02 9 / 2014
The Haber process is the reaction by which ammonia is produced using nitrogen gas and hydrogen gas with the equation;
N2 (g) + 3H2 (g) ↔ 2NH3 (g)
The reaction is reversible as it is in equilibrium. Equilibrium is a condition in which the forward and reverse reactions are occurring at equal rates. The position of the equilibrium can be changed to produce either more product or more reactant to obtain maximum yield by using le Chatelier’s principle. Le Chatelier’s principle states that any change to the system will result in a shift of the reaction to the side which opposes the change.
Increasing the pressure will cause the equilibrium to try to decrease the pressure. The higher pressure on one side means that there are more moles on that side. This shifts the equilibrium to the right to increase the yield because the pressure is lower on the right so it will try to even the pressure out.
This forward reaction is exothermic and increasing the temperature will cause the system to try and decrease the temperature. If we increase the temperature, the yield will decrease because the system opposes the change.
However the rate of reaction is affected by temperature. Therefore a compromise between rate and yield is needed.
Using the graph, we can see that the yield of ammonia is highest at 350 ºC and at 400 atm pressures, however this can be dangerous so pressures of 200 atm and a temperature between 450 °C and 500 °C is most commonly used. A catalyst is also used, which in this case is iron, as this does not affect the position of the equilibrium but does affect the rate that the reaction occurs.
30 4 / 2014
Transition metals are defined as elements which have an incomplete d orbital. Generally, they have high melting & boiling points; they are highly conductive, malleable and they form complex ions. Complex ions are central metal ions which are surrounded by molecules or ligands. Ligands have the ability to donate a pair of electrons which forms a co-ordinate bond (co-ordinate bonding is where an atom shares all the electrons in a bond).
The transition metals react the way they do because of their electronic configuration. The similar energy levels for the 3d and 4s sub shell causes the orbitals to fill in an irregular order. The 4s electron shell is filled first, but when ions are formed, they are the first electrons to leave.
Zinc and Scandium are generally not considered transition metals as their d orbital is full.
12 1 / 2014
The factors that effect the rates of reactions are temperature, concentration, surface area and catalysts. To understand how they work to increase the rate of reaction, firstly we need to consider the collision theory. The collision theory states that in order for particles to react with each other they must collide. For this collision to take place two things are needed; enough activation energy and the orientation of the molecule to be correct. Most of the collisions that occur do not meet the criteria however, so will not react. Activation energy is the minimum energy required for particles to react.
Increasing the temperature of the reactants gives the particles more energy, causing them to speed up. The more the particles are moving, the more they are likely to collide.
Increasing the concentration of one or more of the reactants in the same volume means there is a much higher chance of the particle colliding. However, as the reactants are used up the concentration falls so the rate will begin to slow down as the reaction continues.
As the surface area increases there are more particles readily available to react, meaning there are more collisions. Using a powder instead of a granule means that the reaction will occur quicker.
Finally, using a catalyst will lower the activation energy of the reactants so that more of the particles will be able to react.
28 11 / 2013
Have opened the A Level biology revision blog at the link below
16 10 / 2013
- Electron pair repulsion theory assumes that atoms in a molecule will be arranged in a way that the repulsion of the electron pairs surrounding it will be minimised.
- This theory allows us to predict the shapes of molecules.
lone pair - lone pair
lone pair - bond pair
bond pair - bond pair