Chemical reactions

 Chemical reactions 

They are called: the changes in which new ones (products) with different properties are created from certain initial substances (reactants).

How chemical reactions are symbolized:

Each chemical reaction is symbolized by a chemical equation. In this chemical equation we distinguish two members, connected to each other by an arrow (→). In the first member we write the reactants, while in the second member we write the products. (figure 1.1)

Reactants:

The bodies we have initially, before the reaction starts.

Products: 

The bodies formed during the reaction.

1.1

A chemical equation includes:

➢ the reactants and the products ➢ the

oxidation numbers, so that we put the appropriate coefficients so that the atoms of each element are equal in both sides of the chemical equation.

When does a chemical reaction take place?

For a chemical reaction to take place, according to collision theory, the reactant molecules must collide appropriately (that is, they must have the appropriate speed and a certain orientation). The result of this collision is that the original bonds (of the reactants) are "broken" and new ones (of the products) are created. 

Rate of a reaction:

 Defines the change in concentration of one of the reactants or products, per unit of time.

The speed of a reaction can be increased:

1. By increasing the amount (concentration) of the reactants.

2. With an increase in temperature

3. With the presence of catalysts.

4. By increasing the contact surface of the solid bodies participating in the reaction.

Catalyst: 

It increases the speed of the reaction, without being consumed. Reactions in living organisms are catalyzed by enzymes or biocatalysts

Exothermic:

It is called a chemical reaction that releases heat into the environment (figure 1.2).

 Endothermic:

 It is the reaction that absorbs heat from the environment (figure 1.2).

1.2

How efficient is a reaction?

Many chemical reactions are incomplete, that is, only part of the reactants are converted into products.

General:

The yield of a reaction determines the relationship between the amount of a product that we practically get and the amount that we would get theoretically, if the reaction were complete (one-way).

Increasing the yield of a reaction is done by:

1. the amount (concentration) of the reactants or products

2. the temperature

3. the pressure, as long as gases are involved in the reaction.

Types of chemical reactions:

Chemical reactions can be classified into two major categories, redox and metathesis, where these in turn are broken into parts.

REDOX REACTIONS:

In these reactions the oxidation number of some of the participating elements changes. These are broken down into reactions:

1. Synthesis (eg 2K + Cl2→ 2KCl)

2. Decomposition or decomposition(eg CaCO3 → CaO + CO2)

3. Simple replacement (eg 2Na + FeCl2 → 2NaCl + Fe)

In synthesis reactions:

Chemical element + Chemical element = Chemical

In synthesis reactions:

Chemical compound = Chemical element + Chemical element

In simple replacement reactions:

Chemical element(1) + chemical compound (1') = Chemical element(2) + chemical compound (2')

In this type of reaction (simple replacement), if the Chemical element (1) is a metal or a non-metal it will replace the metal or non-metal of the chemical compound (1') respectively, which will result in a chemical element (2) and a chemical compound (2').

However, for the simple replacement to occur, the chemical element (1), which will be e.g. metal, to be more reactive than the metal of the chemical compound (1').

The order of activity of metals and non-metals follows in the table below (figure 1.3).

( the metals are at the top and the non- metals at the bottom. Also, the increase of reaction goes from the left, which are the most reactive elements, to the right, which are the least reactive ones).

Step by step the creation of a chemical reaction(simple substitution, but these rules apply to all reactions):

e.g. Na + FeCl2 

1.We notice that Na is a metal (this can also be observed from the activity table). So, next we see if it is more reactive than the metal of the chemical compound. If it is, we put an arrow ( → on the arrow X and not continue, thus emphasizing that the reaction does not take place). 

2.Then, after we see that it takes place, we change the positions of the metals between them and finally it is written: Na + FeCl2 → NaCl + Fe. 

3.Next, we need to cross-reference the oxidation numbers to the chemical compound NaCl (from the previous documents I have uploaded, we know that Na has an oxidation number of +1 and Cl -1). After doing this, it results in the right member 

NaCl + Fe. 

4. But for the equation to be balanced, the atoms of the first member must be equal to the atoms of the second member. The reaction we have made up to the third step is: Na + FeCl2 → NaCl + Fe. However, we notice that in the first member there are two Cl, while in the second we have only one Cl. So, we have to put a 2 in front of the Na of the second member (since it is a chemical compound). So it follows: Na + FeCl2 → 2NaCl + Fe. But in this way, Na, and thus we have one in the first member and two in the second. But this can be solved by putting a 2 in the Na of the first member. The final result, after we have checked that all atoms are equal in both members, is: 2Na + FeCl2 → 2NaCl + Fe. 

TRANSPOSITION REACTIONS:

In these reactions the oxidation numbers of all elements participating in the reaction remain constant. These are broken down into:

1. Double replacement (eg AgNO3 + NaCl → NaNO3 + AgCl↓)

 

2. Neutralization (e.g. H+ + OH- → H2O)

In double replacement reactions: Chemical compound (1) + Chemical compound (2) = Chemical compound (3) + Chemical compound (4)

In neutralization reactions: Acid + Base = Salt + Water

In double substitution, the following changes are made: A+B- + C+D- → A+D- + C+B-

e.g. Na2CO3 + Ca(OH)2 → 2NaOH + CaCO3↓

Here we must underline that a double replacement reaction takes place only if one of the reaction products:

1. "falls" as a precipitate (precipitation).

2. it escapes as a gas from the reactive system

3. it is a slightly ionizable compound, that is, it dissociates in a very small percentage.

(The latter case is almost exclusively concerned with neutralization, where the minimum ionizable compound water is formed).

For the other cases we will have to learn to recognize which are sediments and gases . These are tabulated below, (figure 1.4):

Main gases and sediments

GASES: HF, HCl, HBr, HI, H2S, HCN, SO2, CO2, NH3

SEDIMENTS: AgCl, AgBr, Agl, BaSO4, CaSO4, PbSO4

All carbonates except K2CO3, Na2CO3, (NH4)2CO3.

All sulfides except K2S, Na2S, (NH4)2S.

All metal hydroxides except KOH, NaOH, Ca(OH)2, Ba(OH)2

Note: Carbonic acid (H2CO3 ) and sulfuric acid (H2SO3) are unstable compounds, while ammonium hydroxide (NH4OH) is a hypothetical molecule. That is why in the place of the products we write:

CO2↑ + H2O instead of H2CO3

SO2↑ + H2O instead of H2SO3

NH3↑ + H2O instead of NH4OH

During neutralization the anion of acid and the cation of the base form a salt (eg NaOH + HCl → NaCl + H2O)

Acidic oxides:

They behave as acids in their aqueous solutions. 

Basic oxides:

behave as bases in their aqueous solutions

One exception:

In the reactions of NH3 with acids and in the reactions between acidic and basic oxides we do not produce water (eg 2NH3 + H2SO4 → (NH4)2SO4).