more... Flashcards
what are exothermic reactions
reactions that give out heat as the reactants change to products so the temp changes
what are endothermic reactions
reactions that take in heat as reactants change to products so the temp drops
what is enthalpy change
heat energy change measured under a constant pressure
what are standard conditions
100KPa
298K
what is the standard state of an element
physical state under standard conditions
what is the definition of standard enthalpy change of combustion
the enthalpy change when one mole of substance is completely burned in oxygen under standard conditions, with all reactants and products in their standard states
what does standard enthalpy change refer to
heat change measured at 100KPa and 298K
what is the definition of standard enthalpy change of formation
the enthalpy change when one mole of substance is formed from its elements under standard conditions, with all reactants and products in their standard states
what equation would you use to calculate the heat change for a reaction
Q=Mc > T
what is Hess’s Law
the enthalpy change for a reaction is the same, whatever route is taken from reactants to products
how is an enthalpy change different from a heat change
an enthalpy change is an energy change measured under a stated temp and constant stated pressure
what is the unit of energy
Joules
what is the most stable form of Carbon, and what state symbol is it given
Graphite (s)
what value is given to deltaH of elements
zero, no energy is needed to form an element from its element
why will the value calculated for an enthalpy change be different using mean bond enthalpies compared to thermochemical cycles
mean bond enthalpies are averages, and the true value will differ. thermochemical cycle is ‘correct’ answer
are mean bond angels positive or negative? why?
Positive because energy must be put in to break bonds
when drawing enthalpy diagrams, what do we take as 0
elements
what Is meant by the bond dissociation enthalpy
the enthalpy change required to break a covalent bond, with all species in the gaseous state
what is the difference between heat and temp.
temp- average kinetic energy of the particles in a system
heat- total amount of energy of all particles present
do endothermic reactions have positive or negative values
positive
explain the trend in electronegativity that we see in the halogens
halogens become more electronegative as we move up the group because the distance between the nucleus and the bonding electrons decreases, so does shielding
what sort of agents are halide ions
reducing agents
what happens to the ability of halide
ions to act as reducing agents as we move down the group? why?
increases because more shielding and size of atom means outer elecrons are less strongly attracted to the nucleus and so are easily lost
why is the reaction of sodium bromide with concentrated sulphuric acid different from the reaction of sodium chloride with it
bromide ions are stronger reducing agents than chloride ions so are able to reduce the sulphuric acid to sulphur dioxide
how can we predict if one halogen will displace another halogen ion
a halogen will displace the ion of a halogen that is below it in the periodic table
explain the trend in boiling point that we see in the halogens
Bp increases as we move down the group. because larger atoms have more electrons an so stringer van der waals forces between molecules
how do halogens normally react
by gaining electrons
what sort of agent are halogens
oxidising
what happens to the ability of halogens to act as oxidising agents as we move down the group? why?
decreases because increased shielding and size of atom means electrons are less strongly attracted to the nucleus
why are the products from the reaction of NaBr with conc H2SO4 different from Nal?
iodide ions are stronger reducing agents than bromide ions, so reduce the sulphur further
what reagent would you use to distinguish between halide ions
Acidified silver nitrate
what acid would you chose to acidify silver nitrate
nitric acid
why do we acidify silver nitrate before testing halogen ions
to remove ions that would form precipitates and interfere with results eg carbonate
what colour precipitates are formed when halide ions react with acidified silver nitrate
AgF- none
AgCL- white
AgBr- cream
AgI- yellow
what is the trend in solubility of the silver halides as we move down the group
becomes less soluble
AgCl- dissolves in dilute ammonia
AgBr- dissolves in conc. ammonia
AgI- insoluble in conc. ammonia
what do we call a reaction in which an element is both oxidised and reduced
disproportionation
what is the formula of chloric(1) acid
HClO
why is chlorine added to water
it produces chloric (1) acid which kills bacteria by oxidation and is also a bleach
what is the formula of sodium chlorate? what are the oxidation numbers if the elements in it
NAClO-
NA +1
Cl +1
O -2
