Energetics

Question 1

define enthalpy change:

Answer 1

• the heat energy change
  measured under conditions of constant pressure.

Question 2

define exothermic reaction:

Answer 2

when energy is released to the surroundings

Question 3

define endothermic reaction:

Answer 3

when energy is taken in from the surroundings.

Question 4

define standard enthalpy of combustion (∆cHƟ)

Answer 4

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.

Question 5

define standard enthalpy of formation (∆fHƟ).

Answer 5

enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa), all reactants and products being in their standard states.

Question 6

what are the standard conditions?

Answer 6

-temperature: 298K / 25 degrees

pressure: 100kPa / 1atm

Question 7

Calculating the enthalpy change of reaction, H from experimental data

Answer 7

1. Using q = m x cp x T calculate energy change for quantities used
2. Work out the moles of the reactants used
3. Divide q by the number of moles of the reactant not in excess to give H
4. Add a sign and unit (divide by a thousand to convert Jmol-1 to kJmol-1

Question 8

Calculating the enthalpy change of reaction, H from experimental data
-example

Answer 8

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.

Question 9

how to work out the heat change of a reaction?

Answer 9

q= mc∆t

Question 10

define the term bond enthalpy:

Answer 10

the energy required to break a particular covalent bond in one mole of molecule in a gaseous state.