Topic 5: Chemical energetics Flashcards
(14 cards)
What is an exothermic reaction?
A reaction that transfers heat to the surroundings, increasing the surroundings’ temperature.
What is an endothermic reaction?
A reaction that absorbs heat from the surroundings, decreasing the surroundings’ temperature.
In an exothermic reaction, is ∆H positive or negative?
Negative (∆H < 0).
In an endothermic reaction, is ∆H positive or negative?
Positive (∆H > 0).
What is enthalpy change (∆H)?
The amount of heat energy transferred during a chemical reaction.
What is activation energy (Ea)?
The minimum energy that colliding particles must have to react.
What happens to energy in bond breaking?
Energy is absorbed — bond breaking is endothermic.
What happens to energy in bond making?
Energy is released — bond making is exothermic.
What does an exothermic energy level diagram show?
Reactants start higher, products are lower; energy is released.
What does an endothermic energy level diagram show?
Reactants start lower, products are higher; energy is absorbed.
What is the formula for energy change in reactions?
∆H = Bond Breaking – Bond Forming
If the bond breaking energy is greater than bond forming energy, what type of reaction is it?
Endothermic.
If the bond forming energy is greater than bond breaking energy, what type of reaction is it?
Exothermic.
Example: What is the ∆H for this reaction?
H₂ + Cl₂ → 2HCl
H–H = 436 kJ/mol, Cl–Cl = 243 kJ/mol, H–Cl = 432 kJ/mol.
A: ∆H = 679 – 864 = –185 kJ/mol → Exothermic.
