Module 6 Flashcards
Acid
substance that produces hydrogenions in aqueous solution
Keq
equilibrium expression
Alakali
soluable base
small value of Keq
indiactes equilibriumlies tothe left, concentrationreacts >concentration of products
Ka
acid dissociation constant
base
substnace containing oxides of hydroxides
concentration
amount of solute present in a specifed amount. of solution
molarity
number of moles of solute oer litre of solution
mole
Avogardo number of particles
neutralisation
reaction between an acid and a base to form a salt and water
indicator
substance that changes colour in solution dpending on the acidity or bascity of the solution
blue
colour of litmus in alkaline solution
neutral
neither an acid or a base
What are acids?
proton donors
What are bases?
proton acceptors
What are some strong acids?
HCL, HBr, HI, H2SO4, HNO3, HCIO4
What are some weak acids?
CH3COOH (acetic acid), C3H4(COOH) (citric acid)
What is a strong base?
OH-
Dilute acid
a solution that has a small amount of acid molecules
Concentrated acid
a solution that has a large amount of acid molecules dissolved per volume
What does pH measure?
concentration of hydrogen ions
Neutralisation reaction
acid + base -> salt + water
neutralisation with carbonate base
acid + carbonate –> salt + hydrogen
conjugate acid-base pair
a pair of substance that differ by one H+. One is more acidic while
Strength of Conjugate Acid-Base Pairs
conjugates have opposite strength. The stronger acid, the weaker the base
amphiprotic substance
A substance that can behave as either a proton donor or a proton accepter. (act as an acid or base)
pH buffer
a solution that minimise the change in pH.
standard solution
a solution of accurately known concentration
titration
a measured amount of a solution of unknown concentration is added to a known volume of a second solution until the reaction between them is just complete. used to workout the concentration of the unknown solution
equivalence point
the point at which the two solutions used in a titration are present in chemically equivalent amounts for complete neutralisation
end point
when the indicator changes colour and you stop the titration
pH formula
pH= -log[H+]
pOH formula
pOH= -log[OH-]
Lavoiser’s theory
Oxygen was the component responsible for acidity. (later disproved)
Davy’s theory
Hydrogen found in muriatic acid was responsible for acidity rather than oxygen
Arrhenius’s Theory
acid dissociates in water to produce H+
these ions allow electricity to flow
Limitations of the Arrhenius Theory
- acids are only in aqueous solutions. It cannot deal with gases.
- it only accounts for substances which already have hydrogen or hydroxide in their structure
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
Acid + Base =
Salt + water
Acid + Carbonate =
Salt + Water + Carbon Dioxide
Acid + Metal =
Salt + Hydrogen gas
What are some everyday uses of neutralisation reactions?
Antacids
Gardeners
To treat insect bites and stings
How do you calculate the molar heat of neutralisation?
ΔH = q/n
What does q=mcΔT stand for?
q= heat of neutralisation m= mass c= specific heat capacity t= change in temperature
What does ΔH = q/n stand for?
h= molar heat of neutralisation q= heat of neutralisation n = number of moles of water
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 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 triprotic? + example
Triprotic acids give off 3 H+
Eg. Phosphoric acid (H3PO4)
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?
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
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.
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
It 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
