C9 - Enthalpy changes

Question 1

What is enthalpy?

Answer 1

Heat content stored in a chemical system
(H)

Question 2

What is a system?

Answer 2

Chemicals involved in reaction

Question 3

What is enthalpy change?

Answer 3

Difference between enthalpy of products and enthalpy of reactants
(Delta H)

Question 4

Formula for enthalpy change?

Answer 4

Delta H = H (products) - H (reactants)

Question 5

What is the law of conservation of energy?

Answer 5

Energy cannot be created or destroyed

Question 6

What are the surroundings?

Answer 6

Everything that is not the chemical system

Question 7

What is an exothermic reaction?

Answer 7

-Enthalpy of products is smaller than enthalpy of reactants
-Heat loss to surroundings
-Temperature increase
-Delta H negative

Question 8

What is an endothermic reaction

Answer 8

-Enthalpy of products is greater than enthalpy of reactants
-Heat taken in from surroundings
-Temperature decrease
-Delta H positive

Question 9

What is activation energy?

Answer 9

Minimum energy required to start a reaction by breaking bonds
(Ea)

Question 10

Standard condition sign

Answer 10

°/ superscript Plimsoll symbol

Question 11

What is standard pressure?

Answer 11

100kPa

Question 12

What is standard temperature?

Answer 12

298K/25°C

Question 13

What is standard concentration?

Answer 13

1 mol/dm^3

Question 14

What is standard state?

Answer 14

Physical state of a substance under standard conditions
(100kPa and 298K)

Question 15

Units of ethalpy change?

Answer 15

kJ/mol

Question 16

What is the standard enthalpy change of reaction?

Answer 16

Enthalpy change that accompanies a reaction, in the molar quantities, expressed in a chemical equation, under standard conditions
All reactants and products being in their standard forms
Delta(r)H°)

Question 17

What is standard enthalpy change of formation?

Answer 17

Enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions
Delta(f)H°

Question 18

Standard enthalpy change of formation for elements

Answer 18

Enthalpy change of formation of 0kJ/mol

Question 19

What is standard enthalpy change of combustion?

Answer 19

Enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standards conditions
All reactants and products being in their standard states
Delta(c)H°

Question 20

What is the standard enthalpy change of neutralisation?

Answer 20

Enthalpy change that accompanies the reaction of an acid by a base to form one mole of H2O(l), under standard conditions
All reactants and products in their standard states
Delta(neut)H°

Question 21

Degrees to Kelvin

Answer 21

+273

Question 22

Absolute 0 in degrees

Answer 22

-273°C

Question 23

Energy change formula

Answer 23

q = m c deltaT
heat energy (J) = mass (g) x SHC (J/g/K) x change in temperature (°C or K)

Question 24

What is specific heat capacity?

Answer 24

Energy required to raise the temperature of 1g of a substance by 1°C
(c)

Question 25

How to determine enthalpy change if combustion?

Answer 25

1) Calculate energy change (q) in kJ using q=mc(deltaT)
2) Calculate amount (moles) of substance burnt
3) Divide energy change by moles to get the value in kJ/mol

Question 26

Why is the experimental value of enthalpy change of combustion less exothermic?

Answer 26

-Heat loss to surroundings other than water (air)
-Incomplete combustion (CO/C produced)
-Evaporation of substance

Question 27

Why is the experimental value of enthalpy change of combustion more exothermic?

Answer 27

-Non standard conditions

Question 28

How to determine enthalpy change of a reaction for a solid and solution?

Answer 28

1) Calculate energy change (q) in the solution in kJ
2) Calculate amount (in mol) of substance that reacted n=cV
3) Calculate enthalpy change in kJ/mol

Question 29

How to determine enthalpy change of neutralisation?

Answer 29

1) Calculate energy change (q) in the solution in kJ (use total volume of reaction mixture for mass)
2) Calculate amount (in mol) of substance that reacted n=cV (and water?)
3) Calculate enthalpy change in kJ/mol

Question 30

What is an average bond enthalpy?

Answer 30

Average enthalpy change that takes place when breaking 1 mol of a given type of bond (by homolytic fission) in the molecules of a gaseous species

Question 31

What are the limitations of average bond enthalpies?

Answer 31

Actual bond enthalpies vary depending on chemical environment of bond
Calculated by averaging actual bond enthalpies

Question 32

Energy for bond making

Answer 32

Energy released when bonds formed
Exothermic
Delta H negative

Question 33

Energy for bond breaking

Answer 33

Energy required to break bonds
Endothermic
Delta H positive

Question 34

Formula for enthalpy change of a reaction of gaseous molecules of covalent substances

Answer 34

Delta(r)H = Sum of bond enthalpies in reactions - sum of bond enthalpies in products

Question 35

Calculating enthalpy changes from average bond enthalpies

Answer 35

Question 36

What is Hess’ law?

Answer 36

If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route

Question 37

What is an enthalpy cycle?

Answer 37

Diagram showing alternative routes between reactants and products which allows the indirect determination of an enthalpy change from other known enthalpy changes using Hess’s law

Question 38

Enthalpy change of combustion formula

Answer 38

reactants - products

Question 39

Enthalpy change of formation formula

Answer 39

products - reactants