Chemical Reactions 1: Energy & Chemical Dynamics

Chemical Reactions 1: Energy & Chemical Dynamics

Dissolution: An energy phenomenon

Dissolution: An energy phenomenonHeterogeneous mixture: Components can be distinguishedHomogeneous mixture: Components cannot be distinguished

Solutions are a type of homogeneous mixture. Examples of solutions:Running waterAir

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 phenomenonAqueous 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

Dissolution: An energy phenomenon

Dissolution: An energy phenomenonNon-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 phenomenonMolecular 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

Dissolution: An energy phenomenonPolar 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 phenomenonIonic 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

Dissolution: An energy phenomenonStrong electrolyte (Light bulb produces a bright light):

NaOH (s) + H2O Na+(aq) + OH-(aq)100 molecules 100 ions100 ions

Weak electrolyte (Light bulb produces a dim light):CH3COOH (l) + H2O H+(aq) + CH3COO-(aq)100 molecules 10 ions10 ions

Nonelectrolyte (Light bulb does not light):C12H22O11 (s )+ H2O C12H22O11 (aq) 100 molecules 100 molecules

Dissolution: An energy phenomenonSolubility: 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

Dissolution: An energy phenomenonThe 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 phenomenonThe 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 phenomenonIn 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 phenomenonDissolution 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: An energy phenomenon

Dissolution: An energy phenomenonDissolution 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

Dissolution: An energy phenomenon

Dissolution: An energy phenomenonMolar heat of solution: Heat involved in the process of dissolving a mole of solute.

_It is commonly denoted as H (variation of enthalpy). _It is expressed in kJ/mol_It is positive (H > 0) for endothermic reactions_It is negative (H < 0) for exothermic reactions

Dissolution: An energy phenomenon

Dissolution: An energy phenomenon

Dissolution: An energy phenomenonExample: 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 phenomenonExample: Write the H for each of the following dissolutions equations, and state whether the dissolution is exothermic or endothermic.

Dissolution: An energy phenomenonExample: 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 - 49kJ1 mol - X

MM (LiOH): (1*7) + (1*16) + (1*1) = 24g

48g - 49kJ24g X

X = 24.5 kJ Dissolution: An energy phenomenonExample: Calculate the final temperature of a solution formed when 10g of CuSO4 are dissolved in 100ml of water, if Ti is 23C and molar heat of solution of CuSO4 is -68.1 kJ/mol.

Dissolution: An energy phenomenonQs = -QwQs = -mwcwTT = -Qs / mc Tf - Ti = -Qs / mc Tf = -Qs / mc + Ti

Finding Qs : MM(CuSO4) is 160 g/mol 160g -68.1 kJ10g XX = Qs = - 4.27 kJ

Tf = - [- 4.27 * 103 J g C / (100g)(4.18J)] + 23C Tf = 33.2C

Dissolution: An energy phenomenon

Dissolution: An energy phenomenon