Acid, Bases And Kw Flashcards
What is an acid and what is a base
Acid: releases protons
Base: accepts protons
What are Brønsted-Lowry acids?
• Proton donors that release hydrogen ions (H+) when they are mixed with water
What is the end product of hydrogen ions released in water
• You never get H+ ions by themselves in water, they’re always combined with H2O to form hydroxonium ions, H3O+
• HA(aq) + H2O(l) -> H3O+(aq) + A-(aq)
What are Brønsted-Lowry bases
• Proton acceptors
• In solution, they grab hydrogen ions from water molecules
• B(aq) + H2O(l) -> BH+(aq) + OH-(aq)
What difference between Strong acids and weak acids and give examples with equations
• Stong acids: dissociate almost completely in water-nearly all the H+ ions will be released
• Hydrochloric acid is a strong acid
• HCl -> H+ + Cl-
• Weak acids: dissociate only very slightly in water so only small numbers of H+ are formed
• Ethanoic acid or citric acid
• An equilibrium which lies well over to the left
• CH3COOH equilibrium CH3COO-(aq) + H+(aq)
What is the difference between strong bases and weak bases. give examples and equations
• Strong bases ionise almost completely in water
• Sodium hydroxide (NaOH)
• NaOH -> Na+ + OH-
• Weak bases only slightly ionise in water
• Ammonia (NH3)
• Just like weak acids, the equilibrium lies well to the left
• NH3 + H2O equilibrium NH4+ + OH-
What is transferred when Acids and Bases React
Protons
How do acids get rid of their protons and give an example
• They can only get rid of them if there’s a base to accept them
• HA(aq) + B(aq) equilibrium BH+(aq) + A-(aq)
• It’s an equilibrium
Note
The equilibrium’s far to the left for the weak acids, and far to the right for strong acids
What ions does water dissociate into? And give equations
• Hydroxonium and hydroxide ions
• H2O + H2O equilibrium H3O+ + OH-
• H2O equilibrium H+ + OH-
• Equilibrium law can be applied to calculate the equilibrium constant
What are the effects of water dissociating slightly
• There’s always so much water compared to the amount of H+ and OH- ions that the concentration of water is considered to have a constant value, you constant
• Called the ionic product of water and given the symbol Kw
• Kw= Kc x [H2O] = [H+][OH-]
• Kw= [H+][OH-]
• The units for Kw mol2dm-6
What condition affects the value of Kw
Temperature
Note
In pure water, there is always one H+ ion for each OH- ion. if you’re dealing with pure water, then you can say that Kw=[H+]2
What is the purpose of pH scale
The measure of the hydrogen ion concentration
Why is a logarithmic scale used to measure the concentration of hydrogen ion
The concentration of hydrogen ions in a solution can vary enormously
Give the formula for calculations pH
• pH= -log10[H+]
• [H+] is the concentration of hydrogen ions in a solution, measured in mol dm-3
Describe the pH scale
The pH scale normally goes from 0 (very acidic) to 14 (very basic). 0H is regarded as being neutral
How can you calculate the pH from hydrogen concentration?
If you know the hydrogen concentration of a solution, you can calculate the pH using this formula:
• pH= -log10[H+]
How can you calculate the hydrogen ion concentration from pH
[H+] = 10^-pH
Describe strong monoprotic acids
• Such acids such as hydrochloric acid and nitric acid ionise fully in solution
• Monoprotic means that each molecule of acid will release one proton when it dissociates
• This means one mole of acid produces one mole of hydrogen ions.
• So the H+ concentration is the same as the acid concentration
Describe Strong diprotic acids
• A strong diprotic acid releases 2 protons when it dissociates
• Produces 2mol of hydrogen ions for each mole of acid
• Sulfuric acid is an example of a strong diprotic acid
Formula for molar concentration
11.22 g dm-3/ Mr
What is the formula for acid dissociation constant
Ka
Why is the Ka constant used to find pH
Weak acids and bases only dissociate slightly in an aqueous solution, so the H+ is not the same acid the acid concentration
Give the equation for weak acid, HA
HA (aq) equilibrium H+(aq) + A-(aq)
Give the equation for Ka
- Ka= [H+] [A-]/ [HA]
- Ka= [H+]2/[HA]
What are the units for Ka
mol dm-3
What is Ka dependent on?
Ka is dependent on the temperature of a particular acid
Give the formula for pKa
pKa= -log10 Ka
Give the formula for Ka
Ka= 10-pKa
The smaller the pKa…..
The stronger the acid
How do we use titration to find the concentration of an acid or base
- Add a standard solution of acid to measured quantity of base or vice versa
- Pipettes and burettes are used for accurate measurement
- An indicator/ pH meter is added to show you exactly when its neutralized by the acid
What does the vertical on a pH curve indicate
It is here that all the acid is neutralized
When is methyl orange used as an indicator
- For a strong acid and weak base
When is phenolphthalein used as an indicator
- For weak acid and strong base
What indicator is used for a weak acid and weak base
Neither methyl orange and phenolphthalein
What indicator is used for a strong acid and strong acid
Both methyl orange and and phenolphthalein
What is a buffer
A buffer is a solution that resists changes in pH when small amounts of acid or base are added or when its diluted
Give two types of buffers
- Acid buffers
- Base buffers
What are acidic buffers made from?
A weak acid and one of its salts
Describe this reaction:
CH3COOH(aq) equilibrium H+(aq) + CH3COO-(aq)
- Ethanoic acid dissociates slightly
- Salt fully dissociates
- undissociated ethanoic acid molecules
- lots of ethanoate ions from the salt
CH3COOH(aq) equilibrium H+(aq) + CH3COO-(aq), Describe the effect of adding an acidic buffer on adding small amounts of acid
- If you add small amounts of acid, the H+ concentration concentration
- H+ ions react with CH3COO- to form CH3COOH
- Equilibrium shifts to the left reducing H+ concentration
- So pH doesn’t change
Describe the effect of adding an acidic buffer on adding small amounts of base
- Small amount of base, OH- concentration increases.
- OH- ions react with H+ ions to form water
- Removing H+ ions from the solution, causes CH3COOH to dissociate to form H+ ions
- Equilibrium shifts to the right
- The H+ concentration increases so the pH does not change
What are basic buffer made form?
Weak base and one of its salt
Describe the effect of adding a basic buffer on adding small amounts of a base
e.g. NH4Cl(aq) -> NH4+(aq) + Cl-(aq)
NH3(aq) + H2O(aq) equilibrium NH4+(aq) + OH-(aq)
- If a small amount of base is added, the OH- concentration increases
- Solution is more basic
- OH- ions will react with the NH4+ ions to form NH3 and H2O
- Equilibrium will shift to the left
- Removing OH- ions and stopping pH from changing much
Describe the effect of adding a basic buffer on adding small amounts of an acid
e.g. NH4Cl(aq) -> NH4+(aq) + Cl-(aq)
NH3(aq) + H2O(aq) equilibrium NH4+(aq) + OH-(aq)
- Adding small amounts of an acid leads to H+ concentration increasing , solution becomes more basic
- Some of the H+ ions react with OH- ions to make H2O
- Equilibrium shifts to the right to replace the OH- ions that have been used up
- Some of the H+ ions react with NH3
- These will remove the extra H+ ions so the pH won’t change much
What are the purposes of buffers
- Shampoos, contains buffers to prevent human hair from becoming rough if exposed to alkaline conditions
- Enzymes in the body
And buffers in the body to keep the right pH
What is a weak acid?
Acid partially or slightly dissociates in water to form H+ ions
Suggest why the pH probe is washed with distilled water between each of the calibration measurements
• Different solutions must not contaminate each other
• To wash off residual solution/ substance
• To avoid missing end point
Write an expression for the acid dissociation constant Ka for ethanoic acid.
ka= [CH3COO-] [H+]/ [CH3COOH]
Suggest why chloroethanoic acid is a stronger acid than ethanoic acid.
- Cl is (more electronegative so) withdraws electrons
OR negative inductive effect of Cl - Weakens O–H bond
