Chemistry 2012 Help Thread (1 Viewer)

[Living Moment]

Post up any of your Chemistry Questions and they shall be answered

Only HSC/Prelim Questions. Please don't make the thread messy. Only answer and questions and stay on the topic of the thread, Thanks

 

[addoil]

Was waiting for this to be posted back up.

Question:

I know the definition of a buffer solution (a mixture of comparable amounts of a weak acid and its conjugate base such that when significant amounts of acids/bases are added, the pH remains constant), and there examples of buffer solutions include river/lake and blood systems.

But in those 5 marker HSC Qs that ask for an example of how buffer solutions work, how do you answer appropriately WITH chemical equations for each of the specific systems listed above?

 

[Living Moment]

Here was the Q. form the last thread

Q. Assess the significance of early scientists to our understanding of electrochemistry (7 mark)
(the question is from Shipwrecks)

Also, did this question appear in any past HSC or trial exam?
A. Luigi Galvani was an Italian physician in the eighteenth century. He undertook a series of investigation into the twitching of frog leg muscles with a static electricity generator. He also obtained twitching by pressing a brass hook into the frog’s spinal cord and hanging the hook on an iron railing. He concluded that animal tissue contained an “electric fluid”, through which a force, “animal electricity”, could acted upon the tissue. This was the first example in our understanding of electrolytes in electron transfer reactions.

Alessandro Volta was a contemporary of Galvani, who later refuted his theory of “animal electricity”. He believed that the twitching of the frog legs were due to the two different metal pieces holding the legs. In 1800, Volta made his most famous discovery, the voltaic pile, a stack of silver and zinc discs separated by felt pads soaking in brine (salt solution). This discovery led to our understanding of using electron transfer reactions to generate electricity.

Humphry Davy was inspired by work of other scientists who managed to electrolyse and decompose water into hydrogen and water using a voltaic pile. This led to his electrolysis experiments where he decomposed potassium hydroxide (potash) and molten sodium hydroxide (soda) into previously undiscovered elements, potassium and sodium. These investigations led to our understanding of electron transfer reactions as the basis of electrolysis.

Michael Faraday was Davy’s laboratory assistant, and as a result continued Davy’s work on electrolysis. He was the first person to define the laws of electrolysis and quantify electrolysis. He also introduced the terminology used today such as electrolyte, cation and anion. His discoveries led a better understanding of electron transfer reactions in electrolysis.

That's 'describe' but you have to 'assess' yourself self (assess is make a judgement)

 

[deswa1]

Was waiting for this to be posted back up.

Question:

I know the definition of a buffer solution (a mixture of comparable amounts of a weak acid and its conjugate base such that when significant amounts of acids/bases are added, the pH remains constant), and there examples of buffer solutions include river/lake and blood systems.

But in those 5 marker HSC Qs that ask for an example of how buffer solutions work, how do you answer appropriately WITH chemical equations for each of the specific systems listed above?

- Define a bufffer
- Explain what it does
- Give an example of a natural system (such as seawater)
- Write the equilibrium equation for this buffer, identifying the acid and base (depending on which one is conjugate etc.)
- Show that with an introduction of more H+, equilbrium shifts the other way due to LCP, resisting changes in pH. Similar with addition of a base (OH-)

 

[addoil]

- Define a bufffer
- Explain what it does
- Give an example of a natural system (such as seawater)
- Write the equilibrium equation for this buffer, identifying the acid and base (depending on which one is conjugate etc.)
- Show that with an introduction of more H+, equilbrium shifts the other way due to LCP, resisting changes in pH. Similar with addition of a base (OH-)

Yes I know that too, but I don't understand how buffer systems work in rivers/lakes and blood

For example, apparently for the river one you have to talk about the dissolved salts (???)
and for the blood, I don't understand how breathing/exhaling/muscles relate to the H+ ????

 

[hayabusaboston]

Can someone provide me with a description (With notes if possible) on all the pracs in the prelim module "Water" please?

 

[Living Moment]

Was waiting for this to be posted back up.

Question:

I know the definition of a buffer solution (a mixture of comparable amounts of a weak acid and its conjugate base such that when significant amounts of acids/bases are added, the pH remains constant), and there examples of buffer solutions include river/lake and blood systems.

But in those 5 marker HSC Qs that ask for an example of how buffer solutions work, how do you answer appropriately WITH chemical equations for each of the specific systems listed above?

Heres my example try fixing it to your Q.

In aquatic systems, complex buffering systems involving hydrogen carbonate ions and silicates maintain a fairly constant pH of about 8.5 for organisms. Buffer systems are important in biological systems as restricting pH range of organic body fluids ensure body function. Without effective buffer systems, waste acid and bases are produced by the body, and the pH would fluctuate from optimum levels. This results in the decrease and ultimately cessation in enzymic activity as enzymes can work within a narrow range and hence the biochemical and physiological processes in the body are impacted.

For example, human blood has a pH of about 7.4. A condition known as acidosis develops if the pH of blood falls below 7.35. Below 7.0 the person will enter a coma, and if the pH drops below 6.8, death may result. Similarly, if the pH rises above 7.45 a condition known as alkalosis occurs, and above 7.8 this condition is life threatening. The presence of buffers in the blood maintains the pH between 7.35 and 7.45. The main buffer system used to control the pH of blood is the carbonic acid / hydrogen carbonate ion buffer system.

CO2(g)) + H2O(l) H2CO3(aq)
H2CO3(aq) H+(aq) + HCO3-(aq) (If too much acid, goes left. If too much base, goes right.)
H2CO3(aq) + H2O(l) HCO3-(aq) + H3O+(aq)

If hydroxide ions (OH-) are added to the blood, they are neutralised by the carbonic acid part of the buffer (H2CO3). If hydronium ions (H3O+) is added, it is neutralised by the hydrogen carbonate ions (HCO3 -).

Specifically, during inhalation, the binding of oxygen molecules to haemoglobin produces hydronium ions:
HHb4 + H2O(l) + 4O2(g) <--> Hb4O8 + H3O+(aq)
As the equilibrium shifts to the right, the hydrogen carbonate buffer system acts to prevent oxygenated blood from being too acidic. So the H3O+ ions are neutralised by hydrogen carbonate ions. During exhalation, drop in CO2 levels, also cause a drop in carbonic acid levels, prompting equilibrium A to shift to the left, reducing concentration of H3O+ ions, increasing pH.

Therefore, during diffusion of oxygen and carbon dioxide in and out of the lungs, lowers pH with every inhalation, and increases pH with exhalation. This causes problems as pH can fluctuate regularly. However, the complex buffer system outlined above, in addition to several other buffering acid-base pairs, help maintain blood pH within a narrow range around 7.4.