CH3.6 Enthalpy changes for solids and solutions Flashcards
enthalpy change
a way of measuring the change in energy during any chemical or physical change. We can treat energy and enthalpy as the same thing.
principle of conservation of energy
Energy can not be created or destroyed, but may only be converted from one form to another
Hess’s law
The energy change of any chemical reaction is the same regardless of the route taken
Enthalpy change equation
Enthalpy change = (delta)H(for all products)-(delta)H(for all reactants)
exothermic reaction
is one that gives out energy as heat leaving, less chemical eergy in the products that was present in the reactnats
endothermic reaction
is one that takes in energy as heat, leaving ore chemical energy in th eproducts thatn was present in the reactants at the start.
enthalpy change of reaction
is the change in the chemical energy present in substances during a chemical reaction
standard enthalpy change
occurs betwween molar quantities of reactants in thei standard state under standard conditions
what are the standard condtions
temperature 298(K) a concentration of 1moldm-3 for solutioins, a pressure of one atmosphere (1atm) for gases
standard state
is the physical state of a substance under standard conditions.
standard enthalpy change of atomisation
This is the enthalpy change that occurs when one mole o atoms of an element in the gas phase are formed from the element in its standard state under standard conditions
standard enthalpy change of atomisation equation example
Na(s) –> Na(g)
standard enthalpy change of lattice formation
This is the enthalpy change that occurs when one mole of an ionic compound is formed from ions of the element in the gas phase. It is always exothermic and the more exothermic the more stable the ionic compound is.
Standard enthalpy change of lattice formation equation example
Na+(g) + Cl-(g) –> NaCl(s)
standard enthalpy of lattice breaking
the energy chagne that occurs when one mole of an ionic compoundis broke up into ions of the elements in the gas phase
standard enthalpy change of lattice breaking equations example
NaCl(s) –> Na+(g) + Cl-(g)
standard enthalpy change of hydration
This is the enthalpy change that occurs when one mole of an ionic compound in solution is formed from ions of the elements in the gas phase
standard enthalpy change of hydration equation example
Na+(g) + Cl-(g) + aq –> NaCl(aq)
Electron affinity
This is the enthalpy change that occurs when one mole of gaseous negative ions are formed from gaseous atomsof a substance by gaining an electron
Electron affinity equation example
Cl(g) + e- –> Cl-(g)
ionisation energy
This is the enthalpy change that occurs when one mole of gaseous positive ions are formed from gaseous atoms, by the substance losing an electron.
ionisation energy equation example
Na(g) –> Na+(g) + e-
what kind of bond does water create
polar water molecules cause tempory-tempory dipoles with ions
standard enthalpy change of solution
This is the enthalpy cahnge that occurs when one mole of substance dissolves completely in a solvent under standard conditions to form a solution
standard enthalpy change of solutions equations example
M+X-(s) –> M+(g) + X-(g) –> M+(aq) + X-(aq)
enthalpy of lattice breaking, endothermic or exothermic
endothermic
enthalpy of lattice hydration, endothermic or exothermic
exothermic
How enthalpy of lattice breaking and enthalpy of hydration relate to the soubility of salts
(exothermic)
If the enthalpy of lattiec hydration is greater the the enthalpy of lattice breaking then the salt dissolves, if its the other way around the salt will not usually dissolve. The more exothermic the total of the values the more solouble a salt is likely to be.
What will increase the entahlpy of lattice hydration and enthalpy of lattice breaking (2)
increasing the charge on the ions,
decreasing the size of the ions
order of the enthalpy changes in a Born-Haber cycle
Enthalpy of formation, Enthalpy of atomisation of gas, Enthalpy of atomisation of metal, Ionisation energy, Electron affinity, Enthalpy of lattice formation.
calculation
Enthalpy of lattice formation =
Enthalpy of lattice formation = - electron afffinity - ionisation energy - enthalpy of atomisatio - enthalpy of atomisation + enthalpy of formation
what if the enthalpy of formation of a compoundis negative
The energy is given out as the compound is formed its elements, THis tells us that the compound is stbale compared with the elemtns. The more negative the enthaly chagne of formations of a compound the more stable it is.
what if the enthalpy change of formation is positive
The compoundus unstable compared with the elemtns that make it up. This does not mean the compound connot exist, but it means that energy is needed to change the elements into the compound.