Topic 6

  • Created by: Nada_HM
  • Created on: 10-06-18 11:54

Rate of reaction

There are four main factors that affect how quickly a reaction goes: 

Temperature — when the temperature is increased the particles all move quicker. If they're moving quicker, they're going to collide more often and more collisions means a faster rate of reaction

Concentration (or pressure for gases) — if you increase concentration ( or pressure for gases) it means there are more particles of reactant which makes collisions between the reactants more likely. 

Surface area  — if one of the reactants is a solid then breaking it up into smaller pieces will increase its surface area to volume ratio. This means that, for the same volume of solid, the particles around it in the solution will have more area to work on, so there'll be more frequent collisions and the rate of reaction will be faster .

Catalysts — a catalyst is a substance which can speed up a reaction, without being changed or used up in the reaction. They all work by decreasing the activation energy needed for the reaction to occur. They do this by providing an alternative reaction pathway with a lower activation energy.

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Collision Theory

Reaction rates are explained by collision theory. Collision theory just says that the rate of a reaction depends on two things: 

  • The collision frequency of reacting particles (how often they collide). The more there are in a certain amount of time, the faster the reaction is. 
  • The energy transferred during a collision. Particles have to collide with enough energy to be successful.

One way is to increase the frequency of collisions, so that the probability of a successful collision (a collision that results in a reaction) increases. The other way is to increase the energy of the collisions, so that more of the collisions are successful. 

The minimum amount of energy that particles must have in order to react is called the activation energy.

The rate of a reaction is directly proportional to the frequency of successful collisions. This means that, if the frequency of successful collisions doubles, the rate will also double. If the frequency of successful collisions triples, the rate triples, and so on. 

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Reversible Reactions

A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants. Reversible reactions can be represented like this: 

                                                          A + B             C + D

The double arrow means that the reaction can go in either direction. By changing the conditions of a reaction, the overall direction can be changed and the relative amounts of products and reactants can be altered. 

If a reversible reaction takes place in a closed system then a state of equilibrium will always be reached. Equilibrium is when the amounts of reactants and products reach a balance — their concentrations stop changing. 

 As the reactants react in a reversible reaction, their concentrations fall — so the forward reaction will slow down. But as more and more products are made and their concentrations rise, the backward reaction will speed up. After a while the forward reaction will be going at exactly the same rate as the backward one — the system has reached equilibrium. Both reactions are still happening, but the overall effect is nil because the forward and reverse reactions cancel each other out. 

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Energy and reversible reactions

When a reaction's at equilibrium it doesn't mean the amounts of reactants and products are equal: 

  • If the equilibrium lies to the right, the concentration of products is greater than that of the reactants. 
  • If the equilibrium lies to the left, the concentration of reactants is greater than that of the products. 

In reversible reactions, if the reaction is endothermic in one direction, it will be exothermic in the other direction. The energy absorbed by the endothermic reaction is equal to the energy released by the exothermic reaction. 

A good example of a reversible reaction is the thermal decomposition of hydrated copper sulfate. 

If you heat blue hydrated copper sulfate crystals it drives the water off and leaves white anhydrous copper sulfate powder. This is endothermic. If you then add a couple of drops of water to the white powder you get the blue crystals back again and energy is given out. This is exothermic.

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Le Chatelier's Principle

All reversible reactions are exothermic in one direction and endothermic in the other. 

  • If you raise the temperature, the yield of the endothermic reaction will increase and the yield of the exothermic reaction will decrease. If you reduce the temperature, the yield of the exothermic reaction will increase and the yield of the endothermic reaction will decrease. 

Changing the pressure affects reactions where the reactants and products are gases. 

  • Raising the pressure favours the reaction which produces less volume (the fewest number of gas molecules). Lowering the pressure favours the reaction which produces more volume the reatest number of as molecules . 

If you change the concentration of either the reactants or the products, the system will no longer be at equilibrium. 

  • If you increase the concentration of a reactant the system tries to decrease it by making more products. If you decrease the concentration of a product the system tries to increase it again by reducing the amount of reactants. 
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