The magic word here is "dynamic equilibrium". Let's look into that. Show Let's say we got a simple reaction like: $$\ce{A <--> B}$$ Now how fast does A react to B? For this case we assume first order in both directions, which means the reaction rate would be: $$r_1= [\ce{A}] \times k_1$$ and of course for the reaction back it's $$r_2=[\ce{B}]\times k_2$$ So in both cases the reaction rate is dependend on the concentration of the reactant, that's very important. Now let's say we start off with only A, in this case $r_1$ will be large and $r_2$ will be 0, since there's no B there. Now A gets less and B is getting more, so $r_1$ is going down and $r_2$ is going up. At some point $r_1$ will be equal to $r_2$, but this means there will be as much of A produced from B, than A will react to B. And the same for B. So over all the reaction is still going in both directions, but it's in a dynamic equilibrium. Here's an example which describes this pretty well: Let's imagine you got a backyard and right on the boarder to the next one is an apple tree and a lot of old apples are laying on the ground. You don't like those in your garden, so you go around, pick them up and throw them into your neighbours garden. He doesn't like that and starts throwing them back. Now he is old and is moving much slower than you are, so what is happening? At first there are a lot of apples on your side and you might need to walk some meters but then you can pick one up and throw them. So you will throw a lot of apples very fast. On the other hand there aren't many apples on the other side and the old man is slow, so he needs much longer to grab one. So he isn't throwing them back to you very rapidely. But here's the problem: the more apples you throw the less apples there are, and you need to move longer distances to grab one. So the rate in which you are throwing apples decreases. On the other side there are more and more apples, so the old man doesn't need to move far to grab one, so his rate in throwing apples increases. After some time there will be a point where for every apple you throw to your neighbour he will throw one back. And if you count the total number of apples on each side it will pretty much stays the same, even if there are apples flying in both directions the whole time. Skip to content
This is part of the HSC Chemistry syllabus under the topic of Equilibrium Systems HSC Chemistry Syllabus
How does collision theory help us better understand equilibrium systems?This video will explore what collision theory is and how it relates to how we understand equilibrium systems
Collision theory is a theory which states chemical reactions are the result of collisions between molecules or atoms. It utilises our understanding of the particle model to explain that the rate of reaction depends on: 1. Rate of collision between molecules. This is the frequency at which molecules collide. Greater rate of collision leads to greater reaction rate. 2. Activation energy of the reaction. For a reaction to occur, the reactants must overcome a certain amount of activation energy. Even if the collision rate is high, if the particles don’t have enough energy reaction will still not occur.
Figure: Molecular orientation is an important factor of reaction rate as stated by collision theory. If the orientation of reactants is not correct (effective) during collision, reaction does not occur. 3. Molecular orientation of reactants. Rate of reaction can increase if reactants collide in the right orientation. Conversely, rate can decrease if the orientation of molecules is not favourable for the formation of product(s). What Affects Collision Rate?
Collision rate increases at a higher concentration
Collision rate increases at a higher pressure for gases
What Affects Activation Energy?
Molecular Energy Distribution
The effect of increased temperature on molecular energy distribution
The effect of catalyst addition on molecular energy distribution
Collision Theory in Dynamic Equilibrium
$$2NO_{2(g)} \rightleftharpoons N_2O_{4(g)}$$
Figure: change in reaction rate with time. At t1, rate of forward and backward reactions becomes equal, and equilibrium is reached.
Figure: change in concentration of reactants and products of a reversible reaction. [`N_2O_4`] decreases while [`NO_2`] increases. A reaction can reach equilibrium starting with any quantities of reactants and products. Practice QuestionUsing collision theory, explain the changes in [N2O4] and [NO2] shown by the graphs above. Sample answer:
Figure: Reaction between dinitrogen tetroxide and nitrogen dioxide can be monitored by its colour change. Nitrogen tetroxide is colourless whereas nitrogen dioxide has a distinctive brown colour.
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