Lecture 2: Electrolytes PDF

Federal State Budget Educational Institution of Higher Education "URAL STATE MEDICAL UNIVERSITY" OF THE MINISTRY OF HEALTH OF THE RUSSIAN FEDERATION Lecture 2 ELECTROLYTES Questions: 1. Properties of water. pH. 2. Electrolytic theory. 3....

Federal State Budget Educational Institution of Higher Education "URAL STATE MEDICAL UNIVERSITY" OF THE MINISTRY OF HEALTH OF THE RUSSIAN FEDERATION Lecture 2 ELECTROLYTES Questions: 1. Properties of water. pH. 2. Electrolytic theory. 3. Strong electrolytes. 4. Weak electrolytes.  SOLUTIONS Solute Solvent There is interaction between solute and solvent The properties of solutions depend on its composition. Both the nature of the solute and the solvent play an important role. We will primarily consider water (H2O) as a solvent. Properties of water. pH WATER STRUCTURE The Polar Nature of Water  The autoionization of water Н2О + Н2О Н3О+ + ОН- or more simple equation Н2О Н+ + ОН- This equilibrium shifted to the left. Water mainly exists in a form of molecule Н2О. Only some very small amount of water molecules exchanges the proton and transform to Н+ and ОН-  pH definition The molar concentration of Н+ is very small number and it is characterized by pH value pH is the negative decimal logarithm of the molar concentration of protons in an aqueous solution рН = -lg СН+  Decimal logarithms This is the power to which you need to raise the number 10 to get the original number, the logarithm of which we are looking for lg 50 = 1.69 lg5=0.69 lg0.2=-0.69 рН = -lg СН; pOH = -lg СОН- рН + рОН = 14 (25oC) For temperature close to human body temperature рН + рОН = 13,61 (36.6oC≈ 37oC)  Equilibrium concentrations As we consider reversable processes in water solutions we should understand that there exist equilibrium Concentration of each component in solution usually is described in mol/l. That is molar concentration. Molar equilibrium concentration is written in square brackets Not CH+, but [H+], not COH-, but [OH-], etc.  pH scale used to specify the acidity or basicity of an aqueous solution pOH scale is not used very often it is given for better understanding 1) [H+] = [OH-], [H+] = 10-7 mol / l. рН = 7 - environment neutral; 2) [H+] > [OH-], [H+] > 10-7 mol / l. рН < 7 - environment acidic; 3) [H+] < [OH-], [H+] < 10-7 mol / l. рН >7 - environment alkaline. For temperature of human body (36.60C≈ 370C) рН + рОН = 13,61 Neutral рН = рОН = 6,8 !  SUMMARY pH is defined as the negative decimal logarithm of the hydrogen ion (proton) concentration pH decreases with increasing concentration of H+ If pH = 7.0, the solution is neutral. If pH < 7.0, the solution is acidic. If pH > 7.0, the solution is alkaline (basic). Electrolytic theory  Solutions of various compounds conduct electrical current differently: If the solution does not conduct an electrical current, the substance in the solution is considered to be a non-electrolyte. If the solution conducts an electric current, the substance in the solution is considered to be an electrolyte.  ELECTROLYTIC THEORY SVANTE AUGUST ARRHENIUS (19.2.1859–2.2.1927) 1. When dissolved in water, electrolytes dissociate (decompose) into positively and negatively charged ions. 2. The reason for the dissociation of the electrolyte in an aqueous solution is its hydration, that is, the interaction of the electrolyte with water molecules and the breaking of a chemical bond. 3. Under the action of an electric current, ions acquire a directional movement.  ELECTROCONDUCTIVITY DEPENDS ON NATURE OF SUBSTANCE STRONG ELECTROLYTES DISSOCIATE COMPLETELY INTO IONS TO PRODUCE SOLUTIONS THAT CONDUCT ELECTRICITY WELL. WEAK ELECTROLYTES PRODUCE A RELATIVELY SMALL NUMBER OF IONS, RESULTING IN SOLUTIONS THAT CONDUCT ELECTRICITY POORLY. NON-ELECTROLYTES DISSOLVE AS UNCHARGED MOLECULES AND HAVE NO EFFECT ON THE ELECTRICAL CONDUCTIVITY OF WATER.  INTERACTION WITH WATER MOLECULES ELECTROLYTES DISSOLVES IN WATER WITH PRODUCTION OF HYDRATED IONS NaClsolid Naaq+ + Claq- Polar liquids are good solvents for ionic compounds. NON-ELECTROLYTES DISSOLVES IN WATER WITH PRODUCTION OF HYDRATED MOLECULES NON-ELECTROLYTES DO NOT DISSOCIATE  Electrolyte Non-electrolyte NaClsolid Naaq+ + Claq- С6H12O6 (glucose) hydrated ions and molecules hydrated molecules Degree of dissociation (α)–is the ratio of the number of particles disintegrated into ions to the total number of particles For example, the process of dissociation of sodium chloride is written in equation: NaClsolid Naaq+ + Claq- Number of particles is number of moles, and as the volume of the solution doesn’t change we can use molar concentrations to describe number of particles: С(NaCl), С(Na+). In that case degree of dissociation can be written as: С(Na+) , % α= С(NaCl)  ELECTROLYTES medium (3 < α ≤ 33 %) strong (α > 33 %). weak (α ≤ 3 %) Salts Organic ACIDS Alkalis H 2S HCl, H2SiO3 HBr, H2CO3 HCN HI All water insoluble H2SO4 bases: Cu(OH)2, Fe(OH)2 HNO3 NH3∙H2O HClO4  SUMMARY For water solutions all solutes can be classified as electrolytes or non- electrolytes Electrolytes can be divided into 2 groups – weak and strong. It depends on the degree of dissociation. Properties of solutions of strong electrolytes  BEHAVIOR OF STRONG ELECTROLYTES IN SOLUTION Strong electrolytes exist in solution in a form of hydrated ions and their interaction with other substances or ions is influenced by electrostatic attraction Behavior of an electrolyte solution deviates considerably from that an ideal solution! NOT MOLAR CONCENTRATION, BUT ACTIVITY! aion= f·Cion Where: f is the activity coefficient  THE DEBYE-HÜCKEL THEORY IS BASED ON THREE ASSUMPTIONS OF HOW IONS ACT IN SOLUTION: Debye Peter Joseph Wilhelm 1. ELECTROLYTES COMPLETELY DISSOCIATE INTO (24.III.1884– IONS IN SOLUTION. 2.XI.1966) 2. SOLUTIONS OF ELECTROLYTES ARE VERY DILUTE, ON THE ORDER OF 0.01 M. 3. EACH ION IS SURROUNDED BY IONS OF THE OPPOSITE CHARGE, ON AVERAGE.  aion= f·Cion f – activity coefficient It shows how much real solution deviate for ideal solution f