Enthelpy of Combustion for Magnesium

In summary, the speaker is attempting to find the change in enthalpy per mole of magnesium in the reaction Mg + 2HCl --> H2 + MgCl2 using experimental values such as change in temperature, mass of magnesium ribbon, and volume of HCl. They cannot use textbook values and must determine the specific heat capacity of HCl. They mention using the formula Q=mcΔt to determine the heat change in the system and mention the importance of performing the experiment in an adiabatic system to prevent heat loss. They also suggest using the specific heat capacity of water if the reaction is taking place in a diluted solution of HCl. They mention performing the experiment in class and determining that only a small percentage of
  • #1
decamij
53
0
I placed some magnesium ribbon in HCl and measured the temperature change. How would i find the change in enthalpy per mole of magnesium in the following reaction:
Mg + 2HCl --> H2 +MgCl2
If i know the following information: change in temperature = 19C
mass of ribbon = 0.5g
volume of HCl = 100mL

I can't use any known values (i.e. like the ones you'd find in a textbook). I must use the experimental values above. However, i can use specific heat capacity values, c, if i must.
 
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  • #2
[tex]Q=mc\Delta t[/tex]
where Q is the enthalpy change. You cannot just plug numbers into this formula though; remember that it applies to the entire system, and can be used in conjuction with
[tex]\mbox{heat gain}=-\mbox{(heat loss)}[/tex]
How is heat energy transferred in the reaction? (what gains, what loses?)
 
  • #3
Did you perform that experiment in a "bomb calorimeter" (or a suitable improvised version thereof)? If not, the temperature change is meaningless, because an unknown amount of heat was dissipated to the surroundings.
 
  • #4
As Pack_rat2 alluded to, ideally one is doing this in an adiabatic system so that heat is not lost to the environment outside the reaction vessel (presumably test tube). One might have to correct of the mass of the reaction vessel as well if it is heated.

From the mass of HCl solution and temperature, one can determine the change in enthalpy of the solution.

Then assuming that all the heat originated from the chemical reaction - one can determine the energy per unit mass or mole of Mg (the other known quantity).
 
  • #5
But what will i use the specific heat capacity of HCl? I can't find that in my textbook.
 
  • #6
The Specific heat capacity of HCl (s) is 3.93 Jg-1C-1
 
  • #7
decamij said:
But what will i use the specific heat capacity of HCl? I can't find that in my textbook.

I suppose reaction was going on in a relatively diluted solution of the acid. If so, use specific heat of water.
 
  • #8
Yea I would just use the specific heat capacity of water. My class just did this lab the other day. We used 0.5 M HCl and the teacher did some calculations on the board to show us that only about 0.2%(or somewhere around there) of the solution was HCl and the rest was water.
 
  • #9
Gimpinald said:
We used 0.5 M HCl and the teacher did some calculations on the board to show us that only about 0.2%(or somewhere around there) of the solution was HCl

More like 1.8% (closer to 2 then to 0.2).


 

1. What is the enthalpy of combustion for magnesium?

The enthalpy of combustion for magnesium is the amount of energy released when one mole of magnesium reacts with oxygen to form magnesium oxide. It is typically represented as ΔHc, and its value is -601.8 kJ/mol.

2. How is the enthalpy of combustion for magnesium measured?

The enthalpy of combustion for magnesium is measured using a bomb calorimeter, which is a device that allows for the complete combustion of a substance in a closed container. The change in temperature of the surrounding water is then used to calculate the enthalpy of combustion.

3. What factors can affect the enthalpy of combustion for magnesium?

The enthalpy of combustion for magnesium can be affected by the purity of the magnesium, the amount of oxygen available for the reaction, and the temperature at which the reaction takes place. Other factors such as pressure and the presence of impurities can also have an impact.

4. Why is the enthalpy of combustion for magnesium important?

The enthalpy of combustion for magnesium is important because it can be used to calculate the energy released during a chemical reaction involving magnesium. It can also be used to determine the efficiency of a fuel or the amount of energy that can be harnessed from a reaction.

5. How does the enthalpy of combustion for magnesium compare to other elements?

The enthalpy of combustion for magnesium is relatively high compared to other elements, such as carbon and hydrogen. This is due to the high reactivity of magnesium and its ability to easily form stable compounds, resulting in a large release of energy during combustion.

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