heat change (kJ mol-1). -890. -1560. -2220. -2877. You can see that as the alkanes get bigger, more heat is evolved. This sheet explores why that is. ...
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Date: .........................
LONGMAN GCSE CHEMISTRY WORKSHEETS 8: Estimating heat changes during reactions using bond energies
When alkanes burn, energy is released in the form of heat. The table below gives the amount of heat given out per mole of alkane burning. The negative signs in front of the values show that heat is given out. Alkane
CH4
C2H6
C3H8
C4H10
heat change (kJ mol-1)
-890
-1560
-2220
-2877
You can see that as the alkanes get bigger, more heat is evolved. This sheet explores why that is. The equations for the reactions CH4(g) + 2O2(g) CO2(g) + 2H2O(l) C2H6(g) + 3.5O2(g) 2CO2(g) + 3H2O(l) C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(l) C4H10(g) + 6.5O2(g) 4CO2(g) + 5H2O(l) You might think that two of the equations look odd because they contain 3.5 or 6.5 oxygens. Normally the whole equation would be doubled to get rid of this. This time, though, we want to start with 1 mole of the alkane in each case. Here is one of the equations showing the structures of everything:
H
H
H
H
C
C
C
H
H
H
H
(g)
+
5O
O
(g)
3 O
C
O
(g)
+
4H
O
H
(l)
1. Work out the total energy needed to break the molecules on the left-hand side of the equation into separate atoms. (Bond energies (in kJ mol-1): C-C: +347; C-H: +413; O=O: +498) .......................................................................................................................................... .......................................................................................................................................... 2. Work out the energy released when the bonds are reformed on the right-hand side of the equation. (Bond energies (in kJ mol-1): C=O: +805; O-H: +464) .......................................................................................................................................... ..........................................................................................................................................
Remember that energy released is given a negative sign. If you combine your two answers to give the overall energy change, you should get an apparent change of -2054 kJ mol-1. That isn't the same as the value given in the table at the beginning of the sheet. The problem is that bond energies assume that all the substances present are in the gas state. In this case, the water is present in the equation as liquid water. The value of -2054 kJ mol-1 would be the amount of heat evolved if the water were present as steam. When 1 mole of steam condenses to water, an additional 41 kJ of heat is evolved. In this case, there are 4 moles of water in the equation. 3. Calculate the extra amount of heat evolved when this water condenses to a liquid. .......................................................................................................................................... 4. Calculate the overall heat change of this reaction. .......................................................................................................................................... If you have done this correctly, you should have a figure in very good agreement with the one in the initial table. 5. Now complete the following table showing the number of bonds broken and made in burning each of the four hydrocarbons, and use this to calculate the overall heat change in each reaction. Use the equations earlier in the sheet. You can check your final answers because they should be in good agreement with the table on the front of the sheet. Do your calculations on a piece of rough paper. All the calculations are done in the same way as the propane, C3H8, example. hydrocarbon
