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Chemistry part 8, Julia Burdge,2e (2009)

Tham khảo sách 'chemistry part 8, julia burdge,2e (2009)', khoa học tự nhiên, hoá học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 162 CHAPTER 5 Thermochemistry The joule can also be defined as the amount of energy exerted when a force of 1 newton N is applied over a distance of 1 meter. 1 J 1 N m where Because the magnitude of a joule is so small we very often express the energy changes in chemical reactions using the unit kilojoule kJ . 1 kJ 1000 J Sample Problem 5.1 shows how to calculate kinetic and potential energies. a Calculate the kinetic energy of a helium atom moving at a speed of 125 m s. b How much greater is the magnitude of electrostatic attraction between an electron and a nucleus containing three protons versus that between an electron and a nucleus containing one proton Assume that the distance between the nucleus and the electron is the same in each case. Strategy a Use Equation 5.1 Ek imr to calculate the kinetic energy of an atom. We will need to know the mass of the atom in kilograms. b Use Equation 5.2 Ec oc Q Qdd to compare the electrostatic potential energy between the two charged particles in each case. Setup a The mass of a helium atom is 4.003 amu. Its mass in kilograms is .661 X 10-24 s 1kg -27 b The charge on a nucleus with three protons is 3 the charge on a nucleus with one proton is 1. In each case the electron s charge is 1. Although we are not given the distance between the opposite charges in either case we are told that the distances in both cases are equal. We can write Equation 5.2 for each case and divide one by the other to determine the relative magnitudes of the results. Remember that the base units of the joule are kg m2 s2. Think About It We expect the energy of an atom even a fastmoving one to be extremely small. And we expect the attraction between charges of larger magnitude to be greater than that between charges of smaller magnitude. Solution a b Z7 1 2 hk 3 6.649 X 10 c m W in-23 WllUiC 2L I J E i where Z 1 -23 3 -1 The electrostatic potential energy between charges of 3 and 1 is three times that between charges Practice Problem A a Calculate .