OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012

Original post by otrivine

its fine

Describe how PH is controlled including an equation (6)

pH is controlled using a buffer solution. Buffer solutions are solutions that resist changes in pH when small amounts of acid or alkali are added.

A weak acid like ethanoic acid and the salt of the weak acid (sodium ethanoate) make up a buffer solution.

Ch3COOH-> Ch3COO- + H+ (the arrow should be reversible :colondollar:

)

On addition of acid the equilibrium shifts to the left hand side as the extra H+ ions react with the ethanoate ion, removing most of the h+ ions added, preventing a fall in pH.

On addition of alkali the ethanoic acid dissociates and the OH- + H+ -> H20
The equilbirum shifts to the right hand side to restore most of the h+ ions, so a rise in pH is prevented.

Define the term lattice enthalpy with the use of an equation. :smile:

[2 marks]

(edited 11 years ago)

Reply 782

LifeIsGood

12

Original post by The Illuminati

there is!! such as what, I thought F324 was mostly organic?

Stereoisomerism - optical/geometric isomers. There's polymerisation they've asked before.

Reply 783

Original post by The Illuminati

Ion is positively charged, greater attraction between nucleus and outer electron, more energy needed to overcome the attraction

yes 2/2

define the term : standard electrode potential [3 marks]

Reply 784

Original post by Issy123

pH is controlled using a buffer solution. Buffer solutions are solutions that resist changes in pH when small amounts of acid or alkali are added.

A weak acid like ethanoic acid and the salt of the weak acid (sodium ethanoate) make up a buffer solution.

Ch3COOH-> Ch3COO- + H+ (the arrow should be reversible :colondollar:

)

On addition of acid the equilibrium shifts to the left hand side as the extra H+ ions react with the ethanoate ion, removing most of the h+ ions added, preventing a fall in pH.

On addition of alkali the ethanoic acid dissociates and the OH- + H+ -> H20
The equilbirum shifts to the right hand side to restore most of the h+ ions, so a rise in pH is prevented.

Define the term lattice enthalpy with the use of an equation. :smile:

[2 marks]

woow excellent 6/6 marks including QWC

lattice enthalpy is the entahlpy change when one mole of ionic compund is formed from its gaseous ions under standard conditons

For example
Mg(g) + cl2(g) to give mgcl2(s)

Reply 785

Smiley Face :)

Original post by Killjoy-

That's exactly what you are saying - reasoning is the same.
Ratios as two chlorides are released per complex, so 1:2.

Justrealisedthat

Reply 786

Original post by otrivine

woow excellent 6/6 marks including QWC

lattice enthalpy is the entahlpy change when one mole of ionic compund is formed from its gaseous ions under standard conditons

For example
Mg(g) + cl2(g) to give mgcl2(s)

thank you! :smile:

and 1/2 as the mg and cl must be ions, so Mg2+ and 2Cl- -> MgCl2

Reply 787

Original post by Issy123

thank you! :smile:

and 1/2 as the mg and cl must be ions, so Mg2+ and 2Cl- -> MgCl2

snap yes true
Define:enthapy of neutralisation

Reply 788

kurdishboy94

1

Original post by otrivine

woow excellent 6/6 marks including QWC

lattice enthalpy is the entahlpy change when one mole of ionic compund is formed from its gaseous ions under standard conditons

For example
Mg(g) + cl2(g) to give mgcl2(s)

i think your wrong with your equation, isn't the lattice enthalpy equation of MgCl2:

Mg2+(g) + 2Clˉ(g) ==> MgCl2(s)

Reply 789

kurdishboy94

1

Original post by otrivine

snap yes true
Define:enthapy of neutralisation

the energy change that accompanies the neutralisation of an aqueous acid by an aqueous base to form one mol of H20(l) under standard conditions

define a standard conditions?

Reply 790

Original post by otrivine

snap yes true
Define:enthapy of neutralisation

the enthalpy change when an acid and alkali are neutralised to form 1 mole of water.

Why is the enthalpy change of neutralisation of ethanoic acid less exothermic than the enthalpy change of neutralisation of hydrochloric acid? [2marks]

(edited 11 years ago)

Reply 791

The Illuminati

4

I love buffer solution questions, there always seems to be an explanation question that is usually always 5 marks, my teacher told us exactly what to write for 5 marks.

e.g. CH3COOH/CH3COONa buffer system

1. at equlibrium CH₃COOH (equilibrium sign) CH₃COO^- + H⁺
2. adding acid causes CH₃COO^- + H⁺

\rightarrow

CH₃COOH
3. euilibrium shifts to left
4.adding alkali causes OH^- + H⁺

\rightarrow

H₂O
5. equilibrium shifts to right

if there is a sixth mark you can say that there are large reservoirs of CH3COO- and CH3COOH

(edited 11 years ago)

Reply 792

Original post by kurdishboy94

the energy change that accompanies the neutralisation of an aqueous acid by an aqueous base to form one mol of H20(l) under standard conditions

define a standard conditions?

correct

298K,100KPA and 1 mol dm-3

Reply 793

Original post by Issy123

the enthalpy change when an acid and alkali are neutralised to form 1 mole of water.

Why is the enthalpy change of neutralisation of ethanoic acid less exothermic than the enthalpy change of neutralisation of hydrochloric acid? [2marks]

yes is correct but state conditions in case better

Becuase with ehtnaoic acid has weaker bonds less attraction and has OH groups which are weaker when bonding with another molecule and therefore less energy is needed to break the bonds

Reply 794

LifeIsGood

12

Original post by otrivine

yes is correct but state conditions in case better

Becuase with ehtnaoic acid has weaker bonds less attraction and has OH groups which are weaker when bonding with another molecule and therefore less energy is needed to break the bonds

I've never read this, where's it in the textbook/spec?

Reply 795

LifeIsGood

12

Original post by sumsum123

1) Adsorbed onto a material
2) Absorbed into a material
3) Liquify the hydrogen under high pressure

Explain why you cant use hydrochloric acid to acidify potassium manganate (VII) and describe how the titration experiment between potassium manganate (VII) and iron.

What's the answer to this? ^

Reply 796

Original post by otrivine

yes is correct but state conditions in case better

Becuase with ehtnaoic acid has weaker bonds less attraction and has OH groups which are weaker when bonding with another molecule and therefore less energy is needed to break the bonds

okay

And thats true, although i think you have to mention that breaking the OH bond in ethanoic acid requires energy, so energy is used up, so its less exothermic? :smile:

Explain the advantages of Fuel Cell Vehicles over petrol and diesel powered vehicles. (3 marks)

Reply 797

Original post by LifeIsGood

I've never read this, where's it in the textbook/spec?

its the last page on enthalpy of neutralisation

Reply 798