3.3 Enthalpy

Question 1

What is enthalpy ?

Answer 1

Enthalpy is a measure of the heat energy in a chemical system.

Question 2

Enthalpy change

Answer 2

^H = H(products) - H(reactants)
Can be positive or negative depending if reaction is endothermic or exothermic.

Question 3

Exothermic

Answer 3

HEAT LOST TO SURROUNDINGS.
^H is negative
Reactants are higher than products.

Question 4

Endothermic

Answer 4

HEAT INTAKE FROM SURROUNDINGS.
^H is positive.
Reactants are lower than products.

Question 5

Activation Energy

Answer 5

Activation energy is the energy required to break bonds for a reaction to take place.

Question 6

What is the standard enthalpy change of reaction ?

Answer 6

The enthalpy change in a reaction on the molar quantities shown in chemical equation under standard conditions, with all reactants and products in standard state.

Question 7

What is the standard enthalpy change of formation ?

Answer 7

Then enthalpy change when 1 mole of a compound is formed from the elements under standard conditions, with all reactants and products in their standard states.

Question 8

What is the standard enthalpy change of combustion ?

Answer 8

The enthalpy when 1 mole of a substance reacts completely with oxygen under standard conditions, will all reactants and products in their standard states.

Question 9

What is the standard enthalpy change of neutralisation ?

Answer 9

The enthalpy that accompanies reaction of an acid and base to form 1 mole of H2O under standard conditions, with all reactants and products in their standard states.

Question 10

How to find temperature change ?

Answer 10

^T = T(final) - T (initial)

Question 11

How to calculate energy change (q) ?

Answer 11

q = m c ^T
-q= heat energy /J
- m = mass of surroundings that changes temperature /g
- c = specific heat capacity ( 4.18 KJ mol -1)
-^T = temperature change of surroundings

Question 12

What is the specific heat capacity ?

Answer 12

Energy requires to raise temperature of 1g of substance by 1K / 1*C

Question 13

How to work out standard enthalpy change ?

Answer 13

q/n
-q= heat energy /J
-n= number of moles of limiting reactant.

Question 14

What is the average bond enthalpy ?

Answer 14

Is the energy required to break 1 mole of a specified covalent bond (by homolytic fission) in a gaseous molecule.

Question 15

What do bond enthalpies always have ?

Answer 15

Always have a positive enthalpy value and are always endothermic.

Question 16

What energy is greater in bond enthalpies in an endothermic reaction ?

Answer 16

Energy needed to break bonds is greater than energy need to make bonds.

Question 17

What energy is greater in terms of bond enthalpies in exothermic reactions ?

Answer 17

Energy needed to make bonds is greater than energy needed to brake bonds.

Question 18

How to work out standard enthalpy change of a reaction ?

Answer 18

(Sum of enthalpy of reactants) - (sum of enthalpy of products)

Break-Make

Question 19

What are some limitations of average bond enthalpies ?

Answer 19

-All molecules must be gaseous in both reactants and products.
-Uses average bond enthalpy.

Question 20

What is Hess’ Law ?

Answer 20

For a chemical reaction, the enthalpy change is independent of the route taken, provided the initial and final states remain the same.

Question 21

What is an allotrope ?

Answer 21

Different form of the same element.

Question 22

What is indirect determination of enthalpy changes ?

Answer 22

Finding enthalpy change by using 2 common reactions, with same products and finding enthalpy of each reaction to find overall enthalpy indirectly.

Question 23

What are 2 methods of Hess’ Law?

Answer 23

1) Use 2 equations and rearrange to find enthalpy.
2) Use energy profile t find enthalpy change indirectly.