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in this video we're going to talk about how to write the rate law expression and determine the value of the rate constant using the method of initial rates so in this problem we're given a table which shows the concentration of reactants a and B and units of molarity and we also have the initial rate of reaction with the units molarity per minute so how can we determine the rate law expression that fits the experimental data so first we need to find the order of the reaction with reactants a and B so what you want to do is take two trials in which one of the acting is held constant but the other changes so if we pick trials 1 & 2 notice that a is held constant in concentration but the concentration of B doubles so as B doubles from point twenty two point forty notice that the rate increases by a factor of four from point four to one point six so you can write this expression 2 to the what power is equal to four we know that 2 squared is 4 so access to this means that its second order with respect to B so when we write the rate law expression let's say for a we don't know what the order is so I'm just gonna put a why variable but for B we know it's second-order so anytime the concentration doubles and if the rate quadruples you know second later but if you wish to calculate the two if you want to step-by-step process here's what you could do we can use the formula b2 over B 1 raised to the X is equal to r2 of r1 now a is constant so we don't have to worry about aim but for trials 1 and 2 B changes so that's what we have B in this expression so B 2 or the concentration of B for trial two is point four the concentration of beef a trial one is point two and the rate for trial one is 0.4 and the rate for trial 2 is 1 point 6 so 0.4 divided by 0.2 is 2 and 1.6 divided by 0.4 is 4 now if you wish to calculate X it's going to be log 4 divided by log 2 you just got to take the log of those two numbers and this number is always gonna go on the bottom this one's gonna go on the top log 4 divided by log 2 is equal to 2 and so that's how we know it's second order with respect to B so now let's find the order with respect to 8 so notice that if we pick trials 1 & 3 a doubles from point twenty two point forty but B is held constant and notice what happens to the rate the rate doubles from point four two point eight so since a double so we're gonna put a two...