Topic: Chemical bond. Electrolytic dissociation

Lesson: Writing Equations for Ion Exchange Reactions

Let's create an equation for the reaction between iron (III) hydroxide and nitric acid.

Fe(OH) 3 + 3HNO 3 = Fe(NO 3) 3 + 3H 2 O

(Iron (III) hydroxide is an insoluble base, therefore it is not subjected to. Water is a poorly dissociated substance; it is practically not dissociated into ions in solution.)

Fe(OH) 3 + 3H + + 3NO 3 - = Fe 3+ + 3NO 3 - + 3H 2 O

Cross out the same number of nitrate anions on the left and right and write the abbreviated ionic equation:

Fe(OH) 3 + 3H + = Fe 3+ + 3H 2 O

This reaction proceeds to completion, because a slightly dissociable substance is formed - water.

Let's write an equation for the reaction between sodium carbonate and magnesium nitrate.

Na 2 CO 3 + Mg(NO 3) 2 = 2NaNO 3 + MgCO 3 ↓

Let's write this equation in ionic form:

(Magnesium carbonate is insoluble in water and therefore does not break down into ions.)

2Na + + CO 3 2- + Mg 2+ + 2NO 3 - = 2Na + + 2NO 3 - + MgCO 3 ↓

Let's cross out the same number of nitrate anions and sodium cations on the left and right, and write the abbreviated ionic equation:

CO 3 2- + Mg 2+ = MgCO 3 ↓

This reaction proceeds to completion, because a precipitate is formed - magnesium carbonate.

Let's write an equation for the reaction between sodium carbonate and nitric acid.

Na 2 CO 3 + 2HNO 3 = 2NaNO 3 + CO 2 + H 2 O

(Carbon dioxide and water are products of the decomposition of the resulting weak carbonic acid.)

2Na + + CO 3 2- + 2H + + 2NO 3 - = 2Na + + 2NO 3 - + CO 2 + H 2 O

CO 3 2- + 2H + = CO 2 + H 2 O

This reaction proceeds to completion, because As a result, gas is released and water is formed.

Let's create two molecular reaction equations, which correspond to the following abbreviated ionic equation: Ca 2+ + CO 3 2- = CaCO 3 .

The abbreviated ionic equation shows the essence of the ion exchange reaction. In this case, we can say that to obtain calcium carbonate, it is necessary that the composition of the first substance include calcium cations, and the composition of the second - carbonate anions. Let's create molecular equations for reactions that satisfy this condition:

CaCl 2 + K 2 CO 3 = CaCO 3 ↓ + 2KCl

Ca(NO 3) 2 + Na 2 CO 3 = CaCO 3 ↓ + 2NaNO 3

1. Orzhekovsky P.A. Chemistry: 9th grade: textbook. for general education establishment / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. - M.: AST: Astrel, 2007. (§17)

2. Orzhekovsky P.A. Chemistry: 9th grade: general education. establishment / P.A. Orzhekovsky, L.M. Meshcheryakova, M.M. Shalashova. - M.: Astrel, 2013. (§9)

3. Rudzitis G.E. Chemistry: inorganic. chemistry. Organ. chemistry: textbook. for 9th grade. / G.E. Rudzitis, F.G. Feldman. - M.: Education, OJSC “Moscow Textbooks”, 2009.

4. Khomchenko I.D. Collection of problems and exercises in chemistry for high school. - M.: RIA “New Wave”: Publisher Umerenkov, 2008.

5. Encyclopedia for children. Volume 17. Chemistry / Chapter. ed. V.A. Volodin, Ved. scientific ed. I. Leenson. - M.: Avanta+, 2003.

Additional web resources

1. A unified collection of digital educational resources (video experiences on the topic): ().

2. Electronic version of the journal “Chemistry and Life”: ().

Homework

1. In the table, mark with a plus sign the pairs of substances between which ion exchange reactions are possible and proceed to completion. Write reaction equations in molecular, full and reduced ionic form.

2. p. 67 No. 10,13 from the textbook P.A. Orzhekovsky “Chemistry: 9th grade” / P.A. Orzhekovsky, L.M. Meshcheryakova, M.M. Shalashova. - M.: Astrel, 2013.

Basic general education

Line UMK V.V. Lunin. Chemistry (8-9)

Ionic equations

HISTORY OF THE ISSUE

Even the ancient alchemists, carrying out simple chemical reactions in search of the philosopher's stone and recording the results of their research in thick tomes, used certain signs for chemical substances. Each scientist had his own system, which is not surprising: everyone wanted to protect their secret knowledge from the machinations of envious people and competitors. And only in the 8th century did common designations for some elements appear.

In 1615, Jean Begun, in his book “Elements of Chemistry,” which is rightfully considered one of the first textbooks in this section of natural science, proposed using symbols for writing chemical equations. It was only in 1814 that the Swedish chemist Jons Jakob Berzelius created a system of chemical symbols based on one or two of the first letters Latin name element similar to what students are introduced to in class.

In the eighth grade (paragraph 12, textbook “Chemistry. 8th grade” edited by V.V. Eremin), the children learned to compose molecular equations of reactions, where both the reagents and reaction products are presented in the form of molecules.

However, this is a simplified view of chemical transformations. And scientists thought about this already in the 18th century.

As a result of his experiments, Arrhenius found out that solutions of some substances conduct electricity. And he proved that substances with electrical conductivity are in solutions in the form of ions: positively charged cations and negatively charged anions. And it is these charged particles that enter into reactions.

WHAT ARE IONIC EQUATIONS

Ionic reaction equations- these are chemical equations in which the reactants and reaction products are designated as dissociated ions. Equations of this type are suitable for writing chemical substitution and exchange reactions in solutions.

Ionic equations- an integral part of complex and interesting chemical science. Such equations allow you to clearly see which ions undergo chemical transformations. Substances that undergo electrolytic dissociation are written in the form of ions (the topic is discussed in detail in paragraph 10, textbook “Chemistry. 9th grade” edited by V.V. Eremin). Gases, substances that precipitate, and weak electrolytes that practically do not dissociate are recorded in the form of molecules. Gases are indicated by an up arrow (), substances that precipitate are indicated by a down arrow (↓).

The textbook was written by teachers of the Faculty of Chemistry of Moscow State University. M.V. Lomonosov. The distinctive features of the book are the simplicity and clarity of the presentation of the material, a high scientific level, a large number of illustrations, experiments and entertaining experiences, which allows it to be used in classes and schools with in-depth study of natural science subjects.

FEATURES OF IONIC EQUATIONS

1. Ion exchange reactions, unlike redox reactions, occur without disturbing the valence of substances undergoing chemical transformations.

- redox reaction

Ion exchange reaction

2. Reactions between ions occur under the condition that a poorly soluble precipitate is formed during the reaction, a volatile gas is released, or weak electrolytes are formed.

Pour 1 ml of sodium carbonate solution into a test tube and carefully add a couple of drops of hydrochloric acid to it.

What's happening?

Create an equation for the reaction, write the full and abbreviated ionic equations.

Definition

Reactions occurring between ions in electrolyte solutions are called ion exchange reactions(RIO).

During RIO, there is no change in the oxidation states of elements, so RIO is not redox.

The criterion for the irreversibility of ion exchange reactions is the formation of a weak electrolyte.

Berthollet's rule

Ion exchange reactions proceed almost irreversibly if one of the resulting reaction products “leaves” the reaction sphere in the form of:

• gas,
• draft
• or a weakly dissociating electrolyte (for example, water).

If there are no ions in the solution that form a weak electrolyte, the reaction is reversible and in this case its equation is not written, putting the sign “$\ne$”

To write ionic equations, molecular (1), full ionic (2) and short ionic forms of equations (3,4) are used:

$2KOH + H_2SO_4 = K_2SO_4 + 2H_2O \hspace(3cm) (1)$

$2K^+ +2OH^- + 2H^+ + SO_4^(2-) = 2K^+ + SO_4^(2-) +2H_2O \hspace(0.2cm) (2)$

$2OH^- + 2H^+ = 2H_2O \hspace(5cm) (3)$

$OH^- + H^+ = H_2O \hspace(5.5cm) (4)$

Please note that in In a short ionic equation, the coefficients should be minimal. Therefore, in equation (3) all coefficients are reduced by 2, and the resulting equation (4) is considered a short ionic equation.

When drawing up the RIO, it should be remembered that

• water, metals, oxides, gases, precipitation do not disintegrate into ions and are written in all equations in molecular form;
• $H_2SO_3$, $H_2CO_3$, $NH_4OH$, $AgOH$ are unstable and decompose almost instantly when formed:

  $H_2SO_3 = H_2O + SO_2 \uparrow$

  $H_2CO_3 = H_2O + CO_2 \uparrow$

  $NH_4OH = H_2O + NH_3 \uparrow$

  $2AgOH = Ag_2O \downarrow + H_2O$

Algorithm for composing ion exchange reactions

1. Write down the molecular equation and assign the coefficients. When recording chemical formulas reaction products, it is important to remember that the sum of the charges in the molecule must be equal to zero.
2. A complete ionic equation is drawn up, which takes into account the result of dissociation of both the starting substances and the products of the exchange reaction. All soluble compounds are recorded in the form of ions (indicated in the solubility table by the letter “P” (highly soluble in water), with the exception of calcium hydroxide). The formulas of insoluble substances, gases, oxides, and water are written in molecular form. Counting total reaction coefficient, for which we add up all the coefficients on the right and left sides of the equation.
3. To obtain the abbreviated ionic form of the equation, similar ones are given, that is, identical ions are abbreviated before and after the equal sign in the equation. The coefficients must be minimal, and the sums of charges on the left and right sides of the equation must be the same. The total coefficient is calculated in the abbreviated form (similar to the full form).
4. The abbreviated ionic form of the equation reflects the essence of the chemical reaction that took place.

Interaction of basic oxides with acids. Write down the molecular, short and complete ionic equations for the interaction of calcium oxide and hydrochloric acid. Calculate the total coefficients in full and abbreviated form.

Solution

1. Molecular equation:

$CaO + 2HCl = CaCl_2 + H_2O$

2. Complete ionic equation:

$CaO + 2H^+ + \underline(2Cl^-) = Ca^(2+) + \underline(2Cl^-) + H_2O$

The sum of the coefficients is (1+2+2+1+2+1)=9.

3. Abbreviated ionic equation:

$CaO + 2H^+ = Ca^(2+) + H_2O$

The total coefficient is (1+2+1+1)=5.

4. A short ionic equation shows that when calcium oxide reacts with strong acids ($H^+$), the reaction is almost irreversible, resulting in the formation of a soluble calcium salt and a slightly dissociating substance (water)

Interaction of salts with acids. Write down the molecular, short and complete ionic equations for the interaction of potassium carbonate and nitric acid. Calculate the total coefficients in full and abbreviated form.

Solution

1. Molecular equation:

$K_2CO_3 + 2HNO_3 = 2KNO_3 + CO_2\uparrow + H_2O$

2. Complete ionic equation:

$\underline(2K^+) + CO_3^(2-) + 2H^+ + \underline(2NO_3^-) = \underline(2K^+) + \underline(2NO_3^-) + CO_2\uparrow + H_2O$

The sum of the coefficients is (2+1+2+2+2+2+1+1)=13.

3. Brief ionic equation:

$ CO_3^(2-) + 2H^+ = CO_2\uparrow + H_2O$

The sum of the coefficients is (1+2+1+1)=5.

4. A short ionic equation shows that when soluble carbonates (alkali metals) interact with strong acids ($H^+$), the reaction is almost irreversible, resulting in the always formation of carbon dioxide ($CO_2\uparrow$) and a poorly dissociating substance (water )

oxides incl. H 2 O, precipitation (solubility table), weakly dissociating compounds: H 2 S; HNO 2, H 2 SO 3 → SO 2 + H 2 O, H 2 CO 3 → CO 2 + H 2 O, NH 4 OH → NH 3 + H 2 O; CH3COOH; HMnO 4 H 2 SiO 3 , H 3 PO 4

The following have a constant oxidation state:

Group I main subgroup +1, Group II main subgroup +2, H +, O –2, OH –, Al 3+, Zn 2+.

Redox reactions(ORR) are reactions in which elements change their oxidation state (CO) due to the transfer of electrons.

Algorithm for solving redox reactions

We indicate the oxidation number (CO) of each element in the reaction.

We find elements that change their oxidation state.

We select ions or molecules that contain elements with a changed oxidation state.

We sign the oxidizing agent, the reducing agent.

Acidic environment: add nH 2 O, where lack of O → 2nH +

Alkaline environment: add nH 2 O, where excess O → 2nOH –

We equalize each half-reaction (the left side of the half-reaction = the right side), and write down the number of electrons given and received.

We equalize the number of electrons received and given up, and set the coefficients before the half-reactions.

We sign the oxidation process and the reduction process.

We write the total ionic equation taking into account the coefficients.

We transfer the coefficients from the ionic equation to the molecular equation and present similar ones (the left side of the reaction = the right side)

http://ru.wikipedia.org/wiki/%DD%EB%E5%EA%F2%F0%EE%EB%E8%E7

According to Faraday's law: m = EIt/96,500, Q = It, Cl (electricity consumed)

where m is the mass of the substance oxidized or reduced at the electrode; E is the equivalent mass of the substance; I – current strength, A; t – duration of electrolysis, s. Ve N 2 = 11.2 l, Ve O 2 = 5.6 l

To remember cathodic and anodic processes in electrochemistry, there is the following mnemonic rule:

At the anode, the anions are oxidized.

At the cathode, cations are reduced.

In the first line, all words begin with a vowel, in the second - with a consonant.

Or simpler:

CAThode - CATIONS (ions at the cathode)

ANode - ANions (ions at the anode)

Instructions

Before you begin ionic equations, you need to understand some rules. Insoluble in water, gaseous and poorly dissociating substances (for example, water) do not disintegrate into ions, which means write them in molecular form. This also includes weak electrolytes such as H2S, H2CO3, H2SO3, NH4OH. The solubility of compounds can be determined from the solubility table, which is an approved reference material for all types of control. All charges that are inherent in cations and anions are also indicated there. To fully complete the task, you need to write molecular, complete and ionic abbreviated equations.

Example No. 1. neutralization reaction between sulfuric acid and potassium hydroxide, consider it from the point of view of ED (electrolytic dissociation theory). First, write down the reaction equation in molecular form and .H2SO4 + 2KOH = K2SO4 + 2H2O Analyze the resulting substances for their solubility and dissociation. All compounds are soluble in water, which means they are ions. The only exception is water, which does not disintegrate into ions and therefore remains in molecular form. Write the complete ionic equation, find the same ions on the left and right sides and . To cancel identical ions, cross them out.2H+ +SO4 2- +2K+ +2OH- = 2K+ +SO4 2- + 2H2OThe result is an ionic abbreviation equation:2H+ +2OH- = 2H2OCoefficients in the form of twos can also be abbreviated:H+ +OH- = H2O

Example No. 2. Write the exchange reaction between copper chloride and sodium hydroxide, consider it from the point of view of TED. Write the reaction equation in molecular form and assign the coefficients. As a result, the resulting copper hydroxide formed a blue precipitate. CuCl2 + 2NaOH = Cu(OH) 2↓ + 2NaCl Analyze all substances for their solubility in water - everything is soluble except copper hydroxide, which will not dissociate into ions. Write down the complete ionic equation, underline and abbreviate the identical ions: Cu2+ +2Cl- + 2Na+ +2OH- = Cu(OH) 2↓+2Na+ +2Cl- The ionic abbreviated equation remains: Cu2+ +2OH- = Cu(OH) 2↓

Example No. 3. Write the exchange reaction between sodium carbonate and hydrochloric acid, consider it from the point of view of TED. Write the reaction equation in molecular form and assign the coefficients. As a result of the reaction, sodium chloride is formed and released gaseous substance CO2 (carbon dioxide or carbon monoxide (IV)). It is formed due to the decomposition of weak carbonic acid, which breaks down into oxide and water. Na2CO3 + 2HCl = 2NaCl + CO2+H2OAnalyze all substances for their solubility in water and dissociation. Carbon dioxide leaves the system as a gaseous compound, water is a poorly dissociating substance. All other substances disintegrate into ions. Write down the complete ionic equation, underline and abbreviate the identical ions: 2Na+ +CO3 2- +2H+ +2Cl- =2Na+ +2Cl- +CO2+H2O The ionic abbreviated equation remains: CO3 2- +2H+ =CO2+H2O