Fall CHEM 110 Recitation 2 Group Problems 1 In the lab a substance is studied and the following observations are made At room temperature 20 C the substance has the following properties 1 It is a solid that is shiny and silver in color 2 It has a density of 1 738 g cc 3 It is malleable it can be pounded into a flat sheet 4 It is ductile it can be pulled into a thin wire 5 It is a good electrical conductor The substance is subjected to some further experiments and these additional observations are made 6 It melts at 649 C 7 It boils at 1105 C 8 It burns in air glowing with an intense white light 9 It reacts with bromine to give a brittle white solid Which of these characteristics are physical properties and which are chemical properties 2A How many protons neutrons and electrons are present in 235U4 What does the 4 indicate What does the number 235 indicate 2B Which of these species has 45 neutrons A 80Kr B 80Br C 78Se D 79Kr E 80Rb 3 Natural uranium mined from the Earth contains the isotopes uranium 238 uranium 235 and uranium 234 The majority 99 2745 of all the atoms in natural uranium are uranium 238 Most of the remaining atoms 0 72 are uranium 235 and a slight trace 0 0055 are uranium 234 Determine the average atomic weight of uranium 4 Which of the following processes is are endothermic i A system absorbs 50 kJ of heat from the surroundings and does 30 kJ of work on the surroundings ii A system releases 55kJ of heat into the surroundings and 45 kJ of work is done on the system iii q 12 5 kJ w 3500 J HINT Calculate E q w Draw pictures using a box to represent the system and use arrows to show if energy is going into the system or out of the system Recitation 2 Fall CHEM 110 5 Rank these in order of increasing electrostatic potential energy I an electron that is 2 8 nm from a 2 cation II an electron that is 2 8 nm from a 2 anion III an electron that is 2 8 nm from a proton A I III II B I II III C II III I D II I III E III II I Hint draw a picture to help you with this 6 What is the frequency of the wave shown below assuming it is light electromagnetic radiation 7 What is the frequency of light with an energy of 124 kJ mol Challenge question 8 An Argon laser gives off pulses of green light wavelength 514 nm If a single pulse from the laser has a total energy of 10 mJ how many photons are in the pulse Recitation 2