The Rates Of Chemical Reaction
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The rate of a chemical reaction is a measure of how fast the reaction takes place. So, a rapid reaction is completed in a short time. Some reactions may be very slow, e.g. the rusting of steel. A chemical reaction only occurs when particles collide with each other. Particles which move fast are more likely to collide with each other; you can do this by heating them up (raising temperature). This means more particles collide with each other every second, so the rate of reaction increases. There are more particles of hydrochloric acid atoms if there is a higher concentration of them. More particles means there will be a higher amount of collisions so the reaction should be fast.
Other factors that affect the rate of reaction are; temperature, catalysts and surface area. Temperature affects reaction rates as it dictates how much energy the particles in a substance have, at higher temperatures particles have more energy and so move more quickly and collide more. As the particles are moving faster the collisions between them have more energy, this means that they are more likely to create a reaction and so reaction rate increases. Catalysts lower the amount of energy needed for a collision to be successful, so adding a catalyst to a reaction increases the amount of successful collisions and therefore increases reaction rates. The larger the amount of surface area, the faster the reaction rate is.
Independent Variable= Hydrochloric acid concentration
Dependent Variable= Time taken for magnesium to dissolve
To investigate the reaction time between magnesium and hydrochloric acid.
I think that my results will show that the hydrochloric acid with a molarity of 2 will finish reacting the quickest because it is the strongest hydrochloric acid and I predict that the slowest experiment to finish reacting will be the 0.4 molar strength of hydrochloric acid. I think that the results will go slower and slower the nearer they are to the 0.4 strength of hydrochloric acid. The reason that the experiment with hydrochloric acid at 2 molar will dissolve the magnesium ribbon faster is because it is of a stronger concentration and so the molecules in the hydrochloric acid move faster and so react with the magnesium ribbon quicker.
To make sure my experiment was a fair test, I had to keep these factors the same:
Surface area of magnesium
Volume of acid used (10ml)
Used the same type of beaker (80ml)
Length of magnesium (1cm)
Clean the magnesium with emery paper before the experiment
Temperature of the hydrochloric acid
The factor of which I will change is the concentration of the hydrochloric acid.
I used the following equipment:
80ml beakers (x5)
Equal strips of magnesium (x5)
A measuring cylinder
I will need to make my experiment as safe as possible. It will be safe for me and other students around me. Precautions will be made if someone is harmed, such as there will need to be a first aid kit nearby & accessible, also a water tap in case acid touches the skin or eyes. This is what I will need to do:
Care in using glassware as when broken is sharp and can cut the skin
Wear safety goggles as I am using concentrated hydrochloric acid
Care in returning all the equipment at the end of the experiment
Care to eyes and skin plus to be aware of other students around the class
The acid that we are using in this experiment is of 1.0 molar strength, which is strong enough to at least cause irritation to the skin if spilled on it; the hydrochloric acid would also cause irritation to the eyes and mouth if ever spilled into those. Another risk is that magnesium is flammable especially if in powder form, but we are using it in a ribbon form and we are not using any flames in any part of the experiment. The gas which is given off from the reaction between hydrochloric acid and magnesium ribbon is hydrogen which is a flammable gas.
We will reduce the risk of these hazards by wearing protective eyewear for example laboratory goggles, we will also measure out the hydrochloric acid pouring away from the body, over the sink. We will not be using flames in our experiment so the risk is reduced from the hydrogen and magnesium but still other experiments could be using flames so we will still have to be careful and make sure to stay away from any other experiments using flames.
Firstly, we had to record (in a table) the time taken for a 1cm magnesium (Mg) strip to completely dissolve in a beaker containing hydrochloric acid (HCl). I had five different concentrations of the hydrochloric acid: 1.0m, 1.2m, 1.4m, 1.6m, 1.8m and 2.0m. I put 10ml of hydrochloric acid in a glass 80ml beaker, and then one by one I began putting the magnesium strips into each beaker and timing the duration taken for the magnesium to dissolve. The first thing we noticed is that the lower the concentration of the acid, the longer the magnesium took to dissolve. We were trying to find out how the concentration of the acid affected the speed of which the magnesium dissolves.
The equation for this experiment was:
Magnesium + Hydrochloric Acid â†’ Magnesium Chloride + Hydrogen
Mg + 2HCl â†’ MgCl2 + H2
The theory that we use to explain how different variables change the rate of reaction is called the collision theory.
For a reaction to take place, the particles of the substances that are reacting have to collide. If they collide, with enough energy then they will react. The minimum amount of kinetic (movement) energy that two particles need if they are going to react when they collide is called the activation energy.
I needed to collect: Safety goggles, five 80ml beakers, a lab coat, a measuring cylinder, and a pipette. Safety goggles were very important because of the chemicals we were using. Hydrogen gas was also being released, which is highly flammable so we had to be aware if we were to ignite anything near it.
This was the method I used:
Put on safety goggles and a lab coat
Collect (80ml) beakers (x5)
Collect a pipette and a measuring cylinder
Measure out 10ml of hydrochloric acid with a pipette
Release the acid into the beaker
Make sure we have a stopwatch at the ready
Collect magnesium strips
Put the magnesium into the acid and make sure at the exact same time the stop watch has started
The hydrogen produced makes the reaction mixture effervesce (fizz). The faster the reaction, the shorter the time taken for the effervescence to stop.
My results may not be completely accurate, as the person using the stop watch may not react quickly enough to stop the time. Also the amount of milliliters put into the cylinder may not be exact due to the accuracy of the apparatus. Although the results are likely to be reliable as we used the same type apparatus for each experiment, and concentration we tested on, we followed the method exactly the same for each repeat
We used the same method for all five different concentrations of the hydrochloric acid. We used the same pipette and measuring cylinder but made sure they were washed with water and then dried ready for the next concentration.
We repeated the whole experiment for a second time to gain a better insight of our results; this would also allow us to spot outlier's and discuss how we got them. If we found an outlier we repeated the reading.
We then worked out an average between both of our results, although we did not find any outlier's so our average wasn't affected.
Time taken for the magnesium to dissolve (s)
Hydrochloric acid concentration
My graph which is attached to this document, it shows that the slope of the curve was negative due to the huge decrease in the seconds taken for the magnesium to dissolve (negative correlation, as you increase the concentration of hydrochloric acid, the amount of seconds to dissolve decreases). As seen in my graph the difference within the amount of seconds from concentrations 1.0m and 1.2m is the biggest difference of 51.45 seconds. This similarity continues all the way through until the last two concentrations have a small difference of just 4.3 seconds.
To conclude I realised the higher the amount of concentration in the beaker, the faster the magnesium dissolved. A scientific explanation to this would be that there were more particles in the beaker meaning the there was less space resulting to larger amount of collisions. More collisions - The rate of reaction depends on the rate of successful collisions between reactant particles. The more successful collisions there are, the faster the rate of reaction. Surface area could have affected the experiment, but as these factors were controlled my results weren't affected.
I predicted that the higher the concentration the faster the reaction. The reason for this is as the concentration increased the rate of reaction increased. The line showing the results of 1.0 molar acid is the steepest. The reaction is faster with stronger acid because it contains more acid particles. The greater number of acid particles the more chances of a collision between acid and magnesium particles therefore the faster the reaction. The strongest acid also produces the most gas because it contains more reactant acid particles. You can see from the graph below the higher the concentration of acid the faster the rate. There is now evidence to back up my prediction.
My hypothesis has been proven correct, in that beforehand I believed increasing the concentration means that we have more particles in the same volume of solution. This increases the chance of collisions between reactant particles, resulting in more collisions in any given time and a faster reaction. This can show us a general rule about concentrations and rates of reactions.
During my investigation there is a possibility that I may have come across errors. These errors were linked to my measurements. The timing of the magnesium dissolving in the hydrochloric acid could have plus or minus a second due to the reaction speed of the person timing it. Also the (measurements of the hydrochloric acid volumes were accurate to +/- 1cm³.
Many of the errors are down to human judgement,
Measuring of volumes
Reaction speed of the timer
From the results in the table and the graph we can see a steady increase in the rate of reaction as the concentration of the acid increases. This complies with my prediction. The graph shows that there is an increase in the rate of reaction as the concentration increases because the graph has its largest gradient or it is steepest at this point.). Also we can see that as the reaction continues the concentration of the reactants decrease and so does the rate of the reaction as we can see the decreasing gradient on the graph steadily falling and coming to a stop when the reaction is complete and the magnesium has completely disappeared.
I think my experiment went well as a whole. I could have possibly improved the accuracy of some of my results I could have used more concentrations of acid to see if the pattern carried on as it did previously or changed, I could have repeated the readings more times to make them more accurate. In some occasions I could have done things to make my experiment a fairer test. I could have repeated my experiment at least one more time to increase reliability. I could have possibly concentrated more on my experiment than get distracted by peers. If I repeated this experiment I could have tried investigating other aspects of the experiment which could change the rate of reaction. One example could be changing the surface area, the concentration of acid and pressure can affect the reaction. I could have also tried using different acids, such as sulphuric acid.
Different metals could allow me to investigate their (relativity) reactivity and the affect this has on results and reaction rate. The reaction could relate to a real life situation, such as if someone were to walk into a crowded area, there would be more collisions rather than if the place was empty. Therefore people in the area get more annoyed, which results in even more collisions, therefore this example relates to the rates of reaction.
The results lined up very well on the graph with a very close curved line of best fit that all the results fall directly onto or very close to with only two off the line and only 1 second away. This backs up the consistency and reliability of the results, which makes my evidence good enough to support a firm conclusion.
To improve my knowledge of reaction rates I could and how concentration affects them I could conduct experiments using different metals and different acids. I could also use a larger range of concentrations to extend my results and give me more evidence. Using smaller gaps between concentrations would also further my evidence as it would improve my rate graph allowing me to draw a more accurate line of best fit therefore allowing me to make more accurate estimates for results of concentrations not yet tested.
And it could have been improved by the repetition of each concentration 3 times; instead of the one time I was able to conduct, to eliminate all anomalous results. As from 3 sets of readings, an average can be calculated which will provide a more reliable result as it is based upon 3 readings, not just the one reading which may be wrong.
The investigation could be done using one variable and therefore have a set of results which were related in some way. The variables that could be used are:
Particle size/surface area
These variables can 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 one 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 reaction. A rise of 10° C approximately doubles the rate of reaction. When a mixture of substances is heated, the particles move faster. This has two effects. Since the particles are moving faster they will travel greater distance in a given time and so will be involved in more collisions. Also, because the particles are moving faster a larger proportion of the collisions will exceed the activation energy and so the rate of reaction increases.
Looking at the set of results obtained, you can clearly see that they all follow the expected pattern. This is pattern suggests that the reaction rate increase when the concentration of the acid increases because if you increase the concentration of the acid you are introducing more particles into the reaction which will in turn produce a faster reaction because there will be more collisions between the particles which is what increases the reaction rate.
There will always be ways in which you can improve your investigations and the same thing goes to my investigation. Every time I washed a test tube or a measuring cylinder, I did not dry it before using it. This may have affected the rate of reaction, as water would dilute the acid. To improve my results, I could dry the test tubes and the measuring cylinder after they are washed to prevent diluted acids. The size and weight of the magnesium would have affected the rate of reaction. The experiment could be improved by measuring, adjusting and weighing the magnesium ribbons so they all are the same size and weight.
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