There are various forms of energy in nature, such as light energy, heat energy, electric energy, mechanical energy and chemical energy. In the living system, only chemical energy can be directly used as energy for doing work, and other forms of energy can stimulate organisms to do work. For example, they can stimulate animals' sense of balance, vision, warmth, pain and taste respectively. The chemical energy provided for organisms to do work can come from the energy generated by the breaking of chemical bonds of biomolecules due to chemical reactions such as hydrolysis, and can also come from the energy obtained by the change of ion concentration gradient.
For living things, energy stored in chemical bonds is an important free energy. Free energy is energy that can be used to do work. A considerable amount of free energy in food is emitted in the form of heat and can no longer be used to do work. In any case, all forms of energy will eventually be converted into heat energy, so energy is usually measured in units of heat, such as kilojoules (kJ) and kilocalories (kcal). The size of chemical bond energy in biomolecules is related to many factors, the most important of which is the electronegativity difference between atoms connected by bonds (see table). The bond with small bond energy is easy to be destroyed, that is, the bond itself is weak and unstable. In each biochemical reaction, Δ G0' indicates the specific change of standard free energy, "+"indicates that energy is not lost but stored in the product, and "-"indicates that energy is released from the reaction system.
Bond energy is a physical quantity representing the strength of chemical bonds, which can be measured by the energy required for bond breaking. At 10 1.3kPa and 298K, the energy absorbed by 1mol gaseous molecule AB breaking into ideal gaseous atoms is called the dissociation energy of AB (KJ mol- 1), which is usually represented by symbol D(A-B). Namely: AB (G) → A (G)+B (G)
In polyatomic molecules, the energy required to break a bond in a gaseous molecule is called the dissociation energy of this bond in the molecule. For example:
NH3(g)= NH2(g)+H(g)d 1 = 435 kj mol- 1
NH2 (g) = NH (g)+H (g) D2 = 397 kJ mol-1
NH(g)= N(g)+H(g)D3 = 339 kj mol- 1
Although there are three equivalent N-H bonds in NH3 molecule, the energy required to decompose them in turn is different. Bond energy usually refers to the average energy required for each bond when 10 1.3KPa and 298K are used to decompose a gaseous molecule of 1mol into gaseous atoms. Bond energy can be expressed by E. Obviously, for diatomic molecules, bond energy is equal to dissociation energy. For example, at 298. 15K, the bond energy of H is e (h-h) = d (h-h) = 436kJ mol-1; For polyatomic molecules, bond energy and dissociation energy are different. For example, the bond energy of N-H bond in NH molecule should be the average of the dissociation energies of three N-H bonds:
e(N-H)=(d 1+D2+D3)/3 = 1 17 1/3 = 39 1kJ mol- 1
Bond energy is usually obtained by thermochemical methods or spectrochemical experiments, and we often use bond energy to express the strength of a bond.
(Note: Whether a bond can be used to represent the energy of a substance can only represent the difference between the substance and the free energy when it reaches the active state. )
The enthalpy change formula is derived from bond energy: Δ δH = total bond energy of reactants-total bond energy of products.
The relationship between bond energy and matter itself: the greater the bond energy, the lower the self energy, the smaller the bond energy and the higher the self energy. As a reactant, substances need to absorb heat during the reaction. The above reasons are: low energy, stable structure, need to absorb more heat, and large bond energy. High energy, unstable structure, low heat absorption and low bond energy.
breakdown
[Dissociation] The separation of antigen-antibody complex can be divided into free antigen and antibody.
It is often used in biological experiments.
Dissociation refers to the process in which compounds or molecules release ions in a solvent. Dissociation degree can be represented by dissociation degree K.
Generally speaking, dissociation is an endothermic reaction.
Split in life:
In pharmacokinetics, the limiting factors of simple diffusion are fat solubility, molecular size and charge of substances. Generally speaking, gas molecules (such as O2, CO2 and N2), uncharged polar small molecules (such as urea and ethanol) and fat-soluble molecules easily pass through the plasma membrane, while uncharged polar macromolecules (such as glucose) and various charged polar molecules are difficult to pass through the plasma membrane. Most drugs are weakly acidic or alkaline drugs, which will be free in the body and affect absorption. Weakly acidic drugs dissociate less, more indissolubly, and easily pass through biofilm in acidic environment, while weakly alkaline drugs are the opposite.