5.1 Energy Basics

endothermic process

A reaction or change that absorbs heat A cold pack used to treat muscle strains provides an example of an endothermic process. When the substances in the cold pack (water and a salt like ammonium nitrate) are brought together, the resulting process absorbs heat, leading to the sensation of cold.

The specific heat of iron (the material used to make the pan) is therefore: ciron = 18,140 J / (808 g)(50.0 °C) = 0.449 J/g °C The large frying pan has a mass of 4040 g. Using the data for this pan, we can also calculate the specific heat of iron: ciron = 90,700 J / (4040 g)(50.0 °C) = 0.449 J/g °C

Although the large pan is more massive than the small pan, since both are made of the same material, they both yield the same value for specific heat (for the material of construction, iron). Note that specific heat is measured in units of energy per temperature per mass and is an intensive property, being derived from a ratio of two extensive properties (heat and mass). The molar heat capacity, also an intensive property, is the heat capacity per mole of a particular substance and has units of J/mol °C

joule (J)

SI unit of heat, work, and energy is the joule. A joule (J) is defined as the amount of energy used when a force of 1 newton moves an object 1 meter. It is named in honor of the English physicist James Prescott Joule. One joule is equivalent to 1 kg m2 /s2 , which is also called 1 newton-meter.

Temperature

Temperature is a quantitative measure of "hot" or "cold." When the atoms and molecules in an object are moving or vibrating quickly, they have a higher average kinetic energy (KE), and we say that the object is "hot." When the atoms and molecules are moving slowly, they have lower average KE, and we say that the object is "cold"

specific heat capacity (c)

The specific heat capacity (c) of a substance, commonly called its "specific heat," is the quantity of heat required to raise the temperature of 1 gram of a substance by 1 degree Celsius (or 1 kelvin): c = q / mΔT It is an intensive property—the type, but not the amount, of the substance is all that matters.

When one substance is converted into another, there is always an

associated conversion of one form of energy into another. Heat is usually released or absorbed, but sometimes the conversion involves light, electrical energy, or some other form of energy. For example, chemical energy (a type of potential energy) is stored in the molecules that compose gasoline. When gasoline is combusted within the cylinders of a car's engine, the rapidly expanding gaseous products of this chemical reaction generate mechanical energy (a type of kinetic energy) when they move the cylinders' pistons.

Heat flow

increases the thermal energy of one body and decreases the thermal energy of the other. Suppose we initially have a high temperature (and high thermal energy) substance (H) and a low temperature (and low thermal energy) substance (L). The atoms and molecules in H have a higher average KE than those in L. If we place substance H in contact with substance L, the thermal energy will flow spontaneously from substance H to substance L. The temperature of substance H will decrease, as will the average KE of its molecules; the temperature of substance L will increase, along with the average KE of its molecules. Heat flow will continue until the two substances are at the same temperature

Over 90% of the energy we use comes originally from

the sun. Every day, the sun provides the earth with almost 10,000 times the amount of energy necessary to meet all of the world's energy needs for that day

law of conservation of matter

there is no detectable change in the total amount of matter during a chemical change. W

Heat capacity is determined by both the

type and amount of substance that absorbs or releases heat. It is therefore an extensive property—its value is proportional to the amount of the substance.

The Calorie (with a capital C), or large calorie,

commonly used in quantifying food energy content, is a kilocalorie

However, in nuclear reactions, the energy changes are much

larger (by factors of a million or so), the mass changes are measurable, and matter-energy conversions are significant.

When chemical reactions occur, the energy changes are relatively

modest and the mass changes are too small to measure, so the laws of conservation of matter and energy hold well.

: potential energy

the energy an object has because of its relative position, composition, or condition

kinetic energy

the energy that an object possesses because of its motion.

calories (cal) What does it depend on?

A calorie is the amount of energy required to raise one gram of water by 1 degree C (1 kelvin) , this quantity depends on the atmospheric pressure and the starting temperature of the water. The ease of measurement of energy changes in calories has meant that the calorie is still frequently used.

kilojoule

A kilojoule (kJ) is 1000 joules. To standardize its definition, 1 calorie has been set to equal 4.184 joules.

Energy

Energy can be defined as the capacity to supply heat or do work.

work (w)

One type of work (w) is the process of causing matter to move against an opposing force. For example, we do work when we inflate a bicycle tire—we move matter (the air in the pump) against the opposing force of the air already in the tire

heat capacity (C)

The heat capacity (C) of a body of matter is the quantity of heat (q) it absorbs or releases when it experiences a temperature change (ΔT) of 1 degree Celsius (or equivalently, 1 kelvin) C = q / ΔT

exothermic process

a change that releases heat s. For example, the combustion reaction that occurs when using an oxyacetylene torch is an exothermic process—this process also releases energy in the form of light as evidenced by the torch's flame

thermochemistry

area of science concerned with the amount of heat absorbed or released during chemical and physical changes

law of conservation of energy

during a chemical or physical change, energy can be neither created nor destroyed, although it can be changed in form. (This is also one version of the first law of thermodynamics, as you will learn later.)

Energy can be converted from one form into another, but all of the energy present before a change occurs always

exists in some form after the change is completed.

Assuming that no chemical reaction or phase change (such as melting or vaporizing) occurs, increasing the amount of thermal energy in a sample of matter will cause its temperature to And, assuming that no chemical reaction or phase change (such as condensation or freezing) occurs, decreasing the amount of thermal energy in a sample of matter will cause its temperature to

increase decrease

Thermal energy

is kinetic energy associated with the random motion of atoms and molecules

If we know the mass of a substance and its specific heat, we can determine the amount of heat, q, entering or leaving the substance by measuring the temperature change before and after the heat is gained or lost:

q = (specific heat) × (mass of substance) × (temperature change) q = c × m × ΔT = c × m × (Tfinal − Tinitial) In this equation, c is the specific heat of the substance, m is its mass, and ΔT (which is read "delta T") is the temperature change, Tfinal − Tinitial. If a substance gains thermal energy, its temperature increases, its final temperature is higher than its initial temperature, Tfinal − Tinitial has a positive value, and the value of q is positive. If a substance loses thermal energy, its temperature decreases, the final temperature is lower than the initial temperature, Tfinal − Tinitial has a negative value, and the value of q is negative.

Industrial chemical reactions use enormous amounts of energy to produce

raw materials (such as iron and aluminum). Energy is then used to manufacture those raw materials into useful products, such as cars, skyscrapers, and bridges.

Heat (q)

the transfer of thermal energy between two bodies at different temperatures