✅8 - Energetics I

Question 1

What is enthalpy change?

Answer 1

The heat energy change in a reaction at constant pressure

Question 2

What are the units for enthalpy change?

Answer 2

kJmol-1

Question 3

What are the standard conditions?

Answer 3

Elements in their standard states
100kPa
298K

Question 4

What are exothermic reactions?

Answer 4

Give out energy, negative enthalpy change

Question 5

What are endothermic reactions?

Answer 5

Take in energy, positive enthalpy change

Question 6

What do enthalpy level diagrams show?

Answer 6

Overall relative change in a reaction

Question 7

What does an enthalpy level diagram look like for an exothermic reaction?

Answer 7

Reactants higher than products

Question 8

What does an enthalpy level diagram look like for an endothermic reaction?

Answer 8

Products higher than reactants

Question 9

What do reaction profile diagrams show?

Answer 9

Enthalpy changes during a reaction

Question 10

What is the activation energy?

Answer 10

The minimum energy needed for the reaction to occur

Question 11

Why are standard conditions important?

Answer 11

Changes in temperature and pressure can affect enthalpy

Question 12

What is the standard enthalpy change of reaction?

Answer 12

The enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation under standard conditions

Question 13

What is the standard enthalpy change of formation?

Answer 13

The enthalpy change when one mole of a product is formed from its constituent elements in their standard states under standard conditions

Question 14

What is the standard enthalpy change of combustion?

Answer 14

The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions

Question 15

What is the standard enthalpy change of neutralisation?

Answer 15

The enthalpy change when an acid and an alkali react to form one mole of water under standard conditions

Question 16

How would you calculate an enthalpy change?

Answer 16

Mass of solution heated x specific heat capacity of solution x temperature change

Question 17

What is Hess’ Law?

Answer 17

The total enthalpy change of a reaction is always the same, no matter which route is taken

Question 18

What is the enthalpy change of formation for elements?

Answer 18

Zero

Question 19

How can the enthalpy changes of reaction be worked out from enthalpy changes of formation?

Answer 19

Using the formation of the reactants and products, and the enthalpy change of reaction on top

Question 20

How can enthalpy changes be worked out from enthalpy of combustion?

Answer 20

By using the combustion of both reactants and products and putting the unknown on top

Question 21

How can indirect enthalpy changes be found?

Answer 21

By doing the required experiments and then using Hess’ Law

Question 22

Bond breaking is…

Answer 22

…endothermic

Question 23

Bond making is…

Answer 23

…exothermic

Question 24

What is bond enthalpy?

Answer 24

The energy needed to break a mole of a type of bond in a molecule in the gas phase

Question 25

How are mean bond enthalpies calculated?

Answer 25

By taking an average of all the different values fr a bond type

Question 26

How can enthalpy changes be worked out from bond enthalpies?

Answer 26

Sum of bond enthalpies of reactants - sum of bond enthalpies of proudcts