chemical energetics

Question 1

In terms of energy, what happens when a chemical

reaction occurs?

Answer 1

● Energy changes occur, generally in the
form of thermal energy.
● The energy change is either
endothermic or exothermic.

Question 2

What is an endothermic reaction?

Answer 2

● Heat energy is absorbed during the reaction.
● Positive enthalpy change.
● More energy is used to break bonds than is
released in making bonds.

Question 3

What is an exothermic reaction?

Answer 3

● Heat energy is released during the reaction.
● Negative enthalpy change.
● More energy is released making bonds
than is used to break bonds.

Question 4

What is activation energy?

Answer 4

The minimum amount of energy required

to start a reaction.

Question 5

What are standard conditions?

Answer 5

● Pressure: 101 kPa (or 100 kPa) or 1 atm
● Temperature: 298 K or 25℃
● Solution concentrations: 1 moldm-3

Question 6

Define the standard enthalpy change of reaction,

Answer 6

The enthalpy change that accompanies a
reaction in the molar quantities expressed in
an equation under standard conditions, with
all reactants and products in their standard
states.

Question 7

Define the standard enthalpy change of formation

Answer 7

The enthalpy change that occurs when one
mole of product is formed from its
constituent elements under standard
conditions, with all reactants and products in
their standard states.

Question 8

Define the standard enthalpy change of combustion

Answer 8

The enthalpy change that occurs when one
mole of compound completely reacts with
oxygen under standard conditions with all
reactants and products in their standard
states.

Question 9

Define the standard enthalpy change of

neutralisation

Answer 9

The enthalpy change that occurs when
one mole of water is formed in a
neutralisation reaction under standard
conditions.

Question 10

Define the standard enthalpy change of atomisation

Answer 10

The enthalpy change that occurs when 1
mole of gaseous atoms are formed from
an element in its standard state, under
standard conditions.

Question 11

Define the standard enthalpy change of hydration

Answer 11

The enthalpy change that occurs when
one mole of gaseous ions are completely
hydrated by water to form one mole of
aqueous ions, under standard conditions

Question 12

Define the standard enthalpy change of solution

Answer 12

The enthalpy change that occurs when
one mole of ionic compound is completely
dissolved in water to form aqueous ions,
under standard conditions

Question 13

What is bond energy?

Answer 13

● The energy in a covalent bond.
● A measure of the strength of a
chemical bond.

Question 14

How do you directly calculate enthalpy change of a

reaction?

Answer 14

● q = mcΔT
● m - mass of the solution that changes temperature (1g = 1cm3)
c - specific heat capacity (usually of water)
ΔT - temperature change
q - heat energy taken in/ released (joules)
● Divide q (in kJ) by the number of moles of the limiting reactant.
⦵
● Add a sign to show whether ΔHr is exothermic (-) or endothermic
(+).

Question 15

What is Hess’ law?

Answer 15

The enthalpy change of a reaction is

independent of the route it takes.

Question 16

How can you use a Hess cycle to indirectly determine an enthalpy change?

Answer 16

The enthalpy change for fH is equal to H 1 + H 2 - H 3.

Question 17

What is first ionisation energy?

Answer 17

The energy required to remove an electron
from each atom in one mole of gaseous
atoms to form one mole of gaseous 1+ ions
under standard conditions.

Question 18

What is average bond enthalpy?

Answer 18

The average enthalpy change that
occurs when one mole of gaseous
covalent bonds are broken.