# Rection Rates and Equilibrium

• Created by: charley
• Created on: 06-09-18 17:47

## Reaction Rates

Rate of reaction = change in concentration / time (mol dm-3 s-1)

Changes in concentration, pressure, temperature, catalysts and surface ares effect the rate of reaction.

The collision theory = two reacting particles must collide for a reaction to occur. If the particles collide with the correct orientation and they have sufficient energy then the collision will be successful.

Volumes of gas produced and mass lost are proportional to the change in concentration of a reactant or product, over time they both give a measure of the rate of reaction.

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## Catalysts

A catalyst is a substance that changes the rate of reaction without being changed itself. They increase the rate of reaction by providing an alternative reaction pathway with a lower activation energy.

Catalysts aren't used up, may form an intermediate or provide a surface for the reaction to take place. They're also regenerated.

A homogenous catalyst has the same physical state as teh reactants. A heterogeneous catalyst has a different physical state than the reactants.

Autocatalysis is where the product of a reaction acts as the catalyst.

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## The Boltzmann Distribution

The Boltzmann distribution is the spread of molecular energies in gases. Features include; no molecules have 0 energy, the area under the curve = the total number of moleculesa and theres no maximum energy for a molecule.

At higher temperature; more molecules have an energy greater than or equal to the activation energy so the rate of reaction increases. Collisions are more frequent as molecules are moving faster.

With a catalyst; a greater proportion of molecules have an energy greater than or equal to the lower activation energy therefore more molecules will react increasing the rate of reaction.

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## Dynamic Equilibrium and Le Chatelier's Principle

Le Chatelier's principle states that when a system in equilibrium is subjected to an external change, the system readjusts itself to minimise the effect of the change.

Equilibrium is dynamic and must be a closed system. The position of equilibrium indicates teh extent of the reaction.

An increase in concentration shifts equilibrium to the side with more concentration.

An increase in temperature shifts equilibrium in the endothermic direction. A decrease in temperature shifts equilibrium in the exothermic direction.

An increase in pressure shifts equilibrium to the side with fewer molecules. A decrease in pressure shifts equilibrium to the side with more molecules.

A catalyst doesn't change the position of equilibrium but it does increase the rate at which equilibrium is reached.

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## The Equilibrium Constant Kc

An equilibrium constant provides the actual position of equilibrium. The equilibrium law defines the equilibrium constant in terms of concentration.

A Kc value of 1 indicates a position of equilibrium thats halfway between reactants and products.

A Kc value of > 1 indicates a position of equilibrium towards the products.

A Kc value of < 1indicates a position of equilibrium towards the reactants.

Square brackets must be used.

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