Thermochemical Change
Thermochemistry is the study of energy changes by a chemical system during a chemical reaction. Energy is broadly defined as the ability to do work. Energy has no mass and can be differentiated into two forms Kinetic and potential energy (Ek and Ep respectively). The kinetic energy (Ek or KE) of an object is the energy that it possesses due to its motion. Potential energy is the energy held by an object or matter because of its position relative to other objects. The standard unit of measuring energy is Joule (J) or Kilojoule (KJ). Temperature is a property that reflects motion of molecules; therefore, it is fundamentally Kinetic energy. Temperature should be differentiated from Heat, which is the transfer of thermal energy between two objects due to temperature differences.
Thermal energy is energy available from a substance as a result of molecular motion. There are three types of thermal energy:
Solids have mainly vibrational motion. Liquids and gases have all three types.
When energy is transferred from an object to another, there is a change in thermal energies, which results in a change in temperature observed in both objects.
The first law of thermodynamics is also known as Law of Conservation of Energy. It states that energy cannot be created or destroyed in an isolated system.
Energy released = Energy absorbed i.e. Heat lost = heat gained
ΔE 0bject A = ΔE 0bject B
The change in temperature (shortened as ΔT [read as delta T]) of the object depends on-
The exact amount of energy in a system is not known, but the change in energy can be estimated
Remember, thermal energy is affected by m, c and t, so:
mcΔt = mcΔt
Energy transfer will continue until an equilibrium is established.
There are various ways in which the second law is expressed but the fundamental message is that the entropy of any isolated system always increases. Isolated systems spontaneously evolve towards thermodynamic equilibrium, the state with maximum entropy.
One way to understand this is when you look at a teenager's room, if not cleaned and tidied, it will become more messy and disorderly with time regardless of how careful one is to keep it clean.
There are 2 types of thermal energy changes:
Thermal energy (Q) is the total kinetic energy of the entities of a substance and can be calculated using the formula:
Q = mcΔt ...Where -
Q = quantity of thermal energy (J)
m = mass (g)
c = specific heat capacity (J/g.oC)
Δt = temperature change (oC)
An open system refers to a system where energy and matter can flow in and out of it.
A closed system allows energy to flow in and out, but not matter. An isolated system prevents the flow of energy and matter in and out(side) the system.
The total energy of a system is the sum of kinetic and potential energy
Calorimetry is the process of using a calorimeter to measure energy changes of an isolated system. An isolated system does not exchange matter or energy with its outside environment. No calorimeter is 100% sealed and insulated, so calorimeters are not an ideal isolated system but are pretty good.
An Illustration of a calorimeter. (Source: opentextbc.ca)
If the system is endothermic (requires energy from the environment), the water in the calorimeter will lose heat and its temperature will drop and as a result the energy within the system will increase (Hsys is positive.
If the system is exothermic (releases heat energy to its environment), the water in the calorimeter absorbs heat and as a result the temperature of the water increases. The Energy within the system (Hsys) is negative.
Based on the first law of thermodynamics, (Heat loss = Heat gain), if a chemical reaction occurs inside a calorimeter, the enthalpy change of the chemical reaction is equal to the enthalpy change of the water inside the calorimeter.
ΔH = q (water in calorimeter)
nHm system = mcΔT
Hm system = (mcΔT)/n
There are two types of heat capacities:
As previously mentioned, the enthalpy (H) of a system is the sum of the kinetic and potential energy within it. An enthalpy change, ΔH, is the difference between the enthalpy of the products and the enthalpy of the reactants for a system when pressure is kept constant.
ΔH = Hproducts - Hreactants
The actual enthalpy cannot be measured directly, however, the quantity of heat that is released or absorbed by the surroundings of a chemical system can be measured as the change in temperature of the surroundings. The change in potential energy of the chemical system equals the change in kinetic energy of the surroundings.
For endothermic reactions, the energy is listed along with the reactants.
Reactants + Energy -> Products
For exothermic reactions, the energy is listed along with the products.
Reactants -> Products + Energy
In a chemical reaction, potential energy can be defined as the energy stored in chemical bonds both strong covalent or ionic bonds and the relevant intermolecular forces between entities.
Potential energy changes for exothermic and endothermic reactions.
Molar enthalpies refer to energy changes per mole of a specific substance in the system, under constant pressure.
Δr H = nΔrHm
Where:
Δr H = enthalpy of reaction (kJ)
n is the chemical amount (mol)
Δr Hm = molar enthalpy of reaction (kJ/mol)
Molar enthalpy notation.
Hess' law states that regardless of the number of stages or steps of a chemical reaction, the total enthalpy change for the reaction is the sum of all changes. If a chemical reaction is reversed, then the sign of ΔHr is also reversed.
When comparing enthalpy changes for formation reactions of different compounds, there has to be a reference energy state. A formation reaction always begins with elements, so any standard enthalpy of formation reactions are measured from the reference energy state of zero. As an arbitrary convention, for the sake of simplicity, all other enthalpies of compounds are measured relative to that reference energy state.
An illustration of the enthalpy of formation reactions with reactants have the standard reference state of zero.
Substances exist in one of three states: solid, liquid or gases. A phase change is when the state changes with out chemical or temperature changes.
An illustration of the terminologies used to describe the phase changes that can occur to a substance. (Source: Wikipedia-CC BY-SA 3.0)
Illustrations of enthalpy changes of water:
Left - Melting ice, Right - Condensation of steam.
Imagine if you needed to push a ball over a hill, you need energy for the ball to the get to the top of the hill, before it rolls down the other side of the hill. In a similar way, a chemical reaction cannot occur unless the molecules gain sufficient energy to get over the activation energy barrier.
The Activation energy Ea is the minimum amount of energy required to initiate a chemical reaction.
An illustration of a chemical reaction with reactants A and products C. Activation energy is shown in red. (Source: Wikipedia-CC BY-SA 3.0)
Energy is required to break bonds between atoms or ions. This results in an endothermic reaction since the energy is obtained from the surrounding.
Bond energy is released when chemical bonds are formed. The stronger the bond, the more the energy released. Because this energy is released to the surrounding, such reactions result in exothermic reactions.
2H2O(l) -> 2 H2(g) + O2(g) ΔrHo = +571.6 kJ Endothermic reaction
H2(g) + Cl2(g) -> 2 HCl(g) ΔrHo = -184.6 kJ Exothermic reaction
A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the overall process. A catalyst lowers the activation energy for a reaction, which results in a larger number of effective collisions between entities, so the reaction rate increases.
An illustration of the effect of a catalyst on a chemical reaction; reducing the activation energy. (Source: Wikipedia-CC BY-SA 3.0)
Enzymes are catalysts in biological systems. The substrate fits into the active site of the enzyme much like a key fit into a lock causing chemical changes to the substrates and as a result lowering the activation energy. Reactions happen more easily and faster.
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