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Acids and Bases
Chemistry - Ch 15: Acids and Bases
Terms in this set (37)
State the Arrhenius definition of an acid and a base
Acid --> substance that donates the H+ when dissolved in water
Base --> substance that donates the OH- when dissolved in water
What is the Bronsted-Lowry definition of an acid and a base?
Acid --> proton donor
Base --> proton acceptor
Which of the two definitions of an acid and a base are favored? Why?
Bronsted-Lowry; this definition is broader and is not limited to aqueous solutions
How is the hydronium ion formed?
When water acts as a base and accepts an H+ from the acid
Explain the conjugate acid/base pair
For every acid, there is a conjugate base and for every base, there is a conjugate acid
Define amphoteric or amphiprotic substances and give an example
These substances can act as both an acid and a base in the Bronsted-Lowry definition
Water is a amphiprotic substance
What is Kw? Give the equation
The ion-product constant, the product of the molar concentrations of H+ and OH- at a particular temperature.
What is pH and how do we find it?
pH is the measure of acidity
pH = -log [H30+]
How do we find pOH? Explain the connection between pH and pOH.
pOH = -log [OH-]
pH + pOH = 14.00
List the seven strong acids
List the strong bases
NaOH, KOH, LiOH, RbOH, CsOH, Ca(OH)2, Ba(OH)2, Sr(OH)2
What is the strongest acid that can exist in aqueous solution? Strongest base?
H3O+ is the strongest acid and OH- is the strongest base
Can we measure the strength of a strong acid's conjugate base? Why or why not
The conjugate base has no measurable strength. Since a strong acid will completely dissociate, this means the conjugate base will have a small concentration.
Write the equation for Ka
As Ka goes up, the weak acid strength increases. Why?
An increase in Ka means we have more dissociation. The more an acid dissociates, the stronger it becomes.
When calculating Ka or using Ka to find the pH, what must we always keep in mind?
If we make the assumption that x is negligible, we must remember to check to make sure this is true by comparing x to 5% of the initial concentration
Show the equation for percent ionization
As the acid get stronger, what happens to the equilibrium of the equation?
The stronger the acid, the greater the percent ionization, or the further to the right the reaction is at equilibrium
To what extent does a weak acid ionize?
It depends on the initial concentration of the acid.
The more diluted the solution, the greater the percent ionization will be. Recall that when we dilute an acid, the concentration of particles decreases. The equilibrium will shift to the right , resulting in more H+ atoms.
Give the equation for the ionization constant in terms of Ka and Kb
Kw = (Ka)(Kb)
Explain what diprotic and triprotic acids are.
These are acids that may yield more than one H+
How do acids ionize?
In a step-wise function, releasing one H+ at a time.
Which ionization constant is the largest and easiest?
Explain how molecular structure and acid strength affects bond strength.
The stronger the bond between an H and an anion, the weaker the acid. This is because the H does want to break off, so we have less H+ which means a larger pH.
How does oxoacid strength affect bond strength?
For oxoacids having different central atoms (Z) that are from the same group and that have the same oxidation number, acid strength increases with increasing electronegativity of Z. The more electronegative, the easier it will be to pull off an H. This means more dissociations, which results in a stronger acid.
How is bond strength affected by oxoacids with different oxidation numbers?
The higher the oxidation number, the more the central atom will pull on the surrounding atoms. This pulling will make H easier to break off, leading to a stronger acid.
What if we have an oxoacid with the same central atom, but a different number of attached groups?
Using the oxidation numbers, the higher number means the central atom will pull on the attached groups surround it. This pull results in the O-H bond weakening. The H will easily come off, which translates to a strong acid being formed due to the complete dissociation.
Which scientist developed the pH scale? Why?
The pH scale was developed by a biochemist Soren Sorensen as a convenient way of expressing acidity.
In order for a species to act as a Bronsted-Lowry base, it must possess a lone pair of electrons. Why?
This lone pair of electrons will serve as outlet to accept the incoming proton. In a special case such as water, the lone pair may indicate the water acted as a base and donated a proton.
What is the ion product constant for water, and how are the concentrations of H+ and OH- related to one another in solution at 25 degrees C?
The ion product constant for water, at 25 C, is 1 x 10-14. The concentrations of H and OH are related in that, when multiplied, they give the ion product constant. The negative log of OH will give you the pOH, which you can then subtract from 14 to find the pH.
What does the acid ionization constant tell us about the strength of an acid?
As the constant increases, so does the strength of the weak acid. When Ka increases, this means we have more dissociation. The more an acid dissociates, the stronger it becomes.
From the relationship KaKb = Kw, what can you deduce about the relative strengths of a weak acid and its conjugate base?
Given that Kw is a constant, and it is relatively small, you can use the equation to get a relative estimate of the relationship between the acid and base. So, the acid would be strong and the base is weak, or vice versa.
What does it mean to say that carbonic acid is a diprotic acid?
Carbonic acid has the ability to donate two H+ ions
Write all the species that are present in a phosphoric acid solution. Indicate which species act as a acid and which as a conjugate base, and which act as both.
H3PO4(aq) + H2O(l) --> H3O+(aq) + H2PO4-(aq)
Acid - H3PO4 and H3O+
Base - H2O and PO4 -3
Conjugate base and acid - H2PO4- and HPO4 -2
How does the strength of an oxoacid depend on the electronegativity and oxidation number of the central atom?
The acid's strength will increase if the different central atoms are from the same group and have the same oxidation number.
What does percent ionization refer to for an acid?
The percent ionization refers to how strong an acid is. The higher the percentage, the stronger the acid is and the equilibrium point has to shift further to the right.
How many sig figs do we report pH in?
According to her answer key, we report sig figs to two digits past the decimal for pH.
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