Chemical equation

A chemical equation is a writing symbolic system which models the transformation of molecules and atoms at the time of a Chemical reaction. It can be presented in several forms:

reactive → produced (irreversible chemical reaction; form most current in the books of chemistry)
reactive → produced + energy (irreversible chemical reaction)
reactive ⇌ produced + electrons + energy (reversible reaction of oxydoreduction)

For example, the combustion of methane in oxygen is described by:

CH₄ + 2 O₂ → CO₂ + 2 H₂O,

whereas the reversible reaction of the Procédé of Haber is described by:

N₂ + 3:00 ₂ ⇌ 2NH₃ + 92.4 kJ

Sometimes the chemical equations are called equations of reaction or equations assessment .

Description

Each molecule is represented by its Chemical formula: rough, semi-developed or developed planes (see also Représentation of the molecules). The atoms of a molecule are written grouped, the number of atoms within the molecule is indicated after the chemical symbol, in index, for example: the molecule of Eau H₂O is made up of two atoms of Hydrogène and an atom of Oxygène. For a monoatomic crystal, one notes simply the atom, for example Fe for an iron crystal. If a crystal is composed of several chemical species, one indicates a mesh (or a under-mesh) like a molecule, for example Al₂O₃ for a crystal of Alumine.

The chemical reaction is represented by an arrow going towards the line. The reagents are indicated on the left arrow, the products of reaction on the right. If one needs several molecules of comparable nature for the reaction, one indicates this number (whole) before the molecule; this number is called “stoechiometric coefficient”. For example, the chemical equation of the complete Combustion of the Méthane in the Dioxygène is

CH₄ + 2O₂ → CO₂ + 2:00 ₂O

The assessment of the number of atoms on the left and on the right must be balanced above (: on each side of the arrow, there are an atom C, four atoms H and four atoms O).

The stoechiometric coefficients are thus entireties. However, to simplify the writing in certain cases, one divides all the coefficients by the same entirety, one thus has fractional coefficients, but it is about an artifice of notation.

If the reaction can be done in the two directions (balance), one notes two arrows superimposed, an energy towards the line, the other towards the left: “⇆” or “⇋”. When this character is not available, one uses an equal sign “=” or sometimes a double arrow “↔” (although this notation is not very widespread). For example, the dissociation of the Eau can be noted by

2:00 ₂O ⇆ H₃O⁺ + OH^-

2:00 ₂O = H₃O⁺ + OH^-

2:00 ₂O ↔ H₃O⁺ + OH^-

Signs “+” and “-” while exposing indicate the load carried by the Ion S.

In the case of reactions in solid phase, one uses the Notation of Kröger and Vink.

Balancing

The balancing of a chemical equation makes it possible to the students to better include/understand the concept of conservation of the species during a chemical reaction. In practice, a balanced equation makes it possible to predict the quantity of reagents optimal, and the quantities of generated products of reaction, as well as energy necessary to start the reaction, as well as released energy.

There exist various methods to balance the atoms concerned in a reaction chemical:

by gropings
by successive adjustments
algebraically

The first method is effective when there are few atoms and few molecules concerned, the variable word “little” from one individual to another. The second is generally used by people having an unquestionable facility to handle head the numerical expressions. The third leads invariably to a solution, but is more difficult to implement.

Simple chemical reaction

Objective: To balance by successive adjustments the equation of the creation of water.

1. Outline equation

the reagents are H₂ and O₂, whereas the product is H₂O.

One seeks to balance the équation :

H₂ + O₂ → H₂O

2. Starting assumption

One supposes that each coefficient is 1:

1:00 ₂ + 1 O₂ → 1:00 ₂O

3. Balancing from left to right

On the left of the equation, to count the atoms of each chemical element. Is there the same number of this atom on the right?

There are two H on the left, just like on the right. The coefficients are preserved.

There are two O on the left, but only one on the right. Let us change the coefficient by 2 on the right. We are returned to

1:00 ₂ + 1 O₂ → 1 2:00 ₂O

4. Balancing from right to left

a coefficient on the right having changed, to count the atoms of the other chemical element in the molecule. Is there the same number of this atom on the left?

There is now 4:00 on the right (coefficient 2 multiple as well H₂ as O). It is thus necessary to change the coefficient by 2 on the left. We have

1 2:00 ₂ + 1 O₂ → 1 2:00 ₂O

5. Balanced equation?

Vérifier that the account of each atom on the left is the same one as on the right.

After calculations of the atoms, the equation is balanced.

If it were not balanced, to turn over at stage 3 by preserving the coefficients calculated until now.

Complex chemical reaction

Objective: to balance the chemical equation K₄Fe (CN) ₆ + H₂SO₄ + H₂O → K₂SO₄ + FeSO₄ + (NH₄) ₂SO₄ + CO .

It is possible to apply the method of the successive adjustments to this equation, but the risk of error is high. The algebraic method will be preferred to him.

1. To assign a variable to each coefficient

has K₄Fe (CN) ₆ + B H₂SO₄ + C H₂O → D K₂SO₄ + E FeSO₄ + F (NH₄) ₂SO₄ + G CO

2. To impose balance on each atom

the same number of atoms must appear on each side of the equation:

K: 4a = 2d

Fe: 1a = 1e

C: 6a = G

NR: 6a = 2f

H: 2b+2c = 8f

S: B = d+e+f

O: 4b+c = 4d+4e+4f+g

3. To solve the system of equation

(the direct susbstitution is often effective.)

2a

has

6a

3a

6a

Since all the coefficients depend on has, to choose a=1 (more the positive whole small number), from where

1, b6, c6, d2, e1, f3 and g6

4. To register the calculated coefficients

K₄Fe (CN) ₆ + 6:00 ₂SO₄ + 6:00 ₂O → 2 K₂SO₄ + FeSO₄ + 3 (NH₄) ₂SO₄ + 6 CO

Reaction of oxydoreduction

Objective: to write the half-equation for the redox cell IO₃^-/I ^- in basic medium.

1. Outline equation

It acts of the simplest relation between Réducteur and Oxydant. Initially, one does not write a coefficient stoechiometric.

One then seeks to balance the équation :

I^- → IO₃^-

2. Balance atoms of the common element between oxidant and reducing

In our case, it acts of the Iode.

iodine is already balanced in the reaction. One thus does not touch with nothing.

3. Balance Oxygène

Since one is in aqueous solution, water is the most probable supplier of oxygen atoms, one adds Molécule S H₂O to establish the équilibre :

I^- + 3:00 ₂O → IO₃^-

Note : If one is not in an aqueous solution but in the air, one adds O₂ (the Dioxygène is then the most probable supplier of oxygen atoms); let us note that there exists also dioxygene dissolves in water, as well as steam in the air, but we neglect these phenomena for the exercise.

4. Balance Hydrogène

In water, the atoms of hydrogen probably find themselves in the form of Ion oxonium; one thus adds ions H⁺

I^- + 3:00 ₂O → 6:00 ⁺ + IO₃^-

5. Balance electric charges

One adds electron S in order to have electric neutrality

I^- + 3:00 ₂O → 6:00 ⁺ + IO₃^- + 6e^-

Note : the electron S must appear side of the Oxydant. If it were not the case, it must inevitably be an error upstream.

6. Basic medium

One adds OH^- on both sides equation in order to neutralize the Ion S H⁺ :

I^- + 3:00 ₂O + 6 OH^- → 6:00 ⁺ + IO₃^-+ 6e^-+ 6 OH^-

Since:

OH^- + H⁺ → H₂O

One finds:

I^- + 3:00 ₂O+ 6 OH^- → 6:00 ₂O + IO₃^-+ 6e^-

Then after simplification:

I^- + 6 OH^- → 3:00 ₂O + IO₃^-+ 6e^-

7. Validation of the equation

Vérifier that the molecules and the atoms of the departure are present.

Vérifier that the account of the atoms on the left is equal to the account of the atoms on the right.

Anecdote

The discovery of the concept of Stoichiometry, i.e. the fact that the chemicals react in whole proportions, by John Dalton in 1804, was one of the decisive arguments in favor of the atomic Théorie of the matter.

See too

Chemical reaction

External bonds

Online calculator , a computer to calculate the coefficients of a chemical equation
Chemical equation to balance , another computer
Chemical equation balancing program , a computer with coefficients stochiometric which takes account of the electric charges
Online Chemical Equation To balance , another computer

Simple: Chemical equation

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