Different ways to investigate the rate of reaction between Magnesium and Acids

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Fangedbeast
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#1
Report Thread starter 6 years ago
#1
Hi, I'm investigating the rate of reaction between Magnesium and Acids (e.g. hcl and h2so4) in my own time (not for coursework),

One way I'm going to be measuring this will be by changing the concentrations and seeing how much Hydrogen gas is formed...

Question 1:

Are there any other ways of investigating the rate of reaction?

Can I measure it via a pH probe? I was initially thinking no but it suggests here [no. 25] that you can: http://www.york.ac.uk/org/seg/test/A...indaTaylor.htm

and also here: http://www.docbrown.info/page03/AcidsBasesSalts04.htm
" The salt, and its name, depends on the metal and acid used in the reaction and the acid is neutralised in the process. "

Question 2:

" As the reaction is exothermic, students will need to design their experiment very carefully if they wish to investigate the effect of temperature on this reaction. "

If it gives out heat during the reaction, should I measure the temperature rise at room/lab temp, to see how much it raises on average each time (let's say for example +5 degrees Celsius) and then when I do the reaction in a water bath e.g. at 40 degrees Celsius, do I just add on 5 degrees when I plot my rate constant-temperature graph?

Many thanks
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Pigster
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#2
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#2
Q1 How to measure?

Since rate = change in something divided by the time taken, you can fix the change and measure the time OR fix the time and measure the change, e.g. see what volume of gas is made after 20 seconds OR see how long it takes to collect 20 cm3. This applies to pretty much all the other things you could choose to measure.
You can measure the mass of gas lost (not very big since H2 isn't very dense).
The temperature rise - the faster the reaction, the larger the temp. tise
The time taken for all the Mg to disappear
You could drop the Mg in for ten seconds, then remove it, and titrate to find the new conc. and hence how many mol of acid has reacted.

Q2 controlling for temp.

I wouldn't use Mg + acid, since it is very difficult to control for T in a particularly effective way. Another factor is surface area - when the acid reacts, it makes bubbles of H2 on the surface of the solid, which 'protects' the metal from the acid. It also tends to make it float, so only one side of the metal is exposed to the acid. You then have issues with diffusion of the H+ ions in the vicinity of the metal.

Me, I'm all in favour of clock reactions for investigating rates. Mg and acid, would be one of my last choices.
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Fangedbeast
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#3
Report Thread starter 6 years ago
#3
(Original post by Pigster)
Q1 How to measure?

Since rate = change in something divided by the time taken, you can fix the change and measure the time OR fix the time and measure the change, e.g. see what volume of gas is made after 20 seconds OR see how long it takes to collect 20 cm3. This applies to pretty much all the other things you could choose to measure.
You can measure the mass of gas lost (not very big since H2 isn't very dense).
The temperature rise - the faster the reaction, the larger the temp. tise
The time taken for all the Mg to disappear
You could drop the Mg in for ten seconds, then remove it, and titrate to find the new conc. and hence how many mol of acid has reacted.

Q2 controlling for temp.

I wouldn't use Mg + acid, since it is very difficult to control for T in a particularly effective way. Another factor is surface area - when the acid reacts, it makes bubbles of H2 on the surface of the solid, which 'protects' the metal from the acid. It also tends to make it float, so only one side of the metal is exposed to the acid. You then have issues with diffusion of the H+ ions in the vicinity of the metal.

Me, I'm all in favour of clock reactions for investigating rates. Mg and acid, would be one of my last choices.
Thanks for the speedy reply

I think I'll be investigating the RoR by measuring the gas formed in X seconds (probably somewhere in the region of 20 seconds so I can get lots of repeats) and by recording the temperature in X seconds (hopefully my school has a digital thermometer which can be hooked up to a data logger! I'd preferably want to do at least 1 method of automatic data)

Also, how would I use the orders of other acids e.g. the orders ethanoic acid and HCl and use it to figure out the rate equation? As it suggests on the York Investigation help sheet:
" Investigate the effect of other acids (including weak acids) on the rate of the reaction. You will be able to find the order of reaction for each acid and hence the rate equation for the reaction. "
I understand how you get the orders via experiment but how would getting the orders of different acids help me figure the rate equation? Or am I misreading it and is it suggesting "Pick a single acid, find the order and work out the rate equation"?

Regarding controlling the temperature of the Mg + Acid reaction being hard - would it still be theoretically correct/feasible to control the temp of the acid via a water bath? In practical terms, would there be an easier method? I'd like to get data on the activation enthalpy in this investigation, which I can do by changing the temp. of the reaction

Thanks
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Pigster
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#4
Report 6 years ago
#4
I'll try not to just tell you the answers...

Try to ensure that the temperature rise isn't too high, a water bath will help, but I'd guess not too successfully.

Do you know the ionic equation for the various reactions? Do you know the difference between a weak and strong acid? Do you know how the choice of acid should affect the rate equation? I imagine it means (as you say) "Pick a single acid..."
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