Thermodynamics Flashcards
What is heat?
Energy transferred as a result of temperature difference
What is work?
W = F x d
System?
Part of the universe that is of interest
Surroundings?
Rest of the universe
Isolated system?
No exchange of energy or matter with the surroundings
Closed system?
Contains a fixed amount of matter but allows exchange of energy
Open system?
Both matter and energy can be exchanged with the surroundings
Heat capacity?
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
Intensive property?
Property of a substance that doe not depend on the quantity of a substance eg specific heat capacity, density, pressure
Extensive property?
Depends on the quantity of the substance eg mass, length, volume
Enthalpy change?
Heat transferred at a constant pressure by a chemical reaction or process
State function?
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
Hess’s law?
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
Kirchoff equation?
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
Isothermal expansion?
Heat is exchanged between system and surroundings so their temperatures are equal
Work done by systems?
System loses energy and the value of the energy transferred to work therefore has a negative sign
Adiabatic change?
No heat exchange between system and surroundings so their temperatures may not be equal
Sign when heat is absorbed by or work is done on the system?
Energy gained so sign is positive
Sign when heat is related from or work is done by the system?
Energy is lost so sign is negative
Internal energy?
Sum of kinetic and potential energy of molecules
First law of thermodynamics?
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
Bomb calorimeter?
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
How does a bomb calorimeter work?
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
Using internal energy from bomb calorimetry to find enthalpy change what are you assuming?
Assuming the gas obeys the ideal gas equation, the assumption of ideal gas behaviour is reasonable for pressures up to several hundred bar
Enthalpy changes of solution?
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
Accurate solution change?
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
