enthalpy- chapter 9 Flashcards
what is enthalpy
a measure of the heat energy in a chemical system
what is the enthalpy change and the equation
the difference in enthalpies
H(products) - H(reactants)
can be positive or negative
what is the law of conservation of energy
states energy cannot be created or destroyed
what are the two directions of energy transfer
from the system to surroundings- exothermic
from the surroundings to the system- endothermic
what is activation energy
the minimum energy required for a reaction to take place
what are the standard conditions for enthalpy changes (4)
- standard pressure= 101kPa
- standard temperature= 298K
- standard concentration= 1moldm-3
- standard state= the most physical state of a substance under standard conditions
what is the standard enthalpy change of reaction
the enthalpy change that accompanies a reaction in the molar quantities shown in the chemical equation under standard conditions, with all reactions and products in their standard states
what is the 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
what is the standard enthalpy change of combustion
the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states
what is the standard enthalpy change of neutralisation
what does this involve
the energy change that accompanies the reaction of an acid by a base to form one mole of H2O under standard conditions, with all reactions and products in their standard states
the reaction of H+ with OH- to form one mole of H2O
what is the average bond enthalpy
energy is always required to…
bond enthalpies are always…
bond enthalpies always have a…
the energy required to break one mole of a specified type of bond in a gaseous molecule
break bonds
endothermic
positive enthalpy value
how is the average bond enthalpy calculated
from the actual bond enthalpies in different chemical environments
actual bond enthalpycan vary depending on the chemical environment
what is the energy required to break bonds
what is the energy required to form bonds
endothermic, positive
exothermic, negative
what is the calculation of the enthalp change of reaction
bond enthalpies in reactants - bond enthalpies in products
what does Hess’ law state
if a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
can be visualised with an enthalpy cycle
can be extended for any number of enthalpy changes, provided that all the enthalpy changed are known except for one as it can be determined
what is the calculation for the enthalpy change of formation
enthalpy of products - enthalpy of reactant
what is the calculation for the enthalpy change of combustion
enthalpy of reactants - enthalpy of products
what is the energy change of the surroundings calculated by
mass, specific heat capacity and temperature change
what is specific heat capacity
the energy required to raise the temperature of 1g of substance by 1K
what has small values of specific heat capacity
what has large values of specific heat capacity
good conductors of heat such as metals
insulators of heat such as foam plastic
how to calculate temperature change
temperature of final - temperature of initial
how to calculate heat energy (q)
q=m x specific heat capacity x temperature
why is the experimental value for heat capacity lower of methanol than expected (4 reasons)
- heat loss to the surroundings other than the water, this includes the beaker but mainly the air surrounding the flame
- incomplete combustion of methanol= carbon monoxide and carbon being produced, you would see carbon as a black later of soot
- evaporation of methanol from the wick= burner must be weighed as soon as possible after extinguishing the flame, otherwise methanol may have evaporated from the wick
- non standard conditions
