Which changes of state are exothermic

You will thus have -- I'll ignore plasma here and stick to the traditional phase changes. When you're going from particles that are in a higher energy state to particles that are in a lower energy state, you must take away energy. This implies that the process will be exothermic , since heat is being released. You will thus have. And there you have it -- six phase changes, three exothermic and three endothermic, correspond to the three traditional phases of matter , liquid, solid, and gas.

For the three states of matter solid, liquid, and gas there are six possible changes of state. Which changes of state are exothermic and which are endothermic?

Superheating is the reason a liquid heated in a smooth cup in a microwave oven may not boil until the cup is moved, when the motion of the cup allows bubbles to form. At this temperature, the steam begins to condense to liquid water. No further temperature change occurs until all the steam is converted to the liquid; then the temperature again decreases as the water is cooled. This region corresponds to an unstable form of the liquid, a supercooled liquid.

If the liquid is allowed to stand, if cooling is continued, or if a small crystal of the solid phase is added a seed crystal , the supercooled liquid will convert to a solid, sometimes quite suddenly. As the water freezes, the temperature increases slightly due to the heat evolved during the freezing process and then holds constant at the melting point as the rest of the water freezes. Subsequently, the temperature of the ice decreases again as more heat is removed from the system.

For example, supercooling of water droplets in clouds can prevent the clouds from releasing precipitation over regions that are persistently arid as a result. Clouds consist of tiny droplets of water, which in principle should be dense enough to fall as rain. In fact, however, the droplets must aggregate to reach a certain size before they can fall to the ground.

Usually a small particle a nucleus is required for the droplets to aggregate; the nucleus can be a dust particle, an ice crystal, or a particle of silver iodide dispersed in a cloud during seeding a method of inducing rain. One approach to producing rainfall from an existing cloud is to cool the water droplets so that they crystallize to provide nuclei around which raindrops can grow.

This is best done by dispersing small granules of solid CO 2 dry ice into the cloud from an airplane. Solid CO 2 sublimes directly to the gas at pressures of 1 atm or lower, and the enthalpy of sublimation is substantial As the CO 2 sublimes, it absorbs heat from the cloud, often with the desired results.

If a Assume that no heat is transferred to or from the surroundings. The density of water and iced tea is 1. Substitute the given values into the general equation relating heat gained by the ice to heat lost by the tea to obtain the final temperature of the mixture. When two substances or objects at different temperatures are brought into contact, heat will flow from the warmer one to the cooler.

The amount of heat that flows is given by. Eventually, the temperatures of the two substances will become equal at a value somewhere between their initial temperatures. Calculating the temperature of iced tea after adding an ice cube is slightly more complicated. See Subscription Options. Go Paperless with Digital. Highly exothermic chemical reactions are needed to thrust spacecraft into the air. White plumes following the craft are reaction product gases dispersing aluminum oxide. Get smart. Sign up for our email newsletter.

Sign Up. Support science journalism. Knowledge awaits. See Subscription Options Already a subscriber? Create Account See Subscription Options. Because phase changes generally occur at constant pressure i. Hence, fusion, vaporization, and sublimation are all endothermic phase transitions. Hence, freezing, condensation, and deposition are all exothermic phase transitions. The direction of the enthalpy change for each of the phase-transition processes named in Figure 4 is shown in Table 1, below.

Recall that endothermic processes have a positive enthalpy change, and exothermic processes have a negative enthalpy change. The enthalpy change of phase transitions can also be used to explain differences in melting points and boiling points of substances.

A given substance has a characteristic range of temperatures at which it undergoes each of the phase transitions at a given pressure. These temperatures are named for the phase transition that occurs at the temperature e. In general, the greater the enthalpy change for a phase transition is the more heat required for an endothermic transition, or released for an exothermic transition , the greater the temperature is at which the substance undergoes the phase transition.

For example, liquids with strong intermolecular attractions require more heat to vaporize than liquids with weak intermolecular attractions; therefore, the boiling point vaporization point for these liquids will be higher than for the liquids with weaker intermolecular attractions. Now, we shall use our understanding of heat engines and phase transitions to explain how refrigerators work.

The enthalpy changes associated with phase transitions may be used by a heat engine Figure 1 to do work and to transfer heat between 1 the substance undergoing a phase transition and 2 its surrounding environment. In a heat engine, a "working substance" absorbs heat at a high temperature and converts part of this heat to work.

As shown in Figure 2, a refrigerator can be thought of as a heat engine in reverse. The cooling effect in a refrigerator is achieved by a cycle of condensation and vaporization of the nontoxic compound CCl 2 F 2 Freon As shown in Figure 5, the refrigerator contains 1 an electrically-powered compressor that does work on Freon gas, and 2 a series of coils that allow heat to be released outside on the back of the refrigerator or absorbed from inside the refrigerator as Freon passes through these coils.

This is a schematic diagram of the major functional components of a refrigerator. The major features include a compressor containing Freon CCl 2 F 2 gas, an external heat-exchange coil on the outside back of the refrigerator in which the Freon passes and condenses, an expansion valve, and a heat-exchange coil inside the insulated compartment of the refrigerator blue in which the Freon is vaporized, absorbing heat from inside the refrigerator and thus lowering its temperature.

Figure 6 below traces the phase transitions of Freon and their associated heat-exchange events that occur during the refrigeration cycle. The steps of the refrigeration cycle are described below the figure. The numbers in the figure correspond to the numbered steps below. This diagram shows the major steps in the refrigeration cycle. For a description of each step indicated by the green numbers , see the numbered steps below. In this figure, blue dots represent Freon gas, and solid blue areas represent liquid Freon.

Small arrows indicate the direction of heat flow into or out of the refrigerator coils. Please click on the pink button below to view a QuickTime movie showing an animation of the refrigeration cycle shown in the figure above and described below. Click the blue button below to download QuickTime 4. Outside of the refrigerator, the electrically-run compressor does work on the Freon gas, increasing the pressure of the gas. As the pressure of the gas increases, so does its temperature as predicted by the ideal-gas law.

Next, this high-pressure, high-temperature gas enters the coil on the outside of the refrigerator.