Wize High School Grade 12 Chemistry Textbook > Rates of Reactions

Rate of Reaction

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 Reaction Rates  
The reaction rate is a measure of the change in concentration of a reactant or product over time!
ex. In physics we know velocity= Δd/t, in chemistry, a reaction rate=  Δ[ ]/t 
The units of rate would be M/s where M=mol/L 
A general form of a reaction rate is shown below:
aA+bB→cC+dDaA+bB \to cC+dDaA+bB→cC+dD


rate=−1ad[A]dt=−1bd[B]dt=1cd[C]dt=1dd[D]dtrate=-\frac{1}{a}\frac{d[A]}{dt}=-\frac{1}{b}\frac{d[B]}{dt}=\frac{1}{c}\frac{d[C]}{dt}=\frac{1}{d}\frac{d[D]}{dt}rate=−a1​dtd[A]​=−b1​dtd[B]​=c1​dtd[C]​=d1​dtd[D]​

As the reaction proceeds are we getting more/less of the reactant? Less
 More/less of the product? More
 
The - sign indicates that the concentration of reactants are decreasing (losing reactants as the reaction proceeds)
The + sign indicates that the concentration of products is increasing (gaining product as the reaction proceeds )

**Rates must be determined through experimentation. 

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Measuring Reaction Rates
Consider the reaction: A→B\small A\rightarrow BA→B
The reaction rate can be measured using any substance in the reaction. Reaction rate units are always M/s (M stands for concentration, mol/L) 
 Rate=−Δ[A]Δt=ΔBΔtRate=-\cfrac{\Delta\left[A\right]}{\Delta t}=\cfrac{\Delta B}{\Delta t}Rate=−ΔtΔ[A]​=ΔtΔB​
Instantaneous rate: the rate of reaction at an instance in time (the slope of the tangent line to a point in a concentration vs time graph)

Initial rate – instantaneous rate of a chemical reaction at t = 0, immediately after the reaction has begun

Average rate: the rate of reaction over a period of time (the slope of the line connecting 2 points in a concentration vs time graph)

Practice: Rates of Reaction
If N2 is consumed at 2 M/s, what is the rate of production of NH3 and rate of consumption of H2 in the reaction?

N2  + 3H2 → 2 NH3N_2\ \ +\ 3H_2\ \rightarrow\ 2\ NH_3N2​  + 3H2​ → 2 NH3​

Rate of production of NH3 (ans in M/s)

Rate of consumption of H2 (ans in M/s)

I don't know

In the following reaction the average rate of formation of NO(g) is 1.12 mol/Ls. If 2 minutes passes, what is the concentration of O2(g) that is consumed?

4NH3(g) + 5O2(g) --> 4NO(g) + 6H2O(g) 