1.4 Energetics

Question 1

Endothermic reaction - definition and what happens

Answer 1

Energy moves from thermal store of surroudings into chemical energy store of bonds. Temperature decrease. Enthalpy change is positive.

Question 2

Exothermic reaction - definition and what happens

Answer 2

Energy moves from chemical store of bonds to thermal, stores of surroundings. Temperature increases. Enthalpy change is negative

Question 3

Out of bonds breaking and bonds making, which is endothermic and which is exothermic?

Answer 3

Bond breaking - endothermic as energy must be put into breaking bonds
Bond making - exothermic as energetic is released

Question 4

What is enthalpy change?

Answer 4

Change in heat at a constant pressure. Measured in KJ/molm

Question 5

Definition of standard enthalpy change of a reaction

Answer 5

Enthalpy change when the reaction occurs, in molar quantities, shown in chemical equation, under standard conditions, in standard states.

Question 6

Definition of standard enthalpy change of formation

Answer 6

Enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states, under standard conditions.

Question 7

Definition of standard enthalpy change of combustion

Answer 7

Enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions.

Question 8

Definition of standard enthalpy change of neutralisation

Answer 8

Enthalpy change when 1 mole of water is formed from neutralisation of OH- ions with H+ ions under standard conditions

Question 9

Standard enthalpy of atomisation

Answer 9

Enthalpy change when 1 mole of gaseous atoms is formed from elements in its standard state under standard conditions.

Question 10

What are the standard conditions

Answer 10

20-25 degrees and 1 atmospheres of pressure or 100KPa

Question 11

Why can reactions be described as happening at a standard temperature despite the temperature change

Answer 11

The reaction is not thought to be over till the products return to original temperature.

Question 12

What 2 equations are required for enthalpy change?

Answer 12

Change in energy = mass(g) x SHC x change in temperature
Enthalpy change = change in energy / moles

Question 13

What is a calorimeter

Answer 13

Apparatus used to measure heat or enthalpy change

Question 14

What rules are there when calculating Enthalpy change from a calorimeter

Answer 14

• only measure mass of the liquid. If liquid + solid, measure only liquid. If liquid + liquid add tougher.
• assume density of liquid is same as water so 1cm^3 = 1g
• assume no heat is lost to surroundings

Question 15

How do you calculate enthalpy change from a calorimeter when measuring enthalpy change of a fuel?

Answer 15

• weigh water and measure the temperature of change while it is being heated
• calculate change in energy of water
• measure the mass of fuel burned for the specific temperature rise then calculate moles used based on Mr.
• calc Enthalpy change : change in energy of water / moles of fuel burnt

Question 16

What is Hess’s law

Answer 16

The Enthalpy change for a reaction is the same whatever route is take from reactants to products

Question 17

What is bond disassociation enthalpy?

Answer 17

Enthalpy change to break/make the bonds of 1 mole of a compound in its gaseous state

Question 18

What’s more reliable for a specific compound: bond enthalpy or Enthalpy of formation?

Answer 18

Enthalpy of formation because bond enthalpy is the mean enthalpy change across all bonds. Whereas Enthalpy of formation is for that specific compound

Question 19

What’s the difference in drawing Hess cycles with bond enthalpy and any other Enthalpy change?

Answer 19

The elements at the bottom need to be in their gaseous state instead of standard
E.g. 4H instead of 2H2