Thermodynamics

Question 1

What is heat?

Answer 1

Energy transferred as a result of temperature difference

Question 2

What is work?

Answer 2

W = F x d

Question 3

System?

Answer 3

Part of the universe that is of interest

Question 4

Surroundings?

Answer 4

Rest of the universe

Question 5

Isolated system?

Answer 5

No exchange of energy or matter with the surroundings

Question 6

Closed system?

Answer 6

Contains a fixed amount of matter but allows exchange of energy

Question 7

Open system?

Answer 7

Both matter and energy can be exchanged with the surroundings

Question 8

Heat capacity?

Answer 8

Heat needed to raise the temperature of a substance by 1K units of JK-1

Question 9

Specific heat capacity?

Answer 9

Heat needed to raise the temperature of 1g of a substance by 1K units of JK-1g-1

Question 10

Molar heat capacity?

Answer 10

Heat needed to raise the temperature of 1 mole of a substance by 1K units of JK-1mol-1

Question 11

Molar heat capacity at constant pressure Cp?

Answer 11

Heat needed to raise the temperature of 1 mol of a gas by 1K at constant pressure units of JK-1mol-1

Question 12

Molar heat capacity at constant volume Cv?

Answer 12

Heat needed to raise the temperature of 1 mol of a gas by 1K at constant volume units of JK-1mol-1

Question 13

Intensive property?

Answer 13

Property of a substance that doe not depend on the quantity of a substance eg specific heat capacity, density, pressure

Question 14

Extensive property?

Answer 14

Depends on the quantity of the substance eg mass, length, volume

Question 15

Enthalpy change?

Answer 15

Heat transferred at a constant pressure by a chemical reaction or process

Question 16

State function?

Answer 16

Quantity whose value only depends on the initial and final state of the system but not on the route taken to get from the initial to the final state

Question 17

Enthalpy change of fusion?

Answer 17

Transition from solid to liquid (melting) the energy required to melt one mole of a pre substance as its melting point Tm at 1 bar pressure (also called latent hear of fusion)

Question 18

Enthalpy change of vapourisation?

Answer 18

Transition from liquid to gas, energy required to vaporise 1 mole of a pure liquid at it boiling point Tb at 1 bar pressure (also called latent heat of vapourisation)

Question 19

What type of process are fusion and vaporisation?

Answer 19

Endothermic processes values always positive

Question 20

Difference between enthalpy of vapourisation and fusion?

Answer 20

Enthalpy of vapourisation is always larger than enthalpy of fusion this is because more energy is needed to overcome the intermolecular attractions holding the liquid together so the molecules are completely separated as a gas than in melting solid to a liquid, vaporising overcomes all forces between molecules, so can be used as a measure of strength of intermolecular forces in liquid, explains why liquids that form hydrogen bonds have high values for enthalpy of vaporisation

Question 21

Enthalpy change of sublimation?

Answer 21

Solid vaporises directly o gas, thinly change of sublimation is the sum of the enthalpy changes for fusion and vapourisation

Question 22

Hess’s law?

Answer 22

The total enthalpy change for a chemical process is independent of the path by which the reaction occurs provided the starting and finishing states are the same for each reaction path

Question 23

Enthalpy change of formation?

Answer 23

Enthalpy change at 298K when 1 mole of a compound is formed under standard conditions from its constituent elements in their standard states, the enthalpy change for an element in its standard states is zero

Question 24

Enthalpy change of reaction?

Answer 24

sum of enthalpy changes of formation of products - sum of enthalpy changes of formation of reactants

Question 25

Enthalpy change of combustion?

Answer 25

Standard enthalpy change of combustion is the enthalpy change when 1 mol of a substance reacts completely with oxygen gas at 1 bar pressure, usually quoted at 298K

Question 26

Enthalpy change per unit mass of a compound?

Answer 26

Often called energy density and has units of KJg-1 or MJkg-1, it is the enthalpy change on burning 1g or 1kg of a compound and is used as a measure of the efficiency of a fuel which are often purchased by mass

Question 27

Enthalpy change of solution?

Answer 27

When 1 mole of substance dissolves in a large press of pure solvent at 1 bar pressure is called the standard enthalpy change of solution, the value of the enthalpy depends on concentration because the interaction between ions in solution depends on how far the ions are apart which changes with concentration

Question 28

Bond dissociation enthalpy?

Answer 28

Enthalpy change per mole when a particular chemical bond is broken under standard conditions in the gas phase, values are always positive since bond breaking is an endothermic process

Question 29

Mean bond enthalpy?

Answer 29

Mean value of the bond dissociation enthalpy for the bond averaged across a range of related compounds

Question 30

Kirchoff equation?

Answer 30

Allows you to calculate the enthalpy change for a reaction at any temperature providing you know the value of the enthalpy change at one temperature and the heat capacities of the reactants and products

Question 31

Isothermal expansion?

Answer 31

Heat is exchanged between system and surroundings so their temperatures are equal

Question 32

Work done by systems?

Answer 32

System loses energy and the value of the energy transferred to work therefore has a negative sign

Question 33

Adiabatic change?

Answer 33

No heat exchange between system and surroundings so their temperatures may not be equal

Question 34

Sign when heat is absorbed by or work is done on the system?

Answer 34

Energy gained so sign is positive

Question 35

Sign when heat is related from or work is done by the system?

Answer 35

Energy is lost so sign is negative

Question 36

Internal energy?

Answer 36

Sum of kinetic and potential energy of molecules

Question 37

First law of thermodynamics?

Answer 37

Energy can neither be created nor destroyed only introverted between forms The total quantity of energy in the universe is constant The internal energy of an isolated system is constant delta U = 0

Question 38

Bomb calorimeter?

Answer 38

Suitable for studying combustion or other solid gas reactions, volume of system does not change, heat change during a reaction conducted at constant volume is the internal energy change, so a bomb calorimeter measures deltaU

Question 39

How does a bomb calorimeter work?

Answer 39

The heat realised during the reaction raises the temperature of water in the surroundings jacket to relate this temperature change to the heat change during the reaction the calorimeter must be calibrated so the you know how much energy it takes to change the temperature of the water by 1K, the easiest method of doing this is to use a reaction for which the internal energy is already known, the temperature rise produced by burning a known mass of a compound such as benzoic acid or naphthalene is measured and used to calculate the energy realised and the temperature use for the water for the particular calorimeter being used, this relationship is called the calibration factor

Question 40

Using internal energy from bomb calorimetry to find enthalpy change what are you assuming?

Answer 40

Assuming the gas obeys the ideal gas equation, the assumption of ideal gas behaviour is reasonable for pressures up to several hundred bar

Question 41

Enthalpy changes of solution?

Answer 41

Takes place in a container that is thermally insulating the temperature change is measured during the reaction since the calorimeter is not sealed reactions are performed at constant temperature, the relationship between the temperature change and the heat output from the reaction must be established by calibration given by using acid base neutralisation the enthalpy change for this reaction is 56.9KJmol-1 irrespective of the acid or base used as long as they are strong

Question 42

Accurate solution change?

Answer 42

Polystyrene cup not sufficient, commercial apparatus which allows two or more reactants to be placed in an insulated thermochemically controlled chamber until the temperature is constant is needed