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Thermochemistry. Second law of thermodynamics Plan The subject of the thermochemistry. Thermal effect of the chemical reaction. The Hess law and its conclusions. Second low of thermodynamics. Concepts about the entropy. Assistant Kozachok S.S. prepared
Bomb calorimeter for the determination of change in internal energy The process is carried out at constant volume, i.e., ΔV=0, then the product PΔV is also zero. Thus, ΔU=Qv The subscript v in Qv denotes that volume is kept constant. Thus, the change in internal energy is equal to heat absorbed or evolved at constant temperature and constant volume
Bomb calorimeter is made up of heavy steel. The steel vessel coated inside with gold or platinum. It is fitted with a pressure tight screw cap. The two electrodes R1 and R2 are connected to each other through a platinum wire R dipping in the platinum cup C.A small known amount of the substance under investigation is taken in the platinum cup which is supported on the rod R2.The bomb is filled with excess of oxygen under the pressure of 20-25 atmospheres and is sealed. The water bath is also provided with a mechanical stirrer and a Beckmann thermometer which can read correctly upto 0,01 degree.The initial temperature of water is noted and reaction , i.e. combustion is started by passing an electric current through the platinum wire. The heat energy evolved during the chemical reaction raises the temperature of water which is carefully recorded from the thermometer.By knowing the rise in temperature and the heat capacity of the calorimeter, the amount of heat evolved in the reaction can be calculated
Specific heat capacity (often shortened to specific heat) is the measure of the heat energy required to increase the temperature of a unit quantity of a substance by a unit of temperature. For example, at a temperature of 15 Celsius, the heat energy required to raise water’s temperature one kelvin (equal to one degree Celsius) is 4186 joules per kilogram. So this measure would be expressed as 4186 J/(kg·K)or 1000 cal/(kg·K)1 calories = 4,184 joules
Kirchhoff equation Mayerequation ( for mol of ideal gas)
Thermochemistry The study of the energy transferred as heat during the course of chemical reactions. Thermochemical reactions: H2(g) + Cl2(g) = 2HCl;▲ H = -184,6 kJ 1/2 H2(g) + 1/2 Cl2(g) = HCl; ▲ H = -92,3 kJ/mol ▲ H is calculated for 1 mole of product ▲H = ▲U + p▲V ▲H = ▲U + ▲nRT Energy change at constant P = Energy change at constant V + Change in the number of geseous moles * RT
CorrelationU і Н: If υ0, тоНU:СаО + СО2→ СаСО3Ifυ0, тоНU:Na + H2O → NaOH + H2Ifυ=0, тоН=U:H2 + Cl2 → 2HCl
Calculation of standard enthalpies of reactions ▲ H = ( Sum of the standard enthalpies of formation of products including their stoichiometric coefficients ) – ( Sum of the standard enthalpies of formation of reactants including their stoichiometric coefficients ) For elementary substances Н0298 = 0
The Hess’s law Н1 The products of reaction Initial reactans Н2 Н4 Н3 Н1 = Н2 + Н3 + Н4 If the volume or pressure are constant the total amount of evolved or absorbed heat depends only on the nature of the initial reactants and the final products and doesn’t depend on the passing way of reaction.
Conclusions from the Hess law • Нc298(the standard enthalpy of combustion) =-Нf298(the standard enthalpy of formation) 2. Н= ΣnНf298(prod.) - ΣnНf298(reactants) 3. Н= ΣnНс298(reactants) - ΣnНс298(prod.) 4.Н3=Н1-Н2 5.Н1=Н3-Н2 1 1 2 3 2 3
The standard enthalpy of formation ▲ H2980 is defined as the enthalpy change that takes place when one mole of the product is formed under standart condition. C + O2 = CO2▲ Hf1 - ? C + ½O2 = CO ▲ Hf2 CO + ½O2 = CO2 ▲ Hf3 ▲ Hf1 = ▲ Hf2 + ▲ Hf3
The extent of disorder or randomness in a system may be expressed by a property known as entropy.Entropy can be defined asthe property of a system which measures the degree of disorder or randomness in the system Units of entropy joules per mol or calories per degree
- Conformities to the law of heat formation of matters (simple materials) = 0 Berkenheym’s rule of “stars”: heat formation of substances is subjected to the rule of “Stars” by the periodic table (Mg situates between Na and Al that’s why we can use this rule - (organic compounds):
Second law of thermodynamics. 1) Heat cannot be transferred from one body to a second body at a higher temperature without producing some other effect. 2) The entropy of a closed system increases with time 3) The entropy of the universe always increases in the course of every spontaneous (natural) change The second law of thermodynamics introduces the concept of entropy and its relation with spontaneous processes In an isolated system such as mixing of gases, there is no exchange of energy or matter between the system And the surroundings. But due to increase in randomness, there is increase entropy ΔS›0
However, if the system is not isolated, we have to take into account the entropy changes of the system and the surroundings.
Calculation of entropy for the phase passing from the ice to the vapour state at heating. Where Ср’ і Ср’’ – molar heat (capacity)of water at the isobaric process
Free energy and free energy change The maximum amount of energy available to a system during a process that can be converted into useful work It’s denoted by symbol G and is given by ▲G = ▲H - T ▲S where ▲G is the change of Gibbs energy (free energy) This equation is called Gibbs equation and is very useful in predicting the spontaneity of a process. N.B. Gibbs equation exists at constant temperature and pressure
▲ F = ▲ U – T▲S where ▲F is the change of Helmholtz energy N.B. Helmholtz equation exists at constant temperature and volume ΔG would be negative under the following condition:
Effect of Temperature on Feasibility of a Process • Exothermic reactions For exothermic reactions ΔH is always negative, and therefore, it is favourable. b) Endothermic reactions For exothermic reactions ΔH is positive and always opposes the process.
1) Spontaneous (irreversible) process : ▲ G < 0, ▲S > 0, ▲H < 0 2) Unspontaneous (reversible) process : ▲ G > 0, ▲S < 0, ▲H > 0 3) Equilibrium state ▲ G = 0
Third law of Thermodynamics The third law of thermodynamics is a statistical law of nature regarding entropy and the impossibility of reaching absolute zero of temperature. The most common enunciation of third law of thermodynamics is: “As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.”Note that the minimum value is not necessarily zero,although it is almost always zero in a perfect, pure crystal.
The third law helps to calculate the absolute entropies of pure substances at different temperatures. The entropy (S) of the substance at different temperatures T may be calculated by the measurement of heat capacity changes