Every time you combine chemicals, whether in the kitchen or in the laboratory, you create new ones that are called "products". During these chemical reactions, the heat can be absorbed and released from the surrounding environment. The heat exchange between a chemical reaction and the environment is known as the enthalpy of the reaction and is indicated with ∆H. To find the ∆H, start from step 1.
Steps
Step 1. Prepare the reagents for the chemical reaction
To be able to measure the enthalpy of the reaction, first you need to prepare the right quantities of the reactants involved in the reaction itself.
As an example, we want to calculate the enthalpy of the water formation reaction, starting from hydrogen and oxygen: 2H2 (hydrogen) + O2 (oxygen) → 2H2O (water). For the proposed example you can use 2 moles of hydrogen and 1 mol of oxygen.
Step 2. Clean the container
To make sure the reaction occurs without contamination, clean and sterilize the container you intend to use (usually a calorimeter).
Step 3. Place a stirring rod and a thermometer in the container
Be prepared to mix the components, if necessary, and measure their temperature by holding both the stirring rod and the thermometer in the calorimeter.
Step 4. Pour the reagents into the container
Once you have all the tools ready you can pour the reagents into the container. It seals immediately from above.
Step 5. Measure the temperature
Using the thermometer you placed in the container, take note of the temperature as soon as you added the reagents.
For the proposed example, let's say you poured hydrogen and oxygen into the container, sealed it and registered a first temperature (T1) of 150K (which is quite low)
Step 6. Go ahead with the reaction
Leave the two components to act, mix if necessary to speed up the process.
Step 7. Measure the temperature again
Once the reaction has taken place, measure the temperature again.
For the example proposed above, let's say that you have let enough time pass and that the second measured temperature (T2) is 95K
Step 8. Calculate the temperature difference
Subtract to determine the difference between the first and second temperatures (T1 and T2). The difference is indicated as ∆T.
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For the example above, ∆T will be calculated as follows:
∆T = T2 - T1 = 95K - 185K = -90K
Step 9. Determine the total mass of the reagents
To calculate the total mass of the reactants, you will need the molar mass of the components. The molar masses are constant; you can find them in the periodic table of the elements or in the chemical tables.
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In the example above, we used hydrogen and oxygen which have a molar mass of 2g and 32g respectively. Since we used 2 moles of hydrogen and 1 mole of oxygen, the total mass of the reactants will be calculated as follows:
2x (2g) + 1x (32g) = 4g + 32g = 36g
Step 10. Calculate the enthalpy of the reaction
Once you have all the elements, you can calculate the enthalpy of the reaction. The formula is this:
∆H = m x s x ∆T
- In the formula, m represents the total mass of the reactants; s represents the specific heat, which is also constant for each element or compound.
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In the example above, the final product is water which has a specific heat equal to 4, 2 JK-1g-1. Therefore, you will calculate the enthalpy of the reaction in this way:
∆H = (36g) x (4, 2 JK-1 g-1) x (-90K) = -13608 J
Step 11. Make a note of the result
If the sign is negative, the reaction is exothermic: the heat has been absorbed from the environment. If the sign is positive, the reaction is endothermic: the heat has been released from the environment.
