TOPIC 5 SL - ENERGETICS

Question 1

Define enthalpy change

Answer 1

The heat change measured under conditions of constant pressure (in an open vessel)

Question 2

Define standard enthalpy change

Answer 2

Standard enthalpy changes occur at the standard pressure of 100KPa (1bar) and a stated temperature usually 298K [ 25oC – always quoted in K].

Question 3

Define standard enthalpy of combustion

Answer 3

The enthalpy change when ONE MOLE of a substance (reactant) undergoes complete combustion under standard conditions. Forms H2O and CO2.

Question 4

Define standard enthalpy of formation

Answer 4

The enthalpy change when ONE MOLE of a compound is formed in its standard state from its elements in their standard states.

Question 5

Define average bond enthalpy

Answer 5

The amount of energy needed to break 1 mole of (covalent) bonds;
in the gaseous state;

Question 6

How to find bond enthalpy of a reaction

Answer 6

Bonds broken - bonds made

Question 7

What happens to heat in an exothermic reaction?

Answer 7

Heat rises and vice versa

Question 8

Is bond breaking endothermic or exothermic?

Answer 8

Bond breaking is endothermic and bond making is exothermic

Question 9

What values are associated with endothermic and exothermic reactions

Answer 9

Endothermic is positive and exothermic is negative, (endothermic absorbs heat so positive, exothermic loses heat so negative)

Question 10

What is specific heat capacity

Answer 10

The property of a substance which gives the heat needed to raise the temperature of unit mass by 1K. It is represented by c.

Question 11

What does q stand for

Answer 11

Heat change

Question 12

What does change in temperature depend on

Answer 12

Mass of object, heat added and nature of substance

Question 13

What is specific heat capacity dependant on

Answer 13

number of particles present in unit mass sample, which depends on the mass of individual particles

Question 14

q=mct

Answer 14

allows for heat change to be calculated from temperature change

Question 15

How to find enthalpy change per mole in Joules

Answer 15

mct/moles reacting, divided by 1000

Question 16

Why might experimental enthalpy of combustion be different from literature value

Answer 16

Not all the heat is absorbed by water, some is lost to calorimeter or the surrounding environment.

Question 17

Define Hess’s law

Answer 17

Hess’s law states that the enthalpy change for any chemical reaction is independent of the route provided the starting and final conditions, and reactants and products, are the same.

Question 18

What is energy?

Answer 18

The ability to do work

Question 19

What is heat as a form of energy?

Answer 19

Mode of energy transfer which occurs as a result of temperature difference and produces an increase in disorder of how particles behave.

Question 20

What does heat do?

Answer 20

Increases the kinetic energy of particles in a disordered fashion

Question 21

What is enthaply

Answer 21

Measure of the amount of heat energy contained in a substance. It is stored in chemical bonds and intermolecular forces as POTENTIAL energy. When substances react, the difference in enthalpy between reactants and products results in heat change which can be observed.

Question 22

What is temperature?

Answer 22

A measure of average kinetic energy. When temperature decreases, so does KE. The Kelvin scale emphasizes this relationship. Absolute 0 is lowest possible value, at which all kinetic energy stops.