the heat absorbed by 1 mol of a substance in melting from a solid to a liquid Molar Heat of Solidification ( H solid.) = the heat lost when 1 mol of liquid solidifies (or freezes) to a solid

28 Molar Heat of Vaporization ( H vap.) = the amount of heat necessary to vaporize 1 mol of a given liquid. Molar Heat of Condensation ( H cond.) = amount of heat released when 1 mol of vapor condenses to a liquid

29 Standard heat of reaction Hesss law of heat summation states that if you add two or more thermochemical equations you can add the heats of reaction to give the final heat of reaction

30 Standard Heats of Formation The standard heat of formation (ΔH f0 ) of a compound is the change in enthalpy that accompanies the formation of one mole of a compound Standard heat of formation for elements = 0

31 Standard Heat of Reaction H = H (products) – H (reactants) 1.Multiply the standard heat of formation by the number of moles for each reactant and product. 2.Add the reactants together 3.Add the products together 4.Subtract the sum of the reactants from the sum of the products

32 Calculate H for the following reaction. C 2 H 4 (g) + H 2 (g) C 2 H 6 (g) H for C 2 H 4 (g) = 52.5 kJ/mol; H for H 2 (g) = 0 kJ/mol; (free element) H for C 2 H 6 (g) = –84.7 kJ/mol = kJ


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