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Acids and Bases
N/A
66
Chemistry
Not Applicable
10/04/2012

Additional Chemistry Flashcards

 


 

Cards

Term

 

 

 

ACIDS

Definition
An acid is a substance that can donate a proton (H+ ion) 
Term

 

 

BASES

Definition
A base is a substance that can accept protons
Term

 

 

WATER AS A BASE

Definition

Can act as an acid and a base 

base - 

HCl + H2O → H3O+ + Cl-

HCl donates a proton to the water, so the water acts like a base

Term

 

 

WATER AS AN ACID

Definition

H2O + NH→ OH- + NH4+

 

Water is donating a proton to ammonia

Term

 

 

PROTONS

Definition
H+ ions - Hydrogen atom has only one electron, if this is lost all that remains in a proton. In water it's bonded to at least one water molecule to form the ion H3O+
Term

 

 

CALCULATING THE pH OF STRONG ACIDS

Definition

Find concentration c = n / v

 - log [H+] = pH

Term

 

 

 CALCULATE CONCENTRATION FROM pH

Definition
SHIFT log(pH) = [H+
Term

 

 

DILUTING STRONG ACIDS

Definition

Calculate number of moles of acid n = c x v

Find new concentration by divinding moles with new volume c = n / v

Check if the acid is H2SO4 , if so then multiply concentration by 2 

Calculate pH (-log[H+

Term

 

 

CALCULATING pH FOR STRONG ACIDS NEUTRALISED BY A STRONG BASE

Definition

Calculate moles of H+  n = c x v 

Calculate moles of OH- n = c x v

Calculate moles in excess nH+- nOH- 

Calculate concentration of [H+] c = n / v

pH = -log[H+]

 

Term

 

 

Kw

Definition

Kw = [H+] [OH-] / [H2O] 

 

H2O is omitted because it's very small.

This is the equilibrium constant for water 

1 x 10-14

Term

 

 

CALCULATE THE pH WHEN THE CONCENTRATION OF OH- IS GIVEN

Definition
[H+] = Kw (1 x 10-14) / [OH-
Term

 

 

CALCULATING THE pH OF STRONG ACIDS DILUTED WITH WATER

Definition

Calculate moles of OH- n = c x v

Calculate new concentration of [OH-] c = n / v

Calculate [H+] using Kw: [H+] = Kw / [OH-]

Calculate pH: -log[H+]

Term

 

 

CALCULATE pH OF WEAK ACIDS

Definition

Rearrange the equation for Kto make [H+][A-] = Ka [HA] 

[H+] = [A-] So K= [H+]2  =  [H+][A-] = [H+]2

Calculate [H+]2  then √[H+]to find the answer for [H+


Calculate pH -log[H+

Term

 

 

IN A STRONG ACID

Definition
The concentration of [H+] is equal to the concentration of acid
Term

 

 

WHEN DOING A CALCULATION IN REVERSE

Definition
First thing you do when reversed is always the last thing you do
Term

 

 

WEAK ACIDS AND BASES

Definition
Partly dissociate in aqueous solutions
Term

 

 

ENDOTHERMIC PROCESSES

Definition
There will be a higher [H+] if the temperature increases so pH decreases
Term

 

 

EXOTHERMIC

Definition
Lower concentration of H+ when temperature increases, so the pH is higher
Term

 

 

CONJUGATE BASE

Definition

A substance that becomes an acid by gaining a proton

e.g NH3 + HBr ↔ NH4+ + Br-

NH4+ is the conjugate base

Term

 

 

Kw CONCENTRATION OF WATER: WHY IS H2O OMITTED FROM EQUATION? 

Definition
The concentration of water is effectivel constant, becasue it very large so H2O can be cancelled out
Term

 

 

WHY IS WATER ALWAYS NEUTRAL? 

Definition
Becase the [H+] = [OH-] So they have the same number of moles, hence the same concentration
Term

 

 

ESSENTIAL FEATURE OF ACID BASE REACTIONS

Definition
The transfer of protons
Term

 

 

DEFINITION OF Kw

Definition
The ionic product of water
Term

 

 

DEFINITION OF pH

Definition
The scale for measuring acidity and alkalinity. pH = -log [H+]
Term

 

 

H3O+

Definition
Hions combine with water molecules to make the oxonium/hydronium/hydroxium ion
Term

 

 

HIONS

Definition
Have no electrons of thier own so it can only form a bond with another species that has a lone pair of elecetrons
Term

 

 

DIFFERENCE IN pH

Definition
Difference in one pH number is equal to a tenfold difference in [H+
Term

 

 

WHY IS HCl A STRONG ACID? 

Definition

In the gas phase it is covalent

In an aqueous solution it is totally ionic, so there are no molecules left so it's a strong acid

Term

 

 

pKa

Definition

 = -log K

 

This is the antilog of the acid dissociation constant :

K= [H+][A-] / [HA]

Term

 

 

UNITS FOR Ka

Definition

moldm-3 moldm-3  moldm-3


 moldm-3

Term

 

 

VALUES OF pKa

Definition
The smaller the value of pKa the stronger the acid
Term

 

 

TITRATION CURVE - STRONG ACID STRONG BASE*

Definition
Term

 

 

TITRATION CURVE - STRONG ACID WEAK BASE*

Definition
Term

 

 

TITRATION CURVE - WEAK ACID WEAK BASE*

Definition
Term

 

 

TITRATION CURVE - WEAK ACID STRONG BASE*

Definition
Term

 

 

EQUIVILANCE POINT

Definition
Point at which sufficient base/acid has been added to neutralise the acid/base. The pH at equivilance point is not always exactly 7
Term

 

 

TITRATION CURVES

Definition
Except from weak acid and weak base, there is a large and rapid change in pH at the equivilance point even though it may not be at pH 7
Term

 

 

HOW TO CALCULATE CONCENTRATION USING THE EQUIVILANCE POINT

Definition

e.g we find the equivilance point of a titration is when 25cm3 of 0.0150moldm-3 base, neutralised by 15.0cm3 acid. This shows that 25cm3 base has the same number of moles of 15.0cm3 acid. 


n = c x v:so we calculate the number of moles of base 

= 0.0150 x (25 x 10-3) = 3.75 x 10-4 (mol of acid and base)

c = n / v : so cacid = 3.75 x 10-4 / (15 x 10-3) = 0.025moldm-3


Term

 

 

INDICATORS

Definition
To find concentrations of a solution of acid or alkali - equivilance point has exactly the same number of moles of hydrogen ions and hydroxide ions
Term

 

 

END POINT

Definition
The volume of alkali or acid added when the indicator just changes colour
Term

 

 

SUITABLE INDICATORS

Definition

Sharp colour change(1 drop of acid/base) to give full colour change

End point of titration given by indicator  = equivilance point 

Indicator gives distinct colour change 

Term

 

 

COMMON INDICATORS

Definition

Universal - gradual colour change

Methyl orange - changes from red to yellow at pH 3- 5

Bromophenol blue - changes from yellow to blue at pH 4-5

Methyl red - changes from red to yellow at pH 4- 6

Bromothymol blue - changes from yellow to blue at pH 6-7

Phenolphthalein - changes from colourless to pink/red at pH 10 -11

Term

 

 

SUITABLITY OF PHENOLPHTHALEIN AND METHYL ORANGE ON STRONG ACID STRONG BASE*

Definition
Term

 

 

SUITABLITY OF PHENOLPHTHALEIN AND METHYL ORANGE ON WEAK ACID STONG BASE*

Definition
Term

 

 

SUITABLITY OF PHENOLPHTHALEIN AND METHYL ORANGE ON STRONG ACID WEAK BASE*

Definition
Term

 

 

SUITABLITY OF PHENOLPHTHALEIN AND METHYL ORANGE ON WEAK ACID WEAK BASE*

Definition
Term

 

 

HALF NEUTRALISATION POINT

Definition

Point half way between zero and the equivilance point; pH will change very little when we add acid/base up to this point

Here we can find pKof a weak acid because at this point

HA + OH- → H2O + A

At this point: [HA] = [A]

 so, K = [H+] [A-] / [HA

K= [H+

So -logK= -log [H+] So pK= pH

Term

 

 

BUFFER SOLOUTIONS

Definition
They resist changes of acidty and alkalinity when small amounts of acid or alkali are added to them, thier pH remains almost constant; they keep the concentration of hydrogen ions and hydroxide ions almost unchanged; based on equilibrium reaction which moves direction if either ion is added
Term

 

 

ACIDIC BUFFERS

Definition

Made from weak acids because dissociation of weak acids is an equilibrium reaction

Definition: An acidic buffer is made from a mixture of a weak acid and a soluble salt of that acid. It will maintain a pH of below 7 

Term

 

 

HA(aq) ↔ H+(aq) + A-(aq)

Definition

[H+(aq) ] = [A-(aq)]

It's a weak acid, so both are very small because most of the HA is undissociated

Term

 

 

ADDING ALKALI

Definition

HA(aq) + OH-(aq) → H2O(aq) + A-(aq)

 

The OH- is removed so the pH tends to stay the same

Term

 

 

ADDING ALKALI

Definition

Adding Hshifts the equilibrium to the left, so they combine with the Aions to make HA . However, since there is limited supplies of just A-, once no more can be combined and there is an excess in H+ ions, the pH changes. 

Soluble salts of HA can be added that ionise and increases the supply of A-. 

Term

 

 

FUNCTION OF WEAK ACID IN A BUFFER

Definition
Acts like HA which can remove OH-
Term

 

 

FUNCTION OF SALT IN BUFFER

Definition
Source of A- ions that can remove any added H+ ions
Term

 

 

CHANGE IN pH

Definition
Buffers don't ensure NO change in pH at all. Only slight changes however. A buffer can be saturated where all avaliable HA or Ais used up
Term

 

 

MIXTURE OF WEAK ACID

Definition

At half neutralisation where pH = pK

 

neutralise half the acid to get this buffer

Term

 

 

BASIC BUFFERS*

Definition
Resist change, maintian pH above 7; made from a mixture of a weak base and the salt of that base
Term

 

 

EXAMPLE OF A BUFFER

Definition

Blood approx 7.4 because even a small change such as 0.5 can be fatal. It's buffered by this equation 

 H+(aq) + HCO-3(aq)↔ CO2(aq) + H2O(l)

adding extra H+ shifts equilibrium to right

Adding extra OHshifts equilibrium to left

Term

 

 

CALCULATION OF BUFFERS

Definition

HA(aq) ↔ H+(aq) + A-(aq)

K= [H+(aq) ] [A-(aq] / [HA(aq)]

to calculate pH of buffers 

Term

 

 

EXAMPLE OF USING 

K= [H+(aq) ] [A-(aq] / [HA(aq)]

Definition

Buffer contains 0.100moldm-3 ethanoic acid and 0.100moldm-3 sodium ethanoate. Ka = 1.7 x 10-5 and pKa = 4.77 for ethanoic acid. 

K= [H+(aq) ] [A-(aq] / [HA(aq)] Sodium ehtanote - fully dissociated , ethanoic acid, almost undissociated. 

[H+] = Ka[HA] / [A-] = (1.7 x 10-5 x 0.100) / (0.100) = 1.7 x 10-5

-log(1.7 x 10-5) = 4.77

pH = 4.77 pKa = 4.77

pH = pK with equal conc of acid and base

Term

 

 

IN A BUFFER SOLUTION

Definition
[H+] DOESN'T EQUAL [A-] SO SIMPLIFIED EXPRESSION K= [H+]/ [HA] IS NOT VALID
Term

 

 

CALCULATING pH OF A WEAK ACID AND STONG BASE

Definition

Work out moles of both acid and base 

Find XS moles of base

Work out concentration from the moles with the new volume

Rearrange Kw for [H+] and calculate the concentration of hydrogen ions

Calculate pH

 

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