Energy Requirements for the Changes of State

To learn about interactions among water molecules. To understand and use heat of fusion and heat of vaporization. 

Remember that temperature is a measure of the random motions (average kinetic energy) of the particles in a substance. 

Both liquid water and gaseous water contain H2O molecules. In liquid water the H2O molecules are close together, whereas in the gaseous state the molecules are widely separated. The bubbles contain gaseous water. 

It takes energy to melt ice and to vaporize water, because intermolecular forces between water molecules must be overcome. In ice the molecules are virtually locked in place, although they can vibrate about their positions. When energy is added, the vibrational motions increase, and the molecules eventually achieve the greater movement and disorder characteristic of liquid water. The ice has melted. As still more energy is added, the gaseous state is eventually reached, in which the individual molecules are far apart and interact relatively little. However, the gas still consists of water molecules. It would take much more energy to overcome the covalent bonds and decompose the water molecules into their component atoms. 

Intramolecular (bonding) forces exist between the atoms in a molecule and hold the molecule together. Intermolecular forces exist between molecules. These are the forces that cause water to condense to a liquid or form a solid at low enough temperatures. Intermolecular forces are typically much weaker than intramolecular forces. 

Interestingly, water expands when it freezes. That is, one gram of ice at 0 °C has a greater volume than one gram of liquid water at 0 °C. This has very important practical implications. For instance, water in a confined space can break its container when it freezes and expands. This accounts for the bursting of water pipes and engine blocks that are left improtected in freezing weather. 

The expansion of water when it freezes also explains why ice cubes float. Recall that density is defined as mass/volume. When one gram of liquid water freezes, its volume becomes greater (it expands). Therefore, the density of one gram of ice is less than the density of one gram of water, because in the case of ice we divide by a slightly larger volume. For example, at 0 °C the density of liquid water is 

The lower density of ice also means that ice floats on the surface of lakes as they freeze, providing a layer of insulation that helps to prevent lakes and rivers from freezing solid in the winter. This means that aquatic life continues to have liquid water available through the winter. 

It is important to recognize that changes of state from solid to liquid and from liquid to gas are physical changes. No chemical bonds are broken in these processes. Ice, water, and steam all contain H2O molecules. When 

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This equation gives the relation between the electrical potential difference between the copper wires attached to the electrodes when the cell is at equilibrium and the change of the Gibbs energy for the change of state that would take place in the cell if the cell were short-circuited. We point out here that the chemical potentials of electrons refer to 1 mole of electrons or 1 faraday of electricity. Therefore, A G refers to the change of state per faraday. If the change of state requires n faradays,... 

In addition, we usually have a stirrer present in the calorimeter to aid in the mixing of reactants or to hasten thermal equilibration. The mechanical work done on the system by the stirrer results in the continuous addition of energy to the system at a small, approximately constant rate. During the time required for the change in state and thermal equilibration to occur, the amount of energy introduced can be significant. A typical temperature-time variation is shown in Fig. 16, where a greatly expanded temperature... 

Fig. 4 Free energy profile for interaction. Acf and AG are the changes in Gibbs free energy required for the formation of the transition state starting from reactants and complex, respectively. AG° is the change in Gibbs free energy between reactants and complex...

If a reaction can yield products in the ground state or in an electronically excited state, the activation energy for the formation of the latter will therefore be less than that required for the formation of the products in the ground state — provided that there is no significant change in the configuration of the excited-state molecules as compared with the reactant molecules. 

The liquid-liquid interface formed between two immissible liquids is an extremely thin mixed-liquid state with about one nanometer thickness, in which the properties such as cohesive energy density, electrical potential, dielectric constant, and viscosity are drastically changing from those of bulk phases. Solute molecules adsorbed at the interface can behave like a 2D gas, liquid, or solid depending on the interfacial pressure, or interfacial concentration. But microscopically, the interfacial molecules exhibit local inhomogeneity. Therefore, various specific chemical phenomena, which are rarely observed in bulk liquid phases, can be observed at liquid-liquid interfaces [1-3]. However, the nature of the liquid-liquid interface and its chemical function are still less understood. These situations are mainly due to the lack of experimental methods required for the determination of the chemical species adsorbed at the interface and for the measurement of chemical reaction rates at the interface [4,5]. Recently, some new methods were invented in our laboratory [6], which brought a breakthrough in the study of interfacial reactions. 

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