Che201 Assisgnment 2 Solution Fall 2019 2020
Thermochemistry is the study of the heat energy which is associated with chemical reactions or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the system‘s energy exchange with its a surroundings. Thermochemistry is useful in predicting reactant and product quantities throughout the course of a given reaction. In combination
with entropy determinations, it is also used to predict whether a reaction is spontaneous or non-spontaneous, favorable or unfavorable. Thermochemistry is the part of thermodynamics that studies the relationship between heat and chemical reactions. Thermochemistry is a very important field of study because it helps to determine if a particular reaction will occur and if it will release or absorb energy as it occurs. Enthalpy is a thermodynamic property of a system. It is the sum of the internal energy added to the product of the pressure and volume of the system. It reflects the capacity to do non-mechanical work and the capacity to release heat. Enthalpy is denoted as H;
specific enthalpy denoted as h.
Endothermic reactions absorb heat, while exothermic reactions release heat. Thermochemistry coalesces the concepts of thermodynamics with the concept of energy in the form of chemical bonds. The subject commonly includes calculations of such quantities as heat capacity, heat of
Thermochemistry rests on two generalizations. Stated in modern terms, they are as follows:
Hess’ law (1840): The energy change accompanying any transformation is the same whether the process occurs in one step or many.
Hess’ law and thermochemical calculations:
Germain Henri Hess (1802-1850) was a Swissborn professor of chemistry at St. Petersburg, Russia. He formulated his famous law, which he discovered empirically, in 1840.
The enthalpy of a given chemical reaction is constant, regardless of the reaction happening in one step or many steps.
The enthalpy of a reaction is a measure of how much heat is absorbed or given off when a chemical reaction takes place. It is represented by ΔHreaction and is found by subtracting the enthalpy of the reactants from the enthalpy of the products:
ΔHreaction = ΣΔHf products – ΣΔHf reactants
The Greek letter Σ, may be new to you. In mathematics, it is used to represent the phrase “to sum.”
Therefore, this equation is telling us to sum the enthalpy of the products and subtract the sum of the enthalpy of the reactants. Using a table of Standard Thermodynamic Values at 25°C, you may notice that the table, which covers many pages, has five columns. The first column is the formula of an element or compound you are looking up. The second column is its state of matter – which is very important. The third column lists Hformation values, or the enthalpy of formation. This is the amount of energy needed to form one mole of that compound. Most values as you can see are negative because releasing energy (exothermic) is a more common process in nature.
Find sodium sulfide, or Na2S. As you can see, its enthalpy of formation is -373.21 kJ/mol. This means that when one mole of sodium sulfide is formed from its constituent elements (sodium and sulfur), – 373.21 kilojoules of energy is released. Elements in their free state at their state of matter at 25°C (this is called the “standard state”) are assigned a value of 0.0. This is because elements are not formed from anything more basic, therefore no energy must be absorbed or released to create them. When the enthalpy of reaction is calculated, a negative value indicates the reaction is exothermic. A positive value indicates the reaction is endothermic.
The entropy change from a reaction, or Sreaction, is a measure of the dispersal of energy and matter that takes place during a reaction. As far as identifying an increase in dispersal of matter, there are two things that indicate an increase in entropy:
Have more total moles of products than total moles of reactants.
Have products that are in states of matter that exhibit high amounts of freedom for their particles, namely gases and aqueous compounds.
The entropy of a reaction can be calculated using a formula similar to the enthalpy of reaction:
ΔSreaction = ΣΔSproducts – ΔΣSreactants
GIBBS FREE ENERGY
Gibbs Free Energy is a quantity used to measure the amount of available energy (to do work) that a chemical reaction provides. Furthermore, it can be used to determine whether or not a reaction is spontaneous (works) at a given Kelvin temperature. Reactions are very temperature dependent, and sometimes work significantly better at some temperatures than others. The ΔGf° values provided in the table are only viable at 25°C (298.15 K). Similar to the equations for ΔHreaction and ΔSreaction, ΔGreaction is the difference between the sum of the free energy of formation values of the products and reactants:
ΔGreaction = ΣΔGf products – ΔΣGf reactants
A positive ΔGreaction indicates the reaction is nonspontaneous, a negative ΔGreaction indicates the reaction is spontaneous, and a value close to zero indicates an equilibrium. It’s important to note that spontaneous does not necessarily mean fast. A spontaneous reaction is immediate, but like the rusting of metal, may be slow. Reaction rate is governed by other factors that are not related to the thermochemical quantities discussed here.
For all temperatures, including 25°C, the following equation can be used to determine spontaneity:
ΔGreaction = Hreaction – TΔSreaction
In order to use this equation properly, keep these thoughts in mind:
The temperature must be Kelvin, which is done by adding 273.15 to the Celsius temperature.
Sreaction must be converted to kJ/K.
The value calculated for ΔGreaction should be considered an approximate, particularly as the temperature moves further away from 25°C. Both ΔHreaction and ΔSreaction will vary with temperature. Although ΔSreaction tends to vary more, its impact on ΔGreaction tends to be less. This is because ΔSreaction is measured in units of J/K, and when converted to kJ/K (to agree with the units for
ΔReaction and ΔGreaction – kilojoules), it is numerically small. ΔHreaction tends to vary less than ΔSreaction, but because its value is usually several orders of magnitude greater than a kJ/K value for
ΔSreaction, it affects ΔGreaction greatly. Nevertheless, there are some reactions for which the above equation can give a reliable value over a large temperature range.