7: Energy and Chemical Processes

7.1: Energy and Its Units

Energy is the ability to do work. Heat is the transfer of energy due to temperature differences. Energy and heat are expressed in units of joules. 能量是做功的能力。热是由于温差引起的能量传递。能量和热量以焦耳为单位表示。

7.2: Heat and Temperature

1. To relate heat transfer to temperature change.

2. Memorize temperature equations for Celsius, Fahrenheit and Kelvin conversions.

3. Understand how body temperature can vary.

Heat transfer is related to temperature change. Heat is equal to the product of the mass, the change in temperature, and a proportionality constant called the specific heat. 传热与温度变化有关。热量等于质量、温度变化和称为比热的比例常数的乘积。

The fundamental unit of temperature in SI is the Kelvin (K). 

Heat transfer

Heat is a familiar manifestation of energy. 

Differences in temperatures are the ultimate cause of heat transfer.

The direction of heat flow is not shown in heat = mcΔT. If energy goes into an object, the total energy of the object increases, and the values of heat ΔT are positive. If energy is coming out of an object, the total energy of the object decreases, and the values of heat and ΔT are negative.

7.3: Phase Changes

Matter can exist in one of several different states, including a gas, liquid, or solid state. The amount of energy in molecules of matter determines the state of matter: 

A gas is a state of matter in which atoms or molecules have enough energy to move freely. The molecules come into contact with one another only when they randomly collide.

A liquid is a state of matter in which atoms or molecules are constantly in contact but have enough energy to keep changing positions relative to one another.

A solid is a state of matter in which atoms or molecules do not have enough energy to move. They are constantly in contact and in fixed positions relative to one another.

The temperature of a substance does not change as the substance goes from one phase to another. In other words, phase changes are isothermal (isothermal means “constant temperature”).

A phase change depends on the direction of the heat transfer. If heat transfers in, solids become liquids, and liquids become solids at the melting and boiling points, respectively. If heat transfers out, liquids solidify, and gases condense into liquids.

7.4: Bond Energies and Chemical Reactions

1. Bond Energy

Atoms are held together by a certain amount of energy called bond energy. 

Energy is required to break bonds. Energy is released when chemical bonds are formed because atoms become more stable.

Chemical processes are labeled as exothermic or endothermic based on whether they give off or absorb energy, respectively.

2. Enthalpy Change or Heat of Reaction, ΔH

Energy changes in chemical reactions are usually measured as changes in enthalpy.

Note that an exothermic reaction has a negative ΔH value.

3. Endothermic and Exothermic Reactions

endothermic: a process that absorbs energy

exothermic: a process that gives off energy

Endothermic and exothermic reactions can be thought of as having energy as either a reactant of the reaction or a product. Endothermic reactions require energy, so energy is a reactant.

In the course of an endothermic process, the system gains heat from the surroundings and so the temperature of the surroundings decreases (gets cold). 

4. Energy Diagrams

Endothermic and exothermic reactions can be visually represented by energy-level diagrams.

Strong bonds have lower potential energy than weak bonds. 

In endothermic reactions: The energy of the reactants is lower than that of the products. This type of reaction is represented by an "uphill" energy-level diagram.

In an exothermic reaction: The energy of the products is lower than the energy of the reactants. Energetically downhill.

7.5: The Energy of Biochemical Reactions

Energy to power the human body comes from chemical reactions.