Help please! I\'m confused about these questions. Break a bond The stable form o
ID: 907293 • Letter: H
Question
Help please! I'm confused about these questions.
Break a bond The stable form of oxygen at room temperature and pressure is the diatomic molecule O2. What is the sign of delta H for the following process? O2(g) 2 O(g) Plaster of Paris Gypsum is the common name of calcium sulfate dihydrate (CaSO4 . 2 H2O). When gypsum is heated to 150 degree C, it loses most of the water in its formula and forms plaster of Paris (CaSO4 . 0.5 H2O): 2 CaSO4 . 2 H2O(s) 2 CaSO4 . 0.5 H2O(s) + 3 H2O(g) What is the sign of delta H for making plaster of Paris from gypsum? Metallic Hydrogen A solid with metallic properties is formed when hydrogen gas is compressed under extremely high pressures. Predict the sign of the enthalpy change for the following reaction: H2(g) H2(s)Explanation / Answer
5.37 For the process of breaking an oxygen molecule into two atoms you need to break the two bonds (one sigma and one pi) between the oxygen atoms. That is you need to supply energy. thus the process becomes endothermic. For all the processes involving heat/energy to be supplied the thermodyanmic convention is +.thus the delta H for this process has positive sign, endo thermic process.
5.38
When gypsum is heated to 150 degrees it is losing 1.5 water molecules which are present as water of crystallization. Thus we are removing water that is held inside the crysttal lattice by hydrogen bonding. Thus you are again breaking some bonds by supplying energy.(endothermic process, occuring by heating).
Thus the sign for enthalpy change is positive
5.39 Usually a gas is changed into liquid / solid by compressing to high pressues and/or by lowering the temperatures. In the gases the intermolecular attractive forces are very less due to the large distances between the molecules. However in solid state the molecules are very close and the intermolecular attractive forces(vanderWaals forces) are high. When there are more attractions energy is lost. Thus the process of converting gas into liquid/ solid is usually exothermic ( normally you cool them).
Thus the enthalphy change has negtive sign.
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