Chemistry: Enthalpy Flashcards
What is enthalpy?
measure of the heat content of a substance
What is enthalpy change?
change in heat content of a substance at a constant pressure
What are the standard conditions for measuring enthalpy change?
Pressure: 100kPa
Temperature: 298K
What is enthalpy change measured in?
KJmol-1
Define what is meant by the term standard enthalpy of formation
enthalpy change when one mole of substance is formed from its constituent elements under standard conditions with all reactants and products in standard states
How is standard enthalpy of formation represented?
triangle-f-H
What is the equation for calculating standard enthalpy of formation?
=sum of enthalpy formation products - sum of enthalpy formation reactants
What is standard enthalpy of combustion?
enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions with all reactants and products in standard states
How is standard enthalpy of combustion represented?
triangle-c-H
What is the equation for calculating standard enthalpy of combustion?
=sum of enthalpy combustion reactants - sum of enthalpy combustion products
What is temperature related to?
average kinetic energy of particles within a system
What effect does increasing kinetic energy have on temperature?
increases, particles moving faster
What is heat a measure of?
total energy of all particles present in a given amount of substance
What three details are necessary to measure enthalpy change?
-mass of substance heated/cooled -temperature change -specific heat capacity of substance
What is the specific heat capacity of a substance?
amount of heat energy required to raise the temperature of 1g of substance by 1K
What are the units for specific heat capacity?
Jg-1K-1
What is the specific heat capacity of water?
4.18 Jg-1K-1
What equation is used to calculate heat energy given out?
q=m-c-change in t
q: heat energy given out (J)
m: mass of substance (g)
c: 4.18
change in t: change in temperature (K)
What equation is used to find enthalpy change per mole?
q/number of moles reacting
How do we calculate moles?
=mass/mr
What apparatus is used to measure enthalpy of combustion?
flame calorimeter
Using a flame calorimeter, explain how we would measure enthalpy change of combustion
-fixed volume of water (measured) in calorimeter -spirit burner weighed on 2d.p balance
-initial temperature of water measured and recorded -burner ignited, water stirred continuously -flame distinguished after 20C rise -burner re-weighed -final temperature of water measured and recorded
What sources of error are there to the flame calorimeter experiment?
-heat loss to surroundings
-heat absorbed by calorimeter
-incomplete combustion
What steps can we ensure to reduce heat loss?
-calorimeter not placed too far above flame -lid on calorimeter -shielding/draught excluder
What is bond enthalpy?
enthalpy change to break one mole of covalent bonds in the gas phase
What is the value for bond enthalpy?
an average taken from a range of molecules
What equation is used to calculate bond enthalpy?
=sum of enthalpy bonds broken - sum of enthalpy bonds made
How do we conduct an experiment for heat of neutralisation with sodium hydroxide and hydrochloric acid?
-50cm3 1moldm-3 HCl in one polystyrene cup, equal volume of sodium hydroxide in another
-record temperature of each for 4 minutes at every 30 seconds, confirm remain the same
-at 4 don’t record temperature and add solutions together then stir
-record temperature every 30 seconds for further 6 minutes
-draw cooling curve with temperature against time, extrapolate to 4 minutes
-use lines of best fit to find temperature difference
What is Hess’s Law?
states that enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products
