Enthalpy changes

Question 1

What is enthalpy?

Answer 1

A measure of the heat energy in a chemical system

Question 2

What does chemical system refer to?

Answer 2

The atoms, molecules, or ions making up chemicals

Question 3

Can enthalpy be measured?

Answer 3

No

Question 4

Can enthalpy changes be measured?

Answer 4

Yes

Question 5

What is enthalpy change?

Answer 5

The difference in the enthalpies of the reactants and products

Question 6

What is the formula for enthalpy change?

Answer 6

▲H = H (products) - H (reactants)

Question 7

What determines whether the ▲H is positive or negative?

Answer 7

Whether the products have more or less energy than the reactants

Question 8

What does the law of conservation of energy state?

Answer 8

Energy cannot be created or destroyed

Question 9

What happens when there is a chemical reaction involving enthalpy?

Answer 9

Heat energy is transferred between the system and the surroundings

Question 10

What is the system?

Answer 10

The chemicals - reactants and products

Question 11

What are the surroundings?

Answer 11

The apparatus, laboratory, and everything else not in the system

Question 12

Why is the universe everything?

Answer 12

It includes both system and surroundings

Question 13

How can ▲H be determined experimentally?

Answer 13

By measuring the transfer between the system and surroundings

Question 14

What are the two types of energy transfer?

Answer 14

Exothermic and endothermic

Question 15

What is exothermic transfer?

Answer 15

Energy is transferred from the system to the surroundings

Question 16

What is endothermic transfer?

Answer 16

Energy is transferred from the surroundings to the system

Question 17

What do enthalpy profile diagrams show?

Answer 17

The relative enthalpies of the reactants and products and the enthalpy change ▲H

Question 18

What happens during an exothermic reaction?

Answer 18

The chemical system releases energy to the surroundings, increasing the temperature of the surroundings as they gain energy

Question 19

What is the ▲H in an exothermic reaction?

Answer 19

Negative

Question 20

What happens during an endothermic reaction?

Answer 20

The chemical system takes in heat energy from the surroundings, decreasing the temperature of the surroundings as they lose energy

Question 21

What is the ▲H in an endothermic reaction?

Answer 21

Positive

Question 22

What happens during a chemical reaction?

Answer 22

Bonds in the reactants are broken by an input of energy and new bonds in the products form to complete the reaction

Question 23

What is activation energy Ea?

Answer 23

The minimum amount of energy required for a reaction to take place

Question 24

Why do reactions with small activation energies take place rapidly?

Answer 24

Energy needed to break bonds is readily available from the surroundings

Question 25

Why do reactions with large activation energies take place extremely slowly or not at all?

Answer 25

Very large activation energies present a very large energy barrier

Question 26

How is a standard physical value shown in data tables?

Answer 26

Using a special standard sign ⦵

Question 27

What does a standard enthalpy change ▲H⦵ refer to?

Answer 27

An enthalpy change under standard conditions

Question 28

What are the units for standard enthalpy change?

Answer 28

KJ mol -1

Question 29

What does mol -1 refer to?

Answer 29

The amount in mol given by the balancing numbers of the chemicals in a stated equation for the reaction

Question 30

What is standard pressure?

Answer 30

100 kPa

Question 31

What is standard temperature?

Answer 31

Usually 298K (25 oc)

Question 32

What is standard concentration?

Answer 32

1 mol dm-3

Question 33

What is standard state?

Answer 33

The physical state of a substance under standard conditions

Question 34

What is the formula for standard enthalpy change of reaction?

Answer 34

▲rH ⦵

Question 35

What is standard enthalpy change of reaction?

Answer 35

The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states

Question 36

What happens when the equation is balanced with whole numbers?

Answer 36

The amounts are doubled and the enthalpy change is doubled

Question 37

What is the formula for standard enthalpy change of formation?

Answer 37

▲fH ⦵

Question 38

What is standard enthalpy change of formation?

Answer 38

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states

Question 39

What does ▲fH ⦵ for an element refer to?

Answer 39

The formation of one mole of an element from its element

Question 40

What is the formula for standard enthalpy change of combustion?

Answer 40

▲cH ⦵

Question 41

What is standard enthalpy change of combustion?

Answer 41

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states

Question 42

What do products become when an element reacts completely with oxygen?

Answer 42

Oxides of the elements in the substance

Question 43

What is the formula of standard enthalpy change of neutralisation?

Answer 43

▲neutH ⦵

Question 44

What is standard enthalpy change of neutralisation?

Answer 44

The energy change that accompanies the reaction of an acid by a base to form one mole of H2O (l), under standard conditions, with all reactants and products in their standard states

Question 45

Why is the value of ▲neutH ⦵ the same for all neutralisation reactions?

Answer 45

It always involves the same reaction of H+ (aq) with OH- (aq) to form one mole of H2O (l)