Flashcards in Thermodynamics Deck (42)
What is heat?
Energy transferred as a result of temperature difference
What is work?
W = F x d
Part of the universe that is of interest
Rest of the universe
No exchange of energy or matter with the surroundings
Contains a fixed amount of matter but allows exchange of energy
Both matter and energy can be exchanged with the surroundings
Heat needed to raise the temperature of a substance by 1K units of JK-1
Specific heat capacity?
Heat needed to raise the temperature of 1g of a substance by 1K units of JK-1g-1
Molar heat capacity?
Heat needed to raise the temperature of 1 mole of a substance by 1K units of JK-1mol-1
Molar heat capacity at constant pressure Cp?
Heat needed to raise the temperature of 1 mol of a gas by 1K at constant pressure units of JK-1mol-1
Molar heat capacity at constant volume Cv?
Heat needed to raise the temperature of 1 mol of a gas by 1K at constant volume units of JK-1mol-1
Property of a substance that doe not depend on the quantity of a substance eg specific heat capacity, density, pressure
Depends on the quantity of the substance eg mass, length, volume
Heat transferred at a constant pressure by a chemical reaction or process
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
Enthalpy change of fusion?
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)
Enthalpy change of vapourisation?
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)
What type of process are fusion and vaporisation?
Endothermic processes values always positive
Difference between enthalpy of vapourisation and fusion?
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
Enthalpy change of sublimation?
Solid vaporises directly o gas, thinly change of sublimation is the sum of the enthalpy changes for fusion and vapourisation
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
Enthalpy change of formation?
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
Enthalpy change of reaction?
sum of enthalpy changes of formation of products - sum of enthalpy changes of formation of reactants
Enthalpy change of combustion?
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
Enthalpy change per unit mass of a compound?
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
Enthalpy change of solution?
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
Bond dissociation enthalpy?
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
Mean bond enthalpy?
Mean value of the bond dissociation enthalpy for the bond averaged across a range of related compounds