Module 6: Acid-Base Reactions Flashcards
Hydrochloric acid symbol
HCl
HF name
Hydrofluoric Acid
Sulfuric acid symbol
H2SO4
H2SO4 name
Sulfuric acid
Nitrous Acid symbol
HNO2
HNO2 name
Nitrous Acid
HNO3 name
Nitric Acid
Nitric Acid symbol
HNO3
H2CO3 name
Carbonic Acid
Carbonic Acid symbol
H2CO3
Phosphoric Acid symbol
H3PO4
H3PO4 name
Phosphoric Acid
Sodium Hydroxide symbol
NaOH
NaOH name
Sodium Hydroxide
Ca(OH)2 name
Calcium Hydroxide
Calcium Hydroxide symbol
Ca(OH)2
Barium Hydroxide symbol
Ba(OH)2
Ba(OH)2 name
Barium Hydroxide
Na2CO3 name
Sodium Carbonate
Sodium Carbonate symbol
Na2CO3
Sodium Hydrogen Carbonate symbol
NaHCO3
NaHCO3 name
Sodium Hydrogen Carbonate
NH3 name
Ammonia
Ammonia symbol
NH3
Properties of acids:
Dilute solutions have a sour taste
Frequently corrosive to skin
Soluble acids are electrolytes. Electrical conductors in aqueous solutions
Neutralise bases to form salts
Used for cleaning, fire extinguishers, fertilisers, dyes etc
Properties of bases:
Dilute solutions have a bitter taste Frequently corrosive to skin Soluble bases are electrolytes, conductors in aqueous solutions Feel slippery or soapy Neutralise acids to form salts Used in the production of soaps
What are indicators?
Indicators are plant dyes which change colour in response to the degree of acidity or basicity of a solution. Both the acid and base are in equilibrium with each other
Why are indicators important?
Indicators are useful for determining whether a substance is acidic, basic or neutral.
What is an example of a natural indicator?
Red cabbage indicator
Safety for experiments involving HCL and NaOH + why
Wear safety glasses. Sodium hydroxide is caustic and hydrochloric acid is corrosive. Avoid contact with skin. If contact occurs wash with plenty of water
Acid + Base =
Salt + water
Acid + Carbonate =
Salt + Water + Carbon Dioxide
Acid + Metal =
Salt + Hydrogen gas
How do antacids work?
neutralise the acid in the stomach with a base
What are some everyday uses of neutralisation reactions?
Antacids, gardeners use them and to treat insect bites and stings
how do gardeners use neutralisation reactions?
To change the acidity of the soil. Hydrangeas for example change colour depending on the soil PH. Calcium hydroxide can be added to acidic soils and iron salts or gypsum can be added to alkaline soils
How does neutralisation help insect bites?
Treat insect stings to neutralise the toxins and reduce the irritation
Are neutralisation reactions exothermic or endothermic?
Exothermic
How much heat is released in neutralisation reactions?
the amount of heat released is always approximately 57kj per mole of water formed
How do you calculate the heat of neutralisation?
q=mcΔT
What does q=mcΔT stand for?
q= heat of neutralisation m= mass c= specific heat capacity t= change in temperature
How do you calculate the molar heat of neutralisation?
ΔH = q/n
What does ΔH = q/n stand for?
h= molar heat of neutralisation
q= heat of neutralisation
n = number of moles of water
What was Lavoisier’s theory?
1st model
Lavoisier: All acids contain Oxygen
Did Not explain acids such as HCL
What was Davy’s theory?
2nd model
All acid contain hydrogen
couldn’t explain why compounds contained hydrogen but weren’t acids
What was Arrhenius’ theory?
3rd model
That acid formed H+ ions in water and bases formed OH- ions in water
Expanded on both of the above theories but couldn’t explain why not all bases contained OH- as well as why Ammonia is a base
What was Bronsted-Lowry’s theory?
4th model
That acids were proton donors and electrons were proton acceptors
Took into account that bases do not have to contain and OH- ions
What were the order of the theories?
Lavoisier
Davy
Arhenius
Bronsted-Lowry
How do you calculate the pH of a solution?
pH= -log(H+)
How do you calculate (H+)?
(H+) = 10 to the power of -pH
How do you calculate the pOH of a solution?
pOH= -log(OH-)
How do you calculate (OH-)?
(OH-) = 10 to the power of -pOH
What does pH + pOH =?
14
What does pH stand for?
Hydrogen power
What is a hydronium ion?
When a hydrogen ion attached to a water molecule
What is monoprotic? + example
acids give off one H+
Eg. HCl
What is diprotic? + example
Diprotic acids give off 2 H+
Eg. H2SO4
What is triprotic? + example
Triprotic acids give off 3 H+
Eg. Phosphoric acid
How do you work out concentration with more than one H+ ion?
you divide by the number of H+ ions
What factors affect the ph of a solution?
- Is the acid monoprotic or polyprotic?
- is the acid concentrated or dilute?
- is it a strong or a weak acid?
What is a strong acid? + example
one in which all the acid present in a solution has ionized to form hydronium ions with water ions: there are no neutral acid molecules present.
e.g. HCL
It goes to completion (equilibrium)
What is a strong base?
A strong base readily accepts protons
What is a weak acid? + example
A weak acid is one in which only some of the acid molecules present in the solution have ionized to form hydronium ions.
Eg. Ethanoic acid is a weak acid. This means that only some of the ethanoic acid molecules react with water.
It does not go to completion (equilibrium)
What is a weak base?
A weak base does not readily accept protons.
What is a concentrated solution?
A concentrated solution is one in which the total concentration of the solute is high eg. Above 5M
What is a dilute solution?
A dilute solution is one in which the total concentration of the solute is low eg. Below 2M
How does the model you made in class show different strengths of acids and bases?
The model demonstrates a strong acid to be one with many ions and a weak acid to have fewer ions. A dilute strong acid has fewer ions and a dilute weak acid has fewer ions as some molecules may not have dissociated. 4 models of the strength and solution, different sizes to represent different particles, different ions (had charge written on them).
Evaluate your model of the strengths of acids and bases as an instructional tool:
The model is successful at demonstrating the differing strengths and concentrations of acids by simplifying and visually demonstrating what the chemical concept looks like. However, as it is a model it has its limitations. It does not have any water molecules, it is magnified, it is a stagnant piece of time. However, as an instructional tool, it is useful as it is able to adequately teach the concept
What is the correlation between acid strength and dissociation?
The stronger the acid, the more readily it is ionised. The weaker the acid, the less it is ionised.
An equilibrium exists between the acid molecule and its constituent ions.
What equation do you use to calculate ionisation?
[H+] [A-]
Ka = ————
[HA]
What is the correlation between Ka and strength of acid and bases?
The smaller the value of Ka the weaker the acid, the higher the value, the stronger the acid.
The same is true for bases.
What are conjugate acid/base pairs?
Conjugate pairs are species involved in the transfer of protons.
How is the equilibrium position of an acid-base system determined?
The equilibrium position of an acid-base system is determined by the relative strengths of the bases involved.
If the equilibrium lies to the right, strong bases have weak conjugate acids and vice versa.
What is the strength of an acid?
the measure of ionisation
What is the strength of a base?
the measure of dissociation
What does amphiprotic refer too? Provide an example to support
here are some species which are capable of behaving as either a base or an acid according to the Bronsted-Lowry definition given certain circumstances. We say that a species is amphiprotic if it can donate or accept a proton.
e.g. water
What are the steps in calculating the pH of the resultant solution when solutions of acids and/or bases are diluted or mixed?
Number of moles of base The number of moles of OH- (is it mono or polyprotic?) Number of moles of acid Number of moles of H+ Determine LR c=n/v Calculate pH
What is a titration?
a technique where a solution of known concentration is used to determine the concentration if an unknown solution
What is an acid-base titration?
An acid-base titration is a neutralization reaction that is performed in the lab in order to determine an unknown concentration of acid or base.
What is an indicator?
-titrations
An indicator is used to indicate when the acid and base have mixed in exactly the right proportion to neutralize each other. When the indicator changes colour the endpoint of the titration has been reached.
What is a primary standard?
the solution of known concentration.
Outline the Rinsing techniques in titrations:
Burette- Rinsed with water, then the solutions which it will be holding. In order to prevent dilution.
Pipette- Rinsed with water, then the solutions which it will be holding. In order to prevent dilution.
Conical Flask- Rinsed with water, will not change the number of moles
Safety in titrations:
Do not pipette by mouth be sure to use a pipette filler
Many indicators are poisonous and should be treated with care
What is a titration curve?
If the pH of an acid is plotted against the amount of base added during titration, the shape of the graph is called a titration curve.
General acid titration description:
In the beginning, the solution has a low pH and climbs as the strong base is added. As the solution nears the point where all of the H+ are neutralized, the pH rises sharply and then levels out again as the solution becomes more basic as more OH- ions are added.
what is a pH probe?
The pH probe measures changes in the pH as the reaction progresses. How quickly it will rise will depend on the nature of the acid and base.
What is the conductivity meter?
The conductivity meter is measuring the concentration of ions in solution.
Strong acid, strong base titration curve
The graph shows that the pH starts at a very high value, in the basic region. It remains fairly constant until close to the equivalence point where it suddenly decreases. When acid is in excess, the pH stays reasonably constant again, at a very low pH.
The equivalence point is the midpoint of the sudden change in pH.
strong base, weak acid titration curve
The pH starts at a very high value, in the basic region. It remains fairly constant until close to the equivalence point, where it suddenly decreases, although not as much as for the strong acid-base titration. When the acid is in excess, the slope of the pH graph changes and eventually stays reasonably constant at a low pH.
weak base, strong acid titration curve
The pH starts at a high value, in the basic region (although not as high as for the strong base). It decreases slightly, then close to the equivalence point, suddenly decreases, although not as much as for the strong acid-strong acid titration. When the acid is in excess, the pH stays reasonably constant at a very low pH.
weak base, weak acid titration curve
The pH starts a high value in the basic region, although not as high as for a strong base. It remains fairly constant until close to the equivalence point, then it decreases. This change is much smaller than for the other curves. When the weak acid is in excess, the pH decreases reasonably constantly at a low pH.
What is a conductivity curve?
Another method of determining when the equivalence point is reached is by measuring the change in conductivity of the substance during the titration, using a conductivity probe.
Strong acid, strong base conductivity curve
When a strong base is added to a strong acid, the conductance starts at a high value due to the presence of the highly mobile hydrogen ions (from the acid). The hydrogen ions react with the hydroxide ions to form water molecules, so the conductivity of the solution decreases. At the equivalence point, only Na+ and Cl- ions are present. Continued addition of NaOH leads to an increase in conductivity due to an increase in the concentration of Na+ and OH- ions.
strong base, weak acid conductivity curve
When a strong base is added to a weak acid, the conductance starts at a low value. This is due to the low degree of ionisation in a weak acid, resulting in a low concentration of H+ ions. When the strong base is added, the H+ reacts with the OH- to form water, resulting in a slight decrease in conductivity only. However, with a further addition of the strong base, the conductivity increases as hydroxide ion concentration increases.
strong acid, weak base conductivity curve
When a weak base is added to a strong acid, the conductance starts at a high value due to the presence of the highly mobile hydrogen ions (from the acid). The hydrogen ions react with the hydroxide ions to form water molecules, so the conductivity of the solution decreases until it reaches the equivalence point. Unlike the strong base, the continued addition of the weak base does not increase conductivity because the weak base does not ionise enough.
weak acid, weak base conductivity curve
When a weak base is added to a weak acid, the conductance starts at a low value. After the equivalence point, there is little change in conductivity due to the small degree of dissociation of a weak base.
Ka formula symbol meaning
A- = anion HA= molecule
Ka answer meaning
Weak acid= smaller value = partial ionisation
Strong acid= higher value = complete ionisation
pKa formula
pKa = -log10(Ka) -Determines acid strength-
pKa answer meaning
As the acid get weaker pKa increases
Describe a model of neutralisation of strong and weak acids and bases+ explain improvements and accuracy
In class, we used a digital model to demonstrate the neutralisation between a number of strong acids and strong bases. It demonstrated the formation of water as a result of the combination of OH- and H+ ions. It also showed the presence of spectator ions following the formation of water. As it is a model it has many limitations and improvements that can be made. It failed to demonstrate neutralisations involving weak acids and weak bases, It did not portray the water molecules and made them dissolve and it was too slow. As a result of this, it is unable to provide the most accurate representation and is very oversimplified.
What is a back titration?
A back titration is a two-stage technique which can be used in a variety of situations
When is a back titration used?
t may be required if an acid or base is an insoluble salt, in reactions where the endpoint is difficult to observe or if the sample is a gas. It can also be used to conduct chemical analysis of household substances such as soft drink, wine, juice and medicine.
How to calculate back titrations:
Calculate the number of moles of HCl in the initial reaction.
Write a balanced chemical equation between the HCl and calcium carbonate
Write a balanced chemical equation for the reaction between the remaining HCl and the standard NaOH (note that the other products from reaction 1 are still present but do not take part in the titration)
Calculate the number of moles of NaOH in the standard solution
Use your balanced equation to calculate the number of moles of excess HCl that reacted with the NaOH. Note that we only used 25mL of the 250mL solution in this example, so the number of moles of HCl present in this solution is 10x the value calculated!
Calculate the number of moles of HCl that reacted with the calcium carbonate, by subtracting the final amount from the initial amount of HCl.
Use a balanced equation (step 2) to calculate the number of moles of calcium carbonate that reacted with HCl
Use the molar mass of calcium carbonate to calculate the percentage composition
What is a buffer?
is a chemical which is able to minimise a change in the pH if acids or bases are added. A buffer consists of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid
What is a buffer solution?
A buffer solution is an aqueous solution that resists rapid changes in pH when an acid or base is added.
A buffer consists of a weak acid and its conjugate base or a weak base and its conjugate acid. Each buffer has a well-defined pH value.
A buffer can resist a pH change due to the equilibrium between the acid and its conjugate base. When an acid or base is added there is a shift in the equilibrium position to counteract the addition of acid or base. The pH remains relatively constant.
Examples of buffers:
Seawater and the carbonate/bicarbonate buffer system, Blood buffers and the carbonic acid/ hydrogen carbonate ion system.
