An Investigation of factors affecting rate of reaction between an acid and a metal.

An Investigation of factors affecting rate of reaction between an acid and a metal

Plan

I am going to investigate the effect of temperature on the rate of reaction between magnesium ribbon and dilute hydrochloric acid.

For a chemical reaction to take place, particles of the two reactants must collide with each other. From my research on collision theory (from Letts GCSE Chemistry), I also know that the particles must have enough energy to react.

The minimum amount of energy for a reaction to take place is called the “activation energy”.

There are 3 main factors which will affect the rate of my reaction:

Surface area of the Mg ribbon
Concentration of the acid
Temperature of the acid

I am going to study the effect of temperature.

EQUATION:

Magnesium + Dilute Hydrochloric Acid ---→Magnesium Chloride + Hydrogen

Mg (s) + 2HCl (aq) ---→ MgCl2 (aq) + H2

These are the temperatures I am going to heat/cool the HCl to: 5°C 35°C

10°C 40°C

15°C 45°C

20°C 50°C

25°C 55°C

30°C 60°C

My control test will be at room temperature.

HYPOTHESIS:

I hypothesise that the rate of reaction will increase each time the temperature increases, and I shall hopefully see a pattern in my results because when a substance (hydrochloric acid) is heated, the particles in the acid gain more energy which makes them move more and faster. This causes more interaction between the particles in the acid, and those in the magnesium, which means there are more opportunities for the particles to react. The fact the particles are moving faster also means they travel greater distance in a given time and therefore will be able to collide more often with the Mg. This can be proven by Maxwell-Boltzmann distribution theory. They proved that at higher temperatures, more of the molecules gain enough energy to activate a reaction (activation energy). Also, in a previous experiment, sodium thiosulphate solution had reacted quicker in higher temperatures.

I hypothesise that the reaction taking place at 15°C will be twice as slow as the reaction taking place at 30°C and that the reaction at 20°C will be twice as fast as the reaction at 10°C. This is because doubling the temperature will double the speed of the acid particles and so double the number of collisions.

At higher temperatures, more particles will carry the activation energy. This too means that the reaction will be faster because there will be more successful collisions.

FAIR TEST:

I am going to keep the volume of the distilled water (50cm³) and dilute HCl (10cm³) the same in each experiment.

I am going to keep size of the conical flask the same by using the same one in each experiment.

I am going to use 4cm of Mg ribbon each time.

I am going to decide when to acknowledge the Mg has totally reacted, so the experiment is carried out fairly (when all the effervescing has stopped and all the Mg, even the smallest particle, has disappeared)

I am going to take 3 results from every ...

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FAIR TEST:

I am going to keep the volume of the distilled water (50cm³) and dilute HCl (10cm³) the same in each experiment.

I am going to keep size of the conical flask the same by using the same one in each experiment.

I am going to use 4cm of Mg ribbon each time.

I am going to take 3 results from every temperature, and calculate the mean time. If I still end up getting an anomaly, I will then take another 3 results.

APPARATUS:

Conical Flask
Beaker
Measuring cylinder x2 (1 big, 1 small)
Thermometer
Ice
Water bath
Magnesium ribbon
Scissors and ruler (to measure and cut the Mg)

METHOD:

DIAGRAM:

Before starting the experiment, I am going to wear safety goggles so no acid will splash into my eyes and wear/tie-up my overall so I won’t spill anything on my clothing.

CONTROL:

I am going to measure out 50cm³ distilled water using a large measuring cylinder and pour it into a medium sized conical flask.

Then, I am going to cut out 4cm of Mg ribbon, measured by a ruler, and add it into the conical flask the distilled water is in.

I am then going to measure out 10cm³ of HCL using a small measuring cylinder and then measure and record the temperature when it is steady (room temperature) using a thermometer. Then I am going to immediately add the HCL into the conical flask (which the magnesium and water is already in) and start the stop clock. When the magnesium has totally disappeared, and stopped effervescing (I know it is going to effervesce and produce hydrogen gas as magnesium is high up in the reactivity series) I am going to stop the stop clock and record the results (the time it took for the magnesium to stop reacting with the hydrochloric acid)

I am going to repeat this experiment for every temperature (5,10,15,20,25,30,35,40,45,50,55,60°C). I will repeat the experiment 3 times at each temperature to get an average. I am going to use ice to cool the hydrochloric acid to the temperature below the room temperature, and I am going to use a water bath or Bunsen burner to heat the hydrochloric acid above the room temperature.

I am going to record my results in a table, and then draw a graph (x axis= temperature, y axis= time). From the graph, I am going to analyse my results.

Background

On Magnesium

Magnesium is a light, shiny grey metallic element; symbol Mg, atomic number 12, found in group 2 in the periodic table. It is quite reactive giving vigorous reactions towards acids. It is one of the alkaline earth metals, and the lightest of the commonly used metals. It is used in alloys, flash photography, flares, fireworks and flash bulbs because it burns vigorously in air with a bright white light. Magnesium reacts with steam to release hydrogen and it also burn is carbon dioxide gas.

On Hydrochloric Acid

Hydrochloric acid, HCL, is a solution of hydrogen chloride and is corrosive. The acid is a typical strong, monobasic acid forming only one series of salts, the chlorides. Like most acids, it releases hydrogen ions when it is added to water and certain metals, and has a pH of less than 7. Hydrochloric acid is a common laboratory acid.

On Rate of Reaction

The rate of a chemical reaction is a measure of how fast the reaction takes place. It is important to remember that a rapid reaction is completed in a short period. Some reactions are very fast (e.g. the formation of silver chloride precipitate when silver nitrate and hydrochloric acid solutions are mixed). Other reactions are very slow, (e.g. the rusting of iron).

On Collision Theory

The particles of the reacting substances must collide with each other and, secondly, a fixed amount of energy called the activation energy must be reached if the reaction is to take place. If a collision between particles can produce sufficient energy a reaction will take place. Not all collisions will result in a reaction.

A reaction is speeded up if the number of suitable collisions is increased.

This is a diagram of activation energy:

These are the factors which can alter the rate of reaction and can be varied during an investigation of rate and reaction:

Concentration
Particle size/surface area
Temperature
Light
Presence of a catalyst
Pressure

These variables could be used because:

The more concentrated the reactants, the greater the rate of reaction will be. This is because increasing the concentration of the reactants increases the number of collisions between particles and, therefore, increases the rate of reaction.

When on of the reactants is a solid, the reaction must take place on the surface area of the solid. By breaking up the solid into smaller pieces, the surface area is increased, giving a greater area of collisions to take place and so causing an increase in the rate of reaction.

An increase in temperature produces an increase in the rate of the reaction. A rise of 10°C approximately doubles the rate of the reaction. When a mixture of substances is heated, the particles move faster.

This has 2 effects:

Since particles are moving faster they will travel greater distance in a given time and so will be involved in more collisions.
Because the particles are moving faster a larger proportion of the collisions will exceed the activation energy and so the rate of reaction increases.

The rates of some reactions are increased by exposure to light. Light has a similar effect as temperature because it produces heat.

A catalyst is a substance, which can alter the rate of a reaction but remains chemically unchanged at the end of the reaction. Catalysts usually speed up a reaction. A catalyst, which slows down a reaction, is called a negative catalyst or inhibitor. Catalysts speed up reactions by providing an alternative power for the reaction (i.e. one that has much lower activation energy). More collisions will, therefore, have enough energy for this new pathway.

6. When one or more of the reactants are gases an increase in pressure can lead to an increased rate of reaction. The increase in pressure forces the particles closer together. This causes more collisions and increases the rate of reaction.

Obtaining Evidence

I set out the apparatus as shown in the diagram in my plan.

I mostly carried out my plan as I said I would, but I had to make some adjustments to suit the equipment being used.

Concentration

In my plan, I said I was going to use 50cm³ distilled water and 10cm³ HCL. Instead, the chemistry department provided the acids in Molars, so I didn’t need to dilute the HCL. I decided to use 1M of HCL in all my experiments.

Measurement

In my plan, I stated I was going to use 50cm³ of distilled water and 10cm³ HCL (which equals 60cm³ altogether) but I decided to use 20cm³ instead.

The rest of the experiments were carried out as stated in my plan.

I recorded my results in seconds to an integer.

Here are my results:

To find the average rate of reaction, I did ‘rate of reaction 1’ + ‘rate of reaction 2’ + ‘rate of reaction 3’ divided by 3, and I got the average.

As you can see I had an anomaly set of results for the temperature at 60°C, so as I said in my plan, I took another set of results, and this time the results were as I thought they would be.

I have taken 3 readings of time for each temperature to find the average rate of reaction and make the results more accurate. I used a stopwatch to obtain my results, as it is accurate. If/when the stopwatch didn’t start properly; I stopped the experiment and redid it.

I have obtained a sufficient range of results to produce a graph and analyse my results. When drawing my graph I will only use the average rate of reactions.

Analysis

From the experiments I have found out that as the temperature increases, the rate of reaction gets faster.

The graph clearly shows; with a smooth curve that, as the temperature increases, the time for the rate of reaction decreases. This proves that my hypothesis was correct as the highest rate on the graph is at 600secs, 5°C that was the lowest temperature I did my experiment at, and the lowest rate on the graph was 26secs, 60°C that was the highest temperature I did my experiment at. From my results I know that at 5°C, the particles are moving slowly and will travel a smaller distance in a greater time and so won’t be involved in many collisions, but at 60°C

the particles are moving much faster and therefore a larger proportion have collided and surpassed the activation energy, and therefore the rate of reaction is much greater.

I got one set of anomalies in my results, which were for 60°C, so I redid it, and got results I would have expected. I may have got this anomaly because it is hard to judge when the Mg has totally gone

Conclusion?

Scientific conclusion?

Evaluation

The experiment I carried out went very well as I got the results I had expected.

The results could have been more accurate by recording the time to a 1 decimal point, but on a stop clock, it is impossible to stop the clock to a decimal point.

If I could do this experiment, I would improve the reliability of the timing of the Mg, because it is difficult to see when the reaction has finished, by timing the Hydrogen given off (collected).

My method did give me reliable results, as I did three of the same experiment at certain temperatures, and all three of the results were very similar, and therefore could be counted for. Also, my averages I worked out were in a pattern (decreasing as the temperature increased), this also shows my method gave me reliable results.

I got some anomaly results for 60°C (in pink)

They were slower than 55°C when they were meant to be faster. When I redid the experiments, I got the result I expected and they fitted into the pattern. I may have got the anomalies by wrong judgment of when the Mg has totally gone, as it is very difficult to judge.

As I only had one set of anomalies, I had enough correct results to draw a conclusion that the higher the temperature the faster the rate of reaction.

The method could have been improved by timing the hydrogen given off (by collecting it) using a gas syringe. This would have been more a more reliable method, as I would not have had to judge when the Mg had finished reacting each time.

I could extend the investigation by changing the concentration of the HCL and keeping the temperature constant.