Science can intimidate those unfamiliar with its terms and symbols, even when the concept in question is one you can easily grasp. Thermochemistry uses specialised symbols and terms that might confuse people trying to read the equations that describe those kinds of reactions. However, explaining the basic concepts of thermochemistry, and their description in scientific language and symbols, makes it much easier to explain the equations themselves.
- Skill level:
- Moderately Challenging
Note that thermochemical reactions involve both chemical changes and changes in heat. This is why they use the combined word "thermochemical" to describe them.
Describe enthalpy, the transfer of energy. Explain that enthalpy marks an amount of energy you measure over time, usually in joules, and that its symbol is H.
Show that the symbol "H stands for the change in enthalpy in a thermochemical reaction, because " or "delta" is the common scientific symbol for change. Tell the listener that scientists determine the value of "H by experiments, and that the number depends on specific conditions. This means that if any part of a thermochemical reaction changes, all other parts, including "H, change in the corresponding way to balance the equation.
Explain how you can use the sign of "H to determine whether a thermochemical reaction is endothermic or exothermic. Point out that when you require heat to complete a chemical reaction (endothermic), "H is positive -- it has a positive sign and its value is greater than zero. When the reaction gives off heat (exothermic), "H is negative -- it has a negative sign and its value is less than zero.
Review simple examples of thermochemical equations to illustrate the basic formulas for thermochemistry.
For exothermic reactions, write the equation as:
A + B ' C + Heat, and "H should be positive.
For endothermic reactions, it should appear like this:
A + B + Heat ' C, and "H should be negative.
Note the specific conditions under which those reactions take place, and show how each equation balances out any changes and conserves energy.
Tips and warnings
- Explain to the listener that thermochemical reactions conform to the laws of thermodynamics. In particular, the First Law of Thermodynamics -- energy cannot be created or destroyed; it can only go somewhere else or turn into something else -- explains how "H must behave as part of a thermochemical equation.
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