chapter 5 Flashcards
(26 cards)
What is energy in terms of chemistry?
Energy is the capacity to do work or transfer heat.
What’s the difference between heat and temperature?
Heat is energy transfer due to temperature difference (extensive property); temperature is the intensity of heat (intensive property).
What are the two main forms of energy?
Kinetic energy (motion) and potential energy (position/state).
What does the Law of Conservation of Energy state?
Energy cannot be created or destroyed, only transformed; total energy remains constant.
What is the system and surroundings in thermodynamics?
The system is the part under study (e.g., a reaction in a beaker); surroundings include everything else.
Define exothermic vs. endothermic reactions.
Exothermic: heat is released by the system (q is negative).
Endothermic: heat is absorbed by the system (q is positive).
What is specific heat capacity (C)?
The amount of heat required to raise 1 gram of a substance by 1 K (or °C).
Formula for calculating heat transfer (no phase change)?
𝑞=𝐶×𝑚×Δ𝑇
q=C×m×ΔT
What does a negative q value mean?
Heat was released by the system (exothermic).
What is heat of fusion and heat of vaporization?
Heat of fusion is energy required to melt; heat of vaporization is energy required to vaporize.
Formula for heat during phase change?
q=ΔH phase
×mass
When does temperature stay constant?
During a phase change (melting, boiling, etc.).
What is the first law of thermodynamics?
ΔU=q+w — Internal energy change equals heat added plus work done.
What does work (w) depend on in a gas system?
Volume change against constant pressure:
𝑤=−𝑃Δ𝑉
w=−PΔV
What is enthalpy (H)?
A state function representing heat content at constant pressure.
What’s the relationship between ∆H and ∆U?
ΔH=ΔU+PΔV, or approximately
Δ𝐻≈𝑞𝑝
ΔH≈qp
What’s the purpose of a calorimeter?
To measure heat transfer during reactions.
What is Hess’s Law?
The enthalpy change of a reaction is the same, no matter how many steps the reaction takes.
Formula using standard enthalpies of formation?
ΔH
rxn∘=∑nΔH f(products)∘−∑nΔH f(reactants)∘
Is combustion exothermic or endothermic?
Always exothermic (ΔH < 0)
What does a thermochemical equation show?
Reactants, products, and heat change (ΔH) for a balanced reaction.
If ΔH = -483.6 kJ for 2H₂ + O₂ → 2H₂O(g), what’s ΔH for 1 mol H₂O formed?
-241.8 kJ/mol
What is the ΔfH° of elements in standard states (e.g., O₂, H₂)?
Zero
What is the standard enthalpy of formation (ΔfH°)?
The heat required to form 1 mole of a compound from elements in their standard states.
