Kinetics, equilibria and redox reactions

Reaction Rates

Temperature

increasing temperature = increase in rate of reaction 

This is due to the particles having on average more kinetic energy and energy equal to or higher than the activation energy of the reaction. At higher temperatures, the molecules are moving around faster and therefore collide more frequently, making the rate of reaction faster. 

Concentration 

increasing concentration = increase in rate of reaction 

If the concentration is increased, there will be an increased amount of reactant particles and the particles will be closer together. There is also likely to be increased frequency of collisions 

Pressure 

Raising the pressure, pushes all of the gas particles closer together, making them more likely to collide. meaning more frequent successful collisons and therefore increasing the rate of reaction

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Reaction Rates

Catalysts 

A catalyst is a substance that increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy. The catalyst is chemically unchanged at the end of the reaction. 

  • Catalysts can only work on a single reaction 
  • They do not get used up in reactions and only a small amount is needed. They do take part in reactions but they are remade at the end of the reaction. 
  • Because catalysts allow you to make the same amount of product faster (and often at a lower temperature) they are used a lot in industry to save money. 
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Maxwell-Boltzmann distributions

                                                           

The catalyst lowers the activation energy, meaning that there will be more particles with enough energy to react when they collide. It does this by allowing the reaction to go via a different route. So in a certain amount of time, more particles react. 

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Measuring reaction rates

Timing how long a precipitate takes to form 

  •  You can use this method when the product is a precipitate which clouds a solution
  • You watch a mark through the solution and time how long it takes to become unclear 
  • you must use the same observer each time and therefore improve the accuracy 
  • This method is subjective and people may have differing opinions on the exact moment 

Measuring a decrease in mass 

  • When one or more of the products is a gas, you can measure the rate of formation using a mass balance
  • As gas is given off, the mass of the reaction mixture decreases 
  • This method is accurate and easy to do. Must be done in fume cupboard- release of gas 

Measuring the volume of gas given off 

  • This involves using a gas syringe to measure the volume of gas being produced 
  • You can only use this method when one or more products is a gas - accurate method 
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Reversible reactions

Reversible reactions can reach dynamic equilibrium 

Lots of chemical reactions are reversible - they go both ways, to show that the reaction is reversible, you can use the revesible symbol. 

As reactants get used up, the forward reaction slows down- and as more product is formed, the reverse reaction speeds up. 

After a while, the forward reaction will be going at exactly the same rate as the backward reaction meaning that the amounts of reactants and products won't be changing anymore 

This state is called dynamic equilibrium. At equilibrium, the concentrations of reactants and products stay constant. 

A dynamic equilibrium can only happen in a closed system. This means that nothing can get in or out. 

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La Chatelier's principle

La Chatelier's principle predicts what will happen if the conditions are changed

If you change the concentration, pressure or temperature of a reversible reaction, you're going to alter the position of equilibrium. This means that you will end up with different amounts of reactants and products at equilibrium. 

If the position of equilibrium moves to the left, you will get more reactants

If the position of equilibrium moves to the right, you will get more products 

la chatelier's principle states; 

'if a reaction at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium will move to counteract the change' 

so basically, if you raise the temperature, the position of equilibrium will shift to try and cool things down and if you raise the pressure or concentration, the position of equilibrium will shift to try and reduce it again. 

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Effect on La Chatelier's principle

Concentration

  • If you increase the concentration of a reactant, the equilibrium tries to get rid of the extra reactant. It does this by making more product and therefore the equilibrium shifts to the right
  • If you increase the concentration of the product, the equilibrium tries to remove the extra product. This makes the reverse reaction go faster. So the equilibrium shifts to the left. 
  • Decreasing the concentrations has the opposite effect. 

Pressure  (Changing this only effects equilibria involving gases)

  • Increasing the pressure shifts the equilibrium to the side with fewer gas molecules which reduces the pressure
  • Decreasing the pressure shifts the equilibrium to the side with more gas molecules 

Temperature 

  • Increasing temperature means adding heat. The equilibrium shifts in the endothermic direction to absorb this heat. Decreasing the temperature means removing the heat. The equilibrium shifts in the exothermic direction to produce more heat. 
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Industry reaction conditions

Catalysts have no effect on the position of equilibrium. They can't increase yield - but they do mean equilibrium is reached faster. 

Companies have to think about how much it costs them to run a reaction and how much money that they can make from it. This means that they have a few factors to think about when choosing the best conditions for a reaction: 

e.g. Ethanol can be produced via a reversible reaction between ethene and steam - This reaction is carried out at pressures of 60-70 atmospheres and a temperature of 300oC, with a catalyst of phosphoric acid. 

  • An exothermic reaction - lower temperatures favour the forward reactions = better yield 
  • Lower temperature however means a slower rate of reaction- 300oC is a compromise of a reasonable yield and a faster reaction 
  • Higher pressure shifts the equilibrium to the side with fewer molecules, which favours the foward reaction here. High pressure = increase in rate of reaction - too high pressures can be costly for industry to produce and the need for materials in place like strong pipes/containers
  • So the 60-70 atmospheres is a compromise - it gives a reasonable yield at a lower cost 
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