Thermochemistry
The study of the relationships between chemistry and energy.
Energy
The capacity to do work.
Work (w)
The result of a force acting through a distance. When you push a box across a floor you have done work.
Heat (q)
Energy can be transferred through heat (q), the flow of energy caused by a temperature difference. Heat is the exchange of thermal energy between a system and its surroundings caused by a temperature difference.
Kinetic Energy
The energy associated with the motion of an object.
Thermal Energy
The energy associated with the temperature of an object. Thermal energy is actually a type of kinetic energy because it arises from the motions of atoms or molecules within a substance.
Potential Energy
The energy associated with the position or composition of an object. The greater the height of an object the greater the amount of energy is stored as potential energy before the object contains any motion.
Chemical Energy
The energy associated with the relative positions of electrons and nuclei in atoms and molecules, is also a form potential energy. Some chemical compounds, such as the methane in natural gas or the iron in a chemical hand warmer, contain potential energy
The Law of Conservation of Energy
States that energy can neither be created nor destroyed. However, energy can be transferred from one object to another, and it can assume different forms. A good way to understand the track energy changes is to define the
system
under investigation. The s
Energy Exchange
In an energy exchange, energy is transferred between the system and the surroundings. If the system loses energy, the surroundings gain the same amount of energy, and vice versa.
The First Law of Thermodynamics: The Law of Energy Conservation
States that the total energy of the universe is constant. Since energy is neither created nor destroyed, and since the universe does not exchange energy with anything else, the energy content does not change. The first law of thermodynamics has many impli
Internal Energy (U)
The sum of the potential and kinetic energies of all of the particles that compose the system. Internal energy is a
state function
, which means that the value depends only on the state of the system, not on how the system arrived at that state. The state
Summarizing Energy Flow
- If the reactants have a higher internal energy than the products, the change of energy in the system is negative and energy flows out of the system into the surroundings.
- If the reactants have a lower internal energy than the products, the change of e
Thermal Equilibrium
At thermal equilibrium there is no net transfer of heat.
Heat versus temperature
Temperature is a measure of thermal energy with a sample of matter. Heat is the transfer of thermal energy. Thermal energy always flows from matter at high temperatures to matter at lower temperatures.
Temperature Changes and Heat Capacity
When a system absorbs heat, its temperature changes by the change in T. Experiments show that heat absorbed by a system and it's corresponding temperature change are directly proportional. The constant between proportionality between heat and temperature
Heat Capacity (C)
The higher the heat capacity of a system, the smaller the change in temperature for a given amount of absorbed heat. The Specific Heat Capacity of a system is the
quantity of heat required to change its temperature by 1 degree
. Units of het capacity are
Molar Heat Capacity
The amount of heat required to raise the temperature of 1 mole of a substance by 1 degree. Specific heat capacity and molar heat capacity are
intrinsic properties
, they depend on the kind of substance being heated but not the amount.
Specific Heat Capacity
The specific heat capacity of a substance can be used to quantify the relationship between the amount of heat added to a given amount of the substance and the corresponding temperature increase.
Thermal Energy Transfer
When two substances of different temperatures are combined, thermal energy flows as heat from the hotter substance to the cooler one. If we assume that two substances are thermally isolated from everything else, then the heat lost by one substance equals
Pressure-Volume Work
Energy transfer between a system and its surroundings can occur via heat or work. Work is a force acting through a distance. Pressure-Volume work occurs when the force is caused by a volume change against an external pressure.
Calorimetry
Used to measure the heat involved in a chemical reaction. In calorimetry, we measure the thermal energy of the reaction (defined as the system) and the surroundings exchange by observing the change in temperature of the surroundings.
Enthalpy (H)
The enthalpy of a system is defined as the sum of the systems internal energy and the product of its pressure and volume. Since internal energy, pressure, and volume are all state functions, enthalpy is also a
state function
. It is a measure of the heat
Endothermic Reaction
A positive enthalpy indicated that heat flows into the system as the reaction occurs. A chemical reaction with a positive enthalpy is called an endothermic reaction, where heat is absorbed from its surroundings.
Exothermic Reaction
A chemical reaction with a negative enthalpy sign in which heat is given off into the surroundings.
Summarizing Enthalpy
The value of Enthalpy for a chemical reaction is the amount of heat absorbed or evolved in the reaction under conditions of constant pressure.
Enthalpy of Reaction
Also called the heat of reaction, is an
extensive property
that depends on the amount of material undergoing the reaction. The amount of heat generated or absorbed when a chemical reaction occurs depends on the amounts of reactants that actually react.
Hess's Law
The change in enthalpy for a step wise process is the sum of the enthalpy changes of the steps.
Standard State of a Gas
The standard state for a gas is the pure gas at a pressure of exactly one bar.
Standard State for Liquid or Solid
The standard state for a liquid or solid is the pure substance in its most stable form at a pressure of one bar and at the temperature of interest.
Standard State for a Substance in Solution
The standard state for a substance in solution is exactly the concentration per one mole per litre.
Standard Enthalpy Change
The change in enthalpy for a process when all reactants and products are in their standard states.
Standard Enthalpy of Formation for a Pure Compound
The change in enthalpy when 1 mole of the compound forms from its constituent elements in their standard states.
Calorimetry
...