- Parts of a chemical equation
- Location of reagents and products
- Balancing chemical equations
- Physical states of the components of the chemical equation
- Changes in physical state
- Example of chemical equations
- - Photosynthesis
- - Cellular respiration
- - Common element reactions
- Decomposition reaction
- Displacement reaction
- Elimination reaction
- Hydration reaction
- Neutralization reaction
- Synthesis reaction
- Double displacement reaction (metathesis)
- References
The chemical equation is a schematic representation of some of the characteristics of a chemical reaction. It can also be said that the chemical equation describes the changes experienced by the different substances involved in a reaction.
In the chemical equation, the formulas and symbols of the different participating substances are placed, clearly indicating the number of atoms of each element present in the compounds, which appears as a subscript and cannot be changed by balancing the equation.
General chemical equation for a chemical reaction. Reagents and products. Source: Gabriel Bolívar.
The chemical equation must appear balanced, that is, the number of atoms of both reactants and products must be equal. In this way the law of conservation of matter is obeyed. It is desirable that the numbers used in balancing the equations are whole numbers.
These equations do not reveal the consecutive steps, nor by what mechanisms the reactants are transformed into products.
That is why, although they are very useful to understand where a chemical reaction is going, it does not allow us to understand its molecular aspects or how it is affected by certain variables; such as pH, viscosity, reaction time, stirring speed, among others.
Parts of a chemical equation
There are basically three main parts to a chemical equation: the reactants, the products, and the arrow that indicates the direction of the chemical reaction.
Location of reagents and products
All substances that function as reactants and all substances that are products appear in the chemical equation. These groups of substances are separated by an arrow that indicates the direction of the reaction. The reagents are located to the left of the arrow and the products to the right.
The arrow means what is produced and is oriented from left to right (→), although in reversible reactions there are two equivalent and parallel arrows; one directed to the right, and one to the left. The symbol (Δ) is usually placed above the arrow, indicating that heat was used in the reaction.
In addition, the identification of the catalyst is usually placed on the arrow, if possible with its formula or symbol. The different substances that appear as reactants are separated by the sign (+), indicating that the substances react or combine with each other.
In the case of substances that appear as products, the sign (+) does not have the previous connotation; unless the reaction is reversible. It is convenient that the sign (+) is placed equidistant from the substances it separates.
Balancing chemical equations
It is an essential requirement that the chemical equations are properly balanced. For this, a number called the stoichiometric coefficient is placed. Whenever necessary, this coefficient must be before the substances that appear as reactants or products.
This is so to achieve that the number of all the atoms of the elements that appear as reactants is exactly equal to the number of them that appear in the product. The simplest method of balancing chemical equations is trial and error.
Physical states of the components of the chemical equation
In some chemical equations the physical state of substances is marked with a subscript. For this, the following abbreviations are used in Spanish: (s) for the solid state; (l) for the liquid state; (g), gaseous state; and (ac), aqueous solution.
Example: reaction of calcium carbonate with hydrochloric acid.
CaCO 3 (s) + 2 HCl (aq) → CaCl 2 (s) + H 2 O (l) + CO 2 (g)
Changes in physical state
In some cases, it is indicated in the chemical equation if there is production of a gas in the chemical reaction or if there is the precipitation of any of the substances produced.
The presence of a gas is indicated by a vertical arrow with its end pointing upwards (↑), placed on the right side of the gaseous substance.
Example: reaction of zinc with hydrochloric acid.
Zn + 2 HCl → ZnCl 2 + H 2 ↑
If in the chemical reaction one of the substances forms a precipitate, this is symbolized by placing a vertical arrow with its end directed downwards (↓), placed on the right side of the precipitated substance.
Example: reaction of hydrochloric acid with silver nitrate.
HCl + AgNO 3 → HNO 3 + AgCl ↓
Example of chemical equations
- Photosynthesis
Photosynthesis equation
Photosynthesis is a process by which plants capture and transform light energy, coming from sunlight, to generate the energy necessary for their subsistence. Photosynthesis is carried out by some organelles of plant cells called chloroplasts.
Thylakoids are found in the chloroplast membrane, sites where chlorophylls a and b are found, which are the main pigments that capture light energy.
Although photosynthesis is a complex process, it can be outlined in the following chemical equation:
6 CO 2 + 6 H 2 O → C 6 H 12 O 6 + 6 O 2 ↑ ΔGº = 2,870 kJ / mol
C 6 H 12 O 6 is the formula for glucose, a carbohydrate metabolized for the production of ATP; compound that is the main reservoir of energy in most living things. Furthermore, NADPH is generated from glucose, a coenzyme necessary for many reactions.
- Cellular respiration
The cells use oxygen for the metabolism of numerous substances present in the food eaten. Meanwhile, ATP is used as an energy source for the activities carried out by living beings, producing carbon dioxide and water in these processes.
Using glucose as a model for a metabolized substance, respiration can be schematized using the following chemical equation:
C 6 H 12 O 6 + 6 O 2 → 6 CO 2 + 6 H 2 O
- Common element reactions
Decomposition reaction
A compound or compounds dissociate, forming other different compounds with their atoms:
2 KClO 3 (s) → 2 KCl (s) + 3 O 2 (g)
Displacement reaction
A metal reacts with a compound, replacing a metal present in it:
Mg (s) + CuSO 4 (aq) → Cu (s) + MgSO 4 (aq)
Elimination reaction
In this type of reaction, the number of atoms or groups attached to a carbon atom decreases:
CH 3 -CH 2 Br + NaOH → H 2 C = CH 2 + H 2 O + NaBr
Hydration reaction
It is a reaction in which a compound adds a molecule of water. This reaction is important in the preparation of alcohols:
H 2 C = CH 2 + H 2 O → H 2 C-CH 2 OH
Neutralization reaction
A base or alkali reacts with an acid producing a salt and water:
HCl (aq) + NaOH (aq) → NaCl (aq) + H 2 O (l)
Synthesis reaction
In this type of reaction, two or more substances are combined to create a new compound:
2 Li (s) + Cl 2 (g) → 2 LiCl (s)
Double displacement reaction (metathesis)
In this type of reaction there is exchange of positive and negative ions to form new compounds:
AgNO 3 (aq) + NaCl (aq) → AgCl (s) + NaNO 3 (aq)
References
- Flores, J. (2002). Chemistry Edition 1 era. Santillana Editorial
- Mathews, CK, Van Holde, KE, and Ahern, KG (2002). Biochemistry. 3 was Edition. Publisher Pearson Addison Wesley
- Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
- Wikipedia. (2019). Chemical equation. Recovered from: en.wikipedia.org
- Helmenstine, Anne Marie, Ph.D. (September 20, 2019). What Is a Chemical Equation? Recovered from: thoughtco.com