how to calculate rate of disappearance

If needed, review section 1B.5.3on graphing straight line functions and do the following exercise. So we get a positive value Consider that bromoethane reacts with sodium hydroxide solution as follows: \[ CH_3CH_2Br + OH^- \rightarrow CH_3CH_2OH + Br^-\]. For every one mole of oxygen that forms we're losing two moles Again, the time it takes for the same volume of gas to evolve is measured, and the initial stage of the reaction is studied. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Include units) rate= -CHO] - [HO e ] a 1000 min-Omin tooo - to (b) Average Rate of appearance of . Rate of disappearance is given as [ A] t where A is a reactant. So, we said that that was disappearing at -1.8 x 10 to the -5. The ratio is 1:3 and so since H2 is a reactant, it gets used up so I write a negative. Consider a simple example of an initial rate experiment in which a gas is produced. \[\frac{d[A]}{dt}=\lim_{\Delta t\rightarrow 0}\frac{\Delta [A]}{\Delta t}\], Calculus is not a prerequisite for this class and we can obtain the rate from the graph by drawing a straight line that only touches the curve at one point, the tangent to the curve, as shown by the dashed curves in figure $\PageIndex{1}$. The result is the outside Decide math Math is all about finding the right answer, and sometimes that means deciding which equation to use. If it is added to the flask using a spatula before replacing the bung, some gas might leak out before the bung is replaced. Worked example: Determining a rate law using initial rates data Molar per second sounds a lot like meters per second, and that, if you remember your physics is our unit for velocity. 0:00 / 18:38 Rates of Appearance, Rates of Disappearance and Overall Reaction Rates Franklin Romero 400 subscribers 67K views 5 years ago AP Chemistry, Chapter 14, Kinetics AP Chemistry,. To experimentally determine the initial rate, an experimenter must bring the reagents together and measure the reaction rate as quickly as possible. A negative sign is used with rates of change of reactants and a positive sign with those of products, ensuring that the reaction rate is always a positive quantity. Direct link to Oshien's post So just to clarify, rate , Posted a month ago. In each case the relative concentration could be recorded. [ A] will be negative, as [ A] will be lower at a later time, since it is being used up in the reaction. These approaches must be considered separately. Application, Who Direct link to griffifthdidnothingwrong's post No, in the example given,, Posted 4 years ago. Conservation - Calculating background extinction rates This will be the rate of appearance of C and this is will be the rate of appearance of D.If you use your mole ratios, you can actually figure them out. Well notice how this is a product, so this we'll just automatically put a positive here. It is clear from the above equation that for mass to be conserved, every time two ammonia are consumed, one nitrogen and three hydrogen are produced. Rates of reaction are measured by either following the appearance of a product or the disappearance of a reactant. The red curve represents the tangent at 10 seconds and the dark green curve represents it at 40 seconds. Medium Solution Verified by Toppr The given reaction is :- 4NH 3(g)+SO 2(g)4NO(g)+6H 2O(g) Rate of reaction = dtd[NH 3] 41= 41 dtd[NO] dtd[NH 3]= dtd[NO] Rate of formation of NO= Rate of disappearance of NH 3 =3.610 3molL 1s 1 Solve any question of Equilibrium with:- Patterns of problems I do the same thing for NH3. There are several reactions bearing the name "iodine clock." If you take a look here, it would have been easy to use the N2 and the NH3 because the ratio would be 1:2 from N2 to NH3. The practical side of this experiment is straightforward, but the calculation is not. Accessibility StatementFor more information contact us atinfo@libretexts.orgor check out our status page at https://status.libretexts.org. initial concentration of A of 1.00 M, and A hasn't turned into B yet. Now this would give us -0.02. To get reasonable times, a diluted version of the sodium thiosulphate solution must be used. Chemical kinetics generally focuses on one particular instantaneous rate, which is the initial reaction rate, t . This time, measure the oxygen given off using a gas syringe, recording the volume of oxygen collected at regular intervals. We have emphasized the importance of taking the sign of the reaction into account to get a positive reaction rate. The average rate of reaction, as the name suggests, is an average rate, obtained by taking the change in concentration over a time period, for example: -0.3 M / 15 minutes. We Solved Please help for Part C. How do I calculate the | Chegg.com And please, don't assume I'm just picking up a random question from a book and asking it for fun without actually trying to do it. \[ R_{B, t=10}= \;\frac{0.5-0.1}{24-0}=20mMs^{-1} \\ \; \\R_{B, t=40}= \;\frac{0.5-0.4}{50-0}=2mMs^{-1} \nonumber\]. How do you calculate the rate of a reaction from a graph? I couldn't figure out this problem because I couldn't find the range in Time and Molarity. 1 - The Iodine Clock Reaction - Chemistry LibreTexts \[ Na_2S_2O_{2(aq)} + 2HCl_{(aq)} \rightarrow 2NaCl_{(aq)} + H_2O_{(l)} + S_{(s)} + SO_{2(g)}\]. In the example of the reaction between bromoethane and sodium hydroxide solution, the order is calculated to be 2. The react, Posted 7 years ago. Then, log(rate) is plotted against log(concentration). We could do the same thing for A, right, so we could, instead of defining our rate of reaction as the appearance of B, we could define our rate of reaction as the disappearance of A. Direct link to Sarthak's post Firstly, should we take t, Posted 6 years ago. The reaction below is the oxidation of iodide ions by hydrogen peroxide under acidic conditions: \[ H_2O_{2(aq)} + 2I_{(aq)}^- + 2H^+ \rightarrow I_{2(aq)} + 2H_2O_{(l)}\]. The solution with 40 cm3 of sodium thiosulphate solution plus 10 cm3 of water has a concentration which is 80% of the original, for example. How to calculate instantaneous rate of disappearance For example, the graph below shows the volume of carbon dioxide released over time in a chemical reaction. 12.1 Chemical Reaction Rates - Chemistry 2e | OpenStax I find it difficult to solve these questions. During the course of the reaction, both bromoethane and sodium hydroxide are consumed. Calculate, the rate of disappearance of H 2, rate of formation of NH 3 and rate of the overall reaction. 24/7 Live Specialist You can always count on us for help, 24 hours a day, 7 days a week. Are, Learn How to relate rates of disappearance of reactants and appearance of products to one another. of B after two seconds. Everything else is exactly as before. Firstly, should we take the rate of reaction only be the rate of disappearance/appearance of the product/reactant with stoichiometric coeff. We could say it's equal to 9.0 x 10 to the -6 molar per second, so we could write that down here. Then basically this will be the rate of disappearance. We put in our negative sign to give us a positive value for the rate. the balanced equation, for every one mole of oxygen that forms four moles of nitrogen dioxide form. So, we divide the rate of each component by its coefficient in the chemical equation. Rates of Appearance, Rates of Disappearance and Overall - YouTube So the final concentration is 0.02. So, dinitrogen pentoxide disappears at twice the rate that oxygen appears. The concentrations of bromoethane are, of course, the same as those obtained if the same concentrations of each reagent were used. What is disappearance rate? - KnowledgeBurrow.com There are actually 5 different Rate expressions for the above equation, The relative rate, and the rate of reaction with respect to each chemical species, A, B, C & D. If you can measure any of the species (A,B,C or D) you can use the above equality to calculate the rate of the other species. For example if A, B, and C are colorless and D is colored, the rate of appearance of . in the concentration of A over the change in time, but we need to make sure to Transcript The rate of a chemical reaction is defined as the rate of change in concentration of a reactant or product divided by its coefficient from the balanced equation. Because salicylic acid is the actual substance that relieves pain and reduces fever and inflammation, a great deal of research has focused on understanding this reaction and the factors that affect its rate. There are two important things to note here: What is the rate of ammonia production for the Haber process (Equation \ref{Haber}) if the rate of hydrogen consumption is -0.458M/min? Since a reaction rate is based on change over time, it must be determined from tabulated values or found experimentally. Direct link to _Q's post Yeah, I wondered that too. How do you calculate the rate of appearance and disappearance If you take the value at 500 seconds in figure 14.1.2 and divide by the stoichiometric coefficient of each species, they all equal the same value. The table of concentrations and times is processed as described above. Since 2 is greater, then you just double it so that's how you get 20 Molars per second from the 10.You can use the equation up above and it will still work and you'll get the same answers, where you'll be solving for this part, for the concentration A. If starch solution is added to the reaction above, as soon as the first trace of iodine is formed, the solution turns blue. The rate of a chemical reaction is defined as the rate of change in concentration of a reactant or product divided by its coefficient from the balanced equation. The process is repeated using a smaller volume of sodium thiosulphate, but topped up to the same original volume with water. Problem 1: In the reaction N 2 + 3H 2 2NH 3, it is found that the rate of disappearance of N 2 is 0.03 mol l -1 s -1. The general rate law is usually expressed as: Rate = k[A]s[B]t. As you can see from Equation 2.5.5 above, the reaction rate is dependent on the concentration of the reactants as well as the rate constant. Note that the overall rate of reaction is therefore +"0.30 M/s". For 2A + B -> 3C, knowing that the rate of disappearance of B is "0.30 mol/L"cdot"s", i.e. ( A girl said this after she killed a demon and saved MC), Partner is not responding when their writing is needed in European project application. Direct link to putu.wicaksana.adi.nugraha's post Why the rate of O2 produc, Posted 6 years ago. However, using this formula, the rate of disappearance cannot be negative. The quantity 1/t can again be plotted as a measure of the rate, and the volume of sodium thiosulphate solution as a measure of concentration. U.C.BerkeleyM.Ed.,San Francisco State Univ. Human life spans provide a useful analogy to the foregoing. - The rate of a chemical reaction is defined as the change We could say that our rate is equal to, this would be the change So the initial rate is the average rate during the very early stage of the reaction and is almost exactly the same as the instantaneous rate at t = 0. How to set up an equation to solve a rate law computationally? So if we're starting with the rate of formation of oxygen, because our mole ratio is one to two here, we need to multiply this by 2, and since we're losing Why can I not just take the absolute value of the rate instead of adding a negative sign? In most cases, concentration is measured in moles per liter and time in seconds, resulting in units of, I didnt understan the part when he says that the rate of the reaction is equal to the rate of O2 (time. the rate of our reaction. How to calculate instantaneous rate of disappearance concentration of A is 1.00. If someone could help me with the solution, it would be great. Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. So 0.98 - 1.00, and this is all over the final So that would give me, right, that gives me 9.0 x 10 to the -6. So what is the rate of formation of nitrogen dioxide? So for, I could express my rate, if I want to express my rate in terms of the disappearance moles per liter, or molar, and time is in seconds. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Find the instantaneous rate of On that basis, if one followed the fates of 1 million species, one would expect to observe about 0.1-1 extinction per yearin other words, 1 species going extinct every 1-10 years. All rates are positive. When the reaction has the formula: \[ C_{R1}R_1 + \dots + C_{Rn}R_n \rightarrow C_{P1}P_1 + \dots + C_{Pn}P_n \]. However, when that small amount of sodium thiosulphate is consumed, nothing inhibits further iodine produced from reacting with the starch. $r_i$ is the rate for reaction $i$, which in turn will be calculated as a product of concentrations for all reagents $j$ times the kinetic coefficient $k_i$: $$r_i = k_i \prod\limits_{j} [j]^{\nu_{j,i}}$$. The time required for the event to occur is then measured. H2 goes on the bottom, because I want to cancel out those H2's and NH3 goes on the top. 5. However, there are also other factors that can influence the rate of reaction. I'll use my moles ratio, so I have my three here and 1 here. - The equation is Rate= - Change of [C4H9cl]/change of . So, 0.02 - 0.0, that's all over the change in time. of a chemical reaction in molar per second. \[\begin{align} -\dfrac{1}{3}\dfrac{\Delta [H_{2}]}{\Delta t} &= \dfrac{1}{2}\dfrac{\Delta [NH_{3}]}{\Delta t} \nonumber \\ \nonumber\\ \dfrac{\Delta [NH_{3}]}{\Delta t} &= -\dfrac{2}{3}\dfrac{\Delta [H_{2}]}{\Delta t} \nonumber\\ \nonumber \\ &= -\dfrac{2}{3}\left ( -0.458 \frac{M}{min}\right ) \nonumber \\ \nonumber \\ &=0.305 \frac{mol}{L\cdot min} \nonumber \end{align} \nonumber \]. In addition to calculating the rate from the curve we can also calculate the average rate over time from the actual data, and the shorter the time the closer the average rate is to the actual rate. Accessibility StatementFor more information contact us atinfo@libretexts.orgor check out our status page at https://status.libretexts.org. In your example, we have two elementary reactions: $$\ce {2NO -> [$k_1$] N2O4} \tag {1}$$ $$\ce {N2O4 -> [$k_2$] 2NO} \tag {2}$$ So, the rate of appearance of $\ce {N2O4}$ would be This consumes all the sodium hydroxide in the mixture, stopping the reaction. All rates are converted to log(rate), and all the concentrations to log(concentration). Well, if you look at Aspirin (acetylsalicylic acid) reacts with water (such as water in body fluids) to give salicylic acid and acetic acid. This means that the concentration of hydrogen peroxide remaining in the solution must be determined for each volume of oxygen recorded. 2023 Brightstorm, Inc. All Rights Reserved. Why is the rate of disappearance negative? - Chemistry Stack Exchange To get this unique rate, choose any one rate and divide it by the stoichiometric coefficient. From this we can calculate the rate of reaction for A and B at 20 seconds, \[R_{A, t=20}= -\frac{\Delta [A]}{\Delta t} = -\frac{0.0M-0.3M}{32s-0s} \; =\; 0.009 \; Ms^{-1} \; \;or \; \; 9 \; mMs^{-1} \\ \; \\ and \\ \; \\ R_{B, t=20}= \;\frac{\Delta [B]}{\Delta t} \; = \; \; \frac{0.5M-0.2}{32s-0s} \;= \; 0.009\;Ms^{-1}\; \; or \; \; 9 \; mMs^{-1}\]. We will try to establish a mathematical relationship between the above parameters and the rate. dinitrogen pentoxide, we put a negative sign here. Direct link to Omar Yassin's post Am I always supposed to m, Posted 6 years ago. Each produces iodine as one of the products. So, we wait two seconds, and then we measure A reasonably wide range of concentrations must be measured.This process could be repeated by altering a different property. So since the overall reaction rate is 10 molars per second, that would be equal to the same thing as whatever's being produced with 1 mole or used up at 1 mole.N2 is being used up at 1 mole, because it has a coefficient. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Robert E. Belford (University of Arkansas Little Rock; Department of Chemistry). So, NO2 forms at four times the rate of O2. Introduction to reaction rates (video) - Khan Academy Well, this number, right, in terms of magnitude was twice this number so I need to multiply it by one half. All right, finally, let's think about, let's think about dinitrogen pentoxide. The region and polygon don't match. The actual concentration of the sodium thiosulphate does not need to be known. The instantaneous rate of reaction, on the other hand, depicts a more accurate value. So I could've written 1 over 1, just to show you the pattern of how to express your rate. Expert Answer. rate of reaction = 1 a [A] t = 1 b [B] t = 1 c [C] t = 1 d [D] t EXAMPLE Consider the reaction A B Why is 1 T used as a measure of rate? $ rate_{\left ( t=300-200\;h \right )}=\dfrac{\left [ salicylic\;acid \right ]_{300}-\left [ salicylic\;acid \right ]_{200}}{300\;h-200\;h} $, $ =\dfrac{3.73\times 10^{-3}\;M-2.91\times 10^{-3}\;M}{100 \;h}=8.2\times 10^{-6}\;Mh^{-1}= 8\mu Mh^{-1} $. So we just need to multiply the rate of formation of oxygen by four, and so that gives us, that gives us 3.6 x 10 to the -5 Molar per second. Rates Of Formation And Disappearance - Unacademy The reaction can be slowed by diluting it, adding the sample to a larger volume of cold water before the titration. Using Figure 14.4(the graph), determine the instantaneous rate of disappearance of . (e) A is a reactant that is being used up therefore its rate of formation is negative (f) -r B is the rate of disappearance of B Summary. With the obtained data, it is possible to calculate the reaction rate either algebraically or graphically. and calculate the rate constant. Alternatively, relative concentrations could be plotted. Example $\PageIndex{2}$: The catalytic decomposition of hydrogen peroxide. SAMPLE EXERCISE 14.2 Calculating an Instantaneous Rate of Reaction. Find the instantaneous rate of Solve Now. C4H9cl at T = 300s. Stack Exchange network consists of 181 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. Where does this (supposedly) Gibson quote come from? Let's say we wait two seconds. the concentration of A. Reagent concentration decreases as the reaction proceeds, giving a negative number for the change in concentration. So, the Rate is equal to the change in the concentration of our product, that's final concentration Instead, we will estimate the values when the line intersects the axes. So the rate of reaction, the average rate of reaction, would be equal to 0.02 divided by 2, which is 0.01 molar per second. As a reaction proceeds in the forward direction products are produced as reactants are consumed, and the rate is how fast this occurs. Is the rate of disappearance the derivative of the concentration of the reactant divided by its coefficient in the reaction, or is it simply the derivative? How to calculate instantaneous rate of disappearance Joshua Halpern, Scott Sinex, Scott Johnson. If a chemical species is in the gas phase and at constant temperature it's concentration can be expressed in terms of its partial pressure. Instantaneous rate can be obtained from the experimental data by first graphing the concentration of a system as function of time, and then finding the slope of the tangent line at a specific point which corresponds to a time of interest. If humans live for about 80 years on average, then one would expect, all things being equal, that 1 . Direct link to Amit Das's post Why can I not just take t, Posted 7 years ago. What is the rate of reaction for the reactant "A" in figure $\PageIndex{1}$at 30 seconds?. In the second graph, an enlarged image of the very beginning of the first curve, the curve is approximately straight. the average rate of reaction using the disappearance of A and the formation of B, and we could make this a Now to calculate the rate of disappearance of ammonia let us first write a rate equation for the given reaction as below, Rate of reaction, d [ N H 3] d t 1 4 = 1 4 d [ N O] d t Now by canceling the common value 1 4 on both sides we get the above equation as, d [ N H 3] d t = d [ N O] d t In your example, we have two elementary reactions: So, the rate of appearance of $\ce{N2O4}$ would be, $$\cfrac{\mathrm{d}\ce{[N2O4]}}{\mathrm{d}t} = r_1 - r_2 $$, Similarly, the rate of appearance of $\ce{NO}$ would be, $$\cfrac{\mathrm{d}\ce{[NO]}}{\mathrm{d}t} = - 2 r_1 + 2 r_2$$. )%2F14%253A_Chemical_Kinetics%2F14.02%253A_Measuring_Reaction_Rates, $ \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}$ $ \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} $$\newcommand{\id}{\mathrm{id}}$ $ \newcommand{\Span}{\mathrm{span}}$ $ \newcommand{\kernel}{\mathrm{null}\,}$ $ \newcommand{\range}{\mathrm{range}\,}$ $ \newcommand{\RealPart}{\mathrm{Re}}$ $ \newcommand{\ImaginaryPart}{\mathrm{Im}}$ $ \newcommand{\Argument}{\mathrm{Arg}}$ $ \newcommand{\norm}[1]{\| #1 \|}$ $ \newcommand{\inner}[2]{\langle #1, #2 \rangle}$ $ \newcommand{\Span}{\mathrm{span}}$ $\newcommand{\id}{\mathrm{id}}$ $ \newcommand{\Span}{\mathrm{span}}$ $ \newcommand{\kernel}{\mathrm{null}\,}$ $ \newcommand{\range}{\mathrm{range}\,}$ $ \newcommand{\RealPart}{\mathrm{Re}}$ $ \newcommand{\ImaginaryPart}{\mathrm{Im}}$ $ \newcommand{\Argument}{\mathrm{Arg}}$ $ \newcommand{\norm}[1]{\| #1 \|}$ $ \newcommand{\inner}[2]{\langle #1, #2 \rangle}$ $ \newcommand{\Span}{\mathrm{span}}$$\newcommand{\AA}{\unicode[.8,0]{x212B}}$, By monitoring the depletion of reactant over time, or, 14.3: Effect of Concentration on Reaction Rates: The Rate Law, status page at https://status.libretexts.org, By monitoring the formation of product over time. Why do many companies reject expired SSL certificates as bugs in bug bounties? So the formation of Ammonia gas. Here we have an equation where the lower case letters represent the coefficients, and then the capital letters represent either an element, or a compound.So if you take a look, on the left side we have A and B they are reactants. So, now we get 0.02 divided by 2, which of course is 0.01 molar per second. To unlock all 5,300 videos, Then a small known volume of dilute hydrochloric acid is added, a timer is started, the flask is swirled to mix the reagents, and the flask is placed on the paper with the cross. How do you calculate the average rate of a reaction? | Socratic we wanted to express this in terms of the formation For a reaction such as aA products, the rate law generally has the form rate = k[A], where k is a proportionality constant called the rate constant and n is the order of the reaction with respect to A. So, N2O5. To learn more, see our tips on writing great answers. If a very small amount of sodium thiosulphate solution is added to the reaction mixture (including the starch solution), it reacts with the iodine that is initially produced, so the iodine does not affect the starch, and there is no blue color. Data for the hydrolysis of a sample of aspirin are given belowand are shown in the adjacent graph. Direct link to Ernest Zinck's post We could have chosen any , Posted 8 years ago. In this case, this can be accomplished by adding the sample to a known, excess volume of standard hydrochloric acid. Determining Order of a Reaction Using a Graph, Factors Affecting Collision Based Reaction Rates, Tips for Figuring Out What a Rate Law Means, Tips on Differentiating Between a Catalyst and an Intermediate, Rates of Disappearance and Appearance - Concept. So, the 4 goes in here, and for oxygen, for oxygen over here, let's use green, we had a 1. The problem is that the volume of the product is measured, whereas the concentration of the reactants is used to find the reaction order. Now I can use my Ng because I have those ratios here. Using the full strength, hot solution produces enough precipitate to hide the cross almost instantly. Then basically this will be the rate of disappearance. The process starts with known concentrations of sodium hydroxide and bromoethane, and it is often convenient for them to be equal. Suppose the experiment is repeated with a different (lower) concentration of the reagent. On the other hand we could follow the product concentration on the product curve (green) that started at zero, reached a little less than 0.4M after 20 seconds and by 60 seconds the final concentration of 0.5 M was attained.thethere was no [B], but after were originally 50 purple particles in the container, which were completely consumed after 60 seconds. We can normalize the above rates by dividing each species by its coefficient, which comes up with a relative rate of reaction, \[\underbrace{R_{relative}=-\dfrac{1}{a}\dfrac{\Delta [A]}{\Delta t} = - \dfrac{1}{b}\dfrac{\Delta [B]}{\Delta t} = \dfrac{1}{c}\dfrac{\Delta [C]}{\Delta t} = \dfrac{1}{d}\dfrac{\Delta [D]}{\Delta t}}_{\text{Relative Rate of Reaction}}\]. If the reaction had been $A\rightarrow 2B$ then the green curve would have risen at twice the rate of the purple curve and the final concentration of the green curve would have been 1.0M, The rate is technically the instantaneous change in concentration over the change in time when the change in time approaches is technically known as the derivative. Let's calculate the average rate for the production of salicylic acid between the initial measurement (t=0) and the second measurement (t=2 hr). I need to get rid of the negative sign because rates of reaction are defined as a positive quantity. If you're seeing this message, it means we're having trouble loading external resources on our website. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. So I need a negative here. Get Better The two are easily mixed by tipping the flask. Chemical Kinetics - Notes on Rate Of Reaction, Formulas, Questions, - BYJUS This is the answer I found on chem.libretexts.org: Why the rate of O2 produce considered as the rate of reaction ? For example, in this reaction every two moles of the starting material forms four moles of NO2, so the measured rate for making NO2 will always be twice as big as the rate of disappearance of the starting material if we don't also account for the stoichiometric coefficients. Problem 14.6 - Relating rates of disappearance and appearance Is the rate of reaction always express from ONE coefficient reactant / product. There are two different ways this can be accomplished. The iodine is formed first as a pale yellow solution, darkening to orange and then dark red before dark gray solid iodine is precipitated. concentration of our product, over the change in time. [A] will be negative, as [A] will be lower at a later time, since it is being used up in the reaction. I'll show you a short cut now. Just figuring out the mole ratio between all the compounds is the way to go about questions like these. So this is our concentration Let's calculate the average rate for the production of salicylic acid between the initial measurement (t=0) and the second measurement (t=2 hr). Here, we have the balanced equation for the decomposition The change of concentration in a system can generally be acquired in two ways: It does not matter whether an experimenter monitors the reagents or products because there is no effect on the overall reaction.