Strength of Acids and Bases

Strength of Acids and bases

Strength of an Acid/Base

This refers to the extent to which an acid or base dissociates in water and has no impact on concentration or amount.

A small amount of acid/base can still be strong, likewise a large amount of an acid/base can still be weak

Strong Acid:

• A substance that ionizes completely in water to form H₃O+ ions (H+ ions).

• E.g. HNO3(aq) + H2O(l) → H3O+ (aq) + NO− 3 (aq)

• ⇒ A one way arrow is used since this reaction is complete, all of the HNO3 molecules break apart to form H3O+ and NO3 - ions. (Thus, all of the reactants are converted to form products.)

Examples of Strong Acids

• KNOW 7 Strong acids! Memorize these! HCl, HBr, HI, HNO3 , H2SO4 , HClO4 , HCl

Weak Acids

• An acid that ionizes only to a small extent in water.

• E.g. HC₂H₃O₂(aq) + H₂O(l) ↔H₃O⁺ (aq) + C₂H₃O₂ ^- (aq)

• ⇒ A two-way arrow is used since this reaction is not complete but instead consists of an equilibrium mixture of HC₂H₃O_2,, H₃O+ and C₂H₃O₂ ^-

• ⇒ Equilibrium lies to the left since most of the acid molecules have not ionized.

• examples: ethanoic acid (vinegar), citric acid (citrus fruits)

Examples of common Weak acids

• HF, HNO2 , HCN, H3PO4 , H2CO3 , carboxylic acids like CH3COOH and HCOOH

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Strong Bases

• This is a substance that ionizes completely in water to form OH- ions.

• _H2O

  E.g. NaOH(s) → Na+ (aq) + OH- (aq)

• ⇒The reaction goes to completion – all of the Na+ and OH- ions have dissociated from the solid lattice and no undissociated NaOH(s) remains.

Examples of Strong Bases:

• These Group 1A & 2A hydroxides (except Be): LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2 , Ba(OH)2 , Sr(OH)2

• Memorize the 8 SB’s! 

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Weak Bases

• This is a base that ionizes only to a small extent in water.

• NH₃(aq) + H₂O(l) ↔ NH₄⁺ (aq) + OH^- (aq)

• ⇒ Equilibrium lies to the left since most of the ammonia molecules have not accepted protons to form NH + 4 and OH- ions.

• Examples include: ammonia, bleach

Measuring strength

• This can be determined based on the reaction involved or can be measured on a pH scale.

• A strong acid will have a pH nearer to 1 while a weak acid has a pH closer to 7

• A strong alkali (base in solution) has a pH of closer to 14 while a weak alkali has a pH closer to 7

• Remember, pH 7 is neutral! pH less than 7 is acidic and greater than 7 is alkaline!

Buffers

• Weak acids and bases can be used to make buffers.

• A buffer is a solution that is resistant to large changes in pH and so are added to solutions (such as medicines) to make them more stable.

• Buffers can be prepared by mixing a weak acid and its salt or a weak base and its salt. .

• Because a buffer can react with both an acid and a base, its pH remains relatively constant

Concentration of acid/base

• It is important to understand the difference between concentration and strength.

• Concentration is the amount of solute dissolved in a given amount of solution. Strength refers to the solute’s tendency to form ions in water.

• You cannot assume that a strong acid has a low pH, because its concentration also affects pH.

• For instance, a dilute solution of HCl (a strong acid) can have a pH of 6. But a concentrated solution of acetic acid (a weak acid) can have a pH of 3.

Multiple Select

Which of the following statements is correct about the weak base, ammonia (NH₃)?

• It dissociates completely in water to produce a high concentration of hydroxide ions.

• It dissociates partially in water to produce a low concentration of hydrogen ions.

• It dissociates partially in water to produce a high concentration of hydroxide ions.

• It dissociates partially in water to produce a low concentration of hydroxide ions.

Multiple Choice

The higher the concentration of an acid, the stronger the acid.

• True

• False

Multiple Choice

There are 2 types of acids and bases

• Strong and weak

• Strong and basic

• Sour and bitter

• Light and blue

Multiple Choice

The difference between a strong and weak acid is: 

• Strong acids stay intact in water weak acids don't 
• Strong acids are slippery weak acids aren't
• Strong acids break apart in water. Weak acids stay intact
• Weak acids break apart in water most of the time, strong acids do all of the time. 

Multiple Choice

Indigestion and heartburn is causes by an over production of stomach acid. If you were suffering from indigestion, what types of foods should you avoid?

• Foods that have a pH BELOW 7.

• Food that have a pH ABOVE 7.

• Food that have a pH of 7.

• Food has no impact on indigestion.

Multiple Choice

A strong base in a water solution

• is a weak electrolyte

• produces many H⁺ ions

• will not dissolve

• completely dissociates into ions.

Multiple Choice

Why would a potato have a buffering system?

• To survive if its environment changes

• To vary the pH according to the environment

• To keep the pH of the potato low

• To keep the pH very acidic

Multiple Choice

What might happen if buffers did not exist within the human body?

• Our blood and other bodily fluids might become too acidic or basic.

• Our stomach acid would not be able to break down food.

• We would not be able to process glucose within our cells.

• We would not be able to inhale oxygen into our lungs.

Multiple Choice

Buffer is defined as

• ability to resist pH change
• ability to prevent pH from decreasing
• ability to resist a pH increase
• ability to resist pH change when small amount of acid added

Multiple Choice

the more H+ ions the

• stronger the acid

• weaker the acid

• it neutralizes

Multiple Choice

What type of ion does a Basic solution have?

• Hydroxide ion
• Hydronium ion
• Hydrogen ion
• Hydraulic ion

Multiple Select

How would you best describe your science class this year?

• awesome

• very strange

• dull

Multiple Choice

According to the pH scale, which of the following pH values would be the strongest base?

• 0

• 4

• 8

• 14

Multiple Choice

According to the pH scale, which of the following pH values would be the strongest acid?

• 0

• 4

• 8

• 14

Multiple Choice

The pH of a solution is tested, and it is found to be a basic solution. Of the following choices, what could the pH have been?

• 3
• 9
• 7
• 5

Multiple Choice

What does a pH scale do?

• Tells if a substance is an acid or base
• Measures the strength and weakness of acids and bases
• Both statements are correct

Multiple Choice

The pH scale measures...

• ...the strength of an acid.
• ...the strength of hydrogen ions.
• ...the concentration of hydrogen ions.
• ...the concentration of an acid

Multiple Choice

On the pH scale what numbers are bases?

• 8-14
• 0-7
• 7
• 1

Multiple Choice

Which of the following is true about acids and bases?

• The lower the pH, the stronger the acid
• the higher the pH, the stronger the acid
• The lower the pH, the more neutral the acid
• The higher the pH, the weaker the base

Multiple Choice

The higher the concentration of an acid, the stronger the acid.

• True

• False

Multiple Choice

Which beaker, A or B, contains a lower concentration of solution?

(Assuming equal volume of solution in each beaker.)

• A

• B

Multiple Choice

If you have a solution that is very acidic with a pH of 3.5, what can you do to raise the pH? 

• add more acid
• add more water
• increase the temperature
• add a base

Multiple Choice

When making Kool Aid for a group of friends coming over you remember how awesome your science teachers are and decide to do a little experiment with solubility. You get three pitchers and add the same amount of water to each. Then you add in the same amount of flavor powered to each. In pitcher 1, you add in 2 cups of sugar. In pitcher 2, you add 3 cups of sugar and in the last pitcher you put in 5 cups. Which of the answers is TRUE about the solutions you created?

• The solution in pitcher 1 has the greatest concentration.

• The solution in pitcher 2 has the greatest concentration.

• The solution in pitcher 3 has the greatest concentration.

• All the pitchers have the same concentration.

Multiple Choice

In this model what is being changed about the solution?

• There is more solvent being added making it concentrated.

• There is more solute being added making it concentrated.

• There is heat being added to speed up the rate of dissolving.

• There is nothing special about this model.