energetics

Question 1

Enthalpy change

Answer 1

Enthalpy change is the amount of heat energy taken in or given out during any change in a system provided the pressure is constant.

Question 2

Standard Enthalpy Change of Formation

Answer 2

The standard enthalpy change of formation of a compound is the 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 3

Standard Enthalpy Change of Combustion

Answer 3

The 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 4

Incomplete combustion will lead to

Answer 4

soot (carbon), carbon monoxide and water. It will be less exothermic than complete combustion.

Question 5

Standard conditions are:

Answer 5

• 100 kPa pressure
• 298 K (room temperature or 25oC)
• Solutions at 1mol dm-3
• all substances should have their normal state at 298K

Question 6

Measuring the Enthalpy Change for a Reaction Experimentally

equation

Answer 6

Question 7

Calorimetric method

Answer 7

1. washes the equipment (cup and pipettes etc) with the solutions to be used
2. dry the cup after washing
3. put polystyrene cup in a beaker for insulation and support
4. Measure out desired volumes of solutions with volumetric pipettes and transfer to an insulated cup
5. clamp thermometer into place making sure the thermometer bulb is immersed in a solution
6. measure the initial temperatures of the solution or both solutions if 2 are used. Do this every minute for 2-3 minutes
7. At minute 3 transfer second reagent to cup. If a solid reagent is used then add the solution to the cup first and then add the solid weighed out on a balance.
8. If using a solid reagent then use ‘before and after’ weighing method
9. stirs mixture (ensures that all of the solutions is at the same temperature)
10. Record temperature every minute after addition for several minutes

Question 8

Errors in calorimetry method

5

Answer 8

• energy transfer from surroundings (usually loss)
• approximation in specific heat capacity of the solution. The method assumes all solutions have the heat capacity of water.
• neglecting the specific heat capacity of the calorimeter- we ignore any energy absorbed by the apparatus
• reaction or dissolving may be incomplete or slow.
• density of solution is taken to be the same as water

Question 9

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

Answer 9

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-

Question 10

Example 1.

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

Answer 10

Question 11

Example 2.

25.0cm3 of 2.00 mol dm-3 HCl was neutralised by 25.0cm3 of 2.00 mol dm-3 NaOH. The temperature increased 13.5oC Calculate the enthalpy change per mole of HCl.

Answer 11

Question 12

Example 3.

Calculate the enthalpy change of combustion for the reaction where 0.650g of propan-1-ol was completely combusted and used to heat up 150g of water from 20.1 to 45.5oC

Answer 12

Question 13

Errors in this method

Measuring Enthalpies of Combustion Using Calorimetry

Answer 13

• 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 are not carried out under standard conditions as H2O is gas, not liquid, in this experiment

Question 14

Hess’s Law

Answer 14

Hess’s law states that total enthalpy change for a reaction is independent of the route by which the chemical change takes place

Question 15

Combustion calculation

Answer 15

reactants-products

Question 16

Formation calculation

Answer 16

products-reactants

Question 17

Mean Bond energies

Answer 17

The mean bond energy is the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules.

H = Σ bond energies broken - Σ bond energies made

Question 18

Example 8.

Calculate the enthalpy of combustion of propene using the following mean bond enthalpy data

Answer 18

Question 19

Example 9.

Calculate the enthalpy of formation of NH3 using the following mean bond enthalpy data.

½ N2 + 1.5 H2 —– NH3

E(N≡N) = 944 kJ mol-1

E(H-H) = 436 kJ mol-1

E(N-H) = 388 kJ mol-1

Answer 19

Question 20

Answer 20

Question 21

Why experimental enthalpy values will vary from data

Answer 21

If the results are worked out experimentally using a calorimeter the experimental results will be much lower than the calculated ones because there will be significant heat loss. There will also be incomplete combustion which will lead to less energy being released.

Remember that calculated values of enthalpy of combustions will be more accurate if calculated from enthalpy of formation data than if calculated from average bond enthalpies. This is because average bond enthalpy values are averaged values of the bond enthalpies from various compounds.

Question 22

activation energy, E

Answer 22

The activation energy, EA, is defined as the minimum energy which particles need to collide to start a reaction

Question 23

Maxwell Boltzmann Distribution

Answer 23

Question 24

How can a reaction go to completion if few particles have energy greater than EA?

Answer 24

Particles can gain energy through collisions

Question 25

Increasing Temperature

Maxwell Boltzmann distribution

Answer 25

Question 26

sodium thiosulfate and hydrochloric acid

Answer 26

Na2S2O3 + 2HCl —- 2NaCl + SO2 + S + H2O

Question 27

Effect of Increasing Concentration and Increasing Pressure

MAXwell

Answer 27

Question 28

Effect of Increasing Temperature

Answer 28

At higher temperatures the energy of the particles increases. They collide more frequently and more often with energy greater than the activation energy. More collisions result in a reaction. As the temperature increases, the graph shows that a significantly bigger proportion of particles have energy greater than the activation energy, so the frequency of successful collisions increases

Question 29

Effect of Increasing Surface Area

Answer 29

Increasing surface area will cause successful collisions to occur more frequently between the reactant particles and this increases the rate of the reaction.

Question 30

Effect of Catalysts

Answer 30

Definition: Catalysts increase reaction rates without getting used up. Explanation: They do this by providing an alternative route or mechanism with a lower activation energy.