Chapter 9 - Enthalpy Changes

Question 1

Define average bond enthalpy

Answer 1

energy required to break one mole of a specified type of bond in a gaseous molecule.

Question 2

What are the limitations of bond enthalpy

Answer 2

• Actual bond enthalpy can vary depending on the chemical environment of the bond

• Average bond enthalpy is calculated from the actual bond enthalpies in different chemical environments.

Question 3

What is bond breaking

Answer 3

Endothermic

Question 4

What is the H value for endothermic

Answer 4

ΔH is positive

Question 5

What is bond making

Answer 5

Exothermic

Question 6

What is the H value for Exothermic

Answer 6

ΔH is negative

Question 7

Formula for ΔH

Answer 7

ΔH = Σ(bond enthalpies in reactants) – Σ(bond enthalpies in products)

Question 8

What happens to the temp of the surroundings in an Exothermic reaction

Answer 8

Increases

Question 9

What happens to the temp of the surroundings in an endothermic reaction

Answer 9

Decreases

Question 10

What does this mean ΔH⦵

Answer 10

Standard conditions

Question 11

What are the standard conditions

Answer 11

• Pressure: 100 kPa
• Temperature: 298 K
• Concentration: 1 mol dm-3
• State: the physical state under standard conditions

Question 12

Define enthalpy change of reaction

Answer 12

• 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 13

Define enthalpy change of formation

Answer 13

• 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 14

Define enthalpy change of combustion

Answer 14

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

Question 15

Define enthalpy change of neutralisation

Answer 15

• enthalpy 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 16

Formula for calorimetry / heat energy released

Answer 16

q = mcΔT

Question 17

What is q in q = mcΔT

Answer 17

heat energy – J

Question 18

What is m in q = mcΔT

Answer 18

mass of SURROUNDINGS (material changing temp) – g

Question 19

What is c in q = mcΔT

Answer 19

specific heat capacity = water = 4.18 J/gK

Question 20

What is T in q = mcΔT

Answer 20

• Temp change = ΔT = T(final) – T(initial)

Question 21

Accuracy of experimental ΔCH?

Answer 21

Experimental values are LESS exothermic than the data values

Question 22

Why are experimental values are LESS exothermic than the data values

Answer 22

• Heat is lost to surroundings other than water
• Incomplete combustion may occur
• May have evaporation of the fuel from the wick
• Non-standard conditions

Question 23

How can a simply calorimeter be made

Answer 23

from a polystyrene drinking cup, a vacuum flask or metal can

Question 24

Symbol for standard enthalpy change of reaction

Answer 24

Question 25

Symbol for standard enthalpy change of formation

Answer 25

Question 26

Symbol for standard enthalpy change of combustion

Answer 26

Question 27

Symbol for standard enthalpy change of neutralisation

Answer 27

Question 28

Define Hess’ law

Answer 28

states that the overall enthalpy change for a reaction is independent of the route the reaction takes

Question 29

How to draw Enthalpy Cycles – Using ΔfH

Answer 29

1. Write the balanced equation for the reaction at the top 2. Write the balanced elements at the bottom 3. Draw arrows up to the reactants/ products. 4. Add in any ΔH values you know. 5. Use Hess’ Law to calculate any known values.

Question 30

Using vectors, what’s the equation for the Enthalpy Cycles – Using ΔfH

Answer 30

ΔrH = ΣΔfH(products) – ΣΔfH(reactants) A = - b + c A = c - b

Question 31

What’s m in q=mc^T

Answer 31

MASS OF MATERIAL CHANGING TEMP

Question 32

What moles should you calculate in a ΔrH⦵ question when reactants have different number of moles?

Answer 32

Either is fine, but you must use the correct molar ratio Example: if one mole of X reacts with 2 moles of Y: 1X + 2Y → ..... - If you have information for X, work out as normal - If you have information for Y, find n(Y), then you must: - Work as normal finding the enthalpy change per mole of Y - At the end, multiply enthalpy change per mole of Y by number of moles