Energetics

Question 1

What is enthalpy?

Answer 1

It is a measure of the heat content of a substance

Question 2

What is an enthalpy change (∆H)

Answer 2

Change in heat content at constant pressure

Question 3

What are standard conditions?

Answer 3

100 kPa and a stated temperature (usually 298K)

Question 4

What is the energy change in an exothermic reaction?

Answer 4

Energy change is negative

Question 5

What is the heat change in an exothermic reaction?

Answer 5

Heat is given out to the surroundings

Question 6

Whats the energy change in an endothermic reaction?

Answer 6

Energy change is positive

Question 7

What is the heat change in an exothermic reaction?

Answer 7

Heat is absorbed from the surroundings.

Question 8

Define standard enthalpy change of formation

Answer 8

Enthalpy change when 1 mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions.

Question 9

Define standard enthalpy change of combustion

Answer 9

Enthalpy change when 1 mole of a substance is completely burned in oxygen with all reactants and products in standard states under standard conditions.

Question 10

What is the enthalpy of formation of an element?

Answer 10

0

Question 11

Why is the enthalpy of formation of an element 0?

Answer 11

by definition

Question 12

Define standard enthalpy change of neutralisation

Answer 12

Enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions.

Question 13

Which equation is used to find the amount of heat energy given out in calorimetry ?

Answer 13

q=mc∆T

Question 14

In calorimetry, how do you find the heat enthalpy change of a reaction?

Answer 14

∆H = q/moles

Question 15

What are the biggest problems with calorimetry practicals

Answer 15

Heat loss either as draughts or as lack of insulation

Question 16

What is Hess’s Law?

Answer 16

The enthalpy change for a reaction is independent of the route taken

Question 17

How would you find ∆H for a reaction if you were given enthalpy of formation data?

Answer 17

∆H = [SUM of ∆Hf products] – [SUM ∆Hf reactants]

Question 18

How would you find ∆H for a reaction if you were given enthalpy of combustion data?

Answer 18

∆H = [SUM of ∆Hc reactants] – [SUM ∆Hc products]

Question 19

What is the intermediate between reactants and products in the thermochemical cycle if you are given enthalpy of formation data?

Answer 19

the elements that make the reactants and products

Question 20

What is the intermediate between reactants and products in the thermochemical cycle if you are given enthalpy of combustion data?

Answer 20

The oxides of the elements that make up the reactants and products

Question 21

What is a bond enthalpy?

Answer 21

Bond enthalpy is the enthalpy change to break one mole of covalent bonds in the gas phase.

Question 22

How are average bond enthalpies calculated?

Answer 22

The value for the bond enthalpy is an average taken from a range of molecules

Question 23

What can be a problem when using average bond enthalpies?

Answer 23

They are not as accurate as they might be because the values used are averages and not the specific ones for that compound.

Question 24

What phase must compounds be in to use bond energy to calculate ∆H?

Answer 24

They must all be gases

Question 25

If all substances are gaseous, what equation can you use to calculate ∆H?

Answer 25

∆H = (SUM ∆H bonds broken) – (SUM ∆H bonds made)