The inverted vessel at the top of the animation contains hydrogen molecules. It also contains a smaller vessel with porous walls. This small porous vessel contains only oxygen molecules and is connected by a glass tube to the bottle containing water. Some of the oxygen molecules have made their way to the bottom bottle. As the animation proceeds hydrogen molecules diffuse through the porous walls more rapidly than the oxygen molecules diffuse out of the inner vessel. As a result the pressure inside the inner vessel and the bottle builds up and water is forced out of the bottle.

Variation Of Pressure With Number Of Molecules And Temperature

As the molecules of gas strike the walls of the container they exert a force which results in a pressure of 220 torr. When more molecules are admitted to the container, there are more collisions with the walls per square centimeter. Because pressure is force per area, the pressure increases to 270 torr. When the container is then heated, the velocity of the molecules increase, they strike the walls of the container with greater force and the pressure increases to 350 torr.

An Exothermic Reaction

A reaction chamber rests inside a beaker of water. The chamber is designed to efficiently transfer heat to the water. Reactants, the red and blue materials, are added to the reaction chamber and their heat content is shown on the graph to the right. As they react products are formed that have a lower heat content. This difference in energy is released in the form of heat. The heat increases the temperature of the water. The change in enthalpy is defined as the heat content of the products minus the heat content of the reactants.Because the heat content of the products is lower than that of the reactants, the enthalpy change must be negative.

Change In Enthalpy During The Melting Of Ice

As the ice melts it absorbs heat. The enthalpy of the product, liquid water, is therefore greater than that of the ice.

The Entropy Change During The Melting Of Ice

The molecules of water in ice are very nicely ordered, whereas in liquid water they are moving about with considerable disorder. The entropy of the system therefore increases.

The Free Energy Change For The Melting Of Ice At 20 °C

At a constant temperature of 20 °C, the melting of ice is favorable. What is the relationship between the change in free energy and the change in enthalpy and the change in entropy?