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Chemical Reactions 1: Energy & Chemical Dynamics. Dissolution: An energy phenomenon. Dissolution: An energy phenomenon. Heterogeneous mixture: Components can be distinguished Homogeneous mixture: Components cannot be distinguished Solutions are a type of homogeneous mixture.
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Dissolution: An energy phenomenon Heterogeneous mixture: Components can be distinguished Homogeneous mixture: Components cannot be distinguished Solutions are a type of homogeneous mixture. Examples of solutions: • Running water • Air Components: • Solute: The one who dissolves in the “solvent” (e.g. Coffee) • Solvent: The one in which the “solute” dissolves (e.g. Milk)
Dissolution: An energy phenomenon Aqueous solutions: Solutions in which Water acts as the solvent • The polarity of Water makes it possible to dissolve ionic compounds (e.g. Salt) or molecules with a certain polarity • Non-polar substances (e.g. oil) do not dissolve well in Water.
Dissolution: An energy phenomenon Non-polar solutes: Do not dissolve well in water. Dissolution is possible due to energy supplied by collision with water molecules. Dissolution is thus favored by an increase in temperature of the solvent. After a certain time, an equilibrium is established and some molecules go into solution while other return to the solid structure (not visible to human eye).
Dissolution: An energy phenomenon Molecular dissolution: When non-polar solutes dissolve in water, the molecules remain intact (does not dissociate into ions). Non electrolytes (substances that do not conduct electricity while in solution) typically undergo molecular dissolution.
Dissolution: An energy phenomenon Polar solutes: Dissolve well in water. Dissolution is possible due to electrostatic interactions between the charged atoms (or ions) and the slightly charged atoms of the water molecule. After a certain time, an equilibrium is established and some molecules go into solution while other return to the solid structure (not visible to human eye).
Dissolution: An energy phenomenon Ionic dissolution: When polar solutes dissolve in water, the molecules dissociate into ions. Electrolytes (substances that conduct electricity while in solution) typically undergo ionic dissolution. The strength of an electrolyte depends on its ability to dissociate into ions when in solution.
Dissolution: An energy phenomenon Strong electrolyte (Light bulb produces a bright light): NaOH(s)+ H2O Na+(aq) + OH-(aq) 100 molecules100 ions 100 ions Weak electrolyte (Light bulb produces a dim light): CH3COOH (l) + H2O H+(aq) + CH3COO-(aq) 100 molecules10 ions 10 ions Nonelectrolyte (Light bulb does not light): C12H22O11(s ) + H2O C12H22O11(aq) 100 molecules 100 molecules
Dissolution: An energy phenomenon Solubility: Maximum amount of solute that can be dissolved in water at a given temperature. It is usually expressed in g of solvent / 100ml water. *If mass of solute is higher than solubility, a precipitate will be found at the bottom of the solution (too sweet a coffee).
Dissolution: An energy phenomenon The process of dissolution is associated with a change of energy of the system. _In some cases, dissolution of a solvent releases a great deal of energy. This is the case of an exothermic reaction.
Dissolution: An energy phenomenon The process of dissolution is associated with a change of energy of the system. _In some cases, dissolution of a solvent absorbs a great deal of energy. This is the case of an endothermic reaction.
Dissolution: An energy phenomenon In general, for solutes to dissolve, energy is needed to break apart solute particles (molecules or ions) and energy is released when dissolution is achieved (molecules or ions). Both processes occur at the same time, net energy change would depend on which process prevail.
Dissolution: An energy phenomenon Dissolution is exothermic if energy released when it is achieved is greater than energy needed to break apart solute particles. Exothermic reactions increase the temperature of the solution. In general (not only for dissolution) exothermic reactions release energy (T increases)
Dissolution: An energy phenomenon Dissolution is endothermic if energy released when it is achieved is not sufficient to compensate for the energy needed to break apart solute particles. Endothermic reactions decrease the temperature of the solution. In general (not only for dissolution) endothermic reactions absorb energy (T decreases)
Dissolution: An energy phenomenon Molar heat of solution: Heat involved in the process of dissolving a mole of solute. _It is commonly denoted as ΔH (variation of enthalpy). _It is expressed in kJ/mol _It is positive (ΔH > 0) for endothermic reactions _It is negative (ΔH< 0) for exothermic reactions
Dissolution: An energy phenomenon Example: Add the energy term on the appropriate side of each of the following equations, and state whether the dissolution is exothermic or endothermic.
Dissolution: An energy phenomenon Example: Write the ΔH for each of the following dissolutions equations, and state whether the dissolution is exothermic or endothermic.
Dissolution: An energy phenomenon Example: If it is determined that 49 kJ of energy are released when 48 g of LiOH are dissolved, what is the molar heat of solution of this base? 48g - 49kJ 1 mol- X MM (LiOH): (1*7) + (1*16) + (1*1) = 24g 48g - 49kJ 24g – X X = 24.5 kJ
Dissolution: An energy phenomenon Example: Calculate the final temperature of a solution formed when 10g of CuSO4 are dissolved in 100ml of water, if Ti is 23°C and molar heat of solution of CuSO4 is -68.1 kJ/mol.
Dissolution: An energy phenomenon Qs = -Qw Qs = -mwcwΔT ΔT = -Qs / mc Tf - Ti= -Qs/ mc Tf= -Qs/ mc + Ti Finding Qs : MM(CuSO4) is 160 g/mol 160g – -68.1 kJ 10g – X X = Qs= - 4.27 kJ Tf= - [- 4.27 * 103 J g °C / (100g)(4.18J)] + 23°C Tf = 33.2°C
