PROBLEMS 3 BATAA EL GAFAARY

Summary In a reversible process it is possible to return the system and its surroundings to their initial states. Irreversible processes cannot be undone. The work done during a process is equal to the area under the curve in the PV plot. The work done at constant pressure is The work done at constant volume is zero. The work done in an isothermal expansion is

Summary An adiabatic process is one where no heat transfer occurs. The value of the specific heat depends on whether it is at constant pressure or at constant volume. Molar specific heat is defined by: For a monatomic gas at constant volume: For a monatomic gas at constant pressure:

The First Law of Thermodynamics Combining these gives the first law of thermodynamics. The change in a system’s internal energy is related to the heat Q and the work W as follows: It is vital to keep track of the signs of Q and W.

Ideal Gas Processes

Thermal Processes Here is a summary of the different types of thermal processes:

The enthalpy of a given chemical reaction is constant, regardless of the reaction happening in one step or many steps. HESS ΔH°rxn = Σ ΔH°f (products) - Σ ΔH°f (reactants) All the above values have units of kJ/mol because these are standard values. All standard enthalpies have the unit kJ/mol. Remember also that all elements in their standard state have an enthalpy of formation equal to zero.

ΔH°comb = [ 6 (-393.5) + 6 (-285.8) ] minus [ (-1275) + (6) (0) ] 1. Calculate the standard enthalpy of combustion for the following reaction: C6H12O6(s) + 6O2(g) ---> 6CO2(g) + 6H2O(ℓ) To solve this problem, we must know the following ΔH°f values: Since oxygen is an element in its standard state, its enthalpy of formation is zero. ΔH°comb = [ 6 (-393.5) + 6 (-285.8) ] minus [ (-1275) + (6) (0) ] ΔH°comb = -2801 kJ/mol of glucose.

 2. Calculate the standard enthalpy of formation for glucose, given the following values: ΔH°comb, glucose = -2800.8 kJ/mol  ΔH°f, CO2 = -393.5  ΔH°f, H2O = -285.8 Solution: 1) First of all, this is the reaction we want an answer for: 6C(s, graphite) + 6H2(g) + 3O2(g) ---> C6H12O6(s) We know this because the problem asks for the standard enthalpy of formation for glucose. The above chemical reaction IS the standard formation reaction for glucose. We want the enthalpy for it. 2) Here are the reactions to be added, in the manner of Hess' Law: C6H12O6(s) + 6O2(g) ---> 6CO2(g) + 6H2O(ℓ)  C(s, gr.) + O2(g) ---> CO2(g)  H2(g) + 1/2 O2(g) ---> H2O(ℓ) 3) Flip the first reaction and multiply the other two by six. Then add the three reactions together. If you do it right, you should recover the reaction mentioned just above in (1). -2800.8 = [ 6 (-393.5) + 6 (-285.8) ] minus [ (ΔH°f, glucose) + (6) (0) ]

3. Complete combustion of 1 3.Complete combustion of 1.00 mol of acetone (C3H6O) liberates 1790 kJ: C3H6O(ℓ) + 4O2(g) ---> 3CO2(g) + 3H2O(ℓ); ΔH°comb, acetone = -1790 kJ Using this information together with the data below (values in kJ/mol), calculate the enthalpy of formation of acetone. ΔH°f, O2: 0  ΔH°f, CO2: -393.5  ΔH°f, H2O: -285.83 Solution: 1) Hess' Law: ΔH°rxn = Σ ΔH°f, products minus Σ ΔH°f, reactants 2) Sustitute values into equation: -1790 = [ 3 (-393.5) + 3 (-285.83) ] minus [ (ΔH°f, acetone) + (4) (0) ] -1790 = -2037.99 - ΔH°f, acetone 247.99 = - ΔH°f, acetone ΔH°f, acetone = -247.99 kJ/mol

4. Using standard enthalpies of formation, calculate the heat of combustion per mole of gaseous water formed during the complete combustion of ethane gas. The enthalpies of formation needed are: Solution: 1) The balanced equation for the combustion of C2H6 (ethane) is: 2C2H6 + 7O2 ---> 4CO2 + 6H2O 2) The enthalpy of the reaction is: [(4 moles CO2)(-393.5 kJ/mole) + (6 moles H2O)(-241.8 kJ/mole)] – [(2 moles C2H6)(-84.68 kJ/mole) + (7 moles O2)(0 kJ/mole)] 3) However, that's the heat produced when we make 6 moles of H2O(g). Therefore, -…….. kJ / 6 moles H2O = -………. kJ / mole H2O

5. If the temperature of a gas is held constant during compression or expansion it is called Equation of state Isobaric Isothermal Thermal conductivity convection 6. Heat can be transferred by Thermal Conduction Convection Radiation All of the aboveNone of the above None of the above

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