Energetics I Flashcards
what is a negative enthalpy change?
one that gives out heat
what is a positive enthalpy change?
one which takes in heat
give the ‘standard conditions’
a pressure of 1 bar (1000kPa)
a temperature of 298K
define standard enthalpy change
an enthalpy change carried out under standard conditions
describe the definition of the standard enthalpy change of combustion
the enthalpy change when 1 mole of a substance in its standard state burns completely in oxygen under standard conditions of 298K and 1 bar pressure
describe the definition of the standard enthalpy change of formation
the enthalpy change when 1 mole of a substance is formed in its standard state from the pure elements in their standard states under standard conditions of 298K and 1 bar pressure
true or false
‘weak acids fully ionise in solution’
FALSE
In solution, weak acids do not fully ionise
what is calorimetry used for?
to physically measure the changes in enthalpy
what is bomb calorimetry?
it uses a machine called a bomb calorimeter to measure enthalpy changes of combustion
this involves burning a sample of a compound in a sealed vessel and measuring the temperature change
why can bomb calorimetry be inaccurate (3 marks)
heat lost to surroundings
any incomplete combustion that may take place
loss of some reactant that evaporates before it combusts
give the equation to calculate enthalpy change
q = m x c x t
q = heat change
m = mass of the sample
c = specific heat capacity
t = temperature change
Why are the initial and final temperatures in a calorimeter inaccurate?
heat is always being lost from the calorimeter
when is the best time to measure the temperature loss during calorimetry?
after the reaction is complete and extrapolate a line to find the true value
enthalpy = ?
q / moles
What is enthalpy?
When a chemical reaction occurs there is normally a change in energy
What is an endothermic reaction?
Reactions that absorb energy from the surroundings
Products higher in energy than reactants
Delta H is positive
What is an exothermic reaction?
Reactions that release energy to the surroundings
Products lower in energy than reactants
Delta H is negative
describe the standard enthalpy change of neutralisation
the enthalpy change when an acid and alkali react to form 1 mole of water, using standard conditions
describe the standard enthalpy change of a reaction
the enthalpy change of a reaction according to the molar quantities in the equations under standard conditions
how to calculate enthalpy change?
total energy to break bonds - total energy released forming bonds
