Now when dealing with average rate calculations, just remember that knowing how to write expressions for the rate of reaction will help to determine calculations for various time intervals. Remember when we say rate of reaction, that's not tying it to a product or reactant. So in this case, rate would just equal change in concentration over change in time. It's once we react, we combine it to reactives or products that the not negative and positive signs come up in. The coefficients play a role.
So here we're going to say, calculate the average rate of change in concentration of nitrogen monoxide in the 1st 5 seconds of the reaction if the concentration dropped from 1.3 molar to 1.09 molar. All right. So now we're not talking about the rate of reaction. We're talking specifically about this reactant. So now because it's a reactant, it's disappearing, so it has a negative sign. We're talking about exchanging concentration over its change in time. But remember its coefficient has to be included.
So here times change in time, so now that becomes negative. So change in concentration is final minus initial. So that's 1.09 molar - 1.3 molar / 2 × 5 seconds. Remember to set the 1st 5 seconds. So here when we do that, that comes out to be 0.021 molarity per second. Because remember, the top units are molarity, the bottom is seconds.
Here we're not going to look at the five seconds for our number of sig figs. Better to use the concentrations. 1.3 has two, 1.09 has three. You go with the least number of sig figs. So that's why it's 0.021. Notice here that the answer at the end is a positive value. That's because when we subtract, these two get a negative value. Negative times a negative gives me a positive. So although reactants are decreasing over time, their rate is still a positive value. Rate is always positive.
OK, so here the rate of decrease or rate of disappearance for nitrogen monoxide would be 0.021 molar of this reactant disappearing per second.