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October 01, 2025, 10:21:41 pm

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nerd

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Rates Question
« on: June 17, 2009, 04:30:19 pm »
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Consider the reaction between zinc granules (lumps) and hydrochloric acid:
Zn(s)  +  2HCl(aq)    ->    ZnCl2(aq)  +  H2(g)
By altering the conditions under which the reaction is carried out, we can change the rate at which hydrogen has is produced. Which of the following changes will NOT increase the rate of the production of hydrogen gas?
A   Increasing the concentration of the acid
B   Increasing the mass of zinc
C   Increasing the temperature
D   Using powdered zinc instead of zinc granules

Now, the answers say that the answer is B because "changing the total amount of either reactant will not change the rate of the reaction, although it will of course affect the amount of products produced. However, as zinc is a solid reactant, increasing its total surface area will increase its rate of reaction, since the reaction can only occur at its surface."

But, if there is more mass (ie. more reactant), doesn't that mean that there are more particles present per amount of HCl, thus more collisions between particles, hence more successful collisions and a faster rate of reaction?

I'm confused! I got the answer B only because of process of elimination (the others are clearly wrong), but after looking over it again, it doesn't seem to make sense!
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TrueTears

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Re: Rates Question
« Reply #1 on: June 17, 2009, 04:36:21 pm »
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Increasing the mass of Zn does not necessarily mean it will increase the rate of reaction.

The n(Zn) : n(HCl) = 1:2

Once you put more mass of Zn then the required it will just be left in excess, hence won't participate in the reaction.
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nerd

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Re: Rates Question
« Reply #2 on: June 17, 2009, 04:44:05 pm »
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But the question does not specify, so assuming that there is an excess of HCl available would the rate of reaction not increase?
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ilovevce

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Re: Rates Question
« Reply #3 on: June 17, 2009, 04:51:41 pm »
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Whether or not there is an excess of one reactant does not matter in this question.

The rate of reaction depends upon the rate of successful collisions between particles.

- Increasing the concentration of the acid will means more particles per volume and so more collisions per unit time.
- Increasing the temperature will increase the speed at which the particles move and hence more collisions per unit time. The energy of the particles is also greater so more collisions will be successful.
- Using powdered zinc instead of zinc granules increases the surface area of the zinc, so there will be more collisions per unit time.
- Increasing the mass of zinc does not necesarily increase the surface area. If the surface area stays the same when the mass is increased, the rate of reactions remains the same.
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TrueTears

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Re: Rates Question
« Reply #4 on: June 17, 2009, 04:52:23 pm »
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By putting more Zinc will increase the amount of produced, not necessarily the rate of reaction. It will cause more collisions but the amount of successful collisions should be the same as before, the only change is that more products are produced.
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nerd

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Re: Rates Question
« Reply #5 on: June 17, 2009, 05:10:49 pm »
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That doesn't make sense, does it? If there are more collisions, isn't the probability of a successful collision increased?
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TrueTears

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Re: Rates Question
« Reply #6 on: June 17, 2009, 05:13:37 pm »
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Yes there are more collisions but that doesn't mean they are more successful. You haven't done anything to make them "more" successful.
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nerd

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Re: Rates Question
« Reply #7 on: June 17, 2009, 05:21:12 pm »
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So how come an increase surface area increases the rate of reaction? Its the same idea; increased area for particles to collide more collisions between reactant particles more successful collisions faster rate of reaction. Or did I get this all wrong?
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ilovevce

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Re: Rates Question
« Reply #8 on: June 17, 2009, 05:33:01 pm »
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So how come an increase surface area increases the rate of reaction? Its the same idea; increased area for particles to collide more collisions between reactant particles more successful collisions faster rate of reaction. Or did I get this all wrong?

Yes, that's right. Increasing surface area means that the reactants are more exposed to each other and will react faster. The probability of a collision being successful does not increase, but because there are more collisions, the number of successful collisions increases. This means that the rate of reaction is greater.

If you increase the mass of a reactant but not the surface area, you are not increasing the extent to which the reactants are exposed to each other. Therefore, the rate of reaction will be the same, but the reaction will take longer to reach completion since there are more particles that need to react.
« Last Edit: June 17, 2009, 06:25:45 pm by ilovevce »
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Re: Rates Question
« Reply #9 on: June 17, 2009, 06:00:10 pm »
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Yes there are more collisions but that doesn't mean they are more successful. You haven't done anything to make them "more" successful.

generally more collisions --> more successful collisions

What you mean is there are more reactants, but since surface area is the same, the amount of collision over a unit time does not change, but the total number of collisions increase, but this takes place over a much longer time and initial reaction rate does not change.

When dealing with collisions and rate of reactions, we are not dealing with the number, but rather, the rate of collisions per a unit of time.
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nerd

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Re: Rates Question
« Reply #10 on: June 17, 2009, 06:24:14 pm »
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OK - thanks everyone. I get it now. I was thinking about the question too much!

The increase in mass itself does not effect the rate of reaction - it is the increase in surface area that comes about due to the increase in mass that causes the rate to increase. However, an increase in mass does NOT NECESSARILY mean an increase in surface area. You can keep the surface area constant and still have a greater mass. Thus, an increase in mass does not ALWAYS mean that the rate of reaction will increase; it will only increase if the surface area does too.

How does that sound?
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ilovevce

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Re: Rates Question
« Reply #11 on: June 17, 2009, 06:27:01 pm »
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Yes, you've got it now. =)
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