enthalpy change Flashcards
what is H
enthalpy, measure of the het energy in a chemical system
what is the chemical system?
refers to the atoms molecules , or ions making up the chemicals.
enthalpy change?
= H(products)- H(reactants)
change in H can be negative or positive dependin on whether the products contain more or less energy than the reactants.
law of conservation of energy ?
states that energy cannot be created or destroyed
. what is the system in. chemical reaction involving enthalpy changes?
system- the chemicals - the reactants and the products
whats the surroundings in a chem reaction involving enthalpy changes?
surroudnings- the apparatus, the laboratory and evrything thats not the chem system
exothermic reaction
energy transferred from the system to the surroundings
endothermic reaction
energy transferred from surroundings to the system
activation energy
minimum energy required for a reaction to take place
units for standard enthalpy change?
KJmol^-1
standard pressure?
100 kpa
standard temp ?
298K (25 dc)
standard concentration?
1mol dm^-3
standard state?
the physical state of a substance under standard conditions - 100 kPa and 298K
standard enthalpy change of reaction?
the enthalpy change that accompanies a reaction in molar quantities shown n the chemical equation under standard cond. with all products and reactants in their standard states.
standard enthalpy change of formation ?
the enthalpy change that takes place when ONE mole of a compound is FORMED FROM ITS ELEMENTS under standard conditions, with all reactants and products in their standard states.
enthalpy change of combustion ?
enthalpy chnge that takes place when one mol of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their stand. states.
enthalpy change of neutralisation
energy change that accompanies the reaction of an acid by a base to form one mole of H2O (l) udner stand. cond. with all reactants and products in their standard states.
0 kelvins in degrees cels.
-273 degrees
why is a 1k rise in temp the same as 1 degrees rise in temp ?
if use a thermometer graduated in degrees celsius the value of temp change is exacly the same in degrees and K
specific heat capacity (c)
energy required to raise the temp of 1g of a substance by 1K
what is the c of water
4.18Jg^-1K^-1
equation for calculating (heat energy change (q))?
mass x c x change in temp
why may less heat be transferred to water in na experiment that tests the enthalpy change of combustion of methanol ?
heat loss to surroundings otehr than water
incomplete comb. of methanol
evaporation of methanol from the wick
non-standard conditions
