energy - thermochemical equations

Step 1 Calculate the mole of glucose represented by 40 grams.

40/180 =0.22 mole

Step 2 For one mole of glucose 5054kj of energy is released so for 0.22 mole

(0.22/1) X 2803 = 616.67kj.

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Step 1 Calculate the mole of octane represented by 34.5L at SLC.

34.5/22.4 =1.54 mole

Step 2 For one mole of octane 5054kj of energy is released so for 1.54 mole

(1.54/1) X 5054 = 7783.16kj.

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Step 1 Calculate the mole of carbon dioxide

40/44 = 0.91mole

Step 2 For six mole of carbon dioxide,2 803kj of energy is absorbed so for 0.91mole

(0.91/6) X 2803 = 425.12kj.

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Step 1 Calculate the mole of octane needed to produce 760kj. Since 2 mole of octane produces 10108 Kj

(760/10108) X 2 = 0.15mole

Step 2 The mass of 0.075 mole of octane is

0.15 X 114 = 17.1 grams

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Step 1 Calculate the mole of octane needed to produce 340kj. Since 1 mole of octane produces 2803 Kj

(340/2803) X 1 = 0.1213mole

Step 2 The mass of 0.1213 mole of octane is

0.1213 X 180 = 21.83 grams

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Thermochemical equations

Exercises

C₈H₁₈(g) + 12.5O₂(g) => 8CO₂(g)+ 9H₂O(g) ΔH = -5054kj/mol

1) Calculate the energy released when 34.5 litres of octane gas react with oxygen at STP according to the reaction above.
C = 12, H =1, O =16.

Solution

2) Calculate the amount of energy released when 40 grams of glucose burn in oxygen according to the equation below?

C₆H₁₂O₆(aq) + 6O₂(g) => 6CO₂(g)+ 6H₂O(l) ΔH = -2803kj/mol

Solution

3) Calculate the amount of energy absorbed when 40 grams of carbon dioxide are used in photosynthesis to produce glucose according to the equation below.

6CO₂(g)+ 6H₂O(l) => C₆H₁₂O₆(aq) + 6O₂(g) ΔH = +2803kj/mol

Solution

4) Calculate the mass of octane needed to produce 760kj of energy when it reacts with oxygen according to the equation below.

2C₈H₁₈(g) + 25O₂(g) => 16CO₂(g)+ 18H₂O(g) ΔH = -10108kj/mol

Solution

5) Calculate the mass of glucose needed by a plant in order to produce 340kj of energy during respiration.

C₆H₁₂O₆(aq) + 6O₂(g) => 6CO₂(g)+ 6H₂O(l) ΔH = -2803kj/mol

Solution

6) A student experimentally determined the molar enthalpy of combustion(ΔH_c) of ethanol, molar mass of 46.0 g/mol, using the equipment shown in the simplified diagram on the right..

The student made the following experimental measurements

Mass of water in beaker = 105g
Amount of ethanol combusted = 0.920g
Temperature rise of the water = 39.0 ^oC

If the specific heat capacity of water is 4.184 J/g/^oC find the molar enthalpy of combustion of ethanol, calculated from the student's results.
Solution

7) A student measured the heat of combustion of three different fuels using similar equipment as that shown in 6) above. The results are shown in the table on the right. The published value for the heat of combustion of propane is 2217 kJ mol⁻¹.
Which fuel from the table is likely to be propane? Justify your answer .

Solution

8) Instant cold packs are often used by athletes to reduce swelling just after injury. Such cold packs often contain ammonium nitrate. When needed, the ammonium nitrate is mixed with water and the following reaction occurs dropping the temperature.

The above process occurs in two stages. Firstly, energy is required to break the ionic bonds which hold the ions together in the solid state. This energy is taken from the surrounding water. Finally, energy is released when the ions and water molecules form ion-dipole bonds to produce stable complexes.
1) What can you say about the amount of energy required to break the bonds as compared to the amount of energy given out when ion-dipole bonds formed?

2) A cold pack contains 50.0 g of solid ammonium nitrate that is mixed with water when activated. What is the amount of energy that is released or absorbed by the reaction?

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