Investigating the factors the affect the heat change in a neutralisation reaction.
Investigating the factors the affect the heat change in a
neutralisation reaction
Background information
Substances that neutralise acids are called bases. Bases are alkali -
they have pH's above 7; acids are acidic, and have pH's below 7.
Bases that can dissolve in water are also known as alkalis.
Acids
Acids are compounds of non-metals with simple molecular structures.
They all contain hydrogen (H) covalently bonded to other elements,
like HCl, or H2SO4 (hydrochloric and sulphuric acid respectively).
When an acid is dissolved in water, its molecules ionise. The
hydrogen present in every acid has the potential to ionise; if it
does, it becomes an H+ ion. It is the H+ ions that give acids their
acidic properties. The amount of an acid's molecules that ionise in
water determines the strength of its acidity. Vinegar, for instance,
is a weak acid, an only 1 in about 100,000 of its molecules ionise; on
the other hand, almost all molecules of HCl ionise. This means that
in an acid-base reaction involving vinegar, there are less H+ ions
available to react, and so less bonds are made compared to an
acid-base reaction involving HCl. Bond making is exothermic, and so
less energy is given out. Stronger acids (those with a higher number
of H+ particles that can dissolve in water) have a lower pH.
Bases
Just as all acids contain H+ ions, all alkalis contain OH- ions, and
it is this OH- (hydroxide) ion that gives alkalis their alkali
properties.
Bases are usually oxides, hydroxides, or carbonates of metal. Ammonia
is unusual in this respect, as it contains no metal element. Alkali
substances are also very corrosive, and can do even more damage to
living cells than acids.
The neutralisation reaction
NaOH(aq) + HCl (aq) NaCl(aq) + H2O(l)
The above is an example of a neutralisation reaction, involving an
alkali and an acid. The result is a salt and water. In every
neutralisation reaction, the metal in the base (Na+ here) takes the
place of the Hydrogen in the acid, forming a metal compound called a
salt. The term salt is used to describe any compound formed by the
reaction between a base and an acid.
From the above equation, we can break up these molecules into this:
Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq)
Na+(aq) + Cl-(aq) + HHO(l)
The Na+ and Cl- on the left side of the equation are present at the
end of the equation (on the right side), and are known as spectator
ions, as they have not chemically reacted. Therefore, the equation
can be rewritten, with just the reactants, as:
OH-(aq) + H+(aq) H2O(l)
This equation occurs in all acid/alkali neutralisation reactions where
the salt is soluble - hydrogen ions from the acid react with hydroxide
ions from the alkali to form water. The neutralisation reaction is
exothermic because of the bonds being made (OH- + H+ bonding together
to make H2O). This reaction can be generalised to apply to any
reaction between a base and an acid:
acid + base salt + water
If the base happens to be a carbonate, then carbon dioxide is formed
as well (e.g. calcium carbonate, CaCO3):
CaCO3 + 2HCl CaCl2 + CO2 + H2O
Calcium + Hydrochloric Calcium + carbon + water
carbonate acid
chloride dioxide
Moles
A single mole of an element contains 6 × 1023 atoms of that element.
It is a convenient unit of measurement as this amount of atoms has a
mass equal to that of the relative atomic mass of the element in
grams. This number is called the Avogadro ...
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If the base happens to be a carbonate, then carbon dioxide is formed
as well (e.g. calcium carbonate, CaCO3):
CaCO3 + 2HCl CaCl2 + CO2 + H2O
Calcium + Hydrochloric Calcium + carbon + water
carbonate acid
chloride dioxide
Moles
A single mole of an element contains 6 × 1023 atoms of that element.
It is a convenient unit of measurement as this amount of atoms has a
mass equal to that of the relative atomic mass of the element in
grams. This number is called the Avogadro Constant. Therefore one
mole of Hydrogen (6 × 1023 atoms of hydrogen) weighs 1g, and one mole
of Magnesium (6 × 1023 atoms of Magnesium) weighs 24g. When we refer
to solutions as having 1M concentration, we mean that it has 1M of
hydrogen in every litre of the solution.
Planning
I have been asked to find out how different factors affect the energy
released in a reaction between an acid and an alkali.
There are 4 main factors that could be investigated: the temperature
of the acid or alkali, the quantities of the acid or alkali, the
dilution/concentration of the acid or alkali, and the pH of the acid
or alkali (strong/ weak).
The factor I have chosen to investigate is the concentration of the
acid. This is the input/ independent variable.
I will have to keep the controlled variables the same in order to
ensure that any change in readings obtained are due to the change of
my input variable only. To make it a fair test, I will try to keep
the temperature the same throughout the investigation. I will keep it
away in the same place for every experiment ensuring that the room
temperature is the same. I will use a fixed quantity of alkali (20cm3)
and a fixed quantity of acid and water (20cm3). To do this accurately
I will use a burette to measure out the quantities. I will be
measuring the temperature change as the outcome/ dependent variable.
The problem is to find out how much energy is released as the
concentration varies.
I predict that the higher the concentration of the acid solution, the
greater the temperature change will be, and the greater the energy
released.
I have arranged to get sufficiently accurate and reliable results by
using a burette to measure the quantities of solution, I will use
polystyrene cups instead of glass cups and wrap cotton wool around it
for the reactions to reduce temperature loss, and lastly I will repeat
the experiment twice to get results that are more reliable. When using
the burette, I will check the markings before and after at eye level
to get the measurements as accurate as possible. The polystyrene cup
will also have a lid to reduce heat loss.
Linking prediction with theory: The higher the number of H+ and OH-
particles there are, the higher the temperature change. The higher the
concentration of acid, the more the acid (H+) ions the solution
contains. By changing the proportion of water to acid, we can change
the total number of H+ ions, and therefore the density of H+, in any
given area of the solution, in each test. This means that a strong
acid will have more H+ ions in any given area than one with a weaker
acid. So the more the H+ ions there are, the more chance these ions
will come into contact with the OH- ions to react. When they react
together, they make new bonds. Creating new bonds create energy, as
this is an exothermic reaction. This energy will be shown to us as a
temperature rise (change). During the reaction, there will be a sudden
increase in temperature- this is the reaction taking place. There will
eventually be a time when the temperature change decreases in its
acceleration. This slowness in temperature change is when there are no
more H+ ions to react with the OH- ions.
There are risks in this investigation as we are using harmful acids
and alkaline so we must minimise them. Acids are particularly harmful
to the skin and eyes therefore, I will be wearing safety goggles, a
lab coat, and protective gloves. I will also have to wipe up any
spillages.
Method
Equipment list:
Burette
Thermometer
Polystyrene cups
Beakers
Cotton wool
Spillage mats
Glass rod
Procedure:
* Measuring the required amount of acid (hydrochloric) from a
burette into a measuring cylinder Measuring the required amount
(20cm3) of alkali (sodium hydroxide) from a burette into a
measuring
* Pour the acid into a beaker (if necessary) and stir gently with a
glass rod Get the polystyrene cup ready with cotton wool taped
around the sides Pour the alkali into the cup and record the
temperature with the thermometer.
* Add the solution of acid and quickly, put the lid on top of the
cup Stir gently to speed up reaction. When the temperature of the
mixture has stopped increasing (i.e. the reactants have reacted
fully), record the final temperature.
* Repeat the experiment again for results that are more reliable
repeat the experiment for other concentrations and record all
results
Preliminary Results
Sodium Hydroxide Tests
Hydrochloric Acid
1 (degrees c)
2 (degrees c)
3 (degrees c)
Mean
Temperature Increase
0
19
19
19
19
0
0.2
21.5
20.75
20.25
20.5
1.5
0.4
22
21.75
22.5
22
3
0.8
23.25
22.75
23.5
23
4
1
25.25
25.5
26
25.5
6.5
2
29
30
30
30
11
I think that these results are very good and they are what I had
expected. These results show that the higher the concentration of
acid, the higher the energy rises. This means that the more the H+
ions there are, the more the energy there is. I will not change my
method because my current one gives me accurate and reliable results.
My preliminary experiment showed that the different concentrations and
quantities I used worked well to give reliable results. The table
above shows the results for both the 1st results, and the 2nd results-
the repeated ones. I have then taken an average for each of the
different molarities.
To make my experiment more accurate, I did several things. In the
actual experiment, I used a more accurate mercury thermometer to
measure the temperature, a thermometer that measured to the nearest
tenth of a degree. When measuring the solutions from the burettes, I
made sure that I was at eye level to the solution. I.e. my eye was
horizontal to the mark where I wanted. I also did this with the
mercury thermometer. When I was recording the final temperatures of
the reacted/ reacting solution I waited until the temperature was at
its very highest so that I could get maximum results for the
temperatures.
Table of Results
Sodium Hydroxide Tests
Hydrochloric Acid
1 (degrees c)
2 (degrees c)
3 (degrees c)
Mean
Temperature Increase
0
19
19
19
19
0
0.2
20.5
20.25
21.5
20.75
1.75
0.4
21
21.25
21.5
21.25
2.25
0.8
23
22.5
23.5
23
4
1
25
26
25.5
25.5
6.4
2
31
31
31
31
12
Analysing Evidence
To calculate the energy released we have to use the formula:
Energy change = Temperature change × total volume × specific heat
capacity
The specific heat capacity of the acid and alkali is 4.2J /g /°C. The
total volume will always be 60cm3 as we used a total of 60cm3 in each
different molarity.
The results on my graph tell me that as the concentration of acid
increases, the energy released increases. The two axes are directly
proportional to each other- as x increases y increases. My results
agree with my initial prediction that as the concentration of acid
increases, the energy released increases. This is because as the acid
concentration is increased, there are more H+ ions available. With
more H+ ions available, more bonds can be made; the process of making
bonds is exothermic, and so energy is produced, which we measured as
heat energy. Looking at the graph, you can see that it is straight
line through the origin. This tells me that when there is no acid,
there is no energy released. This supports my theory that when there
are no H+ ions, there is nothing to react with the OH- ions therefore
there is no energy given out. From this, we can say that there must be
H+ ions present for there to be any energy change - the temperature
increase we are investigating is dependent upon the bonds formed
between OH- and H+; no bonds can be made without either reactant.
If I were to extend the graph so the x-axis extended past a molarity
of 2.0, we would see a change in the pattern. At the ratio of 20cm3
acid: 20cm3 alkali we get a large output of energy from the reactants.
If there were more acid than alkali, we would expect to get the same
output of energy from the reactants as a ratio of 20cm3 acid: 20cm3
alkali. The line on the graph would start to flatten to a horizontal
line (a gradient of 0).
This is because the H+ ions have fully reacted with the OH- ions;
increasing the number of H+, ions (increasing the concentration) will
have no effect on the energy released, as there will be no more OH-
ions to react with. All the H+ ions will have reacted with the OH-
ions therefore there will be no OH- ions for the H+ ions to react
with.
My results are what I not really what I had expected as I thought that
the graph would be a curve because the H+ ions would run out of OH-
ions to react with. This was the case except it would start to happen
after a molarity of 2.0. Maybe this was the case and my graph did not
show it. If I took more results and repeated them, more I could have
come to a more definite conclusion. Having said this, my current graph
would be a curve if I had extended it like shown in the above diagram.
It is sort of like a curve.
Evaluation
I found my experiment easy to carry out. I did not have to change my
plan at all having done a preliminary. It all went quite smoothly
considering I missed most of the lessons and didn't really know what
was going on. This was the only real problem for me.
I think that I did take enough results and the range of them was large
enough. I had a good set of different molarities that had good regular
intervals between them. If I were to repeat the experiment, I would
repeat the results so that I would have 5 results for each molarity to
even out any anomalies. Looking at my graph, I think I would like to
take some more results near the 2.0 molarity mark to fill in some
gaps. I would look at molarities higher and lower the 2.0 molarity
mark so that I could find out whether the line would curve off and
flatten for definite.
I think that my results were accurate, as most of them lied close to
the line of best fit. If I were to do this experiment again, I would
not change the way I measured things. I measured them accurately.
There was one anomalous result. This is shown ringed on the graph. I
do not really know what had caused it- maybe I had not cleaned the
polystyrene cup out before I added the new solutions. Alternatively,
maybe I was too slow in putting the thermometer into the solution. I
think that my results are consistent with each other. There were not
any that were so different that I should have ignored. The anomalous
result gave me a temperature increase of 12. This is a considerably
large difference but I decided that I would not ignore it. I took the
average like the other readings and plotted, circling it as an
anomalous result. However, I did ignore it when plotting the line of
best fit.
To extend my inquiry I can look at many things. Having looked at
strong acids and alkaline (hydrochloric acid and sodium hydroxide
solution) I can also look at weak acids and alkaline. However, the
results should be the same except I would expect to get less energy
from the weak acids/ alkaline. I could also look at mixing the tow: a
weak acid with a strong alkali or vice versa. This could give some
interesting results although they might not be fair. Looking at
different acids and alkaline could also be done to extend my inquiry.
This also links in with strong and weak acids. We could look at
sulphuric acid or calcium hydroxide solution. This also links in with
pH. We could use acids and alkaline of different pH is to see whether
they follow the same pattern as the one we investigated. Another
possibility would be looking at acids reacting with metals.
