Rate is a measure of a change that happens in a single unit of time. Any suitable unit can be used-a second, minute, hour or even a day. The rate of a chemical reaction is a measure of how fast the reaction takes place. A rapid reaction can be completed in a short period of time. Some reactions are very fast, e.g. (the formation of silver chloride precipitate when silver nitrate and hydrochloric acid solutions are mixed and some can be very slow, e.
g. (the rusting of iron).
The aim of this investigation is to find out the effect of concentration of acid, in the reaction between dilute hydrochloric acid and magnesium ribbon. In this investigation we will test different concentrations of acid reacting with magnesium to see what effect it has on the rate of reaction, this will be a moderate speed reaction.
Before we started the main experiment there were a few things we had to sort out:-
So we set up our experiment as explained later using the cylinder placed in the ice cream tub full of water because we thought it would give the most accurate results out of the two methods available.
We found it to be best to use 2cm of magnesium strip because any more magnesium could be dangerous as it could react violently so we decided to use 2cm, as it seemed the safest.
We also found it to be best to use 50ml of hydrochloric acid at full concentration because again for safety reasons as too much could cause the magnesium to react violently and also because we are measuring concentration not surface area so we don’t want to use too much acid.
We found we would record the data by timing how long it takes for 10ml of hydrogen to be released, starting the stop watch when the magnesium has been placed into the beaker with acid then stopping when the amount of hydrogen released reached the 10ml mark. We found that 10ml was best because otherwise we could be waiting for a long time if we changed the limit to something like 20ml and we didn’t have much time. We’ll record the results in a table and then transfer the results to a graph. Read the answer what is the optimum concentration of pectinase
I predict that as the concentration of the hydrochloric acid increases, the time taken for the magnesium to disappear decreases. This is why I think so:-
The collision theory simply states the rate of reaction simply depends on how often and how hard the reacting particles collide with each other. If the solution is more concentrated then there’ll be more particles of reactant in a smaller space because there’ll be less water molecules making collisions between the Magnesium particles and hydrochloric acid particles more likely. If the acid is less concentrated then there’ll be fewer particles closer together, they’ll be a lot more spread out, away from each other and that means less successful collisions. So basically the higher the concentration of 2HCL you use, the less time it takes for the magnesium to disappear and so the rate of reaction increases because there’ll more successful collisions in a shorter time. Clearly, the time taken for the magnesium to disappear when it is placed in different concentrations of 2HCL, is related. The concentration of 2HCL you use is the independent variable because it will vary, and the time taken for the rate of reaction to take place(i.e. the magnesium to disappear) is the dependant variable because it depends upon the concentration of the hydrochloric acid.
All of these variables could effect the rate of reaction by increasing the number of successful collisions:-
Temperature effects the rate of reaction, when the solutions at a high temperature the particles all move quicker and cover a larger are in a given amount of time which means there’ll be more successful collisions. This is because the heat also increases the energy of the collisions which means the reaction will be quickened. Reactions only happen if the particles collide with enough energy. At a higher temperature there’ll be more particles colliding with enough energy to make the reaction happen. This initial energy is known as activation energy (AE) and is needed to break the initial bonds. If the temperature is not very high then the particles move slower which slows the rate of reaction compared to if the solution was hotter.
Breaking the solid (magnesium) into smaller pieces will increase the surface area which basically means the particles around it in the solution will have more area to work on so there’ll be more successful collisions increasing the rate of the reaction.
A catalyst creates a surface for the reacting particles to stick to where they can collide with each other and this also increases the number of collisions. They lower the activation energy of reactions making it easier for the catalysts to happen which also means lower temperatures can be used.
Variables which will not change to keep the investigation fair
Temperature- the temperature of the solution will not change because it can increase or lower the rate of reaction, how? See above. The temperature will remain at room temperature throughout the investigation to keep it fair.
Surface area- The amount of magnesium used will stay the same (2cm long) throughout the experiment.
Apparatus- The apparatus used will stay the same because differences in size and shape may affect the outcome of the results although the beaker with the acid and reactant will be washed out and cleaned each time we restart the experiment in case traces of solution are left in affecting the outcome of the next experiment.
The volume of the solution will remain the same (50ml) as the volume could also affect the rate of the reaction because if say the solution was 50ml one experiment and 56ml the next it wouldn’t be fair because the particles would be more crowded in the 56ml one and would collide more often. Accuracy is the name of the game, only a few ml above or below 50ml and it would offset the results entirely.
The above variables will not be changed. The time it takes for 10ml of hydrogen to be released by the solution will be measured accurately using a stopwatch it’ll be started as soon as the magnesium is put in the beaker with 2HCL in it. This way most results will be accurate. It is important to keep the reactants separate while setting up the apparatus so that the starting time of the reaction will be measured accurately. We will repeat each experiment 3 times and work out an average to make sure we don’t get any unreliable results.
Before we started the main experiment there were a few things we had to sort out:-
So we set up our experiment as explained later using the cylinder placed in the ice cream tub full of water because we thought it would give the most accurate results out of the two methods available. We found it to be best to use 2cm of magnesium strip because any more magnesium could be dangerous as it could react violently so we decided to use 2cm as it seemed the safest. We also found it to be best to use 50ml of hydrochloric acid at full concentration because again for safety reasons as too much could cause the magnesium to react violently and also because we are measuring concentration not surface area so we don’t want to use too much acid. We found we would record the data by timing how long it takes for 10ml of hydrogen to be released, starting the stop watch when the magnesium has been placed into the beaker with acid then stopping when the amount of hydrogen released reached the 10ml mark. We found that 10ml was best because otherwise we could be waiting for a long time if we changed the limit to something like 20ml and we didn’t have much time. We’ll record the results in a table and then transfer the results to a graph. We’ll use these concentrations for the acid 2, 1.6, 1.2, , 0.8 and 0.4.
Throughout the investigation we wore safety glasses so as to protect our eyes from hydrochloric acid our any other harmful substances coming into contact with our eyes.
First we set up our apparatus as illustrated below:-
As you can see we have filled a beaker 3/4 full, held a paper towel over it and then quickly turned it upside down and placed it in the ice cream tub full of water with one end of the rubber tubing going into it. The hydrogen released from the solution travels down the tube into measuring cylinder, water is denser than hydrogen and so the hydrogen rises to the top of the cylinder. It’s an easy way of measuring how much gas is given off.
Once that was set up we got on with the actual experiment.
We used 2HCL, the most concentrated hydrochloric acid available to year 11 students
An equation for the reaction which will take place:-
Magnesium + hydrochloric acid >magnesium chloride + hydrogen
Mg(s) + 2HCL(aq)>Mgcl2(aq) + H2(g)
1 mole 2 moles 1 mole 1 mole
So we can say that one molecule of magnesium reacts with 2 molecules of hydrochloric acid releasing hydrogen into the air.
We did this experiment three times to prove the results were accurate, if they were inaccurate we repeated the experiment again, or ignored wrong results. We tabulated the results in a clear table as soon as each experiment was completed ,in case we lost them and to check for possible errors or gaps in our results and it helped us to spot patterns in the results. And then we moved on to the next experiment. We measured 40cm3 of acid and then diluted it with10cm3 of water which made the acid less concentrated, we stirred the solution exactly five times (we did this with every experiment because if the water wasn’t diluted properly for some and then was diluted properly for other experiments then the results wouldn’t be fair) and then carried on doing exactly the same as the three steps above. We repeated this same procedure with all 5 experiments from 2-0.4 concentration of acid.
We kept our eyes on the experiment so that we could see exactly when we should’ve started the stop clock and stopped the clock. This way the rate of reaction (the time taken for 10ml of hydrogen to be released) was measured accurately for a fair test. We made sure the amount of magnesium used stayed the same in all the experiments(2cm long).
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 decreases. This complies with my prediction.
increases because the graph has its largest gradient or it is steepest at this point. As the rate of reaction decreases so does the concentration, although there is not a huge difference in the rate of reaction between M1.2 and M2 of the concentration of the acid, you can see the reaction steadily taking longer for M0.8. Then quite a surprise for M0.4 the reaction takes a lot longer (840 seconds) a huge increase and this is because as I’ve explained before there are not are less successful collisions because the particles are spread out a lot more and are not moving very fast because of the water molecules.
The concentration of the acid at M2 takes on average about 7.7 seconds for 10ml oh hydrogen to be released . By this we can determine the rate at which the reaction is taking place by using this formula:-
RATE= 1 =0.0012
There is a big difference in rates of reaction and scientifically proves that by diluting the acid you are decreasing the rate of reaction. These results fit the theory of collision.
I’ve concluded that concentration of an acid does effect the rate of the reaction and the theory on collisions (more collisions increases the rate of reaction) is correct. This applies not only Hydrochloric acid but to all acids.
There were some problems we encountered when carrying out this investigation:-
Replace our beaker in the water with a syringe which would collect the hydrogen gas which solve the turning the beaker upside down problem and would also give more accurate results as it would also be easier to read when 10ml of hydrogen had been released and its more user friendly than the quite fiddly method we used.
Use larger concentrations of acid would give a bigger more accurate conclusion
instead of just using 50ml of solution use 200ml, this way graphs would be more
spaced out and give an accurate form or curve.
I would use a data-logging computer to collect the results because there’d be no human error, I wouldn’t have to be present to record the results, and the results would be extremely accurate.
An alternative way of carrying out this investigation would be to use the gas syringe as explained above or use a mass balance. You’d set the apparatus up so that the hydrogen gas given off would be collected in a beaker on a mass balance, all you would do is time how long it takes for 10ml of hydrogen to be given off from the reactants by just looking at the display on the front. The advantages are that you’re results are going to be a lot more accurate because the mass balance can measure how much gas EXACTLY is in the beaker and so you don’t run the risk of human error.
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