4 - Energetics

Question 1

What is thermochemistry?

Answer 1

The study of thermal changes during chemical reactions.

Question 2

What are exothermic reactions?

Answer 2

When thermal energy is given out (-ΔH)

Question 3

What are endothermic reactions?

Answer 3

When thermal energy is absorbed (+ΔH)

Question 4

Is breaking bonds exo or endo thermic?

Answer 4

Endothermic.

Question 5

Is making bonds exo or endo thermic?

Answer 5

Exothermic.

Question 6

What is the unit of energy?

Answer 6

kJ mol-1
kilojoules per mole

Question 7

What is enthalpy change?

Answer 7

Measuring a thermal energy change at constant pressure.

Question 8

What are the standard conditions for measuring enthalpy changes?

Answer 8

100kPa (1 atm)
298K (25 °C)

Question 9

What is standard state?

Answer 9

The state in which an element exists at standard conditions.

Question 10

What is standard molar enthalpy of formation?

Answer 10

The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions.

Question 11

What is standard molar enthalpy of combustion?

Answer 11

The standard enthalpy change when one mole of a substance is completely burnt in excess oxygen.

Question 12

What is temperature?

Answer 12

The average kinetic energy of particles in a system.

Question 13

What is heat?

Answer 13

A measure of the total energy of all the particles in a given amount of substance.(Depends on number of particles)

Question 14

What is specific heat capacity?

Answer 14

The amount of heat needed to raise the temperature of 1g of a substance by 1K.
Units = Jg-1K-1

Question 15

What is the formula for enthalpy change?

Answer 15

q = mcΔT
mass x specific heat capacity x temperature change.

Question 16

How to reduce heat loss of measuring the enthalpy change?

Answer 16

Use a polystyrene cup around the beaker as they are good insulators. (Prevent heat loss via conduction)
Use a lid on calorimeter to prevent heat loss via convection.
Use larger volumes of reactants as the smaller the effect of heat loss.
Stir the mixture to ensure uniform temperature.

Question 17

What does Hess’ Law state?

Answer 17

The enthalpy change for a chemical reaction is the same regardless of the route taken, given states are constant.

Question 18

Why can the enthalpy change of some reactions not be measured directly?

Answer 18

Slow or complex reaction
Unstable intermediates
High temp or pressure
Phase changes

Question 19

What is a thermochemical cycle?

Answer 19

Diagram that represents Hess’ Law.

Question 20

Which way do arrows point for Hess’s cycle when working out enthalpy of formation?

Answer 20

Down

Question 21

Which way do arrows point for Hess’s cycle when working out enthalpy of combustion?

Answer 21

Down

Question 22

What is the enthalpy of elements in their standard states?

Answer 22

0

Question 23

What is the definition of Bond dissociation enthalpy?

Answer 23

The enthalpy change required to break a covalent bond with all species in the gaseous state.

Question 24

What is the definition of Mean bond enthalpy?

Answer 24

The average energy required, per mole of gaseous compound, to break a particular covalent bond to form gaseous atoms.

Question 25

What is the formula for overall bond enthalpy change?

Answer 25

Bonds broken - bonds formed.
(reactants - products)