B2 Thermodynamics Ideal gas Law Review PV=nRT P = pressure in Pa V = volume in m3 n = # of moles T= temperature in Kelvin R = 8.31 J K -1 mol -1 m = mass of sample M = molar mass N = number of moles N A = 6.02 x n = N/N A = m/M

1st Law of Thermodynamics  U = Q - W  U = change in internal energy Q = heat absorbed by the gas W = work done by the gas on its environment If  U is positive, the gas gains energy, T increases If  U is negative, it loses energy, T decreases If Q is positive, heat energy flows into the gas If Q is negative, heat energy leaves the gas If W is positive, the gas does work on the environment, V increases If W is negative, the environment does work on the gas, V decreases If  U is positive, the gas gains energy, T increases If  U is negative, it loses energy, T decreases If Q is positive, heat energy flows into the gas If Q is negative, heat energy leaves the gas If W is positive, the gas does work on the environment, V increases If W is negative, the environment does work on the gas, V decreases

Work Isobaric Process W = P  V W = p(V 2 –V 1 )  U = Q - P  V This is positive work as the volume increases. T 2 = T 1 (V 2 /V 1 ), so T increases, and  U is positive. Q therefore must be positive also. Heat is added. This is positive work as the volume increases. T 2 = T 1 (V 2 /V 1 ), so T increases, and  U is positive. Q therefore must be positive also. Heat is added.

Work Isothermal Process  U = 0 W = Q P = constant/V This is positive work as the volume increases. Therefore, Q is also positive – heat is absorbed. This is positive work as the volume increases. Therefore, Q is also positive – heat is absorbed.

Work Isovolumetric Process  V = 0 W = 0  U = Q  U and Q are both positive for the process shown. Therefore, T increases as heat is absorbed. The pressure also increases.  U and Q are both positive for the process shown. Therefore, T increases as heat is absorbed. The pressure also increases.

Work Adiabatic Process Q = 0  U = -W PV 5/3 = constant W is positive for the process shown ( V increases). Therefore,  U is negative and the temperature drops. W is positive for the process shown ( V increases). Therefore,  U is negative and the temperature drops.

Process Curves