Chapter 9

Question 1

What is enthalpy H?

Answer 1

a measure of the heat energy in a chemical system

Question 2

What does the chemical system refer to?

Answer 2

the atoms, molecules, or ions making up the chemicals

Question 3

Can enthalpy be measured?

Answer 3

no

Question 4

Can enthalpy changes be measured?

Answer 4

yes

Question 5

What is the enthalpy change ΔH?

Answer 5

the difference in enthalpies

Question 6

ΔH =

Answer 6

H(products) - H(reactants)

Question 7

Is ΔH positive or negative?

Answer 7

it can be either

Question 8

What does the value of ΔH depend on?

Answer 8

whether the products contain more or less energy than the reactants

Question 9

What is the law of conservation of energy?

Answer 9

energy cannot be created or destroyed

Question 10

In a chemical reaction involving an enthalpy change, where is heat energy transferred between?

Answer 10

the system and the surroundings

Question 11

What is the system?

Answer 11

the chemicals - the reactants and products

Question 12

What are the surroundings?

Answer 12

the apparatus, the laboratory, and everything that is not the chemical; system

Question 13

What is the universe?

Answer 13

everything, including the system and surroundings

Question 14

Can an enthalpy change ΔH be determined experimentally?

Answer 14

yes

Question 15

How can an enthalpy change ΔH be determined experimentally?

Answer 15

by measuring the energy transfer between the sustem and the surroundings

Question 16

What two directions can energy transfer be?

Answer 16

1) exothermic - from the system to the surroundings

2) endothermic - from the surroundings to the system

Question 17

What do enthalpy profile diagrams show?

Answer 17

1) the relative enthalpies of the reactants and products

2) enthalpy chaneg ΔH

Question 18

What are the characteristics of an exothermic energy change?

Answer 18

1) chemical system releases heat energy to surroundings
2) energy loss by chemical system is balanced by same energy gain in surroundings
3) ΔH is negative
4) temperature of the surroundings increase as they gain energy

Question 19

What are the characteristics of an exothermic energy change?

Answer 19

1) chemical system takes in heat energy from surroundings
2) energy gain in chemical system is balanced by same energy loss by surroundings
3) ΔH is positive
4) temperature of surroundings decreases as they lose energy

Question 20

What are atoms and ions held together by?

Answer 20

chemical bonds

Question 21

What are the bonds in chemical reactions broken by?

Answer 21

an input of energy

Question 22

What is activation energy Eₐ?

Answer 22

the minimum energy required for a reaction to take place

Question 23

Why do reactions with small activation energies take place very quickly?

Answer 23

the energy needed to break bonds is readily available from the surroundings

Question 24

What does a standard enthalpy change ΔH⦵ refer to?

Answer 24

an enthalpy H change Δ under standard conditions ⦵

Question 25

What are the units of enthalpy change?

Answer 25

kJmol⁻¹

Question 26

What is the standard pressure?

Answer 26

100 kPa

Question 27

What is the standard temperature?

Answer 27

298K (25°C)

Question 28

What is the standard concentration?

Answer 28

1 mol dm⁻³

Question 29

What is the standard state?

Answer 29

the physical state of a substance under standard conditions

Question 30

What is the standard enthalpy change of reaction ΔᵣH⦵?

Answer 30

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 31

What is the standard enthalpy change of formation ΔfH⦵?

Answer 31

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

Question 32

What is the standard enthalpy change of combustion ΔcH⦵?

Answer 32

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 33

What is the standard enthalpy change of formation ΔneutH⦵?

Answer 33

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

Question 34

What three quantities are used to calculate the energy change of the surroundings?

Answer 34

1) mass 2) specific heat capacity 3) temperature change

Question 35

What is mass measured in?

Answer 35

grams (g)

Question 36

What is specific heat capacity?

Answer 36

the energy required to raise the temperature of 1g of a substance by 1K

Question 37

What is the specific heat capacity of water?

Answer 37

4.18 J g⁻¹K⁻¹

Question 38

How is the temperature change ΔT determined?

Answer 38

T(final) - T(initial)

Question 39

What is q?

Answer 39

heat energy

Question 40

q =

Answer 40

mcΔT