Topic 5 & 15 - Energetics

Question 1

What is an open system?

Answer 1

A system that can exchange energy and matter with the surroundings

Question 2

What is a closed system?

Answer 2

A system that can exchange energy but not matter with the surroundings

Question 3

What is an exothermic reaction?

Answer 3

A reaction in which heat is given out by the system

∆H is negative and is treated as a product

Question 4

What is an endothermic reaction?

Answer 4

A reaction in which heat is absorbed by the system

∆H is positive and treated as a reactant

Question 5

What is standard enthalpy change, ∆H?

Answer 5

The heat energy transferred under standard conditions

Question 6

What does the increase in temperature depend on?

Answer 6

1. The mass of the object
2. The amount of heat added
3. The nature of the substance

Question 7

What is the specific heat capacity?

Answer 7

The heat needed to increase the temperature of unit mass by 1 K

Question 8

What kind of processes are combustion and neutralisation?

Answer 8

Exothermic

Question 9

What is the relationship between stability and enthalpy level?

Answer 9

The higher the enthalpy level, the smaller the stability

Question 10

What is enthalpy of combustion?

Answer 10

The heat produced when one mole of a substance is burned in excess oxygen

Question 11

What is Hess’s law

Answer 11

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

Question 12

What is average bond enthalpy?

Answer 12

The energy needed to break one mole of bonds in gaseous molecules under STP

Question 13

What does an endothermic process involve in terms of bonds?

Answer 13

Involves the separation of particles which are held together by a force of attraction (breaking bonds)

Question 14

What does an exothermic process involve in terms of bonds?

Answer 14

The bringing together of particles which have an attractive force between them (forming bonds)

Question 15

What is standard enthalpy change of formation, ∆H_f?

Answer 15

The enthalpy change that occurs when one mole of the substance is formed from its elements in their standard states under STP

Question 16

What is the standard state of a substance?

Answer 16

The pure form of the substance under STP

Question 17

What is the standard enthalpy change of formation for an element?

Answer 17

Zero

Question 18

What is the enthalpy change of formation of a reaction?

Answer 18

∆H_reaction = Σ∆H_f(products) - Σ∆H_f(reactants)

Question 19

What is the standard enthalpy change of combustion, ∆H_c?

Answer 19

The enthalpy change that occurs when one mole of the substance burns completely under STP

Question 20

What is the enthalpy change of a combustion reaction?

Answer 20

∆H_reaction = Σ∆H_c(reactants) - Σ∆H_c(products)

Question 21

What is the first electron affinity, ∆H_e?

Answer 21

The enthalpy change when one mole of gaseous atoms attracts one mole of electrons

Cl(g) + e^–(g) → Cl^–(g)

Question 22

What is the lattice enthalpy, ∆H_lat?

Answer 22

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)

Question 23

What is the enthalpy change of atomisation, ∆H_atom?

Answer 23

The heat change that occurs when one mole of gaseous atoms are formed from the element in its standard state

Na(s) → Na(g)

Question 24

How does lattice enthalpy change with charge and size of particles?

Answer 24

It increases with higher ionic charge and smaller ionic radii due to attractive forces

Question 25

What is the entropy change of a system?

Answer 25

∆S_reaction = Σ∆S(products) - Σ∆S(reactants)

Question 26

What is the relationship between the change in entropy of the surroundings and the change in enthalpy of a system?

Answer 26

Question 27

What is the sign of ∆G_sys for a spontaneous process?

Answer 27

Negative

Question 28

What is the Gibbs’ free energy for a reaction?

Answer 28

∆G = ∆H - T∆S

Question 29

What is the ∆G_sys a measure of?

Answer 29

The spontaneity of a reaction:

• negative → spontaneous
• positive → non-spontaneous

Question 30

How can ∆G_sys be used to predict the feasibility of a reaction?

Answer 30

At low temperature:

∆G_sys ≈ ∆H_sys (T∆S ≈ 0)

At high temperature

∆G_sys ≈ –T∆S_sys (T is high enough to make ∆H negligible)

Question 31

What is the ∆G of a reaction?

Answer 31

∆G_reaction = ∑∆G_f(products) - ∑∆G_f(reactants)

Question 32

What is the ∆G of an element in its standard state?

Answer 32

Zero