Internal Assessment – Investigating the relationship between concentration of reactants and rate of reaction using hydrochloric acid and magnesium strip Research Question
To study the effect of manipulating the concentration of hydrochloric acid on the rate of hydrogen gas production during the reaction with magnesium, using the time taken for the reaction to complete (the fizz to disappear) Introduction
The rate at which w hich a reaction occurs is governed by a variety of factors. These include the concentration of the reactant (or pressure when in gaseous form), the temperature at which the reaction occurs, the surface area of the solid reactants and the presence, or lack of presence, of a catalyst. If any one of these conditions is altered, the rate at which a reaction occurs will be altered. In this experiment the reaction between hydrochloric acid solution and magnesium ribbon is used to investigate the effect of reactant concentration on the rate of reaction. The concentration of hydrochloric acid is manipulated using serial dilution. (adding water to 2 molar HCl in 50 ml solutions) In order for a reaction to occur, there must be a collision between two particles. The rate of reaction is the rate at which these collisions occur. The concentration is an expression of the amount of particles of one substance present. If the concentration of one reactant is increased (and the volume is kept constant) there will be more particles, and hence more collisions between particles. The rate of reaction will then increase. This can be observed through t he rate of evolution of the products of the reaction i.e. the time taken for the reaction to complete. Aim
The aim of this experiment is to investigate the affect of a change in concentration on the rate of reaction. This experiment will be a reaction between a constant amount of magnesium and a constant volume of hydrochloric acid, which will occur in t he following equation:
The independent variable will be the concentration of the hydrochloric acid, the co ncentration of which will vary between amounts 20 ml to 50 ml in 50 ml solution. The rate will be measured by measuring the time for the reaction to complete. The rate of time for completion of reaction will be compared between concentrations, to draw a conclusion about the rate of reaction.
Hypothesis
Increasing the concentration of the HCl means that there will be more particles per dm3 of this substance. The more particles that there are in the same volume, the closer to each other the particles will be. This means that the particles collide more frequently with each other and the rate of the reaction increases. So, when the concentration of HCL is increasing, the rate of reaction will increase, the less time taken for the reaction to occur. Variables Dependent Variable
Independent Variables The concentration of HCl is constant at 2 molar however I will vary its amount in 50 ml solution.
The time taken for the reaction to complete (the bubbles to stop producing)
Controlled Variables
How will it be controlled?
Temperature of the room
The experiment will be conducted in the room in the morning when there is a constant temperature.
Temperature of Hydrochloric Acid
All the acid will be stored in one place prior to the experiment.
Amount of Magnesium Ribbon
A ruler is used to cut 2cm strips of magnesium ribbon.
Volume Of Hydrochloric Acid
A measuring cylinder will be used to measure 50 ml of solution (HCL + water)
20 ml in 50 ml solution 25 ml in 50 ml solution 30 ml in 50 ml solution 35 ml in 50 ml solution 40 ml in 50 ml solution 45 ml in 50 ml solution 50 ml in 50 ml solution
Why does it need to be controlled? The temperature needs to be controlled because it can alter the chemical reaction and thus affect the rate of reaction If the temperature is not constant, it would alter the rate of reaction. The amount of magnesium ribbon used is a major factor that affects the rate of reaction. For a fair test, the same amount is thus used.
If the volume of solution is used, it will affect the rate of reaction.
METHOD Preparation
Using a rule and scissors, 25 2cm strips of magnesium ribbon were cut.
The pieces of magnesium ribbon were then kept in a folder paper to avoid oxidation.
The required 50ml solutions of HCl were prepared.
Experiment
Pour 50 ml of solution (20 ml of 2 molar HCl + 30 ml water) into a conical flask
A 2cm magnesium ribbon was placed into the conical flask of HCl solution.
As soon as the magnesium ribbon was placed into the conical flask, the timer was immediately started.
The time was recorded on the stopwatch when the fizzing had stopped and the magnesium ribbon had disappeared.
Steps above were repeated using 25 ml, 30 ml, 35 ml, 40 m l, 45 ml and 50 ml hydrochloric acid.
RESULTS Data Table 1
HCl volume in 50 ml solution/ml ± 1 ml
Concentration -3 (mol.dm )
T1 (s) ± 2s
T2 (s) ± 2s
T3 (s) ± 2s
T avg (s) ± 2s
20.0
.80
140
143
138
140
25.0
1.0
107
108
110
108
30.0
1.2
73.0
80.0
75.0
76.0
35.0
1.4
53.0
54.0
56.0
54.0
40.0
1.6
46.0
46.0
41.0
44.0
45.0 50.0
1.8 2.0
32.0 24.0
36.0 25.0
30.0 22.0
33.0 24.0
Graph 1
Tavg(s) vs Molarity (mol.dm -3 ) 160 140 s / e t 120 e l p m100 o c o t n 80 o i t c a e R 60 r o f e m i 40 T
Time vs Molarity Expon. (Time vs Molarity)
20 0 0
0.5
1
1.5
2
2.5
Molarity (mol.dm -3 )
ANALYSIS OF GRAPH 1
Graph 1 is a curve graph where the gradient of the curve is steep at t he beginning and progressively becomes horizontal when the concentration is increasing. Based on the Graph 1 that I plotted, when the concentration of HCl is lowered, the time taken for the fizzing of bubbles to stop is longer. This means that the rate of reaction is lower. Graph 1 show that the concentration of HCl is inversely proportional to average t ime taken for the reaction to complete( fizzing to stop and the magnesium strip to disappear) So, to get the straight line graph, I need to plot the graph of 1/time taken for the reaction to complete versus concentration of HCl.
Data Table 2 HCl volume in 50 ml solution (ml) ± 1 ml 20.0 25.0 30.0 35.0 40.0 45.0 50.0
Concentration -3 (mol.dm ) .80 1.0 1.2 1.4 1.6 1.8 2.0
Tavg (s)
1/Tavg (1/s)
140 108 76.0 54.0 44.0 33.0 24.0
.007 .009 .013 .019 .023 .030 .042
Graph 2
1/Tavg(1/s) vs Molarity 0.045
y = 0.028x - 0.0188 R² = 0.9448
0.04 0.035
1/Tavg vs Concentration
0.03
) s / 1 0.025 ( g v a 0.02 T / 1
Linear (1/Tavg vs Concentration )
0.015
0.01 0.005 0 0
0.5
1
1.5
2
2.5
Molarity (mol.dm-3)
Analysis of Graph 2
The graph plotted above shows a linear relationship however the best fit line doesn't pass t hrough the origin, hence I suspect that there was a mistake in my experiment. Graph shows a linear relationship between Concentration of HCl and Rate of reaction (1/time) showing that they are proportional. This also confirms that the order of reaction in respect to Concentration Of HCl is 1.
CONCLUSION
The aim for this investigation is to investigate what is the effect of the concentration of Hydrochloric Acid on the rate of reaction with Magnesium Strip. In my hypothesis statement, I stated “when the
concentration of hydrochloric acid increases, the rate of reaction will increase, the less time taken for the reaction to complete. For the above statement, my hypothesis is proven true because the graph of rate of reaction versus concentration of Hydrochloric Acid is directly proportional as the relationship was linear because the graph shows a straight line. Further calculations indicate that “the order of reaction with respect to Hydrochloric Acid is one”. This statement holds true because the concentration
of Hydrochloric Acid is proportional to the rate of reaction.
EVALUATION AND WAYS OF IMPROVEMENT
The errors in my experiment could be reduced using the following measures 1) I suspect that there were systematic errors in the experiment because the best fit line of graph was shifted by a value of -.018. This could occur because of a human error(hand eye co-ordination). Also at the end of the experiment, the fizzing was sometimes too small to notice thus causing a error in the experiment. 2) The rate and number of stirring in each experiment is different and this affects the time taken for the reaction to complete(the magnesium piece to disappear). To improve the experiment, I could use a magnetic stirrer to make the stirring constant. 3) A significant error is the impurity of water used in the solution of HCl. Solutions of HCl were not pure and there is no guarantee of this. Even exposure of air could result in disruption of purity of substances. The water used in the experiment came from a tap where minerals and chemicals could have potentially affected the results. Therefore to fix the random error, water should be used from a distilled source. 4) Due to time restriction, I could only do 3 trials for each concentration. So to improve the experiment further more trials can be taken to eliminate random errors in my experiment.
