Introduction:
Neutralisation reactions
Acids and alkalis are defined as:
An Acid:
A substance that dissolves in water, producing H+ ions as the only
positive ions.
An acid is a substance, which contains hydrogen, which may be replaced by a metal to form a salt.
Properties:
They change moist litmus paper from blue to red.
They are soluble in water.
They are electrolytes.
They also have a sour or sharp taste.
Cautions:
Some acids are poisonous
Many acids are corrosive and thus dangerous. They burn flesh.
Acids as proton donors:
Acids produce hydrogen ions as the only positive ion. For example when hydrogen chloride dissolves in water the following process occurs.
HCL(aq) → H+ (aq) + CL- (aq)
The hydrogen ion is sometimes called a proton. In water, the proton is combined with water as a result of the following process:
HCL(aq) + H2O(l) →H3O+ (aq) + CL – (aq)
H30+ is known as a hydroxonium ion.
Hydrochloric acid has donated its protons to the water:
H+ (aq) + H2O (l) → H30+ (aq)
All acids are proton donors.
Strong acids are fully ionized in water and are strong electrolytes. A strong acid produces a high concentration of H+ ions in a water solution. E.g. Hydrochloric acid. (HCl). Examples: sulphuric acid, hydrochloric acid and nitric acid. For instance, nitric acid:
HNO3 (aq) + H2O (l) → H3O+ (aq) + NO3- (aq)
Weak acids are partially ionized in water and are weak electrolytes. Examples: Ethanoic acid. A weak acid: Produces a low concentration of H+ ions in a water solution. E.g. Ethanoic acid. (CH3CO2H)
Common strong acids include:
Hydrochloric acid (HCl)
Nitric acid (HNO3)
Sulphuric acid (H2SO4)
Common weak acids include:
Citric acid (H3C6H5O7)
Ethanoic acid (CH3COOH) (vinegar)
Alkalis and Bases
A soluble base is something which produces OH- ions in water.
A Base is a substance, which will react with an acid to form a salt.
A base is a proton acceptor.
An alkali is a base, which is soluble in water.
Properties:
They change litmus paper from red to blue.
They are electrolytes.
In addition many alkalis have a soapy feel.
All bases and alkalis, except ammonia, are metal oxides or metal hydroxides.
CAUTION: Many alkalis may be corrosive and poisonous. Example: sodium hydroxide is often called caustic soda. Caustic means ‘burning’.
A strong alkali:
Produces a high concentration of OH- ions in a water solution. Eg. Sodium hydroxide. (NaOH)
Strong alkalis are fully ionized in water and are strong electrolytes.
Weak alkali:
Weak alkalis are only partially ionized in water and are weak electrolytes. A weak alkali produces a low concentration of OH- ions in a water solution. E.g. Ammonia solution. (NH4OH)
Bases as proton acceptors- when a base reacts with an acid to form a salt, it accepts. Example: magnesium oxide reacts with sulphuric acid to form magnesium sulphate
MgO(s) + H2SO4 (aq) → MgSO4 (aq) + H20 (l)
During this reaction the oxide ion, O2-, of the base accepts 2 protons H+ (O2- (s) + 2H+ (aq) → H2O (l))
Common strong alkalis include:
Sodium hydroxide (NaOH)
Potassium hydroxide (KOH)
Common weak alkalis include:
Ammonium hydroxide (NH4OH)
Aluminium hydroxide (Al(OH)3)
Magnesium hydroxide (Mg(OH)2)
Hydroxide ions:
When alkalis dissolve in water an alkaline solution is formed. Alkaline solutions contain hydroxide ions. Example solid sodium hydroxide produces hydroxide ions when added to water.
NaOH(s) → Na+(aq) + OH(aq)
These hydroxide ions accept protons to form water in the reactions between acids and alkalis.
H+ (aq) + OH- (aq) → H2O (l)
Neutralization reactions
Acids react with bases to form salts.
Acid + Base → salt + water
When aqueous solutions of an acid and a base are combined, a neutralisation reaction occurs. This reaction is characteristically very rapid and generally produces water and a salt. For a strong acid and a strong base in water, the neutralisation reaction is between the hydrogen and hydroxide ions dissolved in solution: H+ + OH- → H2O
Neutralization is the reaction between an acid and a base in such quantities that only the salt + water are produced and no acid or base remain in the solution. When reacting both acid and alkali, both quantities must be as equal as possible, if a neutral solution is desired.
Strong acids and strong bases completely break up, or dissociate, into their constituent ions when they dissolve in water. In the case of hydrochloric acid, hydrogen ions, H+, and chloride ions, Cl-, are formed. In the case of sodium hydroxide, sodium ions, Na+, and hydroxide ions, OH-, are formed. The hydrogen and hydroxide ions readily unite to form water. If the number of hydrogen ions in the hydrochloric acid solution is equal to the number of hydroxide ions in the sodium hydroxide solution, complete neutralisation occurs when the two solutions are mixed.
Heat Involved in Chemical Reactions
The reaction of neutralisation is of course an exothermic reaction. This means that heat is given out during the chemical change that occurs. Along with all neutralisation reactions, all combustion reactions are exothermic, as they of course give out heat. The reactions, which are accompanied by a drop in temperature, are known as endothermic reactions; these reactions take in heat. When using a value of measure to the amount of heat given out, the end result is given a negative value for the change in energy. This may seem a bit odd due to the fact that it is clear that an exothermic reaction emits heat. The reason for giving exothermic reactions a negative ΔH value is because the energy held by the substance has decreased, conversely, in an endothermic reaction, the energy of the actual substance has risen because the energy is held in the bonds.
The reason for heat being released from a reaction is because there are more bonds broken than are made, when bonds are broken, energy is taken in whereas the making of bonds leads to energy being produced. If the reaction is endothermic, there are obviously more bonds to be broken than have been made. It is also the case that stronger bonds take more energy to break than weaker bonds, and when stronger bonds are made, they release a greater amount of energy than when weaker bonds are created. Going by this, it is clear that every single reaction will have, to some degree, an energy change. Another factor discovered is that the amount of energy taken in by breaking bonds equals the amount of energy released through the creating of new bonds.
The amount of energy taken in or released can be expressed in kilojoules or joules, the SI unit for energy. To make the investigation fair I will express my values per mole. The energy changes that occur in reactions can be shown using energy level diagrams. In these diagrams, energy goes on the y-axis, and the x-axis is labelled as the reaction process, which shows the progress of the reaction. These diagrams do not show any numerical values, they are only used to show trends of energy changes in exothermic and endothermic reaction. The enthalpy diagrams are shown below, for both endothermic and exothermic reactions
The equation used to work out the energy transferred is ENERGY (KJ)= S.H.C X MASS (in g) X temperature change (in Kelvin)
Exothermic Reaction Endothermic Reaction
Products
Reactants
Products Reactants
Progress Of reaction Progress Of reaction
I have chosen to vary the type of the acid, for my investigation. I have decided that I am going to have a wide selection of different acids to investigate, but for the alkali, I am only going to have one weak and one strong one. I have chosen to vary the factor of type of acid, rather than volume or concentration (of either acid or alkali), because it would allow a simple and easy procedure. Another reason for choosing this variable is that it will allow a multitude of different combinations that will lead to clear-cut conclusions. The procedure for this variable is also somewhat less complex than most of the others. The acids that I will use for my experiments will be one molar values of sulphuric, hydrochloric, nitric, Ethanoic, methanoic and citric acid. I will use one molar Sodium hydroxide for the course of the investigation.
Preliminary experiment
Aim: to carry out the study on how the change in temperature of a neutralisation reaction is effect by the change in acid used in the reaction. I am going to use 6 different acids and 1 alkali for each experiment.
Prediction: I predict that the stronger acids will produce a higher temperature rise because, first of all, they have no bonds to be broken; in solution they exist as their component ions, completely dissociated. It is known that the breaking of bonds causes energy to be taken in, and when bonds are made, energy is given out. Another reason for a stronger acid producing a higher value for the heat of neutralisation is because it has more free H+ ions. I have deduced this because it is known that in a strong acid, all of the molecules are dissociated into their component ions. When the strong acid is used to neutralise the alkali, a more vigorous reaction would occur as a result of there being more H+ ions in the solution to neutralise the OH- ions in the alkali to give out more heat.
Apparatus:
2* 100 ml beakers
2* 500 ml beaker
2 measuring cylinders
Thermometer
Stirring rod
Method:
Collect the apparatus shown in the list above.
Measure 25 ML of acid.
Then place the acid in a measuring cylinder, to check if the volume is exactly 25 ml.
Then collect 25ml of alkali, and do the same as the acid except in a different measuring cylinder.
If the acid is Diprotic or Triprotic, and you are reacting it with a monoprotic alkali then you must use double or triple the volume of alkali in ratio to the amount of acid. This is to compensate for the extra H+ ions, which if not compensated for would result in an unfair experiment.
Measure the temperature for both acid and alkali and note the value down.
Then pour both into a beaker with the thermometer in the beaker as well.
Record the temperature rise. Perform this experiment for the rest of the acids.
Diagram for preliminary experiment
Results of preliminary experiment
Conclusion of preliminary results
The preliminary experiment performed was fairly well done, but there are several minor adjustments that could be made for when doing the real thing. Firstly, instead of using a beaker to mix the acid and alkali, a polystyrene cup could be used instead. This would stop energy being lost in the form of heat, to the surroundings. Also a lid would be placed on the polystyrene cup when reacting the acid and alkali to prevent further heat loss. I believe these are the only adjustments that need to be made for the real experiment. The prediction made was justified in the results processed, as the stronger acids produced a higher temperature rise due to the fact that, first of all, they have no bonds to be broken; in solution they exist as their component ions, completely dissociated. It is known that the breaking of bonds causes energy to be taken in, and when bonds are made, energy is given out. Another reason for a stronger acid producing a higher value for the heat of neutralisation is because it has more free H+ ions. I have deduced this because it is known that in a strong acid, all of the molecules are dissociated into their component ions. When the strong acid is used to neutralise the alkali, a more vigorous reaction would occur as a result of there being more H+ ions in the solution to neutralise the OH- ions in the alkali to give out more heat. For sulphuric acid there was a temperature change of 12*C, which was expected as it is a strong acid. However, for hydrochloric and nitric acid the temperature change was not as significant as expected. This could be due to wrong amounts of volume being mixed and more care will be taken when doing the actual experiment, to make sure equal amounts of volume are used. To avoid anomalous results the experiment could be repeated and I will do this in the real thing.
Actual Experiment
Aim: to carry out the study on how the change in temperature of a neutralisation reaction is effect by the change in acid used in the reaction. I am going to use 6 different acids and 1 alkali for the whole investigation, all with a concentration of one molar.
Prediction: I predict that the stronger acids will produce a higher temperature rise because, first of all, they have no bonds to be broken; in solution they exist as their component ions, completely dissociated. It is known that the breaking of bonds causes energy to be taken in, and when bonds are made, energy is given out. Another reason for a stronger acid producing a higher value for the heat of neutralisation is because it has more free H+ ions. I have deduced this because it is known that in a strong acid, all of the molecules are dissociated into their component ions. When the strong acid is used to neutralise the alkali, a more vigorous reaction would occur as a result of there being more H+ ions in the solution to neutralise the OH- ions in the alkali to give out more heat.
Apparatus:
2* 100 ml beakers
1* 500 ml beaker
2 measuring cylinders
2 Thermometers
Stirring rod
Polystyrene cup and lid
Method:
Collect the apparatus shown in the list above.
Measure 25 ML of acid.
Then place the acid in a measuring cylinder, to check if the volume is exactly 25 ml.
Then collect 25ml of alkali, and do the same as the acid except in a different measuring cylinder.
If the acid is Diprotic or Triprotic, and you are reacting it with a Monoprotic alkali then you must use double or triple the volume of alkali in ratio to the amount of acid. This is to compensate for the extra H+ ions, which if not compensated for would result in an unfair experiment.
Measure the temperature for both acid and alkali and note the value down.
Then pour both into a beaker with the thermometer in the polystyrene cup as well and seal the cup with the lid quickly and carefully.
Record the temperature rise and perform the experiment two more times. Perform this experiment for the rest of the acids, remembering to repeat it 3 times altogether for each acid.
Results for main experiment
Analysis
As I predicted for the actual experiment’s results, the stronger acids reacted to give a bigger temperature than compared to the weaker acids. However, this was not the case for all of the acids used. Citric acid gave a very high reading of temperature change when it is known that it isn’t a very strong acid when compared with HCL and Nitric acid. But one should also take into account of the high volume citric acid used in reacting with the sodium hydroxide, as 75 cm(squared) of the citric acid were used to compensate for the fact that it is a Triprotic acid, as if the acid is Diprotic or Triprotic, and you are reacting it with a Monoprotic alkali (sodium hydroxide one molar in this case) then you must use double or triple the volume of alkali in ratio to the amount of acid. This is to compensate for the extra H+ ions, which if not compensated for would result in an unfair experiment.
The temperature change for sulphuric acid was recorded as being high, as was expected. As predicted the stronger acids gave higher temperature changes. This is due to the fact that stronger acids produce a higher temperature rise because, first of all, they have no bonds to be broken; in solution they exist as their component ions, completely dissociated. It is known that the breaking of bonds causes energy to be taken in, and when bonds are made, energy is given out. Another reason for a stronger acid producing a higher value for the heat of neutralisation is because it has more free H+ ions. I have deduced this because it is known that in a strong acid, all of the molecules are dissociated into their component ions. When the strong acid is used to neutralise the alkali, a more vigorous reaction would occur as a result of there being more H+ ions in the solution to neutralise the OH- ions in the alkali to give out more heat.
To aid my evaluation of my results I have calculated the enthalpy changes for each of the acids used. I will compare these results with the change in kelvin results.
Change in Joules Graph
Change in Kelvin results graph
Further Analysis
The graphs for both change in Kelvin and Joules, have very similar patterns. The only difference between both is that Citric acid gives 3780.0 joules, which is 1470.0, more joules than sulphuric acid, which has a higher change in Kelvin than compared to citric acid.
The reason for there to be a higher amount of energy maybe due to the fact that a higher volume is used for the citric acid than sulphuric acid, which may link to the fact that a higher volume of acid gives a higher enthalpy change.
I believe my prediction was partially linked to the results recorded, mainly due to the fact that the weaker acids gave higher readings than expected, like Ethanoic and that the stronger acids gave lower than expected readings, like Hydrochloric acid. Overall I believe my results showed the trend that would be expected.
