9 Enthalpy

Question 1

What does ∆H mean? (1 mark)

Answer 1

Heat energy change in constant pressure.

Question 2

What does ∆Hᶱ mean? (1 mark)

Answer 2

Heat energy change in standard conditions.

Question 3

What are the standard conditions? (2 marks)

Answer 3

101kPa and 298K

Question 4

What are the units for enthalpy change? (1 mark)

Answer 4

KJmol-1

Question 5

In an exothermic reaction, are the reactants or products higher? (1 mark)

Answer 5

Reactants

Question 6

What are the two axis on an enthalpy profile diagram? (3 marks)

Answer 6

Enthalpy (KJmol-1)
Reaction progress

Question 7

Define activation energy. (2 marks)

Answer 7

Minimum energy needed to begin breaking bonds and start the chemical reaction.

Question 8

What is the symbol for activation energy? (1 mark)

Answer 8

Ea

Question 9

What is enthalpy of reaction? (2 marks)

Answer 9

∆H when molar amounts of reactants as stated in the equation react under standard conditions.

Question 10

What is the symbol for enthalpy of reaction? (1 mark)

Answer 10

∆rHᶱ

Question 11

What is enthalpy of formation? (2 marks)

Answer 11

∆H when one mole of a substance is formed under standard conditions.

Question 12

What is the symbol for enthalpy of formation? (1 mark)

Answer 12

∆fHᶱ

Question 13

What is enthalpy of combustion? (2 marks)

Answer 13

∆H when one mole of a substance is completely burnt (in oxygen) under standard conditions.

Question 14

What is the symbol for enthalpy of combustion? (1 mark)

Answer 14

∆cHᶱ

Question 15

What is enthalpy of neutralisation? (2 marks)

Answer 15

∆H when one mole of water is formed when an acid and alkali react under standard conditions.

Question 16

What is the symbol for enthalpy of neutralisation? (1 mark)

Answer 16

∆neutHᶱ

Question 17

What is the equation for enthalpy change? (4 marks)

Answer 17

Heat given out = mass of substance heated x specific heat capacity x change in temperature

q= mc∆T

Question 18

What are the units in the equation for enthalpy change? (4 marks)

Answer 18

q- J
m- g
c- Jg-1k-1
T- K or degrees

Question 19

How can you measure ∆H in combustion reactions? (3 marks)

Answer 19

Flammable liquid is burnt inside a flame calorimeter.
The fuel, in a bottle with wick, is burned so that it heats a known mass of water.
The change in temperature of the water is measured.

Question 20

What are 3 possible problems with how we measure ∆H in combustion reactions? ( 3 marks)

Answer 20

Incomplete combustion (less energy given out)
Evaporate
Heat loss to surroundings

Question 21

How can you measure ∆H for reactions in solution? ( 2 marks)

Answer 21

Reaction carried out in an insulated beaker and change in temperature is measured.

Question 22

Name three possible problems with how we measure ∆H for reactions in solutions? (3 marks)

Answer 22

Heat absorbed by container
Heat lost to surroundings
The specific heat capacity is assumed to be 4.185Jg-1k-1

Question 23

What is bond enthalpy? (2 marks)

Answer 23

The energy required to break a covalent bond in the gaseous state. Endothermic. (+)

Question 24

What is average bond enthalpy? ( 2marks)

Answer 24

One mole of covalent bonds is broken in the gas phase averaged over several different conditions.

Question 25

How do you figure average bond enthalpy? ( 2 marks)

Answer 25

∆H of reactants - ∆H of products
or Hess’ cycles

Question 26

What is Hess’ law? (1 mark)

Answer 26

Total enthalpy change for a reaction is independent of the route taken (same conditions).

Question 27

What are the two ‘shortcut’ equations for ∆fHᶱ and ∆cHᶱ? (2 marks)

Answer 27

∆H = ∑∆fH products - ∑∆fH reactants

∆H = ∑∆cH reactants - ∑∆cH products

Question 28

What is the ∆fHᶱ of elements? (1 mark)

Answer 28

0