Energetics Flashcards
define enthalpy change:
- the heat energy change
measured under conditions of constant pressure.
define exothermic reaction:
when energy is released to the surroundings
define endothermic reaction:
when energy is taken in from the surroundings.
define standard enthalpy of combustion (∆cHƟ)
standard enthalpy of combustion of a substance is defined as
the enthalpy change that occurs when one mole of a substance is
combusted completely in oxygen under standard conditions.
(298K and 100kPa), all reactants and products being in their
standard states.
define standard enthalpy of formation (∆fHƟ).
enthalpy change when 1 mole of the compound is formed from its consituent elements under standard conditions (298K and 100kpa), all reactants and products being in their standard states.
what are the standard conditions?
-temperature: 298K / 25 degrees
pressure: 100kPa / 1atm
Calculating the enthalpy change of reaction, H from experimental data
- Using q = m x cp x T calculate energy change for quantities used
- Work out the moles of the reactants used
- Divide q by the number of moles of the reactant not in excess to give H
- Add a sign and unit (divide by a thousand to convert Jmol-1 to kJmol-1
Calculating the enthalpy change of reaction, H from experimental data
-example
Calculate the enthalpy change of reaction for the reaction where 25.0cm3 of 0.200 mol dm-3 copper sulfate was reacted with 0.0100mol (excess of zinc).
The temperature increased 7.0 oC .
Step 1: Calculate the energy change for the amount of reactants in the calorimeter.
Q = m x cp x T
Q = 25 x 4.18 x 7
Q = 731.5 J
Step 2 : calculate the number of moles of the reactant not in excess.
moles of CuSO4 = conc x vol = 0.20 x 25/1000
= 0.005 mol
{If you are not told what is in excess, then you need to work out the moles of both reactants and work outusing the balanced equation which one is in excess.}
Step 3 : calculate the enthalpy change per mole which is often called the enthalpy change of reaction
H = Q/ no of moles
= 731.5/0.005
= 146300 J mol-1 = 146 kJ mol-1
to 3 sf
Finally add in the sign to represent the energy change: if temp increases
the reaction is exothermic and is given a minus sign e.g. –146 kJ mol-1
Remember in these
questions: sign,
unit, same sig figs
as data given.
how to work out the heat change of a reaction?
q= mc∆t
define the term bond enthalpy:
the energy required to break a particular covalent bond in one mole of molecule in a gaseous state.
define Hess’s law
enthalpy change of a reaction for a chemical reaction is independent of the route taken.
give on reason why the bond enthalpy that you calculated in question 2.3 is different from the mean bond enthalpy quoted in a data book
(AS chemistry paper 1 2017 paper)
- the data book value derived from a number of different compounds not just diff NH3 molecules
