9 Enthalpy

Question 1

Is the enthalpy change of an endothermic reaction positive or negative?

Answer 1

Positive

Question 2

Is the enthalpy change of an exothermic reaction positive or negative?

Answer 2

Negative

Question 3

What is activation energy

Answer 3

minimum energy
required for a reaction to take place

Question 4

Define standard enthalpy change of formation

Answer 4

The enthalpy change that takes place when 1 mol of a compound is formed from its elements under standard conditions, with all reactants and products in standard states

Question 5

Define enthalpy change of combustion

Answer 5

The enthalpy change that takes place when 1 mol of a substance completely combusts by completely reacting with oxygen
The standard enthalpy change of combustion is done under standard conditions

Question 6

Define enthalpy change of neutralisation

Answer 6

The energy change that accompanies the reaction of an acid with a base to form 1 mole of H2O (l)

The standard enthalpy change of neutralisation is done under standard conditions

Question 7

State the standard conditions

Answer 7

• Standard pressure = 100 kPa or 1 (atm)
• Standard temperature = a stated temp or 298K (or 25 C)
• Standard concentration (solutions only) = 1 mol dm-3
• Standard state = state of a substance at standard conditions

Question 8

What is the enthalpy change of a reaction

Answer 8

• It refers to a specific equation and the quantities stated in the equation
• the enthalpy change than accompanies a reaction in the molar quantities shown in the chemical equation
• the same reaction but different balancing will produce different enthalpies

(no definition required)

Question 9

Equation to determine enthalpy change

Answer 9

q = mcΔt

enthalpy = mass x SHC x temp change

Question 10

Enthalpy change of formation in terms of products vs reactants

Answer 10

products - reactants

Question 11

Enthalpy change of combustion in terms of products vs reactants

Answer 11

reactants - products

Question 12

Define average bond enthalpy

Answer 12

Average enthalpy change when one mole of (gaseous covalent) bonds are broken (by homolytic fission)

Question 13

Negative enthalpy change means

Answer 13

More energy is released in bond making than taken in in bond breaking

Question 14

What is the relationship between the enthalpy change for a forward and a reverse reaction

Answer 14

The reverse reaction will have the opposite sign than the forward.
Eg, forward = +50 so reverse = -50