Topic: Electrolytic dissociation of hydroxides

Target group

Elementary school student (grades 7. and 8.)

Core curriculum:

Elementary school. Chemistry.

VI. Hydroxides and acids. The student:

4) explains what is the electrolytic dissociation of bases and acids; defines the terms: electrolyte and nonelectrolyte; writes the electrolytic dissociation equations of bases and acids (in a stepwise form for HIndeks dolny 2₂S, HIndeks dolny 2₂COIndeks dolny 3₃); defines acids and bases (according to Arrhenius theory); distinguishes the terms: hydroxide and base.

General aim of education

The student explains what the electrolytic dissociation of bases is and writes the corresponding equations.

Key competences

• communication in foreign languages;

• digital competence;

• learning to learn.

Criteria for success
The student will learn:

• why aqueous solutions of hydroxides conduct electricity;

• what is the electrolytic dissociation process;

• record the electrolytic dissociation equations of hydroxides and present this process using models.

Methods/techniques

• expository

  • talk.

• activating

  • discussion;

  • snowball method.

• exposing

  • exposition.

• programmed

  • with computer;

  • with e‑textbook.

• practical

  • exercices concerned.

Forms of work

• individual activity;

• activity in pairs;

• activity in groups;

• collective activity.

Teaching aids

• e‑textbook;

• notebook and crayons/felt‑tip pens;

• interactive whiteboard, tablets/computers.

Lesson plan overview

Introduction

1. The teacher hands out Methodology Guide or green, yellow and red sheets of paper to the students to be used during the work based on a traffic light technique. He presents the aims of the lesson in the student's language on a multimedia presentation and discusses the criteria of success (aims of the lesson and success criteria can be send to students via e‑mail or posted on Facebook, so that students will be able to manage their portfolio).

2. The teacher together with the students determines the topic – based on the previously presented lesson aims – and then writes it on the interactive whiteboard/blackboard. Students write the topic in the notebook.

3. Health and safety – before starting the experiments, students familiarise themselves with the safety data sheets of the substances that will be used during the lesson. The teacher points out the need to be careful when working with them.

Realization

1. The teacher initiates a short discussion, asking the pupils if water solutions of hydroxides conduct electricity. Students present their opinions.

2. The lecturer refers to the abstracts and asks for familiarization with the experiment 1. Students write a research question and hypothesis, write them in the form. The teacher divides the class into groups, distributes the appropriate equipment, glass and reagents to perform the experiment. Students follow the instructions and write their observations and applications on the form. The teacher asks questions in relation to the observations recorded, complements the students' answers, corrects any mistakes.

3. Students work in the same groups. They carry out experiment 2, according to the same procedures as before.

4. The teacher asks students to formulate definitions of electrolyte and nonelectrolyte. Students work using the snowball method. First, they prepare definitions in pairs, then combine into fours, verify information and create a common position. In the next steps, they merge into more numerous groups, compare studies, verify them until a general class definition is created.

5. Students analyze the illustrations in the abstract, depicting the dissociation of potassium hydroxide and calcium hydroxide. They write their observations on the form. Selected people discuss them on the class forum.

6. The teacher, in reference to the saved and analyzed equations of dissociation of electrolytic hydroxides, asks the question: „How can you demonstrate the dissociation of hydroxides using the general equation?” - asks the willing student to write this equation on the board. Instructs students to define the concept of principles and then refers to the Arrhenius theory. Explains the difference between hydroxide and base.

7. Students, working individually or in pairs, carry out interactive exercises to check and consolidate knowledge learned during the lesson. Selected people discuss the correct solutions for interactive exercises. The teacher completes or straightens the statements of the proteges.

8. The teacher refers students to the abstract and together with the students performs an experiment - an ammonia fountain. Students fill out the form in the abstract. Then the teacher asks willing / selected students to write on the board an equation presenting the reaction of dissociation of ammonia water and explaining its mechanism. If necessary, the Teacher provides additional explanations.

Summary

1. Teacher asks: If there was going to be a test on the material we have covered today, what questions do you think would you have to answer? If the students do not manage to name all the most important questions, the teacher may complement their suggestions.

Homework

1. Listen to the abstract recording at home. Pay attention to pronunciation, accent and intonation. Learn to pronounce the words learned during the lesson.

2. Write a short note about the topics covered in the lesson.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

Electrolytic dissociation of hydroxides

The flow of electric current through the liquid is possible only if there are free ions in it: positive (cations) and negative (anions), i.e. particles that have a deficiency or excess of electrons in comparison with electrically neutral particles. Well‑conducting liquids are aqueous solutions of bases, salts and acids, because in their case the phenomenon of electrolytic dissociation occurs very well.

Sodium hydroxide or porassium hydroxide are electrolytes. In solid form, it will not conduct electricity. When dissolved in water, however, it will. In order to conduct electricity, a substance has to be able to allow an electrical charge to move through it. In water, NaOH dissociates into sodium ions and hydroxide ions. These ions allow electric current to move through the solution.

Distilled water does not conduct electricity. The solutions of the bases conduct electricity - they contain ions that are carriers of electric charge.

Substances whose water solutions conduct electricity, we call electrolytes . Nonelectrolytes are substances whose aqueous solutions do not conduct electricity. Distilled water is nonelectrolyte, and sodium, potassium and calcium hydroxides are electrolytes.

Potassium hydroxide dissociates which means it disintegrates under the influence of water into a single‑positive potassium cation and a single‑negative hydroxide anion. Both the potassium cation and the hydroxide anion are surrounded by water molecules in the aqueous solution (hydration).

Calcium hydroxide dissociates into a double‑positive calcium cation and two single‑negative hydroxide anions. The amount of positive charges arising as a result of dissociation is equal to the amount of negative charges generated.

The theory of the disintegration of substances into ions was developed by Svante Arrhenius. According to it, bases are chemical compounds that dissociate into metal cations and hydroxide anions:

${\text{M(OH)}}_{\text{n}}\stackrel{{\text{H}}_{\text{2}}\text{O}}{\to }{\text{M}}^{\text{n+}}\text{+}{\text{nOH}}^{-}$

Ammonia (${\text{NH}}_{\text{3}}$) is a gas with a characteristic odour, freely soluble in water. Aqueous solution of ammonia (ammonia water) ${\text{NH}}_{\text{3}}\text{·}{\text{H}}_{\text{2}}\text{O}$) is the base, as evidenced by a change in the colour of the indicator, e.g. the colour of a red cabbage brew turned green.

Ammonia water dissociates:

${\text{NH}}_{\text{3}}\text{·}{\text{H}}_{\text{2}}\text{O}\stackrel{{\text{H}}_{\text{2}}\text{O}}{\leftrightarrow }{\text{NH}}_{\text{4}}^{+}\text{+}{\text{OH}}^{-}$

$\text{Ammonia solution}\stackrel{\text{water}}{\leftrightarrow }\text{ammonia cation + hydroxide anion}$

• Water soluble hydroxides disintegrate in water into ions: metal cations and hydroxide anions.

• Hydroxide solutions conduct electricity - they are electrolytes.

• The ion charge is written down by noting the first digit standing after the symbol in the upper index, followed by the ion sign (number one is omitted), e.g. ${\text{OH}}^{-}$, ${\text{Mg}}^{\text{2+}}$. The number of the same ions is noted using the factor in front of the symbol, e.g. $2{\text{Mg}}^{\text{2+}}$.

• The hydroxide anion is always single‑negative.