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Topic: Rate of reaction between Magnesium and Acid  (Read 6065 times)

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Offline FangedBeast

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Rate of reaction between Magnesium and Acid
« on: July 24, 2015, 03:27:50 PM »
Short Intro: Hi, I've been assigned coursework for my A levels [16-18 year olds] where I've been told to start the theory part of my write up during my summer holidays  :-[

We are all investigating rates of reactions investigations and I've been assigned Mg and Acid. My basic aim is to find the rate equation and the activation enthalpy.

I'll be investigating the RoR by using Mg ribbon/powder + different concentrations of the acid and seeing how much gas is formed in X seconds [More gas formed = faster reaction]. Moreover, I will be using different concentrations of the acid and seeing the temp. rise in X seconds [higher temp. rise per time = faster reaction].

I will be finding out the activation enthalpy by seeing the effect of different temperatures on the rate. I will hope to do this by having the reaction take place in a water bath.

I have 3 questions:

1. Although I plan to use the above methods to measure the rate - they will have to be done manually [I don't think my school has electronic thermometers which can be connected to a data logger and I'd much prefer to have at least one method of data which is automatic]. On the help sheet I was given, it was suggested that I can use a different technique [explicitly suggesting conductivity and pH].

Will there be a change in pH when I do the reaction? From googling around [Google says that the acid is neutralised and a salt is made... how is it neutralised?] and it suggesting explicitly on the help page it seems that measuring a pH change is possible/measurable [I just want more of a confirmation really]

2. Will changing the temperature of the reaction via a water bath be a sensible approach to find the activation enthalpy? I know that the reaction is exothermic so  obviously [or rather not] as the reaction carries out, the faster the reaction becomes due to the increase in temperature. But to be honest, it could be counted as only a drift in results as the temperature rise should theoretically be constant in each reaction (and hence its effect on the rate of reaction) for the same concentration. My gut feeling says that it's a perfectly fine method but I asked on another forum and they said:

"...it is very difficult to control for T in a particularly effective way."
and
"... a water bath will help, but I'd guess not too successfully."

I can't think of any large scientific reasoning behind why it would be hard to control the temp. for the reaction and why a water bath would not do it too successfully...

3. Possible comprehension confusion rather than direct chemistry confusion:
In my help sheet it says that a possible investigation would be to:

"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 initially got confused at this, asking myself "Why would getting the orders of multiple different acids help me find the rate equation?" But I understand now that it just means "Pick any acid, focus on it and find the rate equation for it" - am I right?

Many thanks (sorry for the wall of text!) :)

Offline Borek

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Re: Rate of reaction between Magnesium and Acid
« Reply #1 on: July 24, 2015, 03:56:33 PM »
Will there be a change in pH when I do the reaction?

Start with the reaction equation. Seriously.

Quote
Will changing the temperature of the reaction via a water bath be a sensible approach to find the activation enthalpy?


Yes.

However, reaction mixture will not have a constant temperature throughout the experiment, which is why it will be quite difficult to get a correct value.
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Offline Enthalpy

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Re: Rate of reaction between Magnesium and Acid
« Reply #2 on: July 24, 2015, 04:22:49 PM »
Hi FangedBeast, welcome here!

1. With the reaction equation, you can tell how much of a strong acid gets consumed, and consequently, by how much the pH changes. A limit to the method is that, if you put little metal in much of concentrated acid, the pH change will be too small for an accurate measure.

2. It's difficult to stabilize and impose a temperature to some volume of liquid, because an efficient heat transfer with some outside medium needs to be very efficient. For instance, a glass thickness may already insulate too much. You can estimate it by P=ΔT*σ*S/L, where P is the heat power, S the contact area, L the distance to cross (=thickness) and σ the conductivity
https://en.wikipedia.org/wiki/List_of_thermal_conductivities
http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
compare with the the duration and liquid's heat capacity
http://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html
this will tell you how difficult the attempt is, with both liquids moved.

Suggestions:
- you don't need a big accuracy to make your opinion on this
- if you plan a scientific career, you can start collecting data tables now, in well-ordered folders and files.

Imposing a temperature is more difficult when a reaction provides heat. So I understand the suggestion as:
"the temperature will rise anyway, so just observe the effect" or "don't forget to include the effect in your model".

But since the acid cncentration will change at the same time as the temperature, you need several initial concentrations in order to tell what results from the concentration and what comes from the temperature.

3. Acids act on metals not just through the H+ ions. On pure aluminium or its alloys that resist water corrosion, Cl- for instance is especially detrimental. Magnesium can have a similar behaviour if it's pure enough. So you might (or not) find an effect beyond the acid concentration or the pH.

I find interesting to measure the pH during the reaction, because weak acids are partially ionized, and will so-to-say replenish the liquid with H+ as these get consumed.

Offline FangedBeast

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The mechanism for the reaction between magnesium and HCl
« Reply #3 on: January 03, 2016, 10:18:21 AM »
Hi, I've just completed an experiment as part of an investigation to find the rate order w.r.t Hydrochloric acid which I found was 2 (from my graph the rate order is ~2.3 but I guess it's safe to say that it's an order of 2 due to errors)

Anyhow, the rate equation is rate = k[H+]2 (Mg is not included nor investigated for its rate order as it is a solid) and I've come up with a mechanism:

Step 1. 2HCl -> 2H+ + 2Cl-
Step 2. 2H+ + Mg -> H2 + Mg2+
Step 3. Mg2+ + 2Cl- -> MgCl2

2 questions:
1. Does the mechanism proposed look correct?
2. If my rate equation is " rate = k[H+]2 " then H+ must be the only thing my rate-determining step, right? It seems like Step 2 is my RDS as it is the only step with 2H+ in it yet there is also Mg in the equation as well... Am I wrong about "Mg is not included nor investigated for its rate order as it is a solid" and have I f*$#&@ up in not investigating the rate order of magnesium?! (I've already submitted and signed my results  :()

Thanks  :-[
« Last Edit: January 03, 2016, 11:15:37 AM by FangedBeast »

Offline mjc123

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Re: Rate of reaction between Magnesium and Acid
« Reply #4 on: January 04, 2016, 07:02:43 AM »
Steps 1 and 3 are unnecessary as both HCl and MgCl2 are fully dissociated in water.
You are right not to include Mg because it is a solid. Solids are assigned an activity (effective concentration) of 1, and this does not change as it is used up (unlike the concentration of a dissolved species).

Offline FangedBeast

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Re: Rate of reaction between Magnesium and Acid
« Reply #5 on: January 04, 2016, 07:37:07 AM »
Steps 1 and 3 are unnecessary as both HCl and MgCl2 are fully dissociated in water.
You are right not to include Mg because it is a solid. Solids are assigned an activity (effective concentration) of 1, and this does not change as it is used up (unlike the concentration of a dissolved species).

Really? I thought that in the reaction mechanism, because MgCl2 is made as a product of Magnesium ions and Chlorine ions reacting that I include it into the reaction mechanism (as it is not in the solution already before the reaction)...

In regards to the rate equation, would it be effectively be " rate = k [H+]2[Mg]1 " ? This is what is I am most confused about, my mechanism (which I can't think of any other mechanism) is saying one thing and then my rate equation is contradicting it :S

Thanks btw
« Last Edit: January 04, 2016, 08:07:05 AM by FangedBeast »

Offline mjc123

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Re: Rate of reaction between Magnesium and Acid
« Reply #6 on: January 04, 2016, 08:16:43 AM »
Not an order of 1, an activity of 1, i.e. [Mg] = 1 always, therefore we leave it out. Activity is a kind of effective concentration (did you see the words "effective concentration" in brackets?); in non-ideal situations, a reagent behaves as if the concentration had the value of the activity, rather than the actual value of the concentration.
The product is a solution containing Mg2+ and Cl- ions; unless the concentration is very high, these do not produce "MgCl2". Cl- is what is called a "spectator ion" - it is present but takes no part in the reaction.

Offline FangedBeast

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Re: Rate of reaction between Magnesium and Acid
« Reply #7 on: January 04, 2016, 08:55:08 AM »
Not an order of 1, an activity of 1, i.e. [Mg] = 1 always, therefore we leave it out. Activity is a kind of effective concentration (did you see the words "effective concentration" in brackets?); in non-ideal situations, a reagent behaves as if the concentration had the value of the activity, rather than the actual value of the concentration.
The product is a solution containing Mg2+ and Cl- ions; unless the concentration is very high, these do not produce "MgCl2". Cl- is what is called a "spectator ion" - it is present but takes no part in the reaction.

Sorry, I misread and jumped to a conclusion  :-[
Here's my revised mechanism (I've decided to keep the step for dissociation of HCl for personal reasons):

Step 1: HCl -> H+ + Cl-
Step 2: 2H+ + Mg -> Mg2+ + H2

I feel something is wrong because my rate equation does not match with my rate-determining step (step 2)...

Step 2 has Mg in it and my rate equation only H+ and I can't find any explanation for it (aside from my proposed mechanism being wildly wrong)

[An explanation of why I'm stuck, in paint form - http://i.imgur.com/Z9IbsJ4.png]

EDIT: I've just read from an example off of wiki that when the rate equation is something like " rate = k [X]2 " then it means that two molecules of X react together.
Does this mean that my mechanism is different/more complex and the rate determining step is something like:
H3O+ + H3O+[/sup] -> H2 + H2O where instead of using H+ I now use H3O+ (to make it make sense as H+ + H+ -> H2 would not work)

EDIT 2: gaah, this is driving me crazy. So even if I decide to use H3O+ I still get a step where Mg is on the left hand side of the step  >:(:

Step 1: HCl -> H3O+ + Cl-
Step 2: 2H3O+ + Mg -> Mg2+ + H2 + 2H2O

EDIT 3: Ok so maybe this looks better, I've thought up of a mechanism involving just 2H3O+ for the RDS (STEP 1). However, when I balance my equation I get  H2O left over which is supposedly wrong as my equation is meant to balance out to give me my stoichiometric equation (2HCl + Mg -> H2 + MgCl2)
See image: http://i.imgur.com/un8BxeP.png
« Last Edit: January 04, 2016, 10:24:08 AM by FangedBeast »

Offline mjc123

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Re: Rate of reaction between Magnesium and Acid
« Reply #8 on: January 04, 2016, 11:50:41 AM »
Quote
I feel something is wrong because my rate equation does not match with my rate-determining step (step 2)...
YES IT DOES. You have already rightly said that Mg is excluded because it is a solid. Therefore step 2 gives rate = k[H+]2. [Mg] doesn't come into it because [Mg] is not a variable, but a constant, and has no effect on the rate.

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