Problem 21E

Explain the difference between an exothermic and an endothermic reaction. Give the sign of ΔH for each type of reaction.

Solution: Here, we are going to calculate the frequency of the given radiation. Also, we are going to predict the colour associated with the wavelength. Step1: The distance between two adjacent crests or troughs is called wavelength. It is denoted by the greek letter lambda() and is generally expressed in terms of Angstrom units(A ) [1 A = 10 o o -10 m] On the other hand, the number of waves which pass through a given point in one second is known as the frequency. It is denoted by the greek letter nu() and is generally expressed in terms of Hertz(Hz) [1 Hz = 1 sec ] -1 If c is the velocity of light, the the relation between wavelength, frequency and velocity of light is given by: c = x ------(1) Step2: Given, wavelength of the radiation = 650 nm = 650 x 10 m -9 [1 nm = 10 m]-9 -7 = 6.50 x 10 m 8 Velocity of light, c = 3 x 10 m/sec Substituting the values in equation (1), we get, frequency() = ( 3 x 10 m/sec) / (6.50 x 10 m) -7 15 -1 = 0.461 x 10 sec . 14 -1 = 4.61 x 10 sec . Thus, frequency of the radiation is 4.61 x 10 sec . 14 -1 Step3: Given, wavelength of the radiation = 650 nm. From the figure given above, we can say that the colour associated with this wavelength of radiation is red. ------------