M3,S2 Enthalpy Changes

Question 1

Definition of enthalpy change?

Answer 1

Enthalpy change, ∆H (delta H) is the energy transferred in a reaction at constant pressure. The units of ∆H are kJ mol^-1.

Question 2

What causes the change in energy in a chemical reaction?

Answer 2

in a chemical reaction some bonds are broken and some bonds are made causing a change in energy.

Question 3

What doe this symbol mean: ∆Hᶿ ?

Answer 3

It shows all measurements are made under standard conditions and that the elements were their in standard states (i.e. their physical states under standard conditions).

Question 4

What are the standard conditions for enthalpy?

Answer 4

100kPa (about 1 atm) and 298K (25°C)

Question 5

What you need to remember about exothermic reactions (referring to enthalpy)?

give an example of an exothermic reaction.

Answer 5

Question 6

What do you need to remember about endothermic reactions (referring to enthalpy)?

Give an example of n endothermic reaction.

Answer 6

Question 7

What are enthalpy diagrams used to show?

What does an exo. and endothermic reaction profile look like?

Answer 7

Enthalpy profile diagrams show you how the enthalpy (energy) changes during reactions.

Question 8

What is the activation energy of a reaction?

Answer 8

The activation energy, E_a, is the minimum amount of energy needed to begin breaking reactant bonds and start a chemical reaction.

Question 9

What do you need to remember about the enthalpy of a substance?

Answer 9

The less enthalpy a substance has, the more stable it is.

Question 10

Why doesn’t it matter that we can’t directly measure the enthalpy of a system?

Answer 10

It is only ever the enthalpy change you work with.

Question 11

Why do you need to specify the conditions for enthalpy changes?

What are standard enthalpy changes and what do they allow us to do?

Answer 11

Because changes in enthalpy are affected by temperature and pressure.

Enthalpy changes found in data books are usually standard enthalpy changes (changes under the standard conditions: 298k and 100kPa).

Using standard conditions means everyone can know exactly what the enthalpy change is describing.

Question 12

What are the different types of enthalpy change, ∆

What are the different types of ∆H?

Answer 12

There are different types of ∆H (energy change) depending on the reaction.