Oceans Flashcards
Define an acid and an alkali according to the Bonstead Lowry theory
Acid - H+ donor
Alkali - H+ acceptor
Finish the equations
1) Acid + metal
2) Acid + alkali
3) Acid + mental carbonate/ hydrogen carbonate
1) salt + hydrogen (MASH)
2) salt and water
3) salt + water + CO2
Name some common acids and alkalis
Acids
HCl H2SO4 HNO3 H3PO4 all COOH
Alkalis
all group 1/2 hydroxides (NaOH)
all group 1/2 oxides are bases (Na2O)
amines
What are conjugate acid base pairs?
pairs of chemicals that differe by one H+
The conjugate acid turns into its pair when it behaves like an acid (donates a H+)
The conjugate base turns into its pair when it behaves like a base (accepts a H+)
What is the equation for pH?
pH = -log [H+]
How do you calculate the pH of a strong acid?
1) Write a balanced equation for the dissociation
2) Use stoichiometry to find [H+]
3) Use the equation pH = -log [H+]
Example:
What is the pH of H2SO4 with a concentration of 0.523 mol dm-3?
H2SO4 —> 2H+ + SO4 2-
[0.352] [0.704]
1:2
pH = -log[0.704] = 1.5
How do you work out the pH of a strong base?
1) Write disossiation equation
2) Use stiochiometry to find [OH-]
3) Use the ionic product of water equation to find the [H+]
Example:
What is the pH of NaOH with a concentration of 0.01 mol dm-3?
NaOH —> Na+ + OH-
[0.01] [0.01]
1:1
Kw = [H+][OH-]
[H+] = 1x10-14 / 0.01 = 1x10-12
pH = -log[1x10-12] = 12
What is the ionic product of water equation?
Kw = [H+] [OH-]
How do you find the pH of a mixture?
1) Find the limiting moles (moles/ co efficient)
2) Calculate the moles left over of the reactant
3) Calculate the concentration (moles / new total)
4) pH calc as normal
Example:
20cm3 of 0.2 mol dm-3 HCl is mixed with 50cm3 of 0.25 mol dm-3 NaOH what is the pH?
HCl + NaOH ---> NaCl + H2O C [0.2] [0.25] V 0.02dm3 0.05dm3 0.02 + 0.05 = 0.07dm3 (new vol) n 0.004 0.0075 0.0075 - 0.004 = 0.0035 (left over limiting mole of NaOH) (the smallest) NaOH = 0.0035 / 0.007 = 0.005 mol dm-3 NaOH ----> Na+ + OH- (1:1 ratio) [OH-] = 0.005 work out [H+] with Kw then work out pH like normal
How do you calculate the pH of a weak acid?
Ka = [H+]2 (squared) / [HA]
What are the assumptions made when calculating the pH of a weak acid?
H+ and A- are in a 1:1 ratio so [H+] is equal to [A-] so we can use [H+] squared
The equilibrium [HA] and the start [HA] are the same - very little dissociates as it is a weak acid
What is a buffer?
A weak acid and its salt
Resists changes to pH when small volumes of acid or alkali are added
What are the 3 ways you can make a buffer?
1) Weak acid (l) + salt (s)
2) Weak acid (l) + salt (aq)
3) Weak acid x moles + a strong base 0.5x moles
What would happen if you add acid to a buffer?
(CH3COOH)
The extra H+ ions make the POE shift to the left by reacting with CH3COO-
This is possible as the conc of the CH3COO- is high
The conc of the H+ and hence the pH stay constant
What would happen if you add alkali to a buffer?
(CH3COOH)
The OH- reacts with the H+ the POE shifts to the right to replace it
This is possible as the conc of CH3COOH is high
The H+ and hence the pH stays constant
How do you calculate the pH of a buffer?
[H+] = Ka X [HA] / [A-]
What happens when an ionic solid dissolves?
The ionic lattice breaks into gaseous ions
Energy is required to overcome the forces of attraction between the ions
This is the reverse of the lattice enthalpy
Gaseous ions dissolve in water (hydration) due to the attraction between ions and polar water molecules (ion- dipole bonds)
The solid will dissolve if more energy is released when bonds form than is needed to break them
Bonds broken - ionic in the solid and hydrogen in the water
Bonds formed - ion dipole
For something to dissolve the bonds that will be made have to be stronger than the ones broken
What is lattice enthalpy?
enthalpy change on formation of 1 mole of an ionic solid from gaseous ions
always negative because ionic bonds are being made
Magnitude of the lattice enthalpy reflects the strength of the ionic bonds which depends on the charge and size of the ions involved
What is enthalpy change of hydration?
enthalpy change of dissolving 1 mole of gaseous ions in water
always exothermic as ion dipole bonds are being made between ions and polar water molecules
ions become hydrated
What affects lattice enthalpies?
the size and charge of ions
smaller ions are closer together leading to stronger electrostatic attraction between the ions and higher charges lead to a stronger attraction
What affects hydration enthalpies?
size and charge of ions
hydration enthalpy is most exothermic for smaller and highly charged ions as they have a high charge dnesity so can attract polar water molecules easier
What is enthalpy change of solution?
enthalpy change of dissolving 1 mole of a solute forming an infinetly diluet solution
Draw a harber cycle linking the 3 enthalpy changes
What is the solubility product?
gives the maximum concentration that ions can have in a solution
ksp = [positive ions][negative ion] units vary mol dm-3
