March 13th: Collision Theory and Rates of Reaction

In class, we learned about 6.1 Collision Theory and Rates of Reaction

The greater the probability that molecules will collide with sufficient energy and proper orientation, the higher the rate of reaction.

The key understandings for this topic are:

    · Species react as a result of collisions of sufficient energy and proper orientation

    · The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time

    · Concentration changes in a reaction can be followed indirectly by monitoring changes in mass, volume and color

    ·  Activation energy (E_a) is the minimum energy that colliding molecules need in order to have successful collisions leading to a reaction

    · By decreasing E_a, a catalyst increases the rate of a chemical reaction, without itself being permanently chemically changed.

Collision Theory:

·         For a reaction to occur, particles must collide with a certain minimum energy (the activation energy) and have the correct orientation

Factors that affect the rate of a reaction include:

    1.      Concentration

·         Solutions:

·         If you increase the concentration of a solution = more particles in the space & more collisions = faster rate of reaction

·         Changing the volume of a solution WILL NOT affect  the rate of reaction

·         Also, increasing the pressure of a gaseous reaction-

-       If you take a gas and increase its pressure (decrease volume of container) = more collisions = faster rate of reaction

    2.    Surface Area

·         Surface area also plays a role in determining how fast a reaction will occur.

·         For example: Crushing a solid up into a powder will greatly increase its surface area and hence the number of collisions possible between reactant particles. One would also expect to see an increased rate of reaction

    3.    Temperature

·         KE of particles is proportional to temperature in Kelvin

·         Activation Energy: the minimum energy colliding molecules need in order to have successful collisions leading to a reaction

·         A match is needed to start a Bunsen burner

·         Maxwell-Boltzmann Distribution

-       If you increase the temperature, particles will have more energy

-       Drop in curve of T₂ is because the area under the curve is fixed – it has to stay the same. The area under the curve is representing the total number of particles (in the sample)

-       At a higher temperature (T₂), we can see that more particles have an energy greater than or equal to the activation energy

We also did a lab that involved measuring the rate of a reaction (and observing the change in the rate of reaction when the temperature is increased or decreased):

·         When sodium thiosulphate reacts with an acid, a yellow precipitate of sulphur is formed

·         In this experiment, we measured how long it took for the sulphur to form

-       This is done by observing the reaction through a conical flask while viewing a black cross on white paper

-       The “X” is eventually obscured by the sulphur precipitate and the time noted

·         In the experiment, we investigated the effects of temperature of the reactants on the rate of reaction by warming up the solutions beforehand in a water bath

HL Solubility

The Solubility of an ionic compound is determined by the lattice enthalpy and the enthalpy of hydration. It can be shown through a Hess' Cycle

Factors that impact solubility:

Lattice Enthalpy- Increases as size decreases
Hydration Enthalpy- Increases as charge increases and size decreases

HL Energetics of Ionic Bond Formation 09/03/15

Energetics of Ionic Formation

What was covered:

The class started by handing in the previous homework (in this case, the 'Heat Death' poem).

The class then had a quick review on the HL information covered thus far in this chapter about 'Thermochemistry.' The review included a large packet covering thermochemistry and important definitions with their usage/purposes. 

Definitions:

• Atomisation - forming 1 mole of gaseous atoms
• Ionisation Energy - farming 1 mole of uni-positive ions in the gas phase
• Electron Affinity - forming 1 mole of uni-negative ions in the gas phase
• Lattice Energy - forming/ breaking 1 mole of ionic lattice from its constituent ions

Work in class:

In class the 'Born-Harper' cycle was covered step-by-step, on how to construct your own cycle.

A few examples were done in class, 2 with Mr. Carmichael and then some from the packet.

Example of 'Born-Harper Cycle':

To solve the lattice enthalpy, you would treat the cycle with Hess' law, saying enthalpy of formation is equal to the summation of all the other steps, for example:

Enthalpy of formation = (enthalpy of atomisation for Na)+(enthalpy of ionisation of Na)+(enthalpy of atomisation for Cl)+(enthalpy of electron affinity for Cl)+(lattice enthalpy)

-411=109+494+(242*.5)+(-364)+LE -> LE= -1580 kJ/mol

*It's important to note that the arrows pointing upwards will be positive values, and the arrows pointing downwards are of negative value (due to ex/endothermic reactions).

Homework:

The homework assigned for 11/03/15 was to complete the 2 last pages of questions 1-5.

Bibliography:

Born-Harper Cycle Diagram - "Further Enthalpy: Enthalpies, Born-Haber Cycle, Mean Bond Enthalpy." Further Enthalpy: Enthalpies, Born-Haber Cycle, Mean Bond Enthalpy. N.p., n.d. Web. 10 Mar. 2015.