2017 H2 Chemistry JC1 & 2 students post your questions here
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It's all Chemistry, yo. Different colours reflect different pH values in the different regions of the drink in the cup, different degrees of protonation vs deprotonation, different molarities of conjugate acids vs conjugate bases, different extents of molecular conjugation or overlap of p-orbitals with delocalized electrons, different magnitudes of energy differences between lower and higher molecular orbitals, different wavelengths of visible light absorbed when the electrons transit. It's all Chemistry, yo. The most important subject at A levels.
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Hi! This is a relatively general question but how do I predict the products of inorganic reactions given the reactants and I am asked to balance such a reaction equation? Is there a thought process to deduce the products? E.g a reaction between Fe2O3 and CO. Thanks!
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Originally posted by ramsey tang:
Hi! This is a relatively general question but how do I predict the products of inorganic reactions given the reactants and I am asked to balance such a reaction equation? Is there a thought process to deduce the products? E.g a reaction between Fe2O3 and CO. Thanks!
There is no shortcut to predicting products. You've got to apply all your chemistry knowledge from across different topics to determine which (in tougher questions, more than one type of reaction is possible, hence a mixture of different products may be obtained).For Fe2O3 and CO, unless otherwise specified by the (eg. data based) question, otherwise at O and A levels, the only possible reaction (covered by the syllabuses) is redox, where iron is reduced and carbon is oxidized.
Fe2O3 + 3CO --> 2Fe + 3CO2
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Sorry to clarify, assuming I know it is a redox reaction, is there any way of knowing which oxidation state an element would be oxidised/reduced to in order to help predict a product?
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Originally posted by ramsey tang:
Sorry to clarify, assuming I know it is a redox reaction, is there any way of knowing which oxidation state an element would be oxidised/reduced to in order to help predict a product?
For A levels, use the Data Booklet's redox potentials to determine which is the most thermodynamically feasible (ie. change in Gibbs free energy most negative), PLUS use familiarity of expected knowledge within syllabus PLUS look for clues in the question.For O levels, use familiarity of expected knowledge within syllabus, ie. memorize the most stable OSes and expected redox products for commonly tested compounds.
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Alright thank you!
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All JC1 & JC2 students should now be revising for their Mid Yr Exams in end June / early July.
Students with their own tutors are already taken care of. Students without tuition (either due to personal choice or financial constraints) can feel free to post their H2 Chem questions here in this thread (simply register an SgForums account for free if you've not yet done so), and I'll help you out for free.
JC2 students who do not currently have tuition but are considering it, can check out my Special Conditional Offer (limited to JC2 students who join in June 2017 only).
Feel free to post your H2 Chem questions here, but note that I will not spoonfeed you answers or do your homework for you, but instead I will guide you to check your concepts (so don't just post the question, but instead specify your own personal conceptual query, eg. if you don't understand or agree with your school's given answer) and suggest online links with relevant materials for your perusal.
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Though often placed at the top of the alkali metal column in the periodic table, hydrogen does not, under ordinary conditions, exhibit the properties of an alkali metal. Instead, it forms diatomic H2 molecules, analogous to halogens and non-metals in the second row of the periodic table, such as nitrogen and oxygen. Diatomic hydrogen is a gas that, at atmospheric pressure, liquefies and solidifies only at very low temperature (20 degrees and 14 degrees above absolute zero, respectively). However, Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around 4,950,000 bar or 4,890,000 atm or 71,800,000 psi, hydrogen would display metallic properties: instead of discrete H2 molecules (which consist of two bonding pair electrons bound between two nuclei protons), a bulk metallic phase would form with a solid metallic lattice of protons and the electrons delocalized throughout.
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Time yourself. If you can complete this question within 5 mins, you're an A grader. 10 mins, you're a B grader. 15 mins, you're a C grader, and so on.
VJC 2012 P3 Hess Law Qn
ZnO(s) + 3C(s) --> ZnC2(s) + CO(g) ; enthalpy change = + 462 kJ mol-1
ZnO(s) + H2O(l) --> Zn(OH)2(s) ; enthalpy change = - 65 kJ mol-1
ZnC2(s) + 2H2O(l) --> Zn(OH)2(s) + C2H2(g) ; enthalpy change = - 126 kJ mol-1
2C(s) + O2(g) --> 2CO(g) ; enthalpy change = - 221kJ mol-1
2H2O(l) --> 2H2(g) + O2(g) ; enthalpy change = + 572 kJ mol-1
Calculate the formation enthalpy of C2H2(g).
Final Answer = +225.5 kJ mol-1
BedokFunland JC extension :
For each equation, state the type of reaction occurring, and draw the curved-arrow electron-flow reaction mechanism for each reaction.
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[Chemistry / Medicine] -
Just because you didn't know that the salt used at Macs contains aluminium, doesn't make the salt any less toxic for your body and in particular for your brain :
http://www.dailymail.co.uk/health/article-4047426/Aluminium-DOES-cause-Alzheimer-s-Expert-says-new-findings-confirm-metal-devastating-brain-disease.htmlBeing unaware of a problem doesn't make it go away.
McDonalds (including Singapore Macs) uses an aluminium salt, just read the ingredient list on the small Macs salt packs the next time you're at Macs.
https://www.quora.com/What-kind-of-salt-does-McDonalds-use-on-their-fries
At the very least, always ask for UNSALTED fries at Macs and all fast food restaurants. Of course, quite separate from the aluminium problem, are the French Fries itself (or for that matter, all deep fried foods).
http://time.com/4811136/fried-potatoes-mortality-study/
Also see :
http://www.top10grocerysecrets.com/2015-07-02-top-10-toxic-ingredients-used-mcdonalds.html
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[Chemistry / Medical ] - Concerns over Aluminium toxicity and Bisphenol A toxicity from soft drinks, including Coca Cola (btw : all artificial-sweeteners sugar-free 'diet' soft drinks are always more toxic than regular-sugar soft drinks).
"Without the invisible layer of BPA-based epoxy coating, an aluminium can of acidic Coca Cola would disintegrate in just three days!"
https://www.wired.com/2015/03/secret-life-aluminum-can-true-modern-marvel/
Also see :
http://www.coca-colacompany.com/contact-us/faqs
http://www.bisphenol-a.org/human/epoxycan.html
https://www.stevespanglerscience.com/lab/experiments/can-ripper/
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[ Biology / Medical ] - "The Upside of Bad Genes" by Moises Velasquez-Manoff
https://www.nytimes.com/2017/06/17/opinion/sunday/crispr-upside-of-bad-genes.html
Sickle cell was a case in point. The gene is usually found in people who live in, or whose ancestors came from, sub-Saharan Africa, the Arab world and India; in those places, having one copy of the gene can prevent the worst symptoms of malaria. Of every four children our imaginary couple might have, one will probably be afflicted with sickle cell disease, but two would most likely be protected from malarial disease.
Ditto with the gene variants that cause the lung disease cystic fibrosis. In parts of northwest Europe, about 1 in 25 people carries a single copy of the gene. And while two copies cause disease, it has long been hypothesized that having just one protects against tuberculosis — the White Plague that ravaged Europe for a few hundred years.
Carriers of the ApoE4 variant have an up-to-fourfold increased risk of Alzheimer’s disease. In 2010, scientists at Northwestern University found that the gene was more prevalent in tropical and polar populations than in those from mid-latitudes, presumably because it served some function in those regions. Then, strangely, the gene was linked to enhanced cognitive performance in children living in Brazilian slums. And last year, Ben Trumble, an anthropologist at Arizona State University, and colleagues published an intriguing study suggesting that the gene might improve brain function in elderly people living in the Bolivian Amazon. The tribe he studied, called the Tsimane, subsist mostly on what they grow and hunt in the jungle, and about two-thirds have intestinal parasites. Dr. Trumble discovered that if elderly ApoE4 carriers harbored parasites, their cognitive abilities improved relative to noncarriers with parasites. Only carriers without parasites suffered cognitively.
Scientists at Albert Einstein College of Medicine in New York City had recently discovered that ingesting cyanate salts could prevent the “sickling� of red blood cells that leads to the anemia and pain of sickle cell disease. Dr. Jackson knew that a related compound, cyanide, was common in foods across Africa, particularly in the staple crop cassava (which you may know as tapioca). She was also aware that, at the right dose, cyanide could directly protect against the malaria parasite. She realized that regular consumption of cassava — more common in the Southeast than the Northwest — could, by working as an antimalarial drug, affect the prevalence of the sickle cell trait, by making it less advantageous.
Something different was happening in the Northwest, though. Dr. Jackson, who is now at Howard University, thinks that while cassava consumption in the region was insufficient to protect against malaria directly, people who had two copies of the sickle cell gene still ate enough to partly avoid sickling. In that population, diet may have prevented a genetic disease from fully manifesting.
We evolved in environments that are radically different from today’s, and some of our genes may work better in those environments. This complicates the idea of trying to perfect the human genome with technology. Given how much the world has changed in just the past 150 years, and how much it’s likely to change again in the next 150, the question is, “What environment will we optimize our genes for?�
https://www.nytimes.com/2017/06/17/opinion/sunday/crispr-upside-of-bad-genes.html
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I love chem
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Hello! May I ask a question on The Periodic Table?
Qn : When sodium chloride is added to water, a solution is formed which has a pH of 7. When phosphorous pentachloride is added to water, the resulting solution has a pH of 1. Explain these observations, and why these chlorides differ in their reaction with water.
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1) A 4.50g sample of a carbonate of a Group 2 metal (known to be either calcium or strontium) lost 1.34g in mass when heated strongly. Identify the metal, showing clearly your working.
2) Group 2 hydroxides and nitrates decompose to give the same solid residues but different gaseous products.
A group 2 hydroxide exists as a hydrate, M(OH)2.nH2O. When a 1.575g sample of this hydrte was heated, the mass if the sample decreased by 0.720g. Subsequent hearing to constant mass produced a further mass loss of 0.090g. Calculate the value of n, and deduce the identity of the metal M.
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Originally posted by Claresse216:
Hello! May I ask a question on The Periodic Table?
Qn : When sodium chloride is added to water, a solution is formed which has a pH of 7. When phosphorous pentachloride is added to water, the resulting solution has a pH of 1. Explain these observations, and why these chlorides differ in their reaction with water.
Yo!Due to relatively low ionic charge densities, neither the Na+ ion nor the Cl- ion undergoes significant hydrolysis, hence pH of NaCl(aq) remains neutral.
PCl5, being a covalent chloride, is electrophilic (since each Cl is significantly more electronegative than P), hence readily attacked by water nucleophiles to generate 1 mol of the weak acid H3PO4(aq), as well as 5 mol of the strong acid HCl(aq). Hence a strongly acidic solution results.
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Originally posted by Claresse216:
1) A 4.50g sample of a carbonate of a Group 2 metal (known to be either calcium or strontium) lost 1.34g in mass when heated strongly. Identify the metal, showing clearly your working.
2) Group 2 hydroxides and nitrates decompose to give the same solid residues but different gaseous products.
A group 2 hydroxide exists as a hydrate, M(OH)2.nH2O. When a 1.575g sample of this hydrte was heated, the mass if the sample decreased by 0.720g. Subsequent hearing to constant mass produced a further mass loss of 0.090g. Calculate the value of n, and deduce the identity of the metal M.
Q1. From 1.34g of CO2(g), find mol of CO2(g). By stoichiometry, this is also the mol of metal carbonate. Giving sample mass of metal carbonate as 4.50g, and now you know the mol of metal carbonate, you can find the molar mass of the metal carbonate, hence can identify the metal.Q2. The first 0.720g of water lost is from the water of hydration, ie. nH2O. The 2nd 0.090g of water lost is from the decomposition of the hydroxide. From 0.090g of water lost, you can find the mol of water lost, which by stoichiometry is also the mol of the entire hydrated metal hydroxide. Hence n = (mol of water lost 0.720g) / (mol of water lost 0.090g).
Now that you know the value of n, you can work out the molar mass of the hydrated metal hydroxide in terms of an algebraic variable, eg. let M be the molar mass of the metal M. Since you know the mol of the hydrated metal hydroxide, and you know its sample mass is 1.575g, and you have its molar mass (in terms of M), hence you can equate the 3 terms together and solve for M, and therefore identify the metal M.
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Hey UltimaOnline, heres a question from my sch paper
A sample of positively charged chromium ions, Cr^n+, was beamed through an electric field and was deflected by an angle of +8 degrees. A sample of No2^+ was beamed through the same elctric field and was deflected by an angle of +3.01 degrees. With reference to data booklet, calculate the amount of energy required to form 0.2 mol of gaseous Cr^n+ ions from 0.2 mol of gaseous Cr^(n-1)+ ions.
Ans: angle of deflection of Cr^n+=(1/46 divide n/52)x8=3.01
I dont understand why the angle of deflection of Cr is being derived as such. Thanks for the help!
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Originally posted by JJ_DHS:
Hey UltimaOnline, heres a question from my sch paper
A sample of positively charged chromium ions, Cr^n+, was beamed through an electric field and was deflected by an angle of +8 degrees. A sample of No2^+ was beamed through the same elctric field and was deflected by an angle of +3.01 degrees. With reference to data booklet, calculate the amount of energy required to form 0.2 mol of gaseous Cr^n+ ions from 0.2 mol of gaseous Cr^(n-1)+ ions.
Ans: angle of deflection of Cr^n+=(1/46 divide n/52)x8=3.01
I dont understand why the angle of deflection of Cr is being derived as such. Thanks for the help!
Its ok I understood the solution already
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Originally posted by JJ_DHS:
Hey UltimaOnline, heres a question from my sch paper
A sample of positively charged chromium ions, Cr^n+, was beamed through an electric field and was deflected by an angle of +8 degrees. A sample of No2^+ was beamed through the same elctric field and was deflected by an angle of +3.01 degrees. With reference to data booklet, calculate the amount of energy required to form 0.2 mol of gaseous Cr^n+ ions from 0.2 mol of gaseous Cr^(n-1)+ ions.
Ans: angle of deflection of Cr^n+=(1/46 divide n/52)x8=3.01
I dont understand why the angle of deflection of Cr is being derived as such. Thanks for the help!
(+1/46)x = +3.01, hence x = 1.3846x10^2
(+n/52)x = +8, hence n = +3Hence energy required to form 0.2 mol of gaseous Cr^3+ ions from 0.2 mol of gaseous Cr^2+ ions = 0.2 mol x 3rd ionization energy (kJ/mol)