Energetics-Enthalpy ( AS )

Question 1

What energy change is breaking bonds associated with?

Answer 1

Energy is taken in to break bonds—> Endothermic reaction

Question 2

What energy change is making bonds associated with?

Answer 2

Energy is released to make new bonds—> exothermic reaction

Question 3

What is an endothermic reaction?

Answer 3

One with an overall positive enthalpy change—> energy in breaking bonds>energy out making bonds

Question 4

What is an exothermic reaction?

Answer 4

One with an overall negative enthalpy change—> energy in breaking bonds

Question 5

If a reversible reaction is endothermic one way, what type of reaction is the other way?

Answer 5

Exothermic

Question 6

Give 2 examples of exothermic reactions

Answer 6

Combustion of fuels
Neutralisation

Question 7

Give an example of an endothermic reaction

Answer 7

Thermal decomposition

Question 8

Define enthalpy change

Answer 8

Change in heat energy at a constant pressure

Question 9

What are standard conditions?

Answer 9

100kPa/1atm pressure
298K/25°C
A concentration of 1 mol dm-3

Question 10

What does “in standard state” mean?

Answer 10

The state an element/compound exists at in standard conditions

Question 11

Define standard enthalpy of formation

Answer 11

The enthalpy changes for when one mole of a compound is formed from its constituent elements in their standard state

Question 12

Give an example of an equation that represents standard enthalpy of formation

Answer 12

H2(g)+½O2(g)—>H2O(l)

Question 13

Define standard enthalpy of combustion

Answer 13

The enthalpy change for when one mole of a fuel burns completely in oxygen under standard conditions

Question 14

Give an example of an equation that represents standard enthalpy or combustion

Answer 14

C(s)+O2(g)—>CO2(g)

Question 15

How can you calculate enthalpy change from experimental data?

Answer 15

Using the equation Q=mcΔT
m= mass of substance
c= specific heat capacity
ΔT= change in temperature

Question 16

How would you measure the enthalpy change for a reaction occurring in (aq)?

Answer 16

Use an expanded polystyrene cup as a calorimeter(good insulator—> reduces heat loss). Heat is generated in the solution; measure this temperature change. Take heat capacity of solution to be 4.18 and density of solution=1gcm-3

Question 17

What can you use to make experimental determination changes of a reaction more accurate?

Answer 17

Cooling curves

Question 18

What is Hess’s Law?

Answer 18

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

Question 19

Define bond dissociation enthalpy

Answer 19

The enthalpy change required to break a covalent bond, with all species in the gaseous state

Question 20

Define mean bond enthalpy

Answer 20

Average value for bond dissociation enthalpy of a given bond

Question 21

Why may experimental methods for enthalpy determination not be very accurate?

Answer 21

Heat is lost to the surrounding
Not in standard conditions
Reaction may not go to completion