Chemical equation

Chemical equation . It is a simple and schematic way of expressing, through symbols and formulas, the changes that have occurred in the course of a reaction.


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  • 1 Parts of a chemical equation
    • 1 Reactive or reactive substances
    • 2 Substances products
  • 2 Adjustment or balancing
  • 3 Methods for Adjusting (Balancing) Equations
    • 1 Trial Adjustment
  • 4 Interpretation of a chemical equation
    • 1 Qualitative
    • 2 Quantitative
  • 5 External links
  • 6 Sources

Parts of a chemical equation

Reacting or reactive substances

They are the substances that react. They are placed before the arrow or the equals sign.

Substances products

They are the substances that are formed. They are placed after the arrow or the equal sign to (=). Scientists often represent chemical reactions using symbols and formulas of the substances involved and produced.

  • The symbol “+”:among the reactants it is read as “reacts with”, among the product substances it is read as “and”.
  • The arrow or the equals sign:separates the reactants from the product substances. It reads “produces”.
  • A triangle above the arrow or the sign equal to:means “heat”. The reagents must be heated for the reaction to proceed.
  • Sometimes the equation shows the state of aggregation of the substances involved, indicating a lowercase letter in parentheses, after each substance: (s):solid (l): liquid (g): gas, for the substance in solution it uses the symbol (ac) which means aqueous.
  • Coefficients:They are the numbers placed before each substance. They indicate the number of moles of each reactant reacting and the number of moles of each product that form. This is always done due to the Law of Conservation of the mass .

It is customary to omit the number 1. This operation is called adjusting the chemical equation. You should never modify the subscripts of the formulas. Whenever an equation is adjusted, an equal sign is placed, if an arrow is put, it does not show compliance with the Law of Conservation of Mass .

Adjustment or balancing

To adjust or balance a chemical equation is to equalize the number and class of particles ( atoms , molecules or inoes ) reacting with the products, in order to comply with the Law of Conservation of mass.

To adjust or balance a chemical equation, the following should be considered:

  • Know the reactants and products.
  • Check that the equation is complete and correctly written.
  • Check if the equation is already adjusted.
  • The subscripts indicate the amount of the indicated atom in the molecule.
  • The coefficients affect all the substance they precede.
  • Adjustment begins with the metallic element, then the nonmetal (except Oxygen and Hydrogen.
  • The hydrogen and oxygen are equilibrated at the end and in that order, because usually form water (filler). This does not alter the equation, because every reaction is carried out in aqueous solution or produces substances that contain water of crystallization.
  • Use whole numbers and as few as possible to balance.
  • Write the number as a coefficient of the relevant formula; when the coefficient is (1) it is not written, it is understood.
  • You should not change the subscripts of the formulas, nor divide the formula to place the coefficient in the middle of it.
  • Count the number of atoms by multiplying the coefficient by the respective subscripts of the formulas and adding all the atoms of an element that are on the same side of the equation.
  • Check the final balance and readjust, if necessary.

Methods for Adjusting (Balancing) Equations

There are different methods that are used as appropriate, according to the type of reaction, which can occur:

  • Without change of oxidation states in any reacting element:
    • Trial and Error or Trial.
    • Least Common Multiple.
    • Undetermined or Algebraic Coefficients.
  • Some elements change their valence:
    • Ion Electron or Semirreacción: In acid and basic medium.

Trial Fit

As the name implies, it is a way to fit a chemical equation by fumbling. It is used to balance simple equations. The “calculation” is performed trying to equalize both members. For this we will use the following example:

Fit the equation: Al (OH) 3 (ac) + H 2 SO 4 (ac) = Al 2 (SO4) 3 (ac) + H 2 O

  1. We adjust the aluminum metal
  2. We continue with the non-metal sulfur
  3. We continue with hydrogen and oxygen

Fitted equation: 2Al (OH) 3 (ac) + 3H 2 SO 4 (ac) = Al 2 (SO4) 3 (ac) + 6H 2 O

Interpretation of a chemical equation

There are two points of view or criteria when interpreting a chemical equation. These interpretations are called Qualitative and Quantitative .


From a qualitative point of view, they express the nature of the substances that react and are produced.


From a quantitative point of view, they express the relationship between the number of particles ( atoms , molecules , ions ) involved in a chemical reaction.


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