Britical Thinking Questions Answer CTQs 5 - 7 as a team. 5. Is the chemical reaction represented in Figure 3 exothermic or endothermic? 6. Is H for the chemical reaction represented in Figure 3 positive or negative? 7. Provide a chemical equation for a reaction with a value of dH that has the same magnitude of H as the reaction in Figure 3, but has the opposite sign. Model 4: Enthalpy of Atom Combination. Figure 4. The enthalpy of atom combination of CH4(g) at 25C. When a mole of a compound is produced from its constituent atoms in the gas phase at 1 atmosphere pressure and 25C, energy is released as bonds are formed. The standard state heat (or enthalpy) of atom combination, Hac is the difference in enthalpy of product and reactants ( H product H reactants ) when this occurs. Equation 3 is the (hypothetical) chemical equation for this process for CH4(g), and Figure 4 provides a visual representation of this process. C(g)+4H(g)CH4(g) Critical Thinking Questions Answer CTQs 8 - 12 as a team. 8. Based on the information in Model 4, what is the value of H2 for reaction 3 ? END OF WARM-UP Model 3: Endothermic and Exothermic Processes. When chemical processes occur, energy (typically as heat) is either released-an exothermic process, or absorbed - an endothermic process. The breaking of bonds requires energy to pull the atoms apart; bond-breaking is thus an endothermic process. When bonds are formed, energy is released - precisely the same amount of energy which would be required to break those bonds. Thus, the making of bonds is an exothermic process. The quantity of energy released or absorbed in a chemical process can be designated by an enthalpy (energy) change, H, for that process. If there is a release of energy when the reaction occurs, the value of H is negative, and the reaction is exothermic. If the reaction results in a net consumption of energy, then H is positive, and the reaction is endothermic