Equilibrium is when both reactants and products are present in concentrations which have no further tendency to change with time. Usually, this state results when the forward reaction proceeds at the same rate as the reverse reaction. Depending on conditions, a reaction can proceed in the forward or in the reverse direction. In a closed system, reversible reactions undergo dynamic equilibrium. The reaction continues but the concentrations of the reactants and products does not change. If there is a change in the conditions of the equilibrium, the position of equilibrium can change, favoring the products or the reactants. Another way of saying this is that in changing the conditions of the system, it is possible to change the rate of the forward or reverse reaction. Le Chatelier’s Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counter the changes. Another way of saying this is that the system is always seeking equilibrium. In the simplest case, assume the reactants are A and the products are B. By decreasing the concentration of A, the reverse reaction would occur, producing more of the starting materials. Likewise, if the concentration of B increased, the reverse reaction would also occur, producing more of the starting materials. Increasing the temperature of a system in dynamic equilibrium causes the system to favor an endothermic reaction, absorbing the additional heat. Alternatively, decreasing the temperature of a reaction in dynamic equilibrium favors an exothermic reaction, the system produces more heat.

The rate of a reaction is the change in concentration of the reactants or the change in concentration of the products per unit time. The definition of chemical equilibrium when the forward reaction rate and the reverse reaction rate are equal. The result of this equilibrium is that the concentrations of the reactants and the products do not change. The rate constant, k, measures how fast a chemical reaction reaches equilibrium assuming the reactants were supplied with enough activation energy to enable the reaction to proceed in the forward direction from reactants to products. The need for energy is due to the fact that the reactants are unreactive under certain conditions. In this case, the reaction must have some sort of energy input before it can proceed; otherwise, the reactants cannot cross the activation energy threshold and convert to products. The reaction is activated by energy supplied to the reactants by different energy sources. The rate of reaction, the rate constant, and the kinetic energy required for activation of reaction indicate how fast the reaction reaches equilibrium.

For all chemical reactions there is a kinetic and thermodynamic component. The quantity related to kinetics is the rate constant k. The rate constant is associated with the activation energy required for the reaction to proceed according to the reactivity of the reactants. The thermodynamic quantity is the energy difference resulting from the free energy (ΔG) given off during a chemical reaction: how stable the products are relative to the reactants. Although kinetics describes the rate of reaction and how fast equilibrium is reached, it provides no information about conditions once the reaction equilibrates. In the same measure, thermodynamics only gives information regarding the equilibrium conditions of products after the reaction takes place, but does not explain the rate of reaction. Chemical reactivity and stability are two related but distinct concepts 1) kinetics describes whether reactants need to be energized to produce products. It does not consider the overall energy on the activation energy what is required. Conversely, thermodynamics is a state function concerned with the energy of both reactant(s) and product(s). Whether a reaction is spontaneous depends on both the initial and final state. Those reactions with a higher free energy or higher potential chemical energy require energy to drive the reaction as they are not capable of liberating energy and thus being spontaneous.  The most stable states of a kinetic reaction are those of the reactants, in which an input of energy is required to move the reaction from a state of stability, to that of reacting and converting itself to products. Kinetics is related to reactivity. In contrast, the most stable state of a thermodynamically favorable reaction is the products, because the reaction occurs spontaneously, without the need for energy to be added. Thermodynamics is related to stability.

The study of the relationship between heat and work within chemical reactions is known as thermodynamics. Chemical thermodynamics helps to determine whether a reaction proceeds spontaneously as well as determining whether a reaction is reversible or irreversible. Chemical energy is the potential energy with a substance that can be used by making or breaking bonds, which can absorb or evolve heat. As heat and work are both forms of energy, they can be transformed into each other. Work and heat can both be described using the same unit of measure. Sometimes the calorie is the unit of measure, and refers to the amount of heat required to raise one (1) gram of water one (1) degree Celsius. Heat energy is measured in kilocalories, or 1000 calories. Typically, the SI units of Joules (J) and kilojoules (kJ) can be converted into calories. One calorie of heat is equivalent to 4.187 J. The term specific heat is also relevant, the heat required to raise one (1) gram of a material one (1) degree Celsius. Specific heat, given by the symbol “C.”

Chemically, when energy is converted to work, energy in the form of heat moves from one place to another, or energy is stored up in the constituent chemicals. Heat is defined as that energy that is transferred as a result of a temperature difference between a system and its surroundings. Mathematically, we can look at the change in energy of a system as being a function of both heat (q) and work (w). If q is positive, we say that the reaction is endothermic, that is, heat flows into the reaction from the outside surroundings. If q is negative, then the reaction is exothermic, that is, heat is given off to the external surroundings.

Enthalpy is a state property, the word enthalpy comes from the Greek “heat inside”. If a chemical system undergoes some kind of change, but has a fixed volume, the heat output is equal to the change in internal energy (q = ΔE). Define the enthalpy change, ΔH, of a system as being equal to its heat output at constant pressure. So in order to calculate the change in enthalpy, calculate the enthalpy of products minus the enthalpy of reactants.

The process where bonds are broken and/or bonds are formed is a chemical reaction. As the making or breaking of bonds involves the storage or release of energy, a chemical reaction must obey the laws of thermodynamics such as energy cannot be created or destroyed, only transferred. A chemical reaction has the reactants, what atoms or molecules are combined, on the left followed by an arrow to form the products on the right, the atoms and/or molecules that are the outcome of the chemical reaction. A plus sign indicates the combination of two or more reagents or starting materials. As written, the chemical equation must balance: the number of the same type of atoms on the left must equal the number of the same type of atoms on the left.

For example consider the combustion reactions of some simple hydrocarbons:

CH₄                         +             2O₂                           [arrow]                                CO₂                        +             2H₂O

2C₂H₆                     +             7O₂                           [arrow]                               4CO₂                      +             6H₂O

C₃H₈                       +             5O₂                            [arrow]                              3CO₂                      +             4H₂O

2C₄H₁₀                   +             13O₂                         [arrow]                               8CO₂                      +             10H₂O

When dealing with chemical equations with polyatomic ions, where ions are made of more than one atom, there is a special strategy to balance the chemical equation: treat the whole polyatomic ion as if it is only one atom. Let’s take a look at this chemical equation. For example:

Na₃PO₄                 +             MgCl₂                       [arrow]                              NaCl                      +           Mg₃(PO₄)₂

The polyatomic ion in this case is PO₄. Treat this whole polyatomic ion as one atom. On the reactants side there is one PO₄ ion and on the products side, there are two PO₄ ions. The first step is to balance the polyatomic ions on both sides. To do that place a coefficient of 2 in front of Na₃PO₄. The sodium (Na) atoms double and the PO₄ is now balanced on both sides.

Colligative Properties change due to the presence of a solute.  The magnitude of the change is due to the number of particles in the solution and not their chemical identity.  Examples of properties that fall under this category are the vapor pressure, melting and boiling points, and osmotic pressure.  Changes of state depend on vapor pressure, so the presence of a solute affects the freezing point and boiling point of a solvent. In a solution only a fraction of the molecules with energy in excess of the intermolecular force are nonvolatile solute molecules. This reduction in the number of solvent molecules with energies above the intermolecular force lowers vapor pressure of the solution than the pure solvent at all temperatures. The presence of solute particles interferes with the crystallization process and thus the normal melting point is lower; this means more energy needs to be removed to favor crystallization or the formation of a solid. For example, the normal boiling point of a liquid occurs at the temperature where the vapor pressure is equal to 1 atmosphere. A nonvolatile solute elevates the boiling point of the solvent.  The magnitude of the boiling-point elevation depends on the concentration of the particles of solute.  When a solute is dissolved in a solvent, the freezing point of the solution is lower than that of the pure solvent. The equation for freezing-point depression is similar to that for boiling-point elevation.

Osmotic Pressure occurs when a solution and a pure solvent are separated by a semipermeable membrane, which does not allow the solute molecules to pass through only allowing the solvent to cross the membrane. The flow of solvent (in living organisms typically water) is called osmosis. The excess pressure compared to pure solvent is called osmotic pressure. Solutions that have ideal osmotic pressure (at equilibrium) are said to be isotonic. When a solution in contact with pure solvent across a semipermeable membrane is subjected to external pressure greater than the osmotic pressure, then reverse osmosis occurs.

Osmolarity or osmotic concentration is the number of osmoles per Liter of solution (osmol/L or Osm/L). Molarity measure the moles of solute per unit volume of solution, osmolarity measure the number of particles or number of osmoles per unit of volume. When compounds dissociate, the number of osmoles increases. Ionic compounds such as sodium chloride (NaCl) can dissociate in solution into their positively and negatively charged ions. When NaCl dissociates into Na⁺ and Cl⁻ ions, for every 1 mole of NaCl in solution, there are 2 osmoles of solute particles (i.e., a 1 mol/L NaCl solution is a 2 osmol/L NaCl solution). Both sodium and chloride ions affect the osmotic pressure of the solution. Another example is magnesium chloride (MgCl₂), which dissociates into Mg²⁺ and 2Cl^– ions. For every 1 mole of MgCl₂ in the solution, there are 3 osmoles of solute particles. Nonionic compounds do not dissociate, and form only 1 osmole of solute per 1 mole of solute. For example, a 1 mol/L solution of glucose is 1 osmol/L. Multiple compounds in solution will contribute to the osmolarity of a solution. For example, a 4 Osm solution might consist of: 4 moles glucose, or 2.0 moles NaCl, or 2 mole glucose + 1 mole NaCl, or any other type of combination.

Two or more substances in a homogeneous mixture constitute a solution. The substance that is dissolved into another substance is the solute. The substance that dissolves is known as the solvent. Aqueous solutions are the most common solutions in biochemistry. There are several ways to represent the concentration of a solution.

There are several factors affecting solubility. These factors include: intermolecular forces, temperature, pressure. If a solute is going to be soluble in a particular solvent, the interactions between the intermolecular forces holding the solvent molecules together must be overcome to make room for the solute. Likewise, the intermolecular forces holding the solute together must be overcome. Overcoming intermolecular forces requires energy. Solvation energy is the energy released when the solute and the solvent interact. The rule “like dissolves like,” refers to intermolecular forces in a solution (H bonding, dipole-dipole, or London forces. The solubility of a liquid or solid tends to increase with increasing temperature.