CHEMICAL REACTION

A chemical reaction is a natural process that always produces antarubahan chemical compounds. The compound or compounds early in the reaction is referred to as the reactants. Chemical reactions are usually characterized by a chemical change, and will produce one or more products that typically have characteristics that are different from the reactants. Classically, chemical reactions involve changes involving the movement of electrons in the forming and breaking of chemical bonds, although the general concept is basically a chemical reaction can also be applied to the transformation of elementary particles such as the nuclear reaction.Reactions with different chemical used in chemical synthesis to produce the desired compound. In biochemistry, series of chemical reactions catalyzed by enzymes form metabolic pathways, in which the synthesis and decomposition is usually not possible in the cell do. 


The reaction of two or more elements together results in the formation of a chemical bond between atoms and the formation of a chemical compound. But why do chemicals react together? The reason has to do with the participating atoms' electron configurations
In the late 1890s, the Scottish chemist Sir William Ramsay discovered the elements helium, neon, argon, krypton, and xenon. These elements, along with radon, were placed in group VIIIA of the periodic table and nicknamed inert (or noble) gases because of their tendency not to react with other elements . The tendency of the noble gases to not react with other elements has to do with their electron configurations. All of the noble gases have full valence shells; this configuration is a stable configuration and one that other elements try to achieve by reacting together. In other words, the reason atoms react with each other is to reach a state in which their valence shell is filled.
Let's look at the reaction of sodium with chlorine. In their atomic states, sodium has one valence electron and chlorine has seven.

Sodium	Chlorine

Chlorine, with seven valence electrons, needs one additional electron to complete its valence shell with eight electrons. Sodium is a little bit trickier. At first it appears that sodium needs seven additional electrons to complete its valence shell. But this would give sodium a -7 electrical charge and make it highly imbalanced in terms of the number of electrons (negative charges) relative to the number of protons (positive charges). As it turns out, it is much easier for sodium to give up its one valence electron and become a +1 ion. In doing so, the sodium atom empties its third electron shell and now the outermost shell that contains electrons, its second shell, is filled - agreeing with our earlier statement that atoms react because they are trying to fill their valence shell.

Sodium Chloride

This trait, the tendency to lose electrons when entering into chemical reactions, is common to all metals. The number of electrons metal atoms will lose (and the charge they will take on) is equal to the number of electrons in the atom's valence shell. For all of the elements in group A of the periodic table, the number of valence electrons is equal to the group number
Nonmetals, by comparison, tend to gain electrons (or share them) to complete their valence shells. For all of the nonmetals, except hydrogen and helium, their valence shell is complete with eight electrons. Therefore, nonmetals gain electrons corresponding to the formula = 8 - (group #). Chlorine, in group 7, will gain 8 - 7 = 1 electron and form a -1 ion.
Hydrogen and helium only have electrons in their first electron shell.  The capacity of this shell is two.  Thus helium, with two electrons, already has a full valence shell and falls into the group of elements that tend not to react with others, the noble gases.  Hydrogen, with one valence electron, will gain one electron when forming a negative ion.  However, hydrogen and the elements on the periodic table labeled metalloids, can actually form either positive or negative ions corresponding to the number of valence electrons they have.  Thus hydrogen will form a +1 ion when it loses its one electron and a -1 ion when it gains one electron.

Reaction energy

All chemical reactions are accompanied by a change in energy. Some reactions release energy to their surroundings (usually in the form of heat) and are called exothermic. For example, sodium and chlorine react so violently that flames can be seen as the exothermic reaction gives off heat. On the other hand, some reactions need to absorb heat from their surroundings to proceed. These reactions are called endothermic. A good example of an endothermic reaction is that which takes place inside of an instant '"cold pack." Commercial cold packs usually consist of two compounds - urea and ammonium chloride in separate containers within a plastic bag. When the bag is bent and the inside containers are broken, the two compounds mix together and begin to react. Because the reaction is endothermic, it absorbs heat from the surrounding environment and the bag gets cold.
Reactions that proceed immediately when two substances are mixed together (such as the reaction of sodium with chlorine or urea with ammonium chloride) are called spontaneous reactions. Not all reactions proceed spontaneously. For example, think of a match. When you strike a match you are causing a reaction between the chemicals in the match head and oxygen in the air. The match won't light spontaneously, though. You first need to input energy, which is called the activation energy of the reaction. In the case of the match, you supply activation energy in the form of heat by striking the match on the matchbook; after the activation energy is absorbed and the reaction begins, the reaction continues until you either extinguish the flame or you run out of material to react.

Reactions in Solution

Most of the chemical reactions take place in solution. There are three characteristics that the reaction takes place in solution, which precipitates, gas, and electric charge neutralization. All three reactions are generally classified as metathesis reactions involving ions in solution. Therefore, you need to know more about the ions in solution.

1. Equation Ion and Molecule 

During this time, you write the chemical reactions in solution in molecular form. For example, the reaction between sodium carbonate and calcium hydroxide. The equation:Na2CO3 (aq) + Ca (OH) 2 (aq) → 2NaOH (aq) + CaCO3 (s)This equation is called a molecular equation for reacting substances written in the form of the molecule. Molecular equation gives no hint that the reaction involving ions in solution, whereas Ca (OH) 2 and Na2CO3 in water in the form of ions. Ions involved in the reaction is Ca2 + ions and OH-ions from Ca (OH) 2, and Na + ions and CO32-ions derived from Na2CO3. Equation in the form of ions is written as follows.2Na + (aq) + CO32-(aq) + Ca2 + (aq) + 2OH-(aq) → 2Na + (aq) + 2OH-(aq) + CaCO3 (s)This equation is called the equation of ions, ie an equation involving ions in solution. Instructions conversion molecular equation into ionic equation is as follows.1. Ionic substances, such as NaCl is generally written as ions. Characteristics of ionic substances in the equation using phase (aq).2. Substances that are insoluble (precipitate) is written as a formula of the compound. Characteristics of the equation is expressed with the phase (s). 
  
n ionic equations, ions that appear on both sides are called spectator ions (ions audience), the ions are not involved in the chemical reaction. Spectator ions ions can be removed from the equation. For example, as follows.2Na + (aq) + CO32-(aq) + Ca2 + (aq) + 2OH-(aq) → 2Na + (aq) + 2OH-(aq) + CaCO3 (s)So the equation can be written as:Ca2 + (aq) + CO32-(aq) → CaCO3 (s)This equation is called the net ionic equation. In this case, OH-ions and Na + ions considered spectator ions.


a. Precipitation ReactionReaction in solution precipitation reactions considered if one of the reaction products are insoluble in water. Examples of substances that are not soluble in water, ie CaCO3 and BaCO3. To determine the solubility of a substance needed empirical knowledge as a result of measurement of various substances. Consider the reaction between calcium chloride and sodium phosphate follows.3CaCl2 + 2Na3PO4 → Ca3 (PO4) 2 + 6NaClNaCl will dissolve in the water, while Ca3 (PO4) 2 is insoluble. The compounds most soluble phosphate in water, except for phosphate compounds of sodium, potassium, and ammonium. Therefore, the equation can be written as follows.3CaCl2 (aq) + 2Na3PO4 (aq) → Ca3 (PO4) 2 (s) + 6NaCl (aq)By eliminating the spectator ions in the equation, the net ion perasamaan of reaction can be obtained.3Ca2 + (aq) + 2PO43-(aq) → Ca3 (PO4) 2 (s)

b. Reaction Formation GasChemical reactions in solution, but can also form deposits that produce gas. For example, the reaction between sodium and hydrochloric acid to form hydrogen gas. The equation:Na (s) + 2HCl (aq) → 2NaCl (aq) + H2 (g)

2. Acid base neutralization reactionWhat happens when acid is reacted with a base? Suppose we mix a solution of 0.1 M HCl with 0.1 M NaOH solution In the water, strong acid decomposes to form H + ions and the ion residual acid. The presence of H + ions in acidic solution is shown by the low pH values ​​(pH =-log [H +] <7). In alkaline solution OH-ions will be formed and the residual alkali ions. The presence of OH-ions in alkaline solution is shown by the high pH values ​​(pH = 14 - pOH> 7). If a solution of acid and base neutralization reaction will be mixed ion H + and OH-. Evidence neutralization reaction is indicated by the value of H close to 7 (pH ≈ 7). PH value ≈ 7 shows no more H + ions than OH-ions and the acid from a base other than H + and OH-yield water ionization. Thus, essentially acid-base reaction is the neutralization reaction H + and OH-. Molecular equation:HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)Ionic equation:H + (aq) + Cl-(aq) + Na + (aq) + OH-(aq) → Na + (aq) + Cl (aq) + H2O (l)Net ionic equation:H + (aq) + OH-(aq) → H2O (l)Acid-base reaction is called salting reaction because the acid-base reaction is always generated salt. In the reaction of HCl and NaOH produced table salt (NaCl). Some examples of acid-base neutralization reactions or reactions following the formation of salts.a. H2SO4 (aq) + Mg (OH) 2 (aq) → MgSO4 (aq) + 2H2O (l)b. HNO3 (aq) + Ca (OH) 2 (aq) → Ca (NO3) 2 (aq) + H2O (l)c. HCl (aq) + NH4OH (aq) → NH4Cl (aq) + H2O (l)
3. Quantitative Calculation of Reaction in SolutionQuantitative calculations of chemical reactions in solutions typically involve the molar concentration and pH. The things you need to know to learn the solution stoichiometry is known and what is being asked, and then solved by the following four steps.1. Write the balanced chemical equations.2. Change the scale is known to unit mole.3. Use the ratio of the coefficients of the chemical equation to determine the equivalent amount in moles unknown.4. Change units of moles into the scale in question.
4. Calculation of the pH of mixtureIf the solution is acid or base is mixed with a solution of acid or base similar or different types of the concentration of acid or base in a solution that will change. Changes in the concentration of this course will change the pH of the resulting solution mixing.