olvl chem qns
-
Q1.
a) endothermic, since RHS has more energy.
b) exothermic, since ion-dipole bonds are formed betwen the ions and the water molecules, in the crystallization process; and the RHS has less energy.
c) endothermic, since RHS has more energy.
d) combustion of hydrogen is exothermic.
e) since combustion of C is exothermic, the reverse process is endothermic.
f) breaking covalent bond between H and Cl is endothermic. Moreover, free radicals are highly unstable, and hence the RHS clearly has more energy.
g) breaking covalent bond between H and Cl is endothermic. Moreover, free radicals are highly unstable, and hence the RHS clearly has more energy.
Q2.
a) thermal decomposition of calcium carbonate into calcium oxide and carbon dioxide is endothermic. Furthermore, RHS has more energy (gaseous CO2).
b) mixing concentrated acids and water are always exothermic. The saying "Do as you oughta, mix acid with water", is to help students remember not to drop water into concentrated acids, as the strongly exothermic reaction would cause the concentrated acids to start fuming and splashing dangerously, posing a safety hazard.
Originally posted by ultimatenolifer:olvl tys qn topic frm energy changes:
1) which of these equations are exothermic and endothermic rxns? (i have absolutely no idea how to tell when im not given any bond energies to calculate and some dun even have state symbols...zzz)
a)ch4(g) =>c(g) + 4h (g)
b)cuso4(s) + 5h2o(l) => cuso4.5h2o(s)
c)h2 => 2h
d)2h + o => h2o
e)co2 => c + 2o
f)hcl => h + cl
g)cl2 => 2cl
2) which of these does not give out heat to surroundings?
a) caco3(s)=> cao(s) + co2(g)
b)mixing btw sulphuric acid and water
the answer is a becoz i understand that thermal decomposition is endothermic but why is b) wrong? is b) an exothermic rxn?
-
which graph shows how the temp changes as hcl is added slowly to aqueous naoh until the acid is excess? (i will describe the graph in words) (y axis-temp, x axis-vol of hcl)
a) inverse v shaped graph where temp increases sharply den decreases sharply to initial temp OR
b)a sharp increase of temp and stays constant thereafter
-
Originally posted by ultimatenolifer:
which graph shows how the temp changes as hcl is added slowly to aqueous naoh until the acid is excess? (i will describe the graph in words) (y axis-temp, x axis-vol of hcl)
a) inverse v shaped graph where temp increases sharply den decreases sharply to initial temp OR
b)a sharp increase of temp and stays constant thereafter
A is correct. Since after a while, the heat will be dissipated to the surroundings, ie. thermal equilibrium with the environment. -
1)suggest identities for the below oxides
A: a white solid, insoluble in water, forms a colourless soln with both hcl and naoh
B: a white solid, soluble in water, forms a solution which turns blue litmus paper red
C: a white solid dissolves sparingly in water to form a white suspension. the solution turns red litmus paper blue.
2) when iron was added to dilute h2so4, explain why the mixture needs to be gently warmed for rxn to take place
need help on these two qns, thx
-
Originally posted by ultimatenolifer:
1)suggest identities for the below oxides
A: a white solid, insoluble in water, forms a colourless soln with both hcl and naoh
B: a white solid, soluble in water, forms a solution which turns blue litmus paper red
C: a white solid dissolves sparingly in water to form a white suspension. the solution turns red litmus paper blue.
2) when iron was added to dilute h2so4, explain why the mixture needs to be gently warmed for rxn to take place
need help on these two qns, thx
1A) Any of the amphoteric oxides that you're familiar with, as they react with acids and alkalis to form soluble ions and complex ions.
1B) P4O10 phosphorus pentoxide aka phosphorus(V) oxide, which undergoes hydrolysis to generate phosphoric acid.
1C) MgO magnesium oxide, dissolves sparingly in water to generate magnesium hydroxide precipitate (ie. white suspension), which is alkaline.
2) Iron isn't reactive enough to react rapidly with dilute sulfuric acid, unlike for eg. sodium metal. If concentrated sulfuric acid is used, iron reacts rapidly enough. But since dilute sulfuric acid is used, warming is required to speed up the rate of reaction (by increasing the % of reactant particles with energy equals or exceeding activation energy required for the reaction).
-
some qns here again on olvl topic :salts
1) describe 4 ways to test whether a soln is acidic. For each test, explain the observation that proves the soln is acidic. ( i can only tink of litmus paper)
2a) what would you observe when barium chloride soln is added to dilute H2so4? (i thought acids dont react wif salts??)
b) write down the chemical equation for the above rxn.
3) how do u prepare the lead carbonate salt? ( i juz nid the reactants)
4) my own qn: do salts react wif bases?
nid some help. thx .
-
Originally posted by ultimatenolifer:
some qns here again on olvl topic :salts
1) describe 4 ways to test whether a soln is acidic. For each test, explain the observation that proves the soln is acidic. ( i can only tink of litmus paper)
2a) what would you observe when barium chloride soln is added to dilute H2so4? (i thought acids dont react wif salts??)
b) write down the chemical equation for the above rxn.
3) how do u prepare the lead carbonate salt? ( i juz nid the reactants)
4) my own qn: do salts react wif bases?
nid some help. thx .
1) Release of CO2 with sodium carbonate. (protonation of carbonate ion gives carbonic acid which decomposes to carbon dioxide and water)Gives a red, orange or yellow colour with Universal Indicator solution. (protonates the indicator compound to give the relevant indicative colour).
A pH data logger will detect the molarity of protons in solution and give a digital readout. (If the molarity of protons is greater than 1x10^-7 at room temperature, the solution is acidic.)
Dissolves insoluble basic compounds. (protonation of basic group allows ion-dipole interactions with water, making it soluble).
(If some of the above methods are not easily understood at 'O' levels, ignore them and try thinking of other methods, and/or wait for your teacher to go through the answers. You could use different multiple pH indicators, eg. phenolphthalein, methyl orange, methyl red, etc. Google their colours at various pH values.)
2) Ba2+(aq) + SO4 2-(aq) --> BaSO4 (s)
A white precipitate can be observed.
3) aqueous lead(II) nitrate and aqueous sodium carbonate.
4) Only ammonium salts. You get ammonia gas :
NH4+(aq) + OH-(aq) ---> NH3(g) + H2O(l)
-
hi ultimaonline, thx for ur prompt replies. But juz curious, why is bacl2 able to react wif acid. Coz from my acid topic, acids can only react wif metals, carbonates, bases and alkalis. Nothing has been mentioned abt acids reacting wif salts..hope u can enlighten me. thx.
-
Originally posted by ultimatenolifer:
hi ultimaonline, thx for ur prompt replies. But juz curious, why is bacl2 able to react wif acid. Coz from my acid topic, acids can only react wif metals, carbonates, bases and alkalis. Nothing has been mentioned abt acids reacting wif salts..hope u can enlighten me. thx.
Acids are proton (H+) donors. Usually it is the proton (H+) that reacts with metals, carbonates, bases and alkalis.With metals :
Redox reaction. The proton is reduced the hydrogen gas 2H+ + 2e- --> H2 gas, whilst the metal is oxidized (eg. Mg to Mg2+).
With carbonates :
Protonation of carbonate to carbonic(IV) acid which decomposes into carbon dioxide and water. 2H+ + CO3 2- --> H2CO3 --> CO2 + H2O
With bases :
Protonation of base. Eg. NH3 + H+ --> NH4+.
With alkalis :
Protonation of hydroxide ion. H+ + OH- --> H2O.
So the usual reactions of acid are due to the protons (H+ ions).
However, in mixing sulfuric acid and barium chloride, it is the SO4 2- ions, not the protons H+ ions, that combine with the Ba2+ ions to form BaSO4(s) precipitate. It could just as conveniently have been sodium sulfate Na2SO4 instead of sulfuric acid H2SO4. The point is not about the Na+ ions or the H+ ions, the point is the SO4 2- ions which can combine with the Ba2+ ions.
-
im enlightened lol. many thks to ultimaonline again :)
-
Originally posted by ultimatenolifer:
im enlightened lol. many thks to ultimaonline again :)
Most welcome. Keep enjoying Chemistry (and maybe one day you may choose to be a chemistry teacher and share the joy too)!
-
topic on salts:
rxn1: when a few drops of aq. nh3 was added to B, a white ppt of C was observed. It dissolved when excess aq.nh3 was added.
rxn2: when aq.nh3 was added to A, a green ppt D was formed which gradually turned brown on standing in air. A also gave a white ppt, E with acidified BaCl2 soln
rxn3: when aq.Na2Co3 was added to B, a white ppt F was formed. When the residue was washed, filtered and heated strongly, a residue G was obtained. G was yellow but turned white on cooling
rxn4: when aq.Naoh and Aluminium was added to B and warmed gently, a pungent gas was evolved which turned moist red litmus paper blue.
a) Identify all substances from A to G
b) Write the ionic eqn for the formation of ppt C
c)Write a balanced chemical eqn for formation of G
-
Originally posted by ultimatenolifer:
topic on salts:
rxn1: when a few drops of aq. nh3 was added to B, a white ppt of C was observed. It dissolved when excess aq.nh3 was added.
rxn2: when aq.nh3 was added to A, a green ppt D was formed which gradually turned brown on standing in air. A also gave a white ppt, E with acidified BaCl2 soln
rxn3: when aq.Na2Co3 was added to B, a white ppt F was formed. When the residue was washed, filtered and heated strongly, a residue G was obtained. G was yellow but turned white on cooling
rxn4: when aq.Naoh and Aluminium was added to B and warmed gently, a pungent gas was evolved which turned moist red litmus paper blue.
a) Identify all substances from A to G
b) Write the ionic eqn for the formation of ppt C
c)Write a balanced chemical eqn for formation of G
A - Iron(II) sulfate.
B - Zinc nitrate.
C - Zinc hydroxide.
D - Iron(II) hydroxide.
E - Barium sulfate.
F - Zinc carbonate.
G - Zinc oxide (yellow when hot, white when cold)
b) NH3(aq) + H2O(l) <---> NH4+(aq) + OH-(aq)
Zn2+(aq) + 2OH-(aq) <---> Zn(OH)2(s)
c) ZnCO3(s) ---> ZnO(s) + CO2(g)