Enthalpy changes

Question 1

Draw a general exothermic energy profile

Answer 1

*Refer to new enthalpy changes notes

Question 2

Draw a general endothermic energy profile

Answer 2

*Refer to new enthalpy changes notes

Question 3

Definition of standard states

Answer 3

The physical state of a substance under standard conditions.

Question 4

What are the standard conditions?

Answer 4

298 K (25 degrees)
100 kPa (1 atmosphere)
1 moldm^-3 for concentration of solutions.

Question 5

Definition of enthalpy change of reaction

Answer 5

The enthalpy change associated with a stated equation (in their standard states under strandard conditions).

Question 6

Definition of the enthalpy chage of formation

Answer 6

The enthalpy change when one mole of a compound is made from its elements (in their standard states under standard condition).

Question 7

Definition of the enthalpy change of combustion

Answer 7

The enthalpy change whne one mole of a substance is completely combusted with oxygen (in their standard states under standard conditions).

Question 8

Definition of the enthalpy change of neutralisation

Answer 8

The enthalpy change when an aqueous acid reacts with an aqueous base forming one mole of water (in their standard states under standard conditions).

Question 9

2 equations…

Answer 9

Q = -mc(change in temp)
Enthalpy change = Q / n
*Refer to new enthalpy changes notes

Question 10

Why are the values calculated experimentally less than those determined theorteically and where doe these occur?

Answer 10

1. Heat loss
2. Incomplete conbustion
3. Non-standard conditions
   Through the material of the container / through the open top of the conatiner.

Question 11

Ways to minmise heat loss from an experiment

Answer 11

1. Use and insulating lid.
2. Add a lid.

Question 12

Which part of a reaction is endothermic?

Answer 12

When bonds are broken as energy is absorbed from the surroundings into the reaction.

Question 13

Which part of a reaction is exothermic?

Answer 13

When the bonds are made as energy is released from the reaction into the surroundings.

Question 14

Explain an endothermic reaction in terms of making and breaking bonds

Answer 14

If more energy is required to break the bonds in the reactants than is released when the bonds in the products are made, then the overall energy change of the reaction is endothermic. Enthalpy change is always positive.

Question 15

Explain an exothermic reaction in terms of making and breaking bonds

Answer 15

If more energy is released when bonds are made in the products than was required to break bonds in the reactants, then the overall energy change of the reaction is exothermic. Enthalpy change is always negative.

Question 16

Definition of average bond enthalpy

Answer 16

The average bond enthalpy change when breaking one mole of a given type of bond in the molecules of a gaseous species.

Question 17

Calculating enthalpy change using average bond enthalpies equation

Answer 17

Enthalpy change = total of bonds broken in the reactants (endothermic: +ve) — total of bonds made (exothermic: -ve)

Question 18

For Hess’s law: if given combustion data in the question draw arrows going…

Answer 18

…downwards from the reactants and products, to the products of combustion.

Question 19

For Hess’s law: if given formation data in the question, draw arrows going…

Answer 19

…upwards to the reactants and products from the elements that makes them.

Question 20

Definition of Hess’s law

Answer 20

The enthalpy change of a reaction is independent of the route taken providing the initial and final conditions are the same.