A Comparison of Strong and Weak Acids and Bases
DATA COLLECTION
Qualitative Data
Table 1.1: The lists of the solutions being used in this experiment and classifying if it is a strong acid or base or a weak acid or base.
Table 1.2: The appearance of the marble chip used and the magnesium ribbon.
Note: The chemical formula for marble chip is CaCO3(s) and for magnesium ribbon is Mg(s).
Table 1.3: The observations upon reacting different solutions and concentrations of acid with the marble chips, CaCO3(s).
Table 1.4: The order in which the speed of reaction occurs between the acids and the CaCO3(aq).
- Approximately 2.0cm3 of each was used.
Table 1.5: The observations upon reacting different solutions and concentrations of acid with the magnesium ribbon, Mg(s).
Table 1.6: The order in which the speed of reaction occurs between the acids and the magnesium ribbon.
- Approximately 2.0cm3 of each was used.
Table 1.61: The observations made when CuO(s) was added to the acid and the mixture was warmed.
- The Copper (II) oxide, CuO(s), was a black,fine powder.
Fig 1.1: The colour of the acid after CuO(s) was added to the acid.
Quantitative Data
Table 1.7: The concentration of the solutions used in the experiment.
The concentrations of the solutions above are taken as exact values as they are prepared by the lab technician. The columns with light blue shading indicate that there are different concentrations available for the same solution. For example, there are 1.00M , 0.10M and 0.01M of HCl(aq).
Table 1.8: The pH values obtained by ...
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- The Copper (II) oxide, CuO(s), was a black,fine powder.
Fig 1.1: The colour of the acid after CuO(s) was added to the acid.
Quantitative Data
Table 1.7: The concentration of the solutions used in the experiment.
The concentrations of the solutions above are taken as exact values as they are prepared by the lab technician. The columns with light blue shading indicate that there are different concentrations available for the same solution. For example, there are 1.00M , 0.10M and 0.01M of HCl(aq).
Table 1.8: The pH values obtained by the pH probe for CH3COOH(aq), HCl(aq), H2O(l), NH3(aq) and NaOH(aq).
- pH probe used was the Texas Instrument TI-83.
Table 1.9 : The conductivity of the CH3COOH(aq), HCl(aq), H2O(l) and NaOH(aq).
- The range of the conductivity probe for these solution was 0µs – 20,000µs. And the accuracy for the probe was ±1%. This concludes to the uncertainty being ±200µs as
- Due to the different conductivity probe range, a separate table for NH3(aq) was made.
Table 2.0: The conductivity of NH3(aq).
- The range of the conductivity probe for NH3(aq) was 0µs – 2000µs. And the accuracy for the probe was ±1%. This concludes to the uncertainty being ±20µs as
- Conductivity probe used, also for Table 1.9, was Texas Instruments TI – 83.
Data Collection
Theoretical pH values for H2SO4(aq), HCl(aq), HNO3(aq), and NaOH(aq).
Example Calculations (Strong acid):
- 0.01M HCl(aq) was taken as an example calculation
Chemical Equation:
Necessary Formula:
As seen from the equation above, the acid (HCl(aq)) dissociated fully as it is a strong acid. Thus, the concentration of HCl(aq) will equals to the concentration of H+(aq) and Cl-(aq). Therefore, the concentration of H+(aq) is 0.01M. Thus,
Example calculation (Strong base):
- 0.01M NaOH(aq) was taken as an example calculation
Chemical Equation:
Necessary Formula:
As seen from the equation above, the base (NaOH(aq)) dissociated fully as it is a strong base. Thus, the concentration of NaOH(aq) will equals to the concentration of Na+(aq) and OH-(aq). Therefore, the concentration of OH-(aq) is 0.01M. Thus,
Table 2.0: The theoretical pH values of H2SO4(aq), HCl(aq), HNO3(aq), and NaOH(aq) with its respective concentrations.
- No uncertainty was allocated for the pH values as it was taken as an exact value prepared by the technician.
Theoretical pH values for CH3COOH(aq) and NH3(aq).
1.00M of CH3COOH(aq)
Chemical Equation:
Necessary formulas:
From the Data Booklet, pKa of CH3COOH(aq) is 4.76. Thus,
Actually, the formula for dissociation constant for a weak acid is:
However, since the [H+] is very, very, very small, the [H+] in ([CH3COOH(aq)] – [H+(aq)] is ignored. It is suggested that if the [H+] is 10% or less of the concentration of the acid, then it does not cause any major differences. Thus a “check is done”:
1.00M of NH3(aq)
Chemical Equation:
Necessary formulas:
From the Data Booklet, pKb of NH3(aq) is 4.75. Thus,
Therefore,
Actually, the formula for dissociation constant for a weak base is:
However, since the [OH-] is very, very, very small, the [OH-] in ([NH3 (aq)] – [OH-(aq)] is ignored. It is suggested that if the [OH-] is 10% or less of the concentration of the base, then it does not cause any major differences. Thus a “check is done”:
Table 2.1: The theoretical pH values for CH3COOH(aq) and NH3(aq) with its respective concentrations.
- No uncertainty was allocated for the theoretical values as the values used for calculations were taken as exact values prepared by the lab technician.
The pH calculations for weak acids and bases differ to that of strong. This is because in strong acid and bases, it fully dissociates but in weak acids and bases, it only partially dissociates. This also explains why although they have the same concentrations, their pH values are not the same. For example, 1.00M of HCl(aq) and 1.00M of CH3COOH(aq) has the same concentration, however, their pH differs, being pH 0 for HCl(aq) and pH 2.38 for CH3COOH(aq).
Table 2.11: Comparison of theoretical pH value and experimental pH value.
Table 2.2: The conductivity of the CH3COOH(aq), HCl(aq), H2O(l) and NaOH(aq).
Table 2.3: The conductivity of NH3(aq).
Graph 2.3: A comparison of the conductivity of the CH3COOH(aq), HCl(aq) NH3(aq) and NaOH(aq).
Therefore, it is seen from the graph that the acid, HCl(aq) has the highest conductivity at 37701µs for 0.10M and 4898µs for 0.01M of HCl(aq). This may due to its ability to fully dissociates and thus carrying more H+(aq) ions. The base, NaOH(aq) comes next with conductivity of 26531µs for 0.10M NaOH(aq) and 2969µs for 0.01M NaOH(aq). Next comes the weak acid, CH3COOH(aq) at 2005µs for 1.00M of CH3COOH(aq). Its ability to conduct electricity is lower to that of HCl(aq) because weak acid only partially dissociates, thus less H+(aq) ions will be readily available. And finally, comes the weak base at 1138µs for 1.00M of NH3(aq).
Table 2.4: The order in which the speed of reaction occurs between the acids and the CaCO3(s).
Balanced equations:
- 2HNO3(aq) + CaCO3(s) → CO2(g) + H2O(l) + Ca(NO3)2(aq)
- 2HCl(aq) + CaCO3(s) → CO2(g) + H2O(l) + Ca(Cl)2(aq)
- 2CH3COOH(aq) + CaCO3(s) → CO2(g) + H2O(l) + Ca (CH3COO)2 (aq)
- H2SO4(aq) + CaCO3(s) → CO2(g) + H2O(l) + CaSO4(aq)
Although H2SO4(aq) is a strong acid, it however has a lower reaction than CH3COOH(aq) despite the same molarity where as CH3COOH(aq) is only a weak acid. This is because, as H2SO4(aq) reacts with CaCO3(s), it slowly forms a coating of CaSO4(aq) and thus the reaction stops.
Table 2.5: The order in which the speed of reaction occurs between the acids and the magnesium ribbon.
Table 2.6: The balanced equation, reduction and oxidation equation for reaction of magnesium ribbon with each respective acids.
Complex ion equation for reaction of ward acids with CuO(s)
1.00M of HCl(aq)
- [Cu(H2O)6(aq)]2+ → [CuCl4(aq)]2- + [Cu(H2O)6(aq)]2+
- [Cu(H2O)6(aq)]2+ + 4Cl-(aq) CuCl4(aq) + 6H2O(aq)
The colour appears green because of the mixture of colours between [Cu(H2O)6(aq)]2+ which is light blue in colour and [CuCl4(aq)]2- which is yellow in colour, thus you have a green colour.
1.00M of HNO3(aq)
[Cu(H2O)6(aq)]2+ + 2NO3(aq)- → Cu(NO3)2(aq) + 6H2O(l)
1.00M of H2SO4(aq)
[Cu(H2O)6(aq)]2+ + SO4-(aq) → CuSO4(aq) + 6H2O(l)
1.00M of CH3COOH(aq)
[Cu(H2O)6(aq)]2+ + CH3COO- (aq) → Cu(CH3COO)2(aq) + 6H2O(l)
Conclusion
From the experiment, it is seen that there was not much difference between the theoretical pH value and the experimental pH values. For this part of the experiment, a few assumptions were made that would have given errors to the value. They are:
- For the calculations of weak acid, it is assumed that [H+(aq)] = [CH3COO-(aq)]
- For the calculations of weak base, it is assumed that [OH-(aq)] = [NH4+(aq)]
- All the reaction occurred at 25.0°C
- For the reaction of the weak acid, the dissociation of CH3COOH(aq) is so little that its concentration is taken as original acid concentration.
- For the reaction of the weak base, the dissociation of NH3(aq) is so little that its concentration is taken as original base concentration.
In addition, it is known that strong acid tend to be a better conductor compared to strong base, weak acid, and weak base. From Graph 2.3, HCl(aq) has the highest conductivity at 37701µs for 0.10M and 4898µs for 0.01M of HCl(aq). The base, NaOH(aq) comes next with conductivity of 26531µs for 0.10M NaOH(aq) and 2969µs for 0.01M NaOH(aq). Next comes the weak acid, CH3COOH(aq) at 2005µs for 1.00M of CH3COOH(aq). And finally, comes the weak base at 1138µs for 1.00M of NH3(aq). Therefore, it can be concluded that the conductivity of acids or bases comes in the order below:
Strong acid > Strong base > Weak acid > Weak base
As the reacgion of acids with CaCO3(s), it can be seen that the reaction is faster with higher concentration of the acid. This can be seen when 1.00M , 0.10M and 0.01M of HCl(aq) was reacted with CaCO3(s) and 1.00M reacted the fastest whilst 0.01M was the slowest. In addition, it is seen that although H2SO4(aq) was a strong acid, it reacted the slowest among the 1.00M of acids(HCl(aq) and HNO3(aq)) including the weak acid CH3COOH(aq)
As for the reaction of H2SO4(aq), HCl(aq) , HNO3(aq), and CH3COOH(aq) with magnesium ribbon, it is again seen that as concentration increases, so will the speed of reaction. However, for this reaction, H2SO4(aq) was the most reactive, as it produces MgSO4(aq) + H2(g), thus nothing is hindering the reaction from occuring. It is also seen that the weak acid, CH3COOH(aq) has the slowest reaction amongst the 1.00M acids (H2SO4(aq), HCl(aq) and HNO3(aq)).
The colour change that occurred when CuO(s) was heated in warm acid is related to the states of the electrons in the metal. Atoms absorb and emit specific energies of photon corresponding to the metal’s different state of acceptable states for electrons. Therefore, as the mixture was heated, the states available changes and this cause the colour of the solution to change as frequency of transmitted light is different.
