energetics Flashcards
what is enthalpy change?
the amount of heat energy taken in or given out during any change in a system provided the pressure is constant
where is energy transferred in exothermic reactions?
energy is transferred from the system (chemicals) to the surroundings. The products have less energy than the reactants
delta H is negative
what are some examples of exothermic reactions?
combustion of fuels and oxidation of glucose in respiration.
how is energy transferred in an endothermic reaction?
transferred from the surroundings to the system (chemicals). They require an input of heat energy.
Products have more energy than the reactants and delta H is positive
what is an example of an endothermic reaction?
thermal decomposition
what is standard enthalpy of formation?
the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions, with all reactants and products being in their standard states.
what is the standard enthalpy of formation of an element?
0kJ/mol
what is standard enthalpy of combustion?
the enthalpy change when one mole of a substance is combusted completely in oxygen under standard conditions, with all reactants and products being in their standard states.
what is standard enthalpy of neutralisation?
the enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions
enthalpy changes are normally quoted at standard conditions. What are the standard conditions?
100kPa pressure
298K (room temp of 25 degrees C)
solutions at 1mol/dm3
all substances should have their standard state at 298K
what are the two calorimetry equations that can be used to find enthalpy change?
- energy change = mass of solution x heat capacity x temp change
q= mcAT
- enthalpy change= q/ mol
what specific heat capacity does water have?
4.18 J/g/K
what do we assume about solutions in calorimetry calculations?
they have the same density as water, which is 1g/cm3
why do we put the polystyrene cup in a beaker?
for insulation and support
why do we stir the mixture once adding the reactants together in the cup?
ensures that all the solution is at the same temperature
why might exact temperature rises be unclear in calorimetry if the reaction is slow?
cooling occurs simultaneously with the reaction
to counteract this we take readings at regular time intervals and extrapolate the temperature line back to the time the reactants were added together
what are some errors for the calorimetric cup method? (5)
-energy transfer between system and surroundings
-assumptions in SHC of the solution
-ignoring the SHC of the calorimeter- we ignore any energy absorbed by the apparatus
-reaction may be incomplete or slow
-density of solution is taken to be same as water.
basically, how do we measure enthalpies of combustion using calorimetry?
a fuel is burnt and the flame is used to heat water, and temp change of the water is measured
what are the errors of the calorimetry combustion experiment? (6)
-energy losses from the calorimeter
-incomplete combustion of fuel
-incomplete transfer of energy
-evaporation of fuel after weighing
-heat capacity of calorimeter not included
-measurements not carried out under standard conditions as H2O is gas, not liquid, in this experiment.
what is Hess’s law?
the total enthalpy change for a reaction is independent of the route by which the chemical change takes place.
what can arrows on an energy level diagram represent?
direction of arrows can show different routes a reaction can proceed by.
when are Hess cycles used?
to measure enthalpy change for a reaction that cannot be measured directly by experiments. Instead alternative reactions are carried out that can be measured experimentally.
what is the mean bond energy?
the enthalpy needed to break the covalent bond into gaseous atoms, averages over different molecules.
why do we use values of mean bond energy?
every single bond in a compound has a slightly different bond energy
when does the definition of bond energies only apply?
when the substances start and end in the gaseous state
what is the general equation for enthalpy change when all substances are gases?
bond energies broken- bond energies made
why are enthalpy change values calculated from mean bond energies less accurate than using formation or combustion data?
the mean bond energies are not exact
