Endothermic reactions take in energy from the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. The temperature decrease can also be detected using a thermometer.
Our reaction is endothermic.
Orders of reactions
Zero-order reactions (order = 0) have a constant rate. This rate is independent of the concentration of the reactants. A first order reaction (order = 1) has a rate proportional to the concentration of one of the reactants. A second-order reaction (order = 2) has a rate proportional to the concentration of the square of a single reactant or the product of the concentration of two reactants. In other words:
- Zero – rate doesn’t change as concentration increases ( x+y=product)
- First- rate proportional to concentration (y=x)
- Second – rate proportional to concentration or more curvy (y=x²)
Reference:http://www.avogadro.co.uk/kinetics/rate_equation.htm
The Collision Theory
A reaction will only take place when the reactant particles collide with enough energy (activation energy). The activation energy is the energy needed to break the bonds in the molecules so that new bonds can be formed to make the products. Any factor that affects the frequency of collisions or the energy of the reactant particles will lead to an increase in the rate of reaction.
Reference: ,
http://www.talktalk.co.uk/reference/encyclopaedia/hutchinson/m0030471.html
Aim
The rate of reaction measures the change that occurs in a single component of time. When a chemical takes place the particles of the reactants collide. The more often the particles collide the more likely they are to react and so the faster the reaction is. The aim of this experiment is to investigate the rate of reaction between magnesium and hydrochloric acid using the method – Measuring the volume of gas product produced at regular intervals. In other words how the concentration changes whilst measuring the rate of reaction with magnesium and hydrochloric acid.
MASH
Metal + acid → salt + hydrogen
Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)
Magnesium + Hydrochloric Acid Magnesium Chloride + Hydrogen
(s) – Solid
(g) – Gas
(aq) - Aqueous
Magnesium and Hydrochloric Acid Background
Magnesium is a solid which is a greyish-white colour. It is shiny, lightweight and ductile. It tarnishes slightly when exposed to air and because of this magnesium is protected by a thin layer of oxide that is fairly impermeable and difficult to remove. Magnesium is a highly flammable metal when in thin strips or in powder form. Magnesium is in group two of the periodic table. The atomic number is 12. it was discovered by Sir Humphrey Davy in 1808.
Reference for Picture:
Hydrochloric acid, on the other hand, is a colourless and odourless liquid solution (which consists of hydrogen chloride and water) which is corrosive, toxic and is a strong mineral acid. The concentrated acid releases dangerous quantities of hydrogen chloride vapour which if inhaled can be extremely harmful. Also contact with eyes or skin can cause serious permanent damage. In addition it reacts vigorously/violently with most metals.
Prediction
Given the collision theory, I predict that the higher concentration of hydrochloric acid, the faster the reaction. This is due to the fact that there are more molecules in the acid likely to collide with the magnesium. The rate of reaction also relates to the surface area of the magnesium, the bigger the area the greater chance of collision and thus the faster the reaction takes place. I am quite confident with my prediction.
I also believe the investigation I am running will be mostly second order as I am using quite steady concentrations and enough magnesium for me to track and record the results.
Methods for measuring the Rate of Reaction
There are four main methods to measure the reaction rate. They are measuring the volume of gas product produced, measuring the change in the mass of the reactants at regular intervals, timing how long it takes for a small amount of solid reactants to disappear and timing how long it takes for a solution to turn cloudy. I have considered using measuring the volume of gas product produced at regular intervals. The reason being is that you can tell how fast it goes at the beginning, middle and end. You can also get a more accurate picture.
If we collect hydrogen over a period of time, we can get a better rate of reaction. This is because we measure it at closer time intervals. We are not using the “measuring the change in the mass of the reactants at regular intervals” as the mass of hydrogen gas is very small.
Reference for table information and pictures: Twenty First Century Science GCSE Chemistry OCR Oxford, 2006, University of York
Strategy
To measure the volume of gas product produced at regular intervals I must measure the calculated amount of hydrochloric acid using one of the measuring cylinders and pour the acid into the measuring cylinder. Then I have to set up the apparatus using my equipment and fill the bowl with water. After that I have to fill the other measuring cylinder with water, and make sure that it stays filled with water when you turn it upside down (with no air bubbles). I have to then add a 3cm strip of magnesium into the beaker, put the cork (end of delivery tube) back into the flask as quickly as I can, and start the stopwatch. I then must record the volume of hydrogen gas given off at suitable intervals (e.g. 5/10 seconds). I must continue this until no more gas appears to be given off.
Equipment
- Goggles (to protect our eyes from getting acid in them)
- Clamp Stand (to secure the cylinder so it doesn’t accidentally topple)
- Stop watch (to time the reaction at regular intervals)
- Hydrochloric Acid (the reactant)
- Delivery Tube (to collect gas and to make sure none is lost)
- Measuring Cylinder (to measure accurate amounts of acid)
- Water (stays in the beehive shelf so the measuring cylinder become full)
- Sandpaper (to scrap the oxides that exist naturally of the strips of Mg)
- Beehive shelf (to hold the water so the measuring cylinder stays full)
- Pipette (to make sure I got the right amount of acid and to make sure none spilt)
- Beaker (the beaker is used to drop the magnesium into the hydrochloric acid)
Reference for pictures:
Safety
- Wear goggles throughout experiment as I am dealing with hydrochloric acid so it will help protect my eyes from the irritant acid.
- If acid comes into contact with your skin wash of immediately with water.
- Wear gloves to prevent hands burning as the hydrochloric acid is a little corrosive and is an irritant.
- I am using a pipette to prevent the hydrochloric acid from spilling.
- Never sit down whilst doing practical work as if there is a spill or explosion it may be difficult to get away.
- Tie up hair, tuck in ties, put in stools, and put all books and belongings away from the experiment area.
- Clean all surfaces that we have worked on so that none of the acid has been spilt on the table (if so it can irritate pupils who may touch the surfaces).
Reference for pictures: .openschool.bc.ca/elementary/science7/mod1.html
Dilutions
Our concentration intervals shall lie between 2mol to 0.5mol. We were told to keep the same volume of acid the whole time. The minimum was 20cm³ and the maximum was 40cm³. We then decided as a group to go with 30cm³.
To work out the volumes:
Volume of acid = concentrated acid wanted x total volume of solution
Original concentration
Of acid
Volume of water = total volume x volume of acid
We also decided that our time intervals would be every 5 seconds. We decided to do 7 concentrations instead of 6 as we thought that it may be a good idea to have a back concentration if one went wrong. The concentrations are:
- 2 mol/dm³
- 1.75 mol/dm³
- 1.5 mol/dm³
- 1.25 mol/dm³
- 1 mol/dm³
- 0.75 mol/dm³
- 0.5 mol/dm³
Preliminary Test
The preliminary tests that I have done will help me in my experiment because I can learn from my mistakes and improve my original experiment. Hopefully these will give me good results. The preliminary test helps us find the appropriate amount of magnesium ribbon needed to use (3 cm). This helped us ensure that the reaction would not take place to quickly or to slowly. When creating the different concentrations of hydrochloric acid it was essential to try and be as exact as possible. It was also important to bear in mind, to use a fresh reaction flask for each separate concentration of acid to make sure that the subsequent experiment was not contaminated in any way.
____ = Stopped reacting or went off scale (on measuring cylinder).
Dilution table for water
We began using 5 mol but the rate of reaction was too rapid, this didn’t give us the chance to accurately record results. After a couple of attempts with 5 mol we decided to eventually use the 2 mol acid. This gave us the opportunity to record results so we agreed to use the 2 mol for the real experiment. We also tried to use 2cm ribbon of magnesium but it reacted too quickly with both 5 mol and 2 mol. So we decided to use 3cm. on the same hand if the magnesium ribbon were to rust (become covered with magnesium oxide), we would have used sand paper to remove the rust. However the magnesium did not rust for the preliminary test or the real experiment. If it did rust our results would have been affected as another product would have been added to the reaction.
The tables below are the working out my group and I had to do in order to work out the volume of water needed for our experiments – both preliminary and real. My lines of best fit were simple to draw as the points are close to it making my experiment dependable. Most of my error bars were small meaning that my results were fairly accurate. Nevertheless, one thing our group noticed was the fact that the reacted mixture had become hot (temperature), this may have affected my results and made them a little unreliable. In the future, if I were to repeat the experiment, I would use a water bath to cool the mixture down.
Justification of concentrations
The set concentrations I have chosen are: 2 mol mol/dm³, 1.75 mol/dm³, 1.5 mol/dm³, 1.25 mol/dm³, 1 mol/dm³, 0.75 mol/dm³ and 0.5 mol/dm³. These concentrations are high enough and the right speed to react with the magnesium.
