Rates of reaction and factors affecting rate

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General experiment 2: Surface area and reaction rate

Marble (CaCO3) reacts with hydrochloric acid (HCl) to form calcium chloride, water and carbon dioxide gas according to the following equation:

CaCO3(s)+2HCl(ℓ) → CaCl2(s)+H2O(ℓ)+CO2(g)

Aim

To determine the effect of the surface area of reactants on the average rate of the reaction.

Apparatus

2 g marble chips, 2 g powdered marble, concentrated hydrochloric acid (HCl)

one beaker, two test tubes.

Chipa Thomas Maimela‘s insight:

Method

Warning:

Concentrated HCl can cause serious burns. We suggest using gloves and safety glasses whenever you work with an acid. Remember to add the acid to the water and handle with care.

Prepare a solution of hydrochloric acid in the beaker by adding 2 cm3 of the concentrated acid to 20 cm3 of water.

Place the marble chips into one test tube and the powdered marble into a separate test tube.

Add 10 cm3 of the dilute hydrochloric acid to each of the test tubes and observe the rate at which carbon dioxide gas (CO2) is produced (you should see bubbles of CO2).

Results

Note (write down) what you observe.

Questions and discussion

Which reaction proceeds faster?

Can you explain this?

Conclusion

The reaction with powdered marble is faster. The smaller the pieces of marble are (in this case the powdered form is smallest), the greater the surface area for the reaction to take place.

Only the molecules at the surface of the solid can react with the hydrochloric acid. The next layer of molecules can only react once the surface molecules have reacted. That is, the next layer of molecules becomes the surface.

The chips of marble are relatively large, so only a small percentage of the molecules are at the surface and can react initially. The powdered marble has much smaller solid pieces, so there are many more surface molecules exposed to the hydrochloric acid. The more molecules exposed on the surface (the greater the surface area) the faster the reaction will be.

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Responses

Graham Murray

May 30, 2014

The big idea sought from reaction rate studies is the particle scale explanations of why the regular factors control the rate of a reaction. concentrated acid is not desirable for this learning since the same learning can come from dilute and more dilute acid.

The big idea of surface area affecting reaction rate can be learned from a large neatly folded towel which is wet to the point of saturation.
What can be done to increase the rate at which it can be dried?
In particle theory, explain why the changes you make should work?

To dry, water particles have to be able to escape to air.
Options
Place the folded towel on the ground in sunlight
Ditto in a windy location
Unfold the towel; lay it back on the ground
Ditto; suspend from a clothes line (this act alone doubles the area of towel exposed to air.

Explain the above in terms of particles and it becomes easy to explain why a lump of CaCO3 takes longer to react in acid than does a powdered sample of the same mass. For this reaction, there is no advantage from using conc acid.

(there is a case for conc acid, say for the genie from the bottle, ie conc nitric on thiosulphate crystals;fume hood or good ventilation is essential to get the plastic sulphur product; no plastic sulphur from dil nitric).

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Reply
George Viebranz

May 30, 2014

The same can be done with water and effervescent tablets and a discussion of the particle nature of matter. Pulverized tablets complete their reactions to release CO2 much faster than do the intact disk. More particle “collisions” can take place between water and tablet particles.You can also help to demonstrate molecular movement as a function of thermal energy by dropping one tablet in 98 degree F water and another into 35 degree F water. The reaction rates differ not because of the surface area but because of the differences in thermal energy. Helps students to understand why freezers and refrigerators keep food fresh for long periods of time – chemical reactions are slowed by the colder temperatures.

@Graham,
I understand your analogy, but you are using a physical change to supplant chemical change. There is no chemical reaction in evaporation of water. At a certain level, though, the similarities or differences between the ideas becomes a relevant part of the chemistry discussion. When the conversation gets to the point of introducing Boyle’s Law or Charles’ Law then looking at interrelations between pressure, volume, temperature in a closed system becomes a good way to examine both reactions and changes of state.

@Chipa,
If this is going to be a continuing blog, I’d suggest leaving out the conclusion/explanation until some later entry. “How would you explain this” is a good inquiry-based and testable question. I’d want students to wrestle with it rather than “telling” them what happens. The current learning theory in science is that every person constructs their own beliefs about how the world works. Weaker models can be replaced by better models that the student experiences, personally. Simply telling why does not necessarily provide the opportunity for a learner to wrestle with the scientific ideas and explain what their scientific mind is thinking. In that regard, Graham’s recommendation to use the particle nature of matter is a much better starting point than the stoichiometric approach.
Please consider this as words from a “critical friend”. You are to be applauded for your effort to add to the collection of scientific phenomena for minds to consider.
.

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1. CT MAIMELA
  
  May 30, 2014
  
  Much appreciated George and thank you for the advice
  
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2. Graham Murray
  
  May 31, 2014
  
  George
  
  The point about the towel is that a calculation of the area of surface exposed is much more easily calculated for a folded or unfolded towel, than for a tablet before and after being powdered. If the tablet is porous, how can a statement be made made about its surface area?
  
  It is not a chemical process, but it seems to me that describing the change in rate in terms of surface area of a porous substance is a stumbling block for students.
  
  for a chemical process; drop a lighted match into 50 cm3 of kerosene (or alkane with minimum 12 carbons) in a 100cm3 beaker. The match goes out (if you insist on this step with a lighter MR alkane have a cover available to suffocate the flame) Dip a metal ruler in the kerosene then hold it to a candle; the kerosene catches fire. The difference with the kerosene is that on the ruler there is a very small volume spread over a large area, which is much easier for students to appreciate than from the powdered tablet.
  
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Alfred Sebatana Bopape

July 9, 2014

the surface area affect the rate of reaction ,thats a fact but what do they mean by state of division?

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1. CT MAIMELA
  
  July 10, 2014
  
  Take 1 ice cube divide it into 4 pieces you can make as many further small cubes as you may wish
  Try to melt them starting with the first 1 of the original size and the other 4 divided ones etc depending on how much devisions you have.
  The bigger Ice cube 1 will take time to melt than one of the 4 small ice cubes etc and that affect the surface area also.
  
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