Understanding Chemical Reactions: Equilibrium, Acids, Bases, and Redox
Chemical Balance
At the beginning of a reversible reaction, as the concentrations of the reactants are large, the reaction rate will also be great in the sense of the first member to the second. But as you proceed through the process and product concentrations begin to be significant, the reaction rate will increase in the opposite direction, i.e., in the sense of the second member to the first. There will come a time when both speeds are equal. At that moment, as many molecules are recomposed as are destroyed and, therefore, the concentrations of the substances involved in the reaction will not change, remaining constant from that moment on. When this happens, it is said that chemical equilibrium exists or has been reached.
Equilibrium Constant
The law of mass action states: “In any reversible reaction, upon reaching equilibrium, the ratio of the product of the concentrations of the bodies of the second member and the concentrations of the first member bodies, all raised to an exponent equal to the ratio contained in the reaction equation, is a constant for each temperature.”
Concept of Bronsted-Lowry Acid
An acid is any substance that can donate protons (hydrogen ions, H+, are protons). A base is any substance capable of accepting protons. Therefore, for a substance to act as an acid, it must be in the presence of another substance acting on it and accepting protons donated by the acid. In aqueous solutions, water plays this role; it acts as a base against acid, and as an acid compared to bases.
Relative Strength of Acids and Bases in Aqueous Medium
Strong acid: is one that is highly dissociated in solution; there will therefore be a high hydronium ion concentration and a small concentration in molecular form. Weak acid: is a solution that is poorly differentiated. Its molecular concentration is much higher than the ionized form. The degree of dissociation is the mole fraction that is dissociated acid or base, i.e., the mole fraction that is dissociated per initial mol of acid or base.
Hess’s Law
The amount of heat evolved in a chemical reaction depends only on the initial and final states of the system and is independent of the intermediate states, since this amount of heat (at constant pressure) equals the enthalpy change, and this is a state function. That is, if certain initial substances lead to the same end products under the same conditions, the enthalpy has the same value irrespective of the path followed and intermediate products that appear. This law is useful for calculating the heats of reactions that cannot be measured directly.
pH Concept
To avoid the difficulties offered by working with negative powers of 10, Sorensen proposed to express the hydrogen ion concentration through a notation named pH. It also follows that if the solution is acidic, the pH is below 7, and if it is basic, the pH is above 7. For a neutral solution, the pOH is equal to 7; for an acid solution, it is greater than 7; and for a basic solution, the pOH is less than 7. The sum of pH and pOH must be equal to 14.
Electronic Concept of Oxidation-Reduction
Currently, oxidation-reduction relates to the loss and gain of electrons. Oxidation is the loss of electrons. Reduction is the gain of electrons. The processes of oxidation and reduction cannot occur apart from each other, since if a substance loses electrons (is oxidized), there must be one capable of capturing them (reduced). Therefore, any process of oxidation is necessarily linked to another of reduction. It is therefore necessary to speak of oxidation-reduction (redox).
Solubility Concept
We call the solubility of a solute in a given solvent, and, of course, at a certain temperature, the concentration of the solute in a saturated solution. Temperature is a factor that can alter the equilibrium point. In general, solubility increases with temperature.
Factors Affecting Solubility
- Nature of the substance: For a given solvent, there are highly soluble substances and others that are sparingly soluble or insoluble, and between these two limiting cases, a range.
- Nature of the solvent: Some substances are very soluble for a given solvent, while others are very poorly soluble or insoluble.
- Temperature: Generally, the solubility of solids in liquids increases with temperature.
- Presence of a common ion: The solubility of a salt in a solution containing one of its ions is less than in a solution that contains none of the salt ions.
Concept of Elements and Compounds
Matter is all that occupies a place in space and has mass. A material system is a limited portion of matter. Material systems are classified as heterogeneous systems and homogeneous systems. A heterogeneous system is one in which distinct areas with different properties are seen upon direct observation. A homogeneous system is one in which areas with different properties are not appreciated upon direct observation. Those homogeneous systems that cannot be separated into others by purely physical means are called pure substances. Homogeneous systems that can be separated by physical processes into two or more pure substances with different properties are called solutions. Pure substances that cannot be separated into other substances by chemical processes are called elements. Pure substances that can be separated into two or more different elements by chemical processes are called compounds. An atom is defined as the smallest amount of an element that retains the chemical properties of that element. A molecule is the smallest quantity of a pure substance that still retains the chemical properties of that substance.