A reaction in which one or more molecules change into one or more different molecules. The same atoms are present in the products as were in the reactants, but they will be combined differently into molecules and mayhave exchanged electrons. Chemical reactions happen all around us and within us all the time, as part of the normal way things operate.

Distinguished from physical change in which the molecules present don't change, and nuclear reaction in which the exchange of protons causes the atoms present to change.

Chemists study known chemical reactions and devise new ones. Chemical engineers determine how to apply chemical reactions in real life to make desired chemicals.

Yep, another one from the nrt...

Chemical Reactions: Types and Examples

Chemical reactions are described by chemical equations.A chemical equation represents, with symbols and formulas, the reactants and products in a chemical reaction. In this node I will discuss the logistics of chemical equations, the types of symbols in chemical equations, mole ratio, and real life examples of chemical reactions on our planet.

Symbols and terminology in chemical equations

catalyst : Factor that speeds up the chemical reaction but is not permanently consumed in the reaction. In organic reactions, an enzyme is often the catalyst, a protein molecule that is specifically tailored for a specific biological reaction.

coefficient : A small whole number that appears in front of the formula in a chemical equation.

reactant : the substance that undergoes a chemical reaction

product : the substance produced by a chemical reaction

word equation Represents the reactants and products of a chemical reaction by their symbols or formulas.

balanced equation An equation that shows the same number of atoms in each element on both the reactant and product side

mole : The amount of a substance that contains the same number of particles as there are in atoms in exactly 12 g of carbon.

Avogadro’s number : 6.022137 x 10 raised to the 23rd power. The number of particles in 1.0 mol of a pure substance.

mole ratio :

a conversion factor that relates the number of moles of any two substances involved in a single molecule

- The yields symbol, indicates the result of a reaction. There can be arrows going in both directions, indicating a reversible reaction, as well as arrows with symbols that denote the atmospheric pressure of the reaction, the temperature at which the reaction is carried out, or the formula of the catalyst used to alter the rate of the reaction. An arrow pointing upward is an alternative to the (g) symbol and indicates a reactant or product in the gaseous state.

(l) A reactant or product in the liquid state.

(aq) A reactant or product in an aqueous solution (most read this as meaning dissolved in water) .

(g) A reactant or product in the gaseous state

Laws of writing chemical equations

There are several tenets involved when dealing with chemical equations that the chemist and layman must always remember:

1. The equation must represent the known facts. In other words, identify the elements and the exact reaction before writing it out. This information can be obtained by chemical analysis or from other, reliable sources.

2. The equation must contain the correct formulas of the reactants and products. To read or write a chemical equation you must know about the elements and formulas involved: ionic and covalent compounds, diatomic molecules, etc. Each of these elements in their correct form must be represented by their atomic symbol.

The law of conservation of mass must be satisfied. Matter cannot be created nor destroyed. To observe this law, you must balance the equation, making sure that the same number of atoms are represented on both sides, regardless of the chemical change involved. To do this, you can write coefficients in front of the chemical formulas, changing the ratios of atoms to balance the equation. Here’s an example of an unbalanced equation and how to balance it:

CH4(g) + O2(g) -- CO2(g) + H2O (g)

Counting the hydrogen atoms shows that there are four in the reactants but only two in the products. The numbers after the certain elements (such as the second oxygen, for example) are subscripts, not to be tampered with when balancing a chemical equation. A subscript only pertains to that one element and changing it would change the nature of the compound entirely. Remember this: ONLY coefficients can be changed!

Types of reactions

In a chemical reaction, two factors are involved: the reactant and the product . As implied by the name, the reactant (always accompanied by an outside catalyst, usually an enzyme in biological reactions) undergoes a change to yield the product. Because most reactions follow an intrinsic pattern, chemists have identified reactions as belonging to the four following types:

Synthesis Reaction . Two or more substances combine to form a new compound. A common reaction in nature is the combination of an element with oxygen to produce an oxide of the element as represented by this magnesium and oxygen reaction:

2Mg(s) + O2(g) -- 2MgO(s)

Decomposition Reaction . A single compound undergoes a reaction that produces two or more simpler substances. The opposite of synthesis reactions, most take place when energy in the form of heat or electricity is added. Here’s a theoretical example:

AX ---- A + X

Single Replacement Reaction Self-explanatory. One element replaces another element in a compound. Example:

A + BX ---: AX + B

Y + BX --- BY + X

Double Replacement Reaction Again implied by the name, except these reactions almost always involve ionic compounds. Electrolysis , the decomposition of a substance by an electric current, is a type of decomposition reaction. The ions of two compounds exchange places in an aqueous (water containing) solution to form two new compounds. One of the compounds formed is usually a precipitate, (a very slightly soluble compound). Represented algebraically as

AX + BY - AY + BX

And here’s an electrolysis reaction :

2H2O(l) + electric current ---- 2H2(g) + O2(g)

I did lie: there is a fifth type of reaction. In a combustion reaction , a substance combines with oxygen releasing a large amount of energy in the form of light and heat. Hydrocarbons , compounds made up of carbon and hydrogen, are associated with this type of reaction. However, I did not classify it among the four biggies because controversy exists among most chemists as to whether combustion reactions merit their own category. Example:

C3H8 (g) + 5O2 (g) ---- 3CO2(g) + 4H2O (g)

Mole Ratio, and its significance in chemical equations

Mole ratio, or a conversion factor that relates the number of moles of any two substances involved in a chemical reaction, is a pretty big deal in the field of stochiometry (quantitative analysis of chemical reactions) and to those wanting to figure out things like percent yieldand mass-mass relationships. Anyone can obtain the mole ratio simply by looking at a balanced equation:

2Al2O3 (l)------ 4Al(s) + 3O2(g)

By looking at this equation, one can say that 2 moles of aluminum oxide decompose to produce 4 mol of aluminum metal and 3 mol of oxygen gas. These relationships can be expressed in the following mole ratios:

2 mol Al2O3/4 mol AL or vice versa, 2 mol Al2O3/ 3 mol O2 or vice versa, 4 mol AL/3 mol O2 or vice versa.

When solving quantitative problems involving chemical formulas, knowing about mole ratios comes in quite handy.

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