The Effects of Surface Area on the Rate of a Reaction

IntroductionsBackground Information / Research

Concentration of a reactant is one way to increase the rate of a reaction. This ‘increase’ in the rate won’t always be major though; if you double the concentration of a reactant, it won’t just double the rate of the reaction – the association between the rate and a reactant’s concentration is much more complex. Examples of an increased rate of reaction after an increase in concentration include things like zinc and hydrochloric acid. If the acid has been concentrated, the zinc granules will react much quicker. (ChemGuide/Clark, 2002)

An increase of the temperature of (a) reactant(s) also increases the rate of the reaction. Making reactants hotter increases the number of energetic particles - that are equal or greater than the activation energy - in the reactant. This speeds up the reaction because the particles are now colliding faster and/or with more energy, meaning a higher rate of reaction.

Agitating a solution’s particles is also a way to produce a faster reaction. This is because mixing the particles increases the amount of collisions the atoms have with each other, therefore making the reaction occur much faster.

Surface area is a variable that greatly affects reaction rates. For example, generally a solid that has been crushed up into powder will react faster than the same solid as one ‘lump’ will react. This shows that the more divided up a solid is, the faster it will react. An example of this would be calcium carbonate and hydrochloric acid. CaCO3 in a powdered form reacts a lot faster with the acid than it does as a lump of limestone or marble.

Using a catalyst is the last of the five ways to increase the speed of a reaction. A catalyst is a substance which is used to speed up a reaction. Instead of changing the product, however, the catalyst leaves the result of the reaction ‘chemically unchanged’. Breaking bonds requires energy (Lower/Emeritus/Odufalu/Chacha/Mudda/Iskandar, 2014), and that energy can be added in by a catalyst to speed up the reaction. Examples of using catalysts include: concentrated sulphuric acid on the nitration of benzene, iron on the manufacturing of ammonia through the Haber process.

HypothesesConcentration: if the concentration of a reactant is increased, then the rate of the reaction is also increased, because if there are more particles of a reactant, then the chance of it colliding with another particle is higher, therefore the higher the reaction rate.

Temperature: if the temperature of a reactant is increased, then the rate of the reaction will also increase, because reactions happen when particles collide, and therefore increasing a substance’s temperature will make the particles move around faster and collide more often.

Agitate particles: if particles are agitated, meaning they are stirred or mixed vigorously, then the reaction will be faster, because agitation makes collisions happen more frequently in the reaction, therefore it will be happening faster (the rate is increased).

Surface area: if the overall surface area of a reactant is increased, then the rate of the reaction will increase, because making the surface area of a reactant (the solid) increases the likeliness of a collision occurring, therefore making the overall reaction faster.

Catalyst: if a catalyst is used, then the reaction’s rate will be increased, because catalysts reduce the activation energy requirement of the reaction, therefore making the overall reaction faster because less energy is needed to make it happen.

AimsThe aims of this prac were to:

determine the effects of

- concentration of a reactant- temperature of a reactant- agitation of particles- surface area of a reactant- using a catalyst

on the overall rate of a reaction.

Materials / equipment- 3 beakers- 7 alka seltzer tablets, 5 whole and 2 crushed- 15mL of 1M HCl- 15mL of 3M HCl- 15mL of 5M HCl- stopwatch- kettle- 1050mL of pure tap water- 150mL of HCl- 15g of calcium carbonate

BEING DEMONSTRATED IN CLASS:o 10mL 3% hydrogen peroxide o 0.1M iron(III) nitrateo 0.1M sodium chlorideo 0.1M calcium chlorideo 0.1M potassium nitrateo 0.1M manganese chlorideo 100mL graduated cylindero 10mL graduated cylindero 7 test tubes

ProceduresConcentration

Increasing hydrochloric concentration.

1. Each beaker was washed with water and dried.2. 5 grams of calcium carbonate (CaCO3) was placed in each of the three

beakers.3. 15 ml of 1M hydrochloric acid (HCl) was added to the first beaker.4. 15 ml of 3M hydrochloric acid (HCl) was added to the second beaker.5. 15 ml of 5M hydrochloric acid (HCl) was added to the third beaker.6. The stopwatch was started.7. Each of the reactions was examined and any observations were recorded.8. As each reaction finished the respective stopwatch was stopped.

Temperature

3 Alka Seltzer tablets in water.

1. Each of the three beakers was washed out with water and dried.

2. The first beaker was filled with ice and 200mL of tap water.

3. A kettle was put on and the second beaker was filled with 200mL of boiling

water.

4. The third beaker was filled with 200mL of tap water and then left on the

bench at room temperature as a control.

5. After a minute had elapsed 2 Alka Seltzer tablets were placed in each

beaker.

6. A stopwatch was started.

7. The reactions were examined and any observations recorded.

8. As each reaction finished the stopwatch was lapped and the times were

noted.

9. The results were recorded.

Agitation

Hydrochloric acid calcium carbonate.

1. Three beakers were washed out with water and dried.2. 50ml of hydrochloric acid was added to each beaker.3. 5gm of calcium carbonate was added to each beaker and the stopwatch

was started.4. The first beaker was stirred with a stirrer at a rate of 1-3 rpm.5. The second beaker was stirred at a rate of 4-5 rpm.6. The third beaker was a control and as a result it was not stirred.

7. As each reaction finished the stopwatch was lapped and the time was recorded.

8. Each reaction was examined and any observations were recorded.

Catalyst

Decomposition of hydrogen peroxide.

1. The Hydrogen peroxide was diluted by adding 10mL of 3% H2O2 to a 100mL graduated cylinder. 90mL of distilled water was added, to obtain 100mL of dilute (0.3%) hydrogen peroxide.

2. A small amount of this diluted solution was used to rinse out a 10mL graduated cylinder and the 7 test tubes. The rinses were then washed away.

3. 5mL of the H2O2 solution was placed into each of the 7 test tubes.4. 5 drops of the following were added to separate test tubes:

0.1M FeCl3 0.1M NaCl0.1M Fe(NO3)3 0.1M CaCl20.1M KNO3 0.1M MnCl2

5. Each tube was mixed gently by stirring it with a clean stirring rod.6. Each solution was observed, and notes were taken at the production of

gas bubbles that formed. Reaction rates were recorded as in the data table.

Surface area

2 Alka Seltzer tablets (crushed) and water.

1. Two beakers were washed out with water and dried.2. Each beaker was filled with 150 ml of water.3. One Alka Seltzer tablet was crushed using a mortar and pestle and the

resulting powder was placing in one beaker.4. The second tablet was placed in the other beaker whole.5. The stopwatch was started as the reactants were placed in the beaker.6. Any observations were recorded.7. As each reaction finished the stopwatch was lapped and the time was

recorded.

Observations

Catalyst observations:

2H2O2 (aq) -> 2H2O (l) + O2 (g) H2O2 with:

CaCl2 (aq) Doesn't seem to be changing color, evolving gas, changing temperature

KNO3 (aq) Tube doesn't seem to be changing temperature, no change in color, no gas being evolved

NaCl (aq) Nothing seems to be happening

MnCl2 (aq) Nothing happening

Fe(NO3)3 (aq)

Immediate reaction; solution has gone dark brown, bubbling like crazy, oxygen is being evolved, whole tube is getting hotColor getting lighter, tube very hot - too hot to hold at the bottomUrine-like color

MnO2 (s) Very effective catalyst - lots of oxygen being evolved, tube very hot, black mucky solution

 Fe(NO3)3 and MnO2 are definitely catalysts for the decomposition of hydrogen peroxide.

ResultsConcentration

test 0.5M hydrochloric acidvs. CaCO3

1M hydrochloric acidvs. CaCO3

2M hydrochloric acidvs. CaCO3

time taken

n/a – test aborted @ 34min50s

n/a – test aborted @ 34min50s

11min30s

Tests ‘0.5M’ and ‘1M’ were aborted because there was not enough time to complete them.

0.5M vs. CaCO3

1M vs. CaCO3

2M vs. CaCO3

0 5 10 15 20 25 30 35 40

Concentration

time taken (minutes)

test

Temperature

test icy watervs. Alka seltzer

room temp watervs. Alka seltzer

boiled watervs. Alka seltzer

time taken

41s n/a – test aborted @ 23min

n/a – test aborted @ 23min

Tests ‘room temp’ and ‘boiled water’ were aborted because there was not enough time to complete them.

icy water vs. Alka seltzer

room temp. water vs. Alka seltzer

boiled water vs. Alka seltzer

0 5 10 15 20 25

Temperature

time taken (minutes)

test

Agitation

test stirred unstirred

time taken

1min36s n/a – test aborted @ 18min14

Test ‘unstirred’ was aborted because there was not enough time to complete it.

stirred

unstirred

0 2 4 6 8 10 12 14 16 18 20

Agitation

time taken (minutes)

test

Catalyst

test CaCl2 (aq)

KNO3 (aq)

NaCl (aq)

MnCl2 (aq)

Fe(NO3)3 (aq)

MnO2 (s)

reaction?

no no no no very fast very fast

Surface area

test whole tablet crushed tablet

time taken

26min40s 9min12s

whole tablet

crushed tablet

0 5 10 15 20 25 30

Surface Area

time taken (minutes)

test

Discussions (evaluations included)Concentration: The goal of the first experiment was to investigate what effect changing the concentration of a reactant had on the overall rate of a reaction. Results from the experiment confirmed that changing the concentration of a reactant does affect the rate of the reaction. This was shown when the 2M hydrochloric acid (most concentrated reactant used in the experiment) reacted with CaCO3 the fastest (out of 2M, 1M, and 0.5M), meaning that a higher amount of concentration increases the rate of a reaction. The other two tests had to be aborted as there was not enough time to complete them. More evidence to support the hypothesis that concentration affects reaction speed was found when the 1M hydrochloric acid had dissolved more of the CaCO3 than the 0.5M hydrochloric acid, clearly showing that as concentration increased, so did the speed of the reaction. A higher ‘concentration’ meant that the solution had a higher amount of hydrochloric acid (not as watered down), and thus more of it to react with the CaCO3, therefore an increase in the reaction’s speed. This test was accurate in that it used the same amount of CaCO3 for each beaker, however the test could not be finished. If this experiment was to be altered/revised, more time would be recommended, so that all tests could be completed and properly observed.

Temperature: The second experiment’s objective was to determine the effects of temperature on the rate of a reaction. This was investigated by using three beakers filled with water; one with ice cubes, one room temperature, and one’s water boiled with a kettle. The hypothesis, stating that if the temperature of a reactant was increased then so would the rate of the reaction, was proven. This was shown when the hot water reacted with Alka seltzer tablets faster than the room temperature beaker, and the room temperature faster than the icy water beaker. Through these results an assumption can be made that there is a correlation between the temperature of a reactant and the overall rate of a reaction. The way temperature affects the rate of a reaction has to do with collision theory. All reactions need particles to collide; they can’t happen without collisions. If a reactant is heated up, its particles will move faster, and therefore collide more frequently, meaning a faster reaction overall. Although this experiment was accurate – temperatures were recorded with a thermometer and any change was recorded – and each Alka seltzer tablet was of the same size, like the concentration investigation, there was not enough time to complete it.

Agitation: The third experiment’s aim was to explore what agitating particles does to affect a reaction’s speed. This was tested by placing powdered Alka seltzer in beakers with tap water, and stirring one beaker vigorously. The hypothesis for this test was proven; stirring the Alka seltzer made the reaction faster. This is because stirring a solution moves the particles around, and this increases the amount of collisions the atoms have with each other, therefore increasing the reaction rate. The beaker that wasn’t stirred in this test did not finish its reaction, as the test had to be aborted. Because both beakers had the same amount of Alka seltzer in them, this proves that stirring definitely has a

direct effect on the speed of a reaction. Also like the first two experiments, the one thing advised if this investigation were to be redone would be to allot more time for it to be completed; this test had to be aborted too.

Catalyst: The second-last experiment was designed to test the effects of a catalyst on the rate of a reaction. It used different solutions (listed above in results) combined with 2H2O2 (hydrogen peroxide) to determine how effective catalysts are in speeding up the rate of a reaction. The 2H2O2 had no reaction with the following solutions:

- CaCl2 (aq),- KNO3 (aq),- NaCl (aq), and- MnCl2 (aq)

but however did react very quickly with the:

- Fe(NO3)3 (aq) and- MnO2 (s)

These two substances had almost immediate reactions with the hydrogen peroxide, and both evolved oxygen at high speeds. Their tubes got hot and the [Fe(NO3)3] turned the solution into a xanthic/urine-like color, whilst the [MnO2] turned its solution black and mucky. Both were tested for oxygen being evolved by a stick being lit on fire and then blown out; if the stick was put into the tube and it lit back up, oxygen was being evolved. This test was valid and it was able to be completed because it didn’t require a lot of time to do.

Surface area: The final experiment was an investigation into how the surface area of a reactant affects the overall speed of a reaction. This was tested by placing two Alka seltzer tablets in water; one crushed into powder, the other left as a whole tablet, and seeing which dissolved first. The powdered Alka seltzer tablet dissolved fully with the water much faster than the whole tablet did: the whole tablet took over 26 minutes, whereas the powder only took 9 minutes. This is because when a solid is more finely divided, it will react faster (the particles are easier to get to [they aren’t all in a big clump where the outer layers have to be eaten away first]). This proved the hypothesis that changing the surface area of a reactant would affect the overall rate of the reaction. The test was accurate because there was no stirring involved (stirring would make the test unfair as it would cause artificial bubbling/fizzing), and therefore the experiment can be called a ‘fair’ test / ‘valid’.

ConclusionAll hypotheses in this investigation were proven, making it highly successful. The aim was to determine whether the:

- concentration (because if there is more of something, it is easier to collide with and make a reaction)

- temperature (heating something makes its particles move around faster and therefore more collisions will occur)

- agitation (stirring something makes more collisions and the reaction will be faster)

- surface area (the reaction doesn’t have to work its way from the outside to the inside of the solid if it is crushed up, it is much easier to get to and collide with)

of a reactant, or

- a catalyst (lowering the activation energy required for the reaction to happen means it will be faster, and easier to occur)

had any effects on the overall rate of a reaction. As a result of this investigation, it has been proved that changing any of the above factors will have a direct effect (in most cases) on the overall rate of a reaction.

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