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## AP®︎/College Chemistry

### Course: AP®︎/College Chemistry>Unit 5

Lesson 1: Reaction rates

# Introduction to reaction rates

AP.Chem:
TRA‑3 (EU)
,
TRA‑3.A (LO)
,
TRA‑3.A.1 (EK)
,
TRA‑3.A.2 (EK)
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. 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. In most cases, concentration is measured in moles per liter and time in seconds, resulting in units of M/s for the reaction rate. Created by Jay.

## Want to join the conversation?

• I didnt understan the part when he says that the rate of the reaction is equal to the rate of O2 (time ). How do we know that?
Thanks.
• The rate of reaction is equal to the, R = rate of formation of any component of the reaction / change in time. Here in this reaction O2 is being formed, so rate of reaction would be the rate by which O2 is formed.
• why we chose O2 in determining the rate and compared the rates of N2O5 and NO2 with it?
• We could have chosen any of the compounds, but we chose O₂ for convenience.
O₂ has the smallest coefficient.
If we had chosen NO₂, the rate of decomposition of N₂O₅ would have been half as much, and the rate of formation of O₂ would have been one–fourth as much.
Choosing O₂ just avoids having to use fractions.
• So just to clarify, rate of reaction of reactant depletion/usage would be equal to the rate of product formation, is that right?
• Yes, when we are dealing with rate to rate conversion across a reaction, we can treat it like stoichiometry.

This means the difference in rate between different reactants/products is proportional to their coefficients.
• Why do we need to ensure that the rate of reaction for the 3 substances are equal?
• It is the formal definition that is used in chemistry so that you can know any one of the rates and calculate the same overall rate of reaction as long as you know the balanced equation.
• Firstly, should we take the rate of reaction only be the rate of disappearance/appearance of the product/reactant with stoichiometric coeff. as 1? In the video, can we take it as the rate of disappearance of *2*N2O5 or that of appearance of *4*N2O?
Secondly, doesn't that change the rate of reaction by factors of their stoichiometric coeff., which seems quite unreasonable? Does that mean that the rate of reaction varies with how _we_ represent the reaction?
• This is the answer I found on chem.libretexts.org:
Consider now a reaction in which the coefficients are different:
A+3B→2D
It is clear that [B] decreases three times as rapidly as [A], so in order to avoid ambiguity when expressing the rate in terms of different components, it is customary to divide each change in concentration by the appropriate coefficient:
rate=−Δ[A]/Δt=−Δ[B]/3Δt=Δ[D]/2Δt
• Why the rate of O2 produce considered as the rate of reaction ? Is the rate of reaction always express from ONE coefficient reactant / product
• Great question! The reaction rate is always defined as the change in the concentration (with an extra minus sign, if we are looking at reactants) divided by the change in time, with an extra term that is 1 divided by the stoichiometric coefficient. The reason why we correct for the coefficients is because we want to be able to calculate the rate from any of the reactants or products, but the actual rate you measure depends on the stoichiometric coefficient. 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.

Since the stoichiometric coefficient for oxygen is 1, the rate of reaction = (change in O2 concentration/change in time) x (1/stoichiometric coefficient for O2 in balanced reaction). Since the stoichiometric coefficient is 1, the rate just looks like rate = change in O2 concentration/change in time. But we could also write the rate in terms of, say, NO2, as follows: rate = (change in NO2 concentration/change in time) x (1/4), where the 4 comes from correcting for the stoichiometric coefficients.

Hope that helps!
• Am I always supposed to make the Rate of the reaction equal to the Rate of Appearance/Disappearance of the Compound with coefficient (1) ?
thanks
• No, in the example given, it just happens to be the case that the rate of reaction given to us is for the compound with mole coefficient 1. However, the method remains the same. Just figuring out the mole ratio between all the compounds is the way to go about questions like these.
• Why can I not just take the absolute value of the rate instead of adding a negative sign?