Topic 5 & 15 - Energetics Flashcards
What is an open system?
A system that can exchange energy and matter with the surroundings
What is a closed system?
A system that can exchange energy but not matter with the surroundings
What is an exothermic reaction?
A reaction in which heat is given out by the system
∆H is negative and is treated as a product
What is an endothermic reaction?
A reaction in which heat is absorbed by the system
∆H is positive and treated as a reactant
What is standard enthalpy change, ∆H?
The heat energy transferred under standard conditions
What does the increase in temperature depend on?
- The mass of the object
- The amount of heat added
- The nature of the substance
What is the specific heat capacity?
The heat needed to increase the temperature of unit mass by 1 K
What kind of processes are combustion and neutralisation?
Exothermic
What is the relationship between stability and enthalpy level?
The higher the enthalpy level, the smaller the stability
What is enthalpy of combustion?
The heat produced when one mole of a substance is burned in excess oxygen
What is Hess’s law
The enthalpy change for any chemical reaction is independent of the route, provided the starting conditions and final conditions are the same:
∆H1 + ∆H2 = ∆H3
What is average bond enthalpy?
The energy needed to break one mole of bonds in gaseous molecules under STP
What does an endothermic process involve in terms of bonds?
Involves the separation of particles which are held together by a force of attraction (breaking bonds)
What does an exothermic process involve in terms of bonds?
The bringing together of particles which have an attractive force between them (forming bonds)
What is standard enthalpy change of formation, ∆Hf?
The enthalpy change that occurs when one mole of the substance is formed from its elements in their standard states under STP
What is the standard state of a substance?
The pure form of the substance under STP
What is the standard enthalpy change of formation for an element?
Zero
What is the enthalpy change of formation of a reaction?
∆Hreaction = Σ∆Hf(products) - Σ∆Hf(reactants)
What is the standard enthalpy change of combustion, ∆Hc?
The enthalpy change that occurs when one mole of the substance burns completely under STP
What is the enthalpy change of a combustion reaction?
∆Hreaction = Σ∆Hc(reactants) - Σ∆Hc(products)
What is the first electron affinity, ∆He?
The enthalpy change when one mole of gaseous atoms attracts one mole of electrons
Cl(g) + e–(g) → Cl–(g)
What is the lattice enthalpy, ∆Hlat?
The enthalpy change that occurs when one mole of a solid ionic compound is separated into gaseous ions under STP
NaCl(s) → Na+(g) + Cl–(g)
What is the enthalpy change of atomisation, ∆Hatom?
The heat change that occurs when one mole of gaseous atoms are formed from the element in its standard state
Na(s) → Na(g)
How does lattice enthalpy change with charge and size of particles?
It increases with higher ionic charge and smaller ionic radii due to attractive forces
What is the entropy change of a system?
∆Sreaction = Σ∆S(products) - Σ∆S(reactants)
What is the relationship between the change in entropy of the surroundings and the change in enthalpy of a system?
What is the sign of ∆Gsys for a spontaneous process?
Negative
What is the Gibbs’ free energy for a reaction?
∆G = ∆H - T∆S
What is the ∆Gsys a measure of?
The spontaneity of a reaction:
- negative → spontaneous
- positive → non-spontaneous
How can ∆Gsys be used to predict the feasibility of a reaction?
At low temperature:
∆Gsys ≈ ∆Hsys (T∆S ≈ 0)
At high temperature
∆Gsys ≈ –T∆Ssys (T is high enough to make ∆H negligible)
What is the ∆G of a reaction?
∆Greaction = ∑∆Gf(products) - ∑∆Gf(reactants)
What is the ∆G of an element in its standard state?
Zero
