Chapter 3

Heat

Heat, which has units of energy, is the transfer or exchange of thermal energy caused by a temperature difference. For example, when a cold ice cube is dropped into a warm cup of water, heat is transferred from the water to the ice, resulting in the cooling of the water. Temperature, by contrast, is a measure of the thermal energy of matter (not the exchange of thermal energy).

Gaseous Matter Structure

In gaseous matter, atoms or molecules are separated by large distances and are free to move relative to one another. Since the atoms or molecules that compose gases are not in contact with one another, gases are compressible. When you inflate a bicycle tire, for example, you push more atoms and molecules into the same space, compressing them and making the tire harder.

Liquid Matter Structure

In liquid matter, atoms or molecules are close to each other (about as close as molecules in a solid) but are free to move around and by each other. Like solids, liquids have a fixed volume because their atoms or molecules are in close contact. Unlike solids, however, liquids assume the shape of their container because the atoms or molecules are free to move relative to one another. Water, gasoline, alcohol, and mercury are all examples of liquid matter.

Specific Heat Capacity

When the amount of the substance is expressed in grams, the heat capacity is called the specific heat capacity (or sim- ply the specific heat) and has units of joules per gram per degree Celsius (J/g °C).

Chemical Energy

A form of potential energy associated with the positions of the particles that compose the chemical system. For example, the molecules that compose gasoline contain a substantial amount of chemical energy.

Properties of Liquids, Solids, and Gases State

Atomic/Molecular Motion; Atomic/Molecular Spacing; Shape; Volume; Compressibility ----------------------------------------------------------------------------- Solid: Oscillation/ vibration about fixed point, close together, definite, definite, incompressible Liquid: Free to move relative to one another, close together, indefinite, definite, incompressible Gas: Free to move relative to one another, far apart, indefinite, indefinite, compressible

Kelvin (K) Scale

Avoids negative temperatures by assigning 0 K to the coldest temperature possible, absolute zero. Absolute zero

Energy and Chemical Reactions

Chemical reactions that release energy, like the explosion of TNT, are said to be *exothermic*. Some chemical reactions behave in just the opposite way—they absorb energy from their surroundings as they occur. Such reactions are said to be *endothermic*. The use of a chemical cold pack is a good example of an endothermic reaction. When a barrier separating the reactants in a chemical cold pack is broken, the sub- stances mix, react, and absorb heat from the surroundings. The surroundings— possibly including your bruised ankle—get colder.

Work

Defined as the result of a force acting on a distance. For example, if you push this book across your desk, you have done work.

Physical and Chemical Properties

Different substances have unique properties that characterize them and distinguish them from other substances. For example, we can distinguish water from alcohol based on their different smells, or we can distinguish gold from silver based on their different colors.

Law of Conservation of Energy

Energy is neither created nor destroyed. The total amount of energy is constant; energy can be changed from one form to another or transferred from one object to another, but it cannot be created out of nothing, and it does not vanish into nothing.

Physical Change

Matter changes its appearance but not its composition. For example, when ice melts, it looks different—water looks different from ice—but its composition is the same. Solid ice and liquid water are both composed of water molecules, so melting is a physical change. Similarly, when glass shatters, it looks different, but its composition remains the same—it is still glass.

Chemical Change

Matter does change its composition. For example, copper turns green upon continued exposure to air because it reacts with gases in air to form new compounds. Matter undergoes a chemical change when it undergoes a chemical reaction. In a chemical reaction, the substances present before the chemical change are called reactants, and the substances present after the change are called products: Reactants -------------Chemical Change----------> Products

Matter

Matter is defined as anything that occupies space and has mass. Matter is ultimately composed of atoms, submicroscopic particles that are the fundamental building blocks of matter. In many cases, these atoms are bonded together to form molecules, two or more atoms joined to one another in specific geometric arrangements.

Fahrenheit (°F) Scale

On the Fahrenheit scale, water freezes at 32 °F and boils at 212 °F. Room temperature is approximately 72 °F. The Fahrenheit scale was initially set up by assigning 0 °F to the freezing point of a concentrated saltwater solution and 96 °F to normal body temperature (although body temperature is now known to be 98.6 °F).

Celsius (°C) Scale

On this scale, water freezes at 0 °C and boils at 100 °C. Room temperature is approximately 22 °C.

Chemical Property of a Substance

One that a substance displays only through changing its composition. The flammability of gasoline is a chemical property— gasoline does change its composition when it burns. The susceptibility of iron to rust is a chemical property—iron must change into iron oxide to display this property. Chemical properties include corrosiveness, flammability, acidity, and toxicity.

Physical Property of a Substance

One that a substance displays without changing its composition. For example, the characteristic odor of gasoline is a physical property— gasoline does not change its composition when it exhibits its odor. The atomic or molecular composition of a substance does not change when the substance displays its physical properties. For example, the boiling point of water—a physical property—is 100 °C. When water boils, it changes from a liquid to a gas, but the gas is still water. Physical properties include odor, taste, color, appearance, melting point, boiling point, and density.

Classifying Matter According to Composition

Pure Substance: composed of only one type of atom or molecule. Pure substances can themselves be divided into two types: elements (a substance that cannot be broken down into simpler substances- copper) and compounds (a substance composed of two or more elements in fixed definite proportions- water, table salt, sugar). Mixture: composed of two or more different types of atoms or molecules combined in variable proportions. In a heterogeneous mixture, such as oil and water, the composition varies from one region to another. In a homogeneous mixture, such as salt water or sweetened tea, the composition is the same throughout. Homogeneous mixtures have uniform compositions because the atoms or molecules that compose them mix uniformly.

Units of Energy

SI Unit: (J) Joules A second unit of energy is the calorie (cal), the amount of energy required to raise the temperature of 1 g of water by 1 °C. A calorie is a larger unit than a joule: 1 cal = 4.184 J. A related energy unit is the nutritional or capital C Calorie (Cal), equivalent to 1000 little c calories. Electricity bills usually come in yet another energy unit, the kilowatt-hour (kWh).

Solid Matter Structure

Solid Matter: Ice, diamond, quartz, and iron are examples of solid matter. Solid matter may be crystalline, in which case its atoms or molecules arrange in geometric patterns with long-range, repeating order, or it may be amorphous, in which case its atoms or molecules do not have long-range order. Examples of crystalline solids include salt and diamond; the well-ordered, geometric shapes of salt and diamond crystals reflect the well- ordered geometric arrangement of their atoms. Examples of amorphous solids include glass, rubber, and plastic.

Energy Associated with Chemical and Physical Changes

Systems with high potential energy—like the raised weight—have a tendency to change in a way that lowers their potential energy. For this reason, objects or systems with high potential energy tend to be unstable. A weight lifted several meters from the ground is unstable because it contains a significant amount of localized potential energy. Unless restrained, the weight will fall, lowering its potential energy. Some chemical substances are like the raised weight just described. For example, the molecules that compose TNT (trinitrotoluene) have a relatively high potential energy—energy is concentrated in them just as energy is concentrated in the raised weight. TNT molecules therefore tend to undergo rapid chemical changes that lower their potential energy, which is why TNT is explosive.

Temperature

The atoms and molecules that compose matter are in constant random motion— they contain thermal energy. The temperature of a substance is a measure of its thermal energy. The hotter an object, the greater the random motion of the atoms and molecules that compose it, and the higher its temperature. We must be careful to not confuse temperature with heat.

Energy

The capacity to do work. Like matter, energy is conserved. The total energy of a sample of matter is the sum of its kinetic energy, the energy associated with its motion, and its potential energy, the energy associated with its position or composition. For example, a moving billiard ball contains kinetic energy because it is moving at some speed across the billiard table. Water behind a dam contains potential energy because it is held at a high position in the Earth's gravitational field by the dam. When the water flows through the dam from a higher position to a lower position, it can turn a turbine and produce electrical energy.

States of Matter

The common states of matter are solid, liquid, and gas. In solid matter, atoms or molecules pack close to each other in fixed locations. Three states of matter Water exists as ice (solid), water (liquid), and steam (gas). In ice, the water molecules are closely spaced and, although they vibrate about a fixed point, they do not generally move relative to one another. In liquid water, the water molecules are also closely spaced but are free to move around and past each other. In steam, water molecules are separated by large distances and do not interact significantly with one another.

Electrical Energy

The energy associated with the flow of electrical charge.

Thermal Energy

The energy associated with the random motions of atoms and molecules in matter. The hotter an object, the more thermal energy it contains.

Heat Capacity

The heat capacity of a substance is the quantity of heat (usually in joules) required to change the temperature of a given amount of the substance by 1 °C.