This is the fourth of five Branches of Chemistry, which is physical chemistry. Thus, physical chemistry that focuses on phenomena macroscopic, miscroscopic, atomic, subatomic particles, and chemical processes through the system based on the physics concepts and principles. Meanwhile, there are some sub-branches of physical chemistry that need to be explained. Thus, physical chemistry meant to create greater applications for life.
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1. Thermodynamic & Thermochemistry
Some studies show that heat is the most important factor in chemical reaction. Heat drives some reaction to occur but for some cases it retains reaction. The study about learning how to measure the heat transfer in chemical reaction is a branch of physical science called thermodynamic, which describes the meaning of heat and gives procedures for measuring heat transfer quantitatively. Heat is just another form of energy which known as thermal energy
Thermodynamic have wide range of application but in this article we limit our study for certain aspect that is chemical. At this point, we know that the transfer of heat lesson called thermodynamic which is more a domain of physical. Then, we can say the thermodynamic of chemical aspect is called thermochemistry.
Because chemical reactions are usually studied in constant pressure, heat transfer is reaction are measured at constant pressure. To obtain a number of heat transfer at constant-pressure, a chemical reaction is needed and the amount of the heat transferred symbolized as qp.
This reaction called reaction enthalpies which as formulation as:
[tab title=”Enthalpies Formula
qp = ΔH = Hf – Hp = Hproduct – Hreactan = ΔHreaction
When carbon monoxide burns in oxygen to produce carbon dioxide, the heat is release or given off into the environment, it has a negative sign. When heat is given off by reaction (ΔH is negative), the reaction is said to be exothermic. The contrary reaction of exothermic which heat is taken up showed by ΔH positive are called endothermic. The example of endothermic is the preceding reaction written in the opposite direction or reverse direction of carbon dioxide.
Thermodynamic unable us to predict ΔH of decomposition reaction with complete confidence, even if calorimetric experiment is never actually done of performed for it.
Standard State Enthalpies
The absolute values of the enthalpy of an element behave the same like internal energy means it can not be measured or calculated. Only the changes in enthalpy can be measured. It is necessary to adopt a reference state for enthalpies of substances therefore with this problem chemist define standard states for principles of modern chemistry as follows :
- for solid and liquid, the standard state is the thermodinamycally stable state at a pressure of 1 atm and at a specified temperature.
- for gases, the standard state is the gaseous phase at a pressure of 1 atm, at specified temperature and exhibiting ideal gas behavior.
- for dissolved species, the standard state is a 1-m solution at a pressure of 1 atm, at specified temperature and exhibiting ideal solution behavior.
The most common temperature choosen for specified temperature is 298,15 K (25 C exactly). So, when the temperature standard is not stated therefore the temperature is 298,15 K. Once the standards have been defined, it can be concluded that the chemical element in their standard states at 298,15 K have zero enthalpy.
The delta enthalpy or the change in enthalpy when the conditions are in standard. It means all reactants and products are in their standard state with the specified temperature called standard enthalpy symbolized as ΔHof . The enthalpy changes for the reaction that produce 1 mol of the compound from its element in their stable states is called standard enthalpy of formation concept ΔHof of compound.
To form in a new arrangement of the atom a chemical reaction requires an existing bond to break. Chemists have found that in highly reactive intermediate species there could be more than one bond have been broken and find the amount of energy required to break a particular bond.
Once such important measurable quantity is the enthalpy changes when a bond is broken in the gas phase, called the bond enthalpy. This change is invariably positive because atoms bonded together have lower energy than when it’s separated. For instance, the reaction happen in bond enthalpy of a C- H in methane need 438kJ mol-1, when measured in standard enthalpy condition.
A process happens if one or more properties of a system changes. Process could be divided into three categories:
- Isobaric means the process occurs when the pressure of system remaining constant
- Isothermal means the process occurs when the temperature of system remaining constant
- Adiabatic means process occurs without any heat exchange.
Electrochemistry is the branch of physical chemistry concerned with the inter conversion of chemical and electrical energy through oxidation-reduction reaction. Electrons are transferred between two hall-reaction through external electrical energy. Electrical energy can be used to drive chemical reactions that are not naturally spontaneously occurred.
Electrochemical methods are very widely used in medical diagnostic, where they routinely measure very low concentration of molecules of clinical significance. Other applications of electrochemistry are found in solar energy, conversion, fuel cells and energy storage technology.
The discussion about electrochemistry started by reviewing redox reaction, aqueous solution, introducing the components of electrochemical cells and distinguishing galvanic cells from electrolytic cells which not all of these subject will be explained in this article. Some subject will be explained briefly.
Galvanic cells is one of electrochemical cells, electrochemical itself is the location where electrochemical reaction interconvert chemical and electrical energy through coupled reduction oxidation reaction in which half-reaction are separated.
According to Gibbs theory, in galvanic cells free energy made available in spontaneous reduction oxidation reaction and can be converted into electrical energy which can be useful for work. The example of galvanic cell happen on reduction-oxidation reaction of copper metal to form copper ions, sand silver ion have been reduced to silver metal. A wire connects the two metal electrodes, allowing electrons to flow between them and the ionic conductor connecting cells is called a salt bridge.
The other concept must be known is anode and cathode. Anode is the location of electrode when it experience an oxidation and cathode is the location where electrode experience reduction. These terms and concept was stated by Michael Faraday.
In galvanic cell, all reaction described as spontaneously occurred and the energy change is written negative. This thermodynamic driving force related to an electrostatic driving force. The system composed of charged particles spontaneously seek the of the lowest electrostatic potential energy just like mechanical system spontaneously seek states of the lowest mechanical potential energy. In other word, this electrolytic cell will be activated when an energy applied under reduction-oxidation reaction occur.
Three major parts of electrolytic are the substance that produce an electrically conducting solution or called as electrolyte and the other are electrodes such as anode and cathode. So, we could conclude that Electrolytic cell is working in contrary way of galvanic cell or batteries.
In 1833, Michael Faraday found and demonstrated quantitative relationship between the amount of charge that followed and the quantities of material product or consumed in electrochemical reactions. Therefore the relationship he discovered known as Faraday’s Law which state as follows :
- the mass of a given substance that is produce or consume in electrochemical reaction is proportional to the quantity of electric charge passed.
- equivalent masses of different substances are produces or consumes in electrochemical reaction by given quantity of electric charge passed.
These laws has successfully summarize the stoichiometry of electrochemical. Not only stated the law Faraday also measured the charge mass ratio of the hydrogen ion, the value he obtained is about 1000 times smaller than e/me , then conclude that “ the charge on the electron must be very large or the mass of the electron must be very small, compared to the hydrogen ion” .
The charge of one mole electron has been given a special name and symbol as Faraday constant, which has amount as, F = 96,485.34 C mol-1
3. Chemical Kinetic
Chemical study major subject is reaction rates which involves the explanation of how do catalyst increase rates of chemical reaction. Why do small changes in temperature often have such large effect on the cooking rate of food. How does the way molecules combine to form of product and so on of these kind of question make this study becoming so complex.
The primary goal of chemical kinetics is to reduce the mechanism of reaction from experimental studies of its rates. The reaction mechanism itself is the slow or fast a reaction could produce its final products.
Rates of Chemical Reactions
The speed of reaction depend on many factors. Concentration of the reacting compound is the one who have significant effect. The other factor that effect is temperature which mean that careful control of temperature is critical for quantitative measurement in chemical kinetic. Finally, the rate often depend crucially on the physical form if reactants.
A kinetics experiment involves measuring the rate of change of substance who participating in chemical reaction. A series of measurement which use different method for different condition could reveal the rate of change of the concentration. For low speed, we can use low temperature condition and in very fast reaction often used a flash of light whose rate is then tracked by measuring the absorption at particular wave length.
In previous explanation, we conclude that once product from reaction is formed it can react back to give original reaction, the net rate is the difference which written in specific equation such as:
Net rate = forward rate – reverse rate
This means measurement of concentration give the net rate rather than simply the forward rate. In addition, many reactions go to completion mean they have a measurable rate only in forward direction or else the experiment can be arranged so that the product are removed as they are formed. The reaction between the rate of a reaction and concentration is called rate law.
Many reaction do not occur in single step, but rather than it sequence in steps to arrive at the product. Each of this step called as elementary reaction and occurs through the collision of atom, ions, or molecules. A unimolecular elementary reaction involves only a single reactant molecule. The example for this reaction is the dissociation of energized N2O3 molecules in gas phase. The most common type of elementary reaction involves the collision of atom, ions, or molecule which called biomolecular the example of this reaction is collision of NO + O3 in gas phase which is the reverse reaction of N2O3 .
The term termoecular is used for a reaction step that involves the simultaneous collision of three molecules, which is a much less most likely reaction for instance the recombination if iodine atoms in the gas phase to form iodine molecules. A reaction mechanism is a detailed sequence of elementary reactions, the details provide the rate of the molecule combined to yield the overall reaction. A reaction intermediate is a chemical species that is formed and consumed in the reaction but does not appear in the overall balanced chemical equations.
There is a direct relation between rates for elementary steps in a chemical reaction mechanism and the overall equilibrium constant K. This principle called detailed balance which states that at equilibrium the rate each elementary process is balanced by the rate of its reverse process. The prove of this principal is that rate constants of three steps elementary reaction are k1,k,k3 while the reverse of the three steps elementary reaction are k-1,k-2,k-3.
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4. Spectroscopy & Photochemistry
The key concept or the backbone of spectroscopy is quantization of energy. Energy quantization is a revolutionary concept that took most the early twentieth century to discover. Spectroscopy itself covers study about measures various form of light energy that are absorbed o emitted only at specific wave light.
Spectroscopy involves measurement of electromagnetic radiation as emitted from atoms and molecules between definite energy levels and can be characterized by wave length or frequency. Light energy is proportional to frequency with, Ɛ = hv and angular momentum is quantized as well (mvr = nh)
Photochemistry is a study of chemical reaction produced by light, because the absorption of a photon from light a molecule may raise to an excited electronic state where it will be more likely to react than in the ground electronic state. In photochemical reactions, the activation energy is supplied by absorption of light. This concept is the contrary of thermal reaction that activation energy is supplied by intermolecular collisions.
The energy of photon can be determined by equation of Ephoton = hv = hclλ. So, the energy of one mole of photon is NAhv. According to the Stark-Einstein law of photochemistry, the number of photon absorbed equals the number of molecules making transition to an excited electronic state. The term fluorescence does mean there is no change in spontaneous emission of radiation by an electronic transition in which the total electronic spin. In other hand phosphorescence mean emission that follow electronic excitation by absorption of light. And the term luminescence refers to any emission of light by electronically excited species, and includes fluorescence and phosphorescence.
The example of photochemistry that happen in plant generally in leaves specially is photosythesis which is the process where plant uses ultaviolet energy to convert dioxide and water into glucose and oxygen. As mention above, the concept of photochemistry is about light, the photosythesis it self use light as it input and water to produce glucose and oxygen each in solid or fluid form and gas form.
5. Quantum Mechanic
Quantum mechanic provides a firm conceptual foundation for understanding several phenomena that chemist seek to understand. Such as, the atomic interaction that form molecule and extended solid structure, intermolecular interactions, and the chemical reactivity.
With a lot of development in quantum chemistry, today quantum chemistry provides very good approximate solutions to Schrodinger equation for molecules. It also provides a qualitative concept for representing and describing chemical bonds, molecular structure and chemical reactivity.
The Born- Oppenheimer approximation is the starting point for quantum mechanic. The points of Born- Oppenheimer approximation are nuclei are so much heavier than electrons that may be considered fixed space while the electron move rapidly around nuclei. The fact that electrons move so much faster than nuclei allows us to treat electronic motion independently of nuclear motion by solving an electronic Schrodinger equation for each value in inter nuclear separation.
6. Surface Chemistry
In this branches of physical chemistry, according to its title it means this study is about chemical reaction happen on the surfaces and interfaces. This study focusing on how molecule or atom interact with each other on the surface. The key point of studying molecule interaction is to understand the desirable chemical reactions for instance heterogeneous catalysis.
The purpose of studying the chemical reaction is in case to modify the chemical reaction so it can produce a various desired effect and or improvement locate on surface and interfaces. There are several fields who are fond of this study such as catalysis, electrochemistry, and geochemistry.
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- Branches of Biochemistry
- Branches of Analytical Chemistry
- Branches of Chemistry
- Branches of Organic Chemistry
Meanwhile, there are sub branches of physical chemistry that used in daily life to compound many basic of industrial, laboratory, and many studies.