The Energy of Physical, Chemical,
& Nuclear Processes
The study of energy and energy transfer is known as thermodynamics. When this study of energy transfer is specific to energy involved in chemical reactions it is called thermochemistry.
Studying Energy Changes
The Law of Conservation of Energy states that the total energy of the universe is constant. (Energy can neither be created nor destroyed.
Energy can be transferred from one substance to another, and it can also be converted to different forms.
In the study of energy changes it is important to define a frame of reference in order to clearly define what part of the universe is being discussed. Generally the universe is divided into 2 parts: (the system which is the part of the universe being studied and observed (the reactants and products of a chemical reaction). (the surroundings which is everything else (the container in which the reaction is taking place, the air surrounding the container, etc.).
Heat and Temperature
Heat (Q) is defined as the transfer of kinetic energy (unit of joules; J). Heat is transferred spontaneously from warmer objects to cooler objects.
Temperature (T) is a measure of the average kinetic energy of the particles that make up a substance (unit of Celsius; oC or Kelvin; K). [pic]
Enthalpy and Energy Change
The enthalpy (H) of a substance is defined as the total internal energy of that substance at a constant pressure. However, there is no way to determine the absolute enthalpy of a substance because there is no reference which can be used as a zero point.
Instead enthalpy change (ΔH) which accompanies a process is studied. Therefore, the enthalpy of the reactants with respect to the enthalpy of the products is the overall enthalpy change. The enthalpy change of a process is equivalent to its heat change at constant pressure.
Enthalpy Changes in Chemical Reactions
Chemical bonds are a source of energy:
(breaking a bond requires energy.
(creating a bond releases energy.
If more energy is released during the formation of product bonds than was required to break the reactant bonds, then the process is said to be exothermic.
If it requires more energy to break the reactant bonds than is released from the formation of the product bonds, then the reaction is said to be endothermic.
Representing Enthalpy Changes
The enthalpy change in a chemical reaction is known as the enthalpy of reaction (ΔHrxn).
The standard enthalpy change of reaction (ΔHorxn) indicates that a reaction has taken place at standard conditions; 100 kPa and 25oC.
Representing Exothermic Reactions
1. Using a thermochemical equation where the enthalpy of the reaction appears on the product side of the reaction. H2(g) + ½O2(g) ( H2O(l) + 285.8 kJ
2. Indicate the enthalpy of reaction as a separate expression beside the chemical equation. H2(g) + ½O2(g) ( H2O(l) ΔHorxn= -285.8 kJ
For exothermic reactions, ΔHorxn values are always negative.
3. Represent the enthalpy of reaction on an enthalpy level diagram.
Representing Endothermic Reactions
1. Using a thermochemical equation where the enthalpy of the reaction appears on the reactant side of the reaction. MgCO3(s) + 117.3 kJ ( MgO(s) + CO2(g)
2. Indicate the enthalpy of reaction as a separate expression beside the chemical equation. MgCO3(s) ( MgO(s) + CO2(g) ΔHorxn= +117.3 kJ
For endothermic reactions, ΔHorxn values are always positive.
3. Represent the enthalpy of reaction on an enthalpy level diagram Stoichiometry and Thermochemical Equations
The enthalpy of reaction is linearly dependent on the quantity of reactants/products.
MgCO3(s) ( MgO(s) + CO2(g)
ΔHorxn = +117.3 kJ then
2MgCO3(s) ( 2MgO(s) + 2CO2(g)
2(ΔHorxn = +117.3 kJ)
= +234.6 kJ
Given: N2(g) + 3H2(g) ( 2NH3(g) + 92.5 kJ, what is the enthalpy change when 1.0 kg of...
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