Thermochemistry - Lecture notes 3 PDF

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Prepared by: Mary Sheenalyn P. Rodil, Ph.D ...

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THERMOCHEMISTRY Prepared by: Mary Sheenalyn P. Rodil, Ph.D TERMS: heat - defined as the transfer of thermal energy - defined as the total kinetic energy and internal energy of a substance. System - the particular area of interests Surroundings - everything outside the particular interests Open systems allow the transfer of mass and energy Isolated systems do not allow the transfer of mass and energy. closed systems allow the flow of energy but not mass. Transfer of Energy Practice Exercise: Classify the following processes as endothermic or exothermic. 1. Melting of butter 2. Rubbing the hands with alcohol 3. Burning of gasoline 4. Mixing of HCl with water 5. Subliming of naphthalene 6. Photosynthesis 7. Ice or snow forming in clouds FIRST LAW OF THERMODYNAMICS

 The first law states that energy is neither created nor destroyed but is converted to another form.  The change in internal energy is also related to the energy exchanges that occur as heat (q) and work (w). Mathematically, it is expressed as

ΔE = q + w SAMPLE PROBLEM: (1) What is the change in internal energy of a system that absorbs 523 J of heat and does 452 J of work as a result? (2) If the internal energy of a system increases by 20 J and the quantity of work done on a system is 50 J. Is the heat absorbed or given off and how much?

Enthalpy of Chemical Reactions Enthalpy - a thermodynamic quantity used to describe heat changes taking place at constant pressure. The equation is H = E + PV Thus, change in enthalpy is ΔH = ΔE + Δ(PV) Since ΔE = q – PΔV, then ΔH = (q – PΔV) + Δ(PV) If the pressure is held constant, this becomes ΔH = q – PΔV + PΔV or ΔH = qp Overall, we can state that at constant volume, the heat flow is equal to ΔE while under constant pressure, the heat flow is equal to ΔH. Thermochemical Equations The heat of a reaction is the heat associated with the transformation of the reactants to products. It is the amount of heat released or absorbed when specified amounts of substances react. The balanced equation showing the associated heat of reaction is called a thermochemical equation. Practice Exercise: Using the thermochemical equations, the heat evolved or gained of a given quantity of a substance can then be determined.

Example: Thermochemical Equation and Heat of Reaction Hess’s Law and Heat of Reaction Hess’s Law states that ΔH is independent of the number of steps involved in a reaction. It is the same whether the reaction occurs in one step or in a series of steps. Another way of looking at it is to remember that ΔH is a state function, so it is independent of the route the reaction is carried out. Hess’s Law and Heat of Reaction Enthalpy of Formation CALORIMETRY Calorimetry - the measurement of the heat released or absorbed during a physical or chemical process and is carried out within a device called calorimeter. Calorimeter contains water and/or other materials which can absorb heat and its outside walls are insulated. The amount of heat changes depend on the specific heat or heat capacity of the substance. Specific heat (s) measures how much heat is required to raise the temperature of one gram of a material by 10C. Heat capacity (C) is the heat required to raise the temperature of a given quantity of material by 10C. Heat capacity is C = ms m -mass of the material in grams

If the temperatures change (ΔT=Tfinal-Tinitial) is also given, then heat change (q) can now be determined following the equation: q = msΔT or q =CΔT Like enthalpy change (ΔH), q is positive for endothermic process and negative for exothermic process. Problems:

(1) An 11.25 g ethanol is heated from 25.00C to 52.10C. Calculate the amount of heat absorbed by the ethanol. (2) Copper metal with a mass of 7.56 g cools from 65.10C to 11.20C. Find the heat released by the metal. CONSTANT- VOLUME CALORIMETRY In this device, the reaction is carried out at constant volume and with no heat flow between the calorimeter and the outside. The heat changes occur only among the sample, closed bomb and the water. The heat given off (or absorbed) by the sample is absorbed (or released) by the water and the bomb. 1. Calculate the heat change of the combustion of 1.78 g benzoic acid (C6H5COOH) in a constant-volume bomb calorimeter. As measured, the temperature rises from 15.170C to 28.520C and the heat capacity of the bomb plus water is 5.714 kJ/ 0C. Express in kJ/g and kJ/mol. 2. A 0.3786 g unknown substance is burned in a constant-volume bomb calorimeter that has a heat capacity of 1.173 kJ/0C. After the reaction, the temperature increases by 1.5620C. Find the heat given off by the burning of the unknown, on kJ/g. A 23.5 g sample of unknown metal at 79.50C was placed in a constant-pressure bomb calorimeter containing 50.0 g water at 30.00C. The final temperature of the system was found to be 32.60C. What is the specific heat of the metal?...