2.1 Thermochemistry

Question 1

What are standard conditions

Answer 1

• 298K
• 101kPa
• 1moldm3

Question 2

Delta H equation

Answer 2

-mc deltaT
—————
n

Question 3

Define enthalpy change of formation

Answer 3

The enthalpy change when one mole of a substance is formed from its constituent elements under standard cknditioms

Question 4

Enthalpy change of combustion

Answer 4

The enthalpy change when one mole of a substance is completely combusted under standard conditions

Question 5

What does θ represent

Answer 5

Standard conditions

Question 6

Define Hess’ law

Answer 6

No matter the route of a reaction, the overall enthalpy change will remain the same

Question 7

Enthalpy change is

Answer 7

sum of anti-clockwise arrows - sum of clockwise arrows

Question 8

Positive value is …

Answer 8

Endothermic

Question 9

Negative value is …

Answer 9

Exothermic

Question 10

Which way do arrows go when you have combustion data?

Answer 10

Toward element, away from equation

Question 11

Where do arrows go when you have formation data

Answer 11

Toward equation, away from elements

Question 12

How do literature enthalpy change values differ from experiment?

Answer 12

• lit are numerically larger
• due to there being heat loss and/or incomplete combustion in experiment

Question 13

In terms of bond strengths, explain why enthalpy changes of combustion are negative

Answer 13

-ve enthalpy means energy in bonds broken is less than energy in bonds made

Question 14

Explain why the enthalpy change of combustion of propanol is more negative than that of ethanol

Answer 14

More binds broken and made in propanol and therefore more energy is released

Question 15

Economic and environmental factors of gasification and catalysed reactions to make glucose to ethanol

Answer 15

Both conserve carbon, but use non-renewable fuel sources
Gasification: lower atom economy, CO is toxic and CO2 contribute to climate change. 700°C require a lot of energy
Ethanol: higher atom economy, ethanol can be used in cars, no harm to enviro, catalyst lowers AE so cheaper

Question 16

Why is it important that HCL and NaOH is at the same temp

Answer 16

Otherwise a temp change would occur when adding the acid which would not be related to the reaction, confounding results

Question 17

State what is meant by molar mass

Answer 17

Mass of one mole of a compound

Question 18

Why is the standard enthalpy of formation of both hydrogen and nitrogen gases 0?

Answer 18

High elements are in their standard states

Question 19

Give a reason why the sign of enthalpy value calculated is different from the sign of the temperature change measured

Answer 19

Enthalpy measures chemical energy and so as heat energy increases, chemical energy must decrease

Question 20

Explain what is meant by bond enthalpy

Answer 20

Energy required to break bonds / energy released by making 1 mole of bonds

Question 21

In order to measure enthalpy change, the reaction must …

Answer 21

Involve a solution

Question 22

Why can MgCl2 not be used to get enthalpy change

Answer 22

No solution present as forms a solid

Question 23

Why are bond enthalpies described as average

Answer 23

Each individual bond will be in a different environment and therefore have different strengths

Question 24

Why is the boiling temp of methanol higher than chloromethane

Answer 24

• methanol has hydrogen bonding between molecules
• chloromethane has VdW between molecules
• hydrogen bonding stronger than VdW