In Chemistry for you, Lawrie Ryan Stanley Thornes 1996 Page 235, they did the same experiment in the same conditions, and below are their results and it clearly shows that the rate is proportional to the concentration and therefore repeating the experiment should obtain the similar kind of results.
As the rate should be proportional to the concentration of hydrochloric acid, it can be deduced that Rate ≈ [HCl]1
Instead of the proportionality sign, a constant is usually introduced, Rate = k[HCl]1
The constant is known as the rate constant and the square brackets mean concentration in mol dm-3. The power to which the reactant is raised to is the order of reaction with respect to the reactant, which in this case, is hydrochloric acid.
Reactive metals such as zinc and magnesium react with acids to produce a salt and hydrogen gas.
Metal + Acid → Salt + Hydrogen
Magnesium + Hydrochloric Acid → Magnesium Chloride + Hydrogen
The hydrogen in this test will travel through the delivery tube into the measuring cylinder, as this cylinder is filled with water, the hydrogen forces water out to allow required space. This is how the amount of hydrogen in the cylinder is recorded, by measuring the space of unfilled water.
Apparatus
Measuring cylinder
Beehive shelf
Trough
Conical flask
Magnesium
Hydrochloric acid
Bung
Delivery tube
Water
Method
- You require magnesium ribbon 3cm long
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Measure 50cm3 of hydrochloric acid with a measuring cylinder before pouring it into a flask.
- Arrange the equipment as shown in the apparatus diagram on the previous page
- And when the magnesium is added, the bung must be replaced as quickly as possible and the stopwatch is started simultaneously.
- When the reaction has finished (no more hydrogen “bubbles up” through the measuring cylinder) the stopwatch is stopped.
- The space left over in the measuring cylinder (absent of water) is hydrogen and this is calculated and recorded.
- Each experiment should be repeated three times for accuracy.
This experiment is to be carried out of different concentrations of the acid. With the three sets of results for each concentration, an average can worked out. There are nine concentrations to take, these are as follows:
- 2.0 [m]
- 1.8 [m]
- 1.6 [m]
- 1.4 [m]
- 1.2 [m]
- 1.0 [m]
- 0.8 [m]
- 0.6 [m]
- 0.4 [m]
The main variable of this experiment is the concentration of the hydrochloric acid; the amount of each reactant is constant. From the experiment, the main things that are being tested for are the time taken for the reaction and the volume of hydrogen produced. This much information is then enough to find the rate of reaction. Rate = Volume / Average Time.
By keeping it a fair test, the amount of reactant is always constant because an increasing amount means more particles and more successful collisions affecting the rate. Also, the surrounding temperate should be about constant; with a high temperature the particles move around more quickly. This causes them to collide more often and therefore react more quickly. (Page 66 Chemistry for OCR A by Lees and Payne)
The safety measures/precautions taken are to wear goggles as protective eyewear, hydrogen is a flammable gas, also, hands should be washed after the experiment is over and no other chemical should be touched with unwashed hands.
Conclusion
From the results obtained, my graph shows that when the concentration increases, the rate of reaction also increases. This means the stronger the acid, the faster the reaction, this can be explained using the collision theory. As the concentration increased the particles became closer to one another and susceptible to successful collisions and so a faster rate takes place, my graph shows this.
When referring this graph to the prediction, we can see that it is not a 1st order reaction but a 2nd order reaction. This means that as the concentration goes up the rate of reaction also goes up by the square of the concentration y=x2
The rate expression has actually turned out to be Rate = k[HCl]2
The reason for the change of order is because at higher concentrations, the rate of reaction increases, but because the reaction is exothermic at higher concentrations, temperature plays it’s part and the particles move around more quickly. This causes then to collide more often, and therefore react more quickly (Page 66 Chemistry for OCR A by Lees and Payne)
It was first predicted that the result graph would show the proportionality between the two factors, but as it is shown, the concentration is not proportional to the rate of reaction.
Analysing this investigation:
- In the reaction between magnesium and hydrochloric acid, the higher the concentration of the acid results in the higher rate of reaction.
- If the concentration is higher, there is a higher rate of reaction and as the reaction is exothermic, the particles gain energy and collide more often and so the rate increases further.
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Because of the reaction being exothermic at the higher concentrations, it turned out to be 2nd order as opposed to a 1st order reaction.
Evaluation
Most of my results are accurate; the few that may not be very accurate are only marginally off the mark. The method was fair because we kept the amount of each reactant the same. Having more of a reactant means there are more particles and more chance of successful collisions (faster rate.) Also, the timing had to be accurate; the stopwatch had to be started simultaneously as the magnesium was dropped into the hydrochloric acid for all of the concentrations. This is because the magnesium starts reacting as soon as the strip has made contact with the acid.
My results are fairly accurate because it shows a strong correlation and clearly shows the relationship involved. There was an anomaly in the graph, for the 1.4[m] concentration, the point on the graph didn’t take part in the general trend. I would expect the rate to be slightly higher than it was. This could have occurred because of human mistake, maybe the timing wasn’t accurate or the amount of acid might have been a little more than what was required.
The results obtained seem reliable beacause the line of best fit clearly shows the correlation, if there were numerous anomalies, then the graph wouldn’t prove to be reliable, but as these results have only minor inaccuracies, it can show the correct trend.
Because the reaction begins as soon as magnesium is added, the fact that putting on the bung after the reaction had started may lose gas that could affect the reliability of the results and could be prevented if a divided flask was used. A divided flask contains the reactants and is airtight, all you have to do is to tip it so both reactants combine and the experiment would be more accurate.
We could improve our method by using a water bath, where the temperature is a constant. This would help us because it would improve accuracy, as the temperature doesn’t come into effect. This method would prevent the effect of an exothermic reaction on the overall results.
A downfall was that we didn’t know exactly when the reaction actually stopped. This was because we were only confirmed that the reaction was taking place by ocassional bubbling and because this is only occasional, when the reaction had stopped in the real world; we were still waiting for another bubble (and once we found out none came, we stopped the stopwatch some 10- 15 seconds later.) If we made this mistake for all the concentrations with approxiamately the same number of seconds later, this wouldn’t be an issue but because at lower concentrations there is a low rate; the hydrogen produced is also slower. This decreases accuracy and reliability of the experiement.
The method I have used is suitable because it is set in the form of instructions to someone who is about to carry out the experiment. Also, it is formed in bullet points making it more straightforward. When we practically carried out the method it was all done in a logical and organised order repeadedly (9 concentrations, 3 times each)
