Chapter 4 - Energetics

Question 1

What are standard conditions?

Answer 1

100kPa and 298 Kelvin (kelvin is the celsius temperature +273)

Question 2

What is the definition of enthalpy change(deltaH)

Answer 2

The heat energy change measured under a constant pressure

Question 3

define standard enthalpy of combustion

Answer 3

Enthalpy of combustion is the energy change when one mole of fuel is burnt completely

Question 4

Define standard enthalpy of formation

Answer 4

Enthalpy change of formation is the energy change when one mole of substance is formed from its constituent elements

Question 5

What is the equation required to calculate heat change?

Answer 5

q = mcDeltaT, where m is mass, c is specific heat capacity and delta T is temperature change

Question 6

What does Hess’s law state?

Answer 6

Hess’s law states that the enthalpy change for a reaction will always be the same, no matter what route is taken for the reaction (The best way to learn Hess’s law is to do many many practice questions on it)

Question 7

Define “mean bond enthalpy”

Answer 7

Mean bond enthalpy is the amount of energy required to break or amount of energy released when a bond is made. It is calculated using Hess’s Law, due to all the bonds requiring slightly different amounts of energy to break depending on the rest of the molecule

Question 8

When is the enthalpy of something equal to 0?

Answer 8

All elements are considered to be at enthalpy 0 when they are in their standard state at room temperature and pressure, for example H2 or C(s, graphite)