The First Law of Thermodynamics states that energy cannot be created or destroyed, only transferred or transformed. The total energy within a substance is called its Internal Energy (U). This energy changes when heat (q) is added/removed or when work (w) is done on/by the system.
ΔU = q + w
In most chemistry labs, reactions occur at constant pressure (open to the atmosphere). Under these conditions, the heat flowing in or out of the system is equal to the change in a property called Enthalpy (ΔH).
Consider the thermochemical equation for the formation of water:
H2(g) + ½ O2(g) ➞ H2O(l) ΔH = −286 kJ
Step 1 (Part A): Identifying the Flow
Based on the sign of ΔH, is this reaction releasing heat to the environment or absorbing it? Click for Part A Answer
Since ΔH is negative (−286 kJ), the reaction is Exothermic (releases heat).
Step 2 (Part B): Scaling the Reaction
Enthalpy is an extensive property (it depends on amount). If you double the reactants to produce 2 moles of liquid water, what is the new ΔH? Click for Part B Answer
You must multiply the ΔH by the same factor (2):
ΔH = 2 × (−286 kJ) = −572 kJ
A student reacts 0.0500 mol of HCl with 0.0500 mol of NaOH. The lab technician measures that 2.9 kJ of heat is produced.
Step 1 (Part A): Finding Heat per Mole
Using the data above, calculate the enthalpy change (ΔH) for exactly 1.00 mole of HCl. Click for Part A Answer
ΔH = (Heat / Moles) = −2.9 kJ / 0.0500 mol = −58 kJ/mol
Step 2 (Part B): Writing the Equation
Write the complete balanced thermochemical equation using the value from Part A. Click for Part B Answer
HCl(aq) + NaOH(aq) ➞ NaCl(aq) + H2O(l) ΔH = −58 kJ
When you eat a gummy bear (sucrose), your body performs a complex series of reactions to extract energy. On paper, the combustion of 1 mole of sucrose releases a massive 5,960 kJ of energy. This energy is what your cells "capture" to perform biological work, such as muscle contraction and nerve impulses.