Acid, Bases and Kw Flashcards

1
Q

Bronsted-Lowry acids are what?

A

Bronsted-Lowry acids are proton donors - they release hydrogen ions (H+) when they’re mixed with water. You never get H+ ions by themselves in water, they’re always combined with H2O to form hydroxonium ions H3O+

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2
Q

Bronsted-Lowry bases are what?

A

Bronsted-Lowry bases are proton acceptors. When they’re in solution, they grab hydrogen ions from water molecules

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3
Q

How do strong acids and bases react in water?

A

Strong acids dissociate (or ionise) almost completely in water - nearly all the H+ will be released

Strong bases ionise almost completely in water too

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4
Q

How do weak acids and bases react in water?

A

Weak acids dissociate only very slightly in water - only a small amount of H+ ions are formed

Weak bases only slightly ionise in water

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5
Q

Where does the equilibrium lie in strong and weak acid/base reactions with water?

A

In strong acids and bases the equilibrium lies extremely far to the right

In weak acids and bases the equilibrium lies extremely far to the left

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6
Q

What happens when acids and bases react?

A

Acids can only donate their protons if there’s a base to accept them - in this reaction the acid, HA, transfers a proton to the base, B:

HA + B —– BH+ + A-

It’s an equilibrium, so if you add more HA or B, the position of equilibrium moves to the right. If you add more BH+ or A-, the equilibrium will move to the left

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7
Q

How does an acid donate protons in water?

A

When acid is added to water, water acts as a base and accepts the proton:

HA + H2O —– H3O+ + A-

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8
Q

Does water dissociate?

A

Water dissociates slightly into hydroxonium ions and hydroxide ions. So this equilibrium exists in water:

H2O —– H+ + OH-

You can write an expression for Kc using this equilibrium reaction

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9
Q

Where does the equilibrium lie in water?

A

Water only dissociates a small amount so equilibrium lies well over to the left

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10
Q

Explain Kw

A

There is so much water compared to the amounts of H+ and OH- ions that the concentration of water is considered to have a constant value. So, if you multiple the expression for Kc by H2O you get the ionic product of water

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11
Q

Give the expression for Kw

A

Kw = Kc x [H2O] = [H+][OH-]

or simply: Kw = [H+] [OH-]

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12
Q

How does temperature affect Kw?

A

Kw always has the same value for an aqueous solution at a given temperature. The value of Kw changes as temperature changes

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13
Q

What can you say about Kw in pure water?

A

In pure water, there is always one H+ ion for each OH- ion. So, [H+] = [OH-]. This means that if you are dealing with pure water, then you can say that Kw = [H+]^2

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14
Q

What are the units of Kw?

A

mol^2 dm^-6

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