[BLANK_AUDIO]. Now we can consider another way of measuring the course of a reaction. If we have a process where we have a change in the number of ions, we can look at the electrochemical conductance. So let's consider the process which we looked at before of ethyl acetate plus sodium hydroxide giving ethanol and sodium acetate. In this particular case, the conduct, conductance is decreasing as the hydroxide ions are converted into acetate ions. Let's first, we'll just simplify this whole process, and let's just say that on the left we've got ions of type A and on the right we've got ions of type B. And then we're going to measure the conductance of the solution. So, at time t we're going to measure the total conductance of the solution. So that's given the simple capital G here. We also know that the initial conductance when we only have A ions which we'll call g zero that must be equal to the conductance of a. If we allow the reaction to proceed to an infinite time. So we call that g infinity. Then the conductance must be that of B ions because all the A ions should conducted, converted into B ions. So we can get to relationship between all of these because we can see that the total conductance is going to be made up of the conductance from both the A ions, and the B ions. So at any given time t, that's going to be the conductance of a multiplied by the fraction of A ions at that time. That will be contribution from the A ions plus the contribution from the B ions which are going to be the conductance of B times a fraction of B ions. So we can write this out as an expression. So we've got total conductance, which we know is G, and that's going to be equal to the fraction which is A ions. So that's the concentration of A ions divided by the total concentration of ions, which is A plus B, multiplied by the conductance of A. We know the conductance of a because that's just equal to G0. Plus fraction of B ions, that's going to be the concentration of B divided by the concentration of A plus B, times the conductance of B, and the conductance of B is given by G infinity. So, that's our starting point. [BLANK_AUDIO]. So taking this equation and rearranging it, bringing the A plus B up to the left hand side, the equation. We end up with A plus B times G equals concentration of a times the conductance of A, which is G0, plus the concentration of B times the conductance of B, which is G infinity. But we have some other things which we know. So if A ions are going into B ions then the total concentration of a plus the concentration of B is going to be equal to the initial concentration of A ions. So the total ions we have here is equal to ions we have at the beginning. And also if we rearrange that, we can get concentration of B is equal to concentration of A initial minus the concentration of A. So using these equalities we can plug them back into the original expression, so instead of A plus B, we can put in A0. So I've got concentration of A zero, times con, conductance g equals concentration of a times conductance G zero plus B we can replace with B here. So we'll replace it with this expression here which is concentration A 0minus concentration of a all multiplied by G infinity. So, this expression here can be rearranged to give us what we want. For our rate which is the concentration of a, so we make the concentration of A the subject of this equation. We end up with an expression, g minus g infinity, divided by g initial minus g infinity. All multiplied by the initial concentration A0. So here we have a function which we can now plot to make, to determine whether it's first or second order, zero order. So for example, if it's a first order plot we plot the natural log of this function, G minus G infinity over G0 minus G infinity times the initial concentration of A0, versus the time T. Another concept which is related to order, which we need to consider, is something called molecularity. This is often confused with order when looking at reaction kinetics. The order that we've been discussing is a quantity which we've shown how to determine this. And this always relates to the overall reaction. However, when a reaction proceeds it can be complex in nature. In fact, most reactions are complex. And by complex, we mean that the reaction occurs in a sequence of steps. This is called the reaction mechanism. And the molecularity refers to these individual steps. Rather than the overall reaction. So, you could have a step in the reaction which is unimolecular. And if that was the case, then this, a single molecule, would be reacting all by itself without collision with another molecule. A much more common process would be a bimolecular step, and this would be where a pair of molecules collide. Now if we want to determine the molecularity, then this will necessitate determining the reaction mechanism. The two are closely linked to one another. The reason that these things get confused order and molecularity, is if we just consider a single step of a process. And let's suppose it's a simple biomolecular pro, process. Two molecules coming together, reacting to give a product. Then, if it is a simple bimolecular step, the kinetics of that step are also second order. But the reverse is not true, so if you have something that shows second order kinetics, this does not imply that it is a simple bimolecular process. It might've arrived at the second order kinetics by a sequence of the steps; in other words, the reaction could still be complex. [BLANK_AUDIO]