Energetics 1

Question 1

standard conditions

Answer 1

298K and 100KPa

Question 2

enthalpy change

Answer 2

heat energy change measured at constant

pressure

Question 3

exothermic

Answer 3

when more energy is given out from bonds being formed than is taken in by bonds being broken

Question 4

endothermic

Answer 4

When the breaking of bonds in a reaction requires more energy than is given out by the formation of new bonds

Question 5

energy diagrams for endo and exo

Answer 5

exo= negative, products lower than reactants
endo= positive, products high than reactants

Question 6

Enthalpy of formation, ∆Hθf

Answer 6

The enthalpy change that takes place when mole of substance is formed from its elements in their standard state at a temperature of 298K and pressure of 100kPa

Question 7

Enthalpy of combustion,∆Hθc

Answer 7

The enthalpy change that takes place when mole of substance is completely burned in oxygen at a temperature of 298K and pressure of 100kPa

Question 8

Enthalpy of neutralisation, ∆Hθc

Answer 8

The enthalpy change that takes place when one mole of water is formed in a neutralization reaction at a temperature of 298K and pressure of 100kPa

Question 9

Enthalpy of reaction, ∆Hθr

Answer 9

The enthalpy change that takes place when the number of moles of substances in the equation react at a temperature of 298K and pressure of 100kPa

Question 10

energy transferred equation

Answer 10

mass of water x 4.2 x change in temperature

Question 11

∆H

Answer 11

heat energy/moles

Question 12

using a calorimeter

Answer 12

1. A measuring cylinder is used to measure out 100cm3 of water.
2. The water is placed in a calorimeter (basically a metal can).
3. The liquid to be investigated is placed in a spirit burner. The spirit burner is weighed.
4. The temperature of the water is noted
5. The apparatus is set up as shown below.
6. The liquid is ignited and allowed to heat the water.
7. After a temperature rise of around 20 oC, the flame is extinguished.
8. The exact temperature is noted
9. The burner is weighed to find out how much liquid has been burned.

Question 13

two main reasons for inaccurate values for enthalpy of combustion

Answer 13

Heat loss to the surroundings

Incomplete combustion

Question 14

Hess’ law

Answer 14

For a given chemical change, the overall energy change will always be the same whether the change takes place in one step or a number of steps

Question 15

combustion Hess at the bottom

Answer 15

oxidised elements

arrows going downwards

Question 16

Formation Hess at the bottom

Answer 16

elements

arrows going upwards

Question 17

bond enthalpy

Answer 17

∆H when one mole of a bond is broken and reactants and products are gaseous, always endothermic

Question 18

mean bond enthalpy

Answer 18

∆H for the breaking of a particular type of bond in a particular molecule, averaged out over all those bonds

Question 19

limits of mean BE

Answer 19

bond energies are averages

everything has to be gaseous

Question 20

bond energy

Answer 20

broken - made

Question 21

reality of standard enthalpy of combustion

Answer 21

exothermic make temp higher than 298K

effects the states of products

Question 22

Reasons enthalpy change of a reaction is different to the data book

Answer 22

• Heat loss/gain to/from surroundings
• approximation in specific heat capacity of solution
• neglecting specific heat capacity of calorimeter/apparatus
• reaction may be incomplete/slow
• temp change is very small
• density of solution is taken as the same as water
• conditions not standard