Rate and Extent of Chemical Change


Rate of Reaction

The rate of a chemical reaction is how fast the eactants are changed into products. Examples of slow reactions include the rusting of iron and chemical weathering. The metal magnesium reacting with acid experimentis an example of a moderate reaction, it produces a gentle stream of bubbles, and burning and explosions are fast reactions, they produce a lot of gas.

You can find the rate of a reaction by recording the amount of product formed, or reactant used up. The steeper the line, the faster the reaction. Faster reactions will produce a flat line. Reactions with the same initial amount of reactant will steady off at the same place on the graph.

Collision theory explains the rate of reacton and what it depends on:

  • The collision frequency of reacting particles - the more collisions, the faster the reaction. They are directly proportional.
  • The energy transferred during a collision - particles have to colide with enough energy for it to be successful.

Activation energy is the amount of energy needed to break the bonds in the eactants so that the reaction can start.

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Factor Affecting Rates of Reaction

The rate of reaction depends of four things:

  • Temperature - As the temp increases, the particles gain more energy and mve faster, resulting in an increase in frequency of collisions and more collisons will have enough energy to be successful and make the reaction happen.
  • The concentration of a solution of the pressure of a gas - An increase in concentration means there are more particles in the same volume of solvent. An increase in pressure is the same amount of particles in a smaller space. This makes collisions more frequent.
  • Surface area of a solid - Breaking up a solid increases the surface are to volume ratio
  • The presense of a catalys - A catalyst is a substance that speeds up a chemical reaction without taking part or beng used up. They work by decreasing the activation energy needed for the reaction to occur by providing an alternative reaction pathway with a lower activation. Enzymes are biological catalyst, the catalsyt reactions in living things.
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Finding Reaction Rates from Graphs

To find the mean rate of a whole reaction, work out the overall change in y-values and then divide this by the total time taken for the reaction. To find the mean between two points, do the same within the time interval.

To find the raet of rection draw a tangent and calculate the gradient.

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Reversible reactions

A + B <-> C + D

- here the products can react to form reactants. As the reactants react, their concentrations fall, so the forward  reaction will slow down, but as more and more products are made, their concentration rises and so the backwards reaction speeds up.

After a while, the forward reaction will be oing at exactly the same rate as the backward reaction, the system is at equillbrium. A dynamic equillibrium both reactions are happening but with no overall effect. This means the concentrations of reactants and products have reached a balance and won't change.

Equillibrium is only reached if the reversible reaction takes place in a 'closed system' - nothing can escape or enter.

In reversible reacton, one direction is endothermic and the other is exothermic. The enregy transferred from the surroundings is equal to that released back, for example thermal decmposition of hydrated copper sulfate is endothermic in the forward reaction and exothermic backwards:

  • hydrated copper sulfate <-> anhydrous copper sulfate + water.

If you heat blue hydrated copper (II) sulfate crystals, it drives the water off and leaves white anhdrous coper (II) sulfate powder, this is endothermic. If you then add a couple of drops of water you get the blue crystals back, this is exothermic.

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Le Chatelier's Principle

Le Chatelier's principle is the idea that if you change the conditions of a reversible reaction at equillibrium, the system will try to counteract that change. It can be used to predict the effect of any change you make to a sytem:

  • Change in temperature - If you decrease the temperature, the equillibrium will will move in the exothemic direction to produce more heat, therfore you will get more products for the exothermic reaction and fewer products for the endothermic reaction. If you increase the temperaute the reaction will shift to the endothermic direction.
  • Change in pressure - If you increase the pressure, the equillibrium tries to reduce it and moves to the direction where there are fewer molecules (use the balanced symbol equation and observe which side has the greatest number of each molecule (the big number)). If you decrease the pressure, equillibrium tries to increase it and moves to where there are more molecules.
  • Change in concentration - Changing the concentration will take the reaction out of equillibrium, so the system tries to counteract this. If you increase the concentration of the reactans the sytem tries to decease it by making more products. If you increase the concentration of products, it reduces the amount of reactants.
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