Equilibria.doc

Kinetics •

Reactions only happen when: Particles collide in the correct orientation, and they possess the activation energy (minimum amount of kinetic energy particles need to react). This is the collision theory.

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Enthalpy profile diagram:

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Factors affecting the rate of reaction: concentration, temperature, pressure, surface area catalysis.

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Maxwell-Boltzmann distribution: •

Shows distributions of molecular energies in a gas

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When temperature is increased, particles will have more kinetic energy and move faster. This means that more particles will have energies greater than the activation energy and will react. This changes the shape of the Maxwell Boltzmann distribution curve pushing it to the right, with a peak lower than the original. Lower temperature Higher temperature

Factor Concentratio n (solution) Pressure (gas) Surface area (solids)

Temperatur e Catalyst

How it affects rate Increasing conc./pressu re increases rate Increasing surface area increases rate Increasing temperature increases rate Speeds up the reaction

Explanation

Only molecules in this region can react – molecules have a higher energy than the activation energy

The particles become more crowded, therefore collide more times which increases the reaction rate. The smaller the size of reacting particles, the greater the total surface area. Increasing surface area means larger area is exposed for reaction and more collisions. Increasing temperature means the average speed of reacting particles increases, therefore more collisions per second. Lower the activation energy by providing an alternative route. If activation energy is lower, more particles will have enough energy to react.

Total number of gas molecules under the curve

Catalysts: •

Increase the rate of a reaction by providing an alternative reaction pathway with a lower activation energy. It is chemically unchanged at the end of the reaction.

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Homogenous catalysts: in the same state as the reactants.

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Forms intermediates with the reactants, which the products are then formed from.

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The activation energy needed to form the intermediates and the products from the intermediates is lower than that needed to make the products directly from the reactants.

Chemical Equilibia

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Many reactions do not go to completion because the reaction is reversible

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Dynamic equilibrium: When the rates of the forward and reverse reactions are equal. It’s dynamic because individual molecules react continuously. It is at equilibrium because no net change occurs (overall concentrations remain constant

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Equilibrium can only happen in a CLOSED system.

The effect of conditions on the position of equilibrium: •

Controlled by Le Chatelier’s principle: When a system at equilibrium is subjected to a change, it will behave in such a way to counteract that change.

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Temperature is a very important way to control industrial processes, because it is the most effective factor (general rule – increase in 10K doubles the rate of reaction.

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Pressure is very expensive to use in equilibrium processes.

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The red-brown gas NO2 exists in equilibrium with pale yellow N2O4 :

N2O4

2NO2

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The forward reaction is endothermic.

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If the position of equilibrium shifts to left the mixture pales

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If the position of equilibrium shifts to right the mixture darkens