Speed of Reaction Application Question

[nicholaschean]

Homework Statement

The apparatus below shows an experiment to investigate the speed of reaction between lithium and water. A piece of lithium is wrapped in steel gauze and placed in a beaker of ]water. The volume of gas collected in the measuring cylinder is measured at 10s time interval.

Suggest two possible reasons why the shape of the graph from Student Y's results differ from that of the graph of Student X's results.(assume that both students recorded their results correctly)

Homework Equations

The Attempt at a Solution

Student Y could have added more lithium pieces as the reaction proceeds for the first 37s as the gradient is increasing which signifies speed of reaction increasing and concentration of reactants increasing. He could also have used water of lower temperature as gradient of the graph is still decreasing at 50s which means reaction has not stopped.

I am quite confident with my first reason but not so with my 2nd where it suggest water of lower temperature. Please do enlighten me. Thank you!

 

[Borek]

It is mostly about surface in contact with water. Temperature plays some role, and mentioning it won't hurt, but somehow I feel like like speed with which water gets to the metal surface through the fizzling bubbles is a limiting factor.

One important thing you have not mentioned is oil on the lithium surface - if it was not removed, it will slow down the reaction, especially at the beginning. Lithium is stored in oil and it has to be dried out before it can be used.

 

[nicholaschean]

well ok let's assume that everything is done properly for them including the drying of oil before these students carry out the experiment. I went to check bacck my notes, a high/low temperature will only slow/speed the reaction but concentration of reactants will still decrease, hence the graph should no matter what be still like X. But Y's graph seems to be increasing in speed for the first 40s which means increasing amount of reactants. So, the first reason I stated where he added in more could be right. But i guess the 2nd reason has to be another.

 

[nicholaschean]

hmm i asked a senior and he suggested the 2nd reason to be particle size. As Student Y could be using granulated zinc whereby the reaction speed increases as it goes cos need break down the granulated into powdered then once its powdered it becomes fast shortly due to larger surface area and then it decreases after 40s due to usage of reactants?

 

[Borek]

Why zinc if lithium? And surface size was the first thing I suggested. Note that small chunk - if its shape is more or less spheric - can only get smaller, however, if initial shape was irregular, it can break, and total surface can change as "internal" surface gets exposed.

Granules getting smaller will still have smaller surface, just like a single sphere does.

Besides, assuming "everything was done correctly" you are rejecting some of the possible explanations. That's not a correct approach IMHO.

 

[nicholaschean]

Sorry for the confusion, was a typo i meant granulated lithium. Well, most granulated reactant provided are usually not spherical to perfection and are mostly irregularly. And i guess that among all the factors that affect speed of reaction, I think surface area is really the desirable one that fits in here as the 2nd reason

 

[Borek]

Non-spherical doesn't necessarily mean irregular in this context. Cube will behave the same way, in fact surface goes down with size for most of the shapes. You need a particularly specific situation for the available surface to grow during reaction. That's why oil (or oxide on the surface) is an important case.

 

[nicholaschean]

Hmm but how does oil cause reactant amount to increase cos looking at the graph, for the first 40s, a increasing gradient is observed which signifies a increase in amount of reactants where there are more and more.

 

[Borek]

Water gets under the oil surface on the edges, and small droplets are removed, exposing more and more surface. However, IMHO exact effects are completely random and unpredictable.

 

[nicholaschean]

Being in Secondary 4 this year, could i rephrase this portion as presence of impurities in the lithium?

 

[Borek]

That would be something different. If you want it to sound more serious, call it "incorrect sample preparation".

 

[nicholaschean]

From my current syllabus, all i know is that the factors affecting speed of reaction are:
-Surface Area(Powdered vs Granulated)
-Concentration(High=faster,low=slower)
-Temp(High=faster,low=slower)
-Pressure(High=faster,low=slower)
-Catalysts(High=faster,low=slower)

And i also know that as a reaction proceeds, its concentration decreases due to usage of reactants hence speed decreases through reaction just like X.

However, Y seems something out of my syllabus where the concentration increases due to more and more reactants hence speed increases through the first 40s. In my current level, I can only explain one reason which is the addition of more reactants(lithium/water). As for the 2nd reason,Im quite clueless.

 

[Borek]

In my current level, I can only explain one reason which is the addition of more reactants(lithium/water). As for the 2nd reason,Im quite clueless.

That's not the problem of the level, that's problem of blindly applying the rule. The higher the surface on which reaction takes place, the faster the reaction. It doesn't mean it applies ONLY to granulated vs powdered material. Any situation in which exposed surface changes will yield the same effect, no matter what is the reason behind.

 

[nicholaschean]

I see..Sorry but could you please explain again what the surface area does?

 

[Borek]

Reaction takes place at the surface, so the higher the surface in contact with water, the faster the reaction, as long as water is able to touch the surface.

For a given amount of material you get the lowest surface to volume ratio for a single sphere. If you take many smaller spheres, surface/volume ratio gets higher, that's why using dust gives faster reaction than using large granules. It hold also for slightly irregular shapes.

Note: that doesn't mean when you put some granulated metal into acid reaction gets faster when granules become smaller - surface/volume ratio gets higher, but as the amount of the substance gets smaller, total surface is smaller as well, so the reaction slows down.

However imagine you have a large chunk, that is broken - there is a crack inside. You put it in the water. Initially it reacts mainly on the external surface, as crack is very small and speed at which water can get inside is low. But when water gets inside produced gas pushes crack walls away, to the moment when the granule breaks up - at this moment effective surface get substantially higher, and reaction speeds up. This is still easily understandable in terms of speed vs surface relationship.

Imagine you have a large chunk, that is covered with oil. Oil separates surface from the water, so the reaction is limited only to these parts of the surface which are not covered in oil. However, vigorous gas production removes oil droplets from the surface (on the edges of oil stain), exposing more and more surface, and thus speeding up the reaction. Again, the effect is still easily understandable in terms of speed vs surface relationship.

 

[nicholaschean]

Hmm so all around, how do i put all this as a sentence to conclude the 2nd possible reason?

 

[Borek]

Up to you, English is not my first language

 

[nicholaschean]

So cna i say that Student Y probably used lithium with a larger surface area

 

[Borek]

That won't be exactly true, what matters is the fact surface area changed in an atypical way during the Y student experiment.