topic 4: energetics Flashcards
(17 cards)
enthalpy change definition
enthalpy change is the heat energy change measured under conditions of constant pressure.
endothermic reaction
-endothermic reactions ABSORBS energy from surroundings
-temperature of surroundings DECREASE
-energy change is POSITIVE
-the enthalpy of the system INCREASES
REACTIONS EXAMPLE: thermal decomposition, photosynthesis
exothermic reaction
-exothermic reactions RELEASES energy to the surroundings
-temperature of surroundings INCREASE
-the enthalpy change is NEGATIVE
-the enthalpy of the system DECREASES
REACTIONS EXAMPLE: oxidation, combustion, neutralization
standard enthalpy of combustion definition
the enthalpy change when one mole of a substance is completely burned in oxygen with all reactants and products in their standard states under standard condition
standard enthalpy of formation definition
the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions, at 298K and 100KPa
heat change equation
q=mc△t
hess’s law
the overall enthalpy change for a chemical reaction is independent of the route taken from reactants to products
mean bond enthalpy definition
The average enthalpy change when one mole of a specified covalent bond is broken, averaged out across the range of compounds.
mean bond enthalpy formula
In general (if all substances are gases)
∆ H = Σbond enthalpies broken (reactants) - Σbond enthalpies made (products)
RP2 - outline the method of how you would measure the enthalpy of combustion for a reaction using flame calorimetry
- Fuel is burnt and the energy released is used to heat a set volume of water.
- Fuel continues to burn until the temperature of water rises by a set change (20C etc.).
- Mass of fuel used then measured using a balance.
- Calculation.
RP2 - what are the main errors in the experimental procedure when measuring enthalpies of combustion?
- Heat loss to surroundings (air/flask)
- Incomplete combustion.
- High temperature changes lead to evaporation of water.
RP2 - outline how to carry out a practical to measure enthalpies of neutralisation/displacement.
- weigh out 3.90-4.10g of anhydrous copper sulfate in dry, stoppered weighing bottle and record precise mass
- construct a table so you can record temps at minute intervals up to 15 minutes
- place 25cm3 of deionised water into polystyrene cup and record initial temperature
- record the temperature for every min for 3 mins
- at the 4th minute add the powdered anhydrous copper sulfate to the water in the polystyrene cup
- at 5th minute record temperature and do this at every minute up to 15 mins, stirring the solution
- graph of temp against time and draw two separate lines of best fit
- extrapolate both lines to the 4th minute
- determine the temp change at the 4th min then use calculation to work out enthalpy change
RP2 - why cant the heat of hydration of anhydrous copper sulfate not be measured directly?
it is impossible to add the exact amount of water to hydrate the copper salt and it is not easy to measure the temperature change of a solid turning into another solid.
RP2 - what are the assumptions made when doing calorimetry?
solution has same density as water, specific heat capacity of solution is the same as water, that all the heat given out when the fuel burns is used to heat the water
RP2 - how can we reduce heat loss to the surroundings in a calorimetry practical in solution?
use an insulated container - polystyrene cup with a lid for support and insulation, make sure temperature difference between calorimeter and surroundings isn’t too different
RP2 - other than heat loss, how can the accuracy of the calorimetry experiment be improved?
stir solution, digital thermometer, read thermometer at eye level, use greater concentrations and masses
RP2 - why does using greater concentrations and masses lead to an increased accuracy in the calorimetry practical?
leads to greater temperature change so smaller uncertainty
