This is a popular topic in chemistry. It involves a lot of comprehending, but becomes easy after we guide you through it. Remember, only the chem files can make it easy for you to understand, nothing else. Kinetics deals with how fast reactants are converted into products. Equilibrium involved the extent to which reactants are converted into products.
The speed with which reactants are converted into products is simply defined as the reaction rate (the rate of the reaction, right?). Let’s talk about the following reaction:
Here is how the reaction works:
First: Them molecules speed toward each other.
Second: The collision results in the breaking of bonds of H and I and new H-I bonds form. Just before H-I bonds just start to form, there is a lot of energy given off. This point is called the activated complex or transition state, the point before products are formed.
Finally: H-H and I-I bonds are completely broken and H-I bonds are formed, thus producing hydrogen iodide.
There are many factors that affect the reaction rate:
Concentration: the more concentrated the reactants the form collisions will form, thus increasing the reaction rate.
Surface Area: the more space the reactants cover, the greater the collision, thus the greater the reaction rate.
Temperature: if temperature increases then molecules will be flying around faster and thus have a higher chance of hitting each other, so temp. increases the reaction rate.
Catalysts change the rate of a chemical reaction without being consumed by the reaction itself. There are some that increase the rate of reaction and some that slow it down.
Now let’s look at a potential energy diagram.
As you can see the reactants start at a bit of potential energy, then work their way up just before they become products. One they are products the potential energy is lower than that of the reactants.
Notice that activation energy, symbolized with Ea, is the difference between the energies of activated complex and the reactants.
EQUILIBRIUM
When a reaction is in equilibrium, the forward and reverse reaction rates are equal. That means the concentrations are constant: they don’t change. Now you have to note that equilibrium does not mean that the concentrations of both reactants and products are equal. Absolutely NOT! Now what do we mean when we say the reaction proceeds in the reverse direction? We mean it proceeds in the reverse direction, duh! In other words, the products are now forming back into reactants. Up above with the hydrogen-iodide reaction you can see that is goes in the forward direction to synthesize into Hydrogen Iodide. In the reverse direction the hydrogen iodide decomposes into hydrogen and iodine.
Something closely associated with equilibrium is a law called Le Chatelier’s Principle. Let’s look at this equation:
So far this is a reversible reaction so A and B can make C and D, or C and D can make A and B.
If we add more A to the reaction system, we are going to get more C and D at equilibruim.
If we add more B, then we still are going to get more C and D.
So adding more of A or B will increase the production of C and D; this is called driving the equilibrium to the right. (it’s moving in that direction)
Adding more C or D will obviously increase A and B. Here we are driving the equilibrium to the left.
Another way of looking at this is in term of crowding. When we add more A to the system, things are crowded on the left side, so the system relieves itself of the problem by shifting toward the right. Neat analogy, eh? Similarly, if we add more C or D, then things get crowded on the right side, and so it moves toward the left.
What if we increase the concentration of one species on the left side, what will happen to the other species? Its concentration will decrease. (Sociologists call this "brain drain", the INS(Immigration and Naturalization Service) calls it "illegal entry" but what do we care? We're the Chem FIle experts!) Let’s say we add more A and we know that C and D will increase. But B will go down. The reason is that we did not add any B to the reaction system. Thus the increased collisions between A and B particles and the increased production of C and D will tend to reduce the concentration of B at equilibrium. Here is another thought: adding more A crowds things on the left. In order to relieve themselves of the crowding some A particles and B particles form more C and D. So there is less of B now, than A.
What if we take away some A, B, C, or D? Using the crowding principle, the equilibrium will shift to the side where something has been taken away. So in the above equation, if A is taken away, the equilibrium will shift to the left to compensate for the loss of A.
What happens when we increase or decrease the temperature? When the temperature is increased, the reaction will proceed to the endothermic side. (the side where heat is not given off but absorbed) Conversely, if we decrease the temperature the reaction will proceed in the exothermic direction.
Note: the reaction will tell you which side is endo or exo. They usually have at the end of the reaction + heat to indicate that that side is exothermic while the other is endothermic.
What happens if we increase volume or decrease it? When the volume is increased, the reaction will go in the direction that produced more moles of gas. The opposite happens when the volume is decreased: goes to the side with fewer moles of gas. Let’s look at this equation:
This reaction is known as the Haber process: synthesizing ammonia from nitrogen and hydrogen. Now if the volume is increased, the equilibrium will move to the left side since it has more moles. 1 + 3 = 4, rather than 2 on the right. Note that if there is no gas involved in the reaction or there is equal number of moles on both side, then volume has no effect.
What happens if we increase pressure? Same as volume. Remember volume is inversely proportional to pressure. So if pressure increase, volume decreases, so it will move to the fewer moles of gas side.
Before we end the equilibrium chapter, one more thing you need to know: the law of mass action as it applies to equilibrium. The law of mass action is basically a formula that is represented like this: If we have a reaction like:
A stands for the mole coefficient and a stands for the element. Now the law of mass action is simply this: You take the the concentrations of the products side raised to the powers of their coefficients divided by the same thing for the reactants. Wow what a sentence. Let’s see this in mathematical terms:
Keq(equil expression) = | ---------------
|
[ ] = concentration
That’s it! This formula is useful in telling which side the equilibrium is leaning towards. If the answer is greater than one, the forward reaction is favored because the concentrations of the products exceeds those of the reactants. If it is less than one, the reverse reaction is favored; the reactants’ concentrations exceed those of the products’.
Don’t worry too much about this formula. The main thing is understanding the concepts more than the math.