Topic 8- Chemical Energetics

Question 1

Definition for the standard enthalpy change of a reaction (r)

Answer 1

The enthalpy change measured at 100kPa and a stated temperature (usually 298k) when the number of moles of substances in the equation as written react

Question 2

What does system mean in a chemical reaction?

Answer 2

the atoms and bonds involved in a chemical reaction

Question 3

Explain the law of conservation

Answer 3

the amount of energy in an isolated system remains the same. energy cannot be destroyed or created, it can only be transferred from one to the another

Question 4

What energy change is breaking bonds associated with?

Answer 4

endothermic reaction –> energy is taken in to break bonds

Question 5

What energy change is making bonds associated with?

Answer 5

exothermic reaction –> energy is released to make bonds

Question 6

What is an endothermic reaction?

Answer 6

a reaction with an overall positive energy change (enthalpy of products is greater than enthalpy of reactants)

Question 7

What is an exothermic reaction?

Answer 7

a reaction with an overall negative enthalpy change (enthalpy of products is lower than the reactants)

Question 8

What does activation energy mean?

Answer 8

the minimum energy required for a reaction to take place

Question 9

Which way does the activation energy arrow point on an enthalpy profile diagram?

Answer 9

always points upwards

Question 10

What are the standard conditions?

Answer 10

100kpa

298k

Question 11

What does ‘in standard state’ mean?

Answer 11

the state an element/compound exists in standard conditions

Question 12

Define enthalpy change of formation (f)

Answer 12

the energy change that takes place when one mole of a compound is formed from its constituent elements in their standard state under standard conditions

Question 13

Define enthalpy change of combustion (c)

Answer 13

the energy change that takes place when one mole of a substance is completely burned in oxygen

Question 14

Define enthalpy change of neutralisation

Answer 14

the energy change that takes place when one mole of water is formed from a neutralisation reaction

Question 15

Give examples of enthalpy change of formation and combustion

Answer 15

formation: H2 (g) + 0.5O2 (g) —> H2O (l)
combustion: C(s) + O2(g) —> CO2 (g)

Question 16

How can you calculate enthalpy change from experimental data?

Answer 16

Q=mc/\T
m=mass of substance heated (usually water)
c=specific heat capacity of water
/=temperature change

Question 17

What does a simple calorimeter include?

Answer 17

• thermometer
• beaker with known mass of water
• gauze
• tripod
• spirit burner containing substance to be tested
• heatproof mat

Question 18

Why might experimental methods for enthalpy determination not be accurate?

Answer 18

• heat lost to surroundings
• not in standard conditions
• reaction not gone to completion

Question 19

What does average bond enthalpy mean?

Answer 19

mean energy required to break 1 mole of bonds in gaseous molecules

Question 20

Why will using bond enthalpies not be as accurate as using standard enthalpy of formation/combustion?

Answer 20

bond enthalpies are a mean for the same bond across different molecules whereas standard enthalpy of combustion and formation apply just to that molecule- therefore more accurate

Question 21

How do you calculate enthalpy change using just bond enthalpies?

Answer 21

sum of (bond enthalpies of reactants)- sum of (bond enthalpy of products)

Question 22

Give examples of exothermic processes

Answer 22

• freezing water
• condensing water vapour
• dissolving sodium hydroxide in water
• dilute HCL + NaOH (aq)
• combustion of petrol

Question 23

Give examples of endothermic processes

Answer 23

• melting ice
• evaporating water
• dissolving ammonium nitrate in water
• dilute ethanoic acid + solid sodium hydrogencarbonate
• photosynthesis

Question 24

Give the method for the experimental way to determine the enthalpy change of combustion of a substance

Answer 24

1) a spirit burner containing the liquid under test is weighed
2) a known volume of water is added to a copper can
3) the temperature of the water is measured
4) the burner is lit
5) the mixture is constantly stirred with the thermometer
6) when the temperature of the water has reached approximately 20 degrees above its initial temperature, the flame is extinguished and the burner is immediately reweighed
7) the final temperature is measured

Question 25

How do you calculate the enthalpy change of combustion in 3 stages?

Answer 25

stage 1: calculate the heat energy transferred to the water (Q) Q=mc x temp change
stage 2: Calculate the moles of substance burned (n) n=m/Mr
stage 3: calculate the enthalpy change of combustion
enthalpy change= -Q/n

Question 26

State the sources of error and assumptions made in the combustion experiments

Answer 26

• some of the heat energy is transferred to the air, not the water
• some of the substance may not burned completely and so incomplete combustion occurs where soot forms at the bottom of the can
• some of the heat energy is transferred to the copper can and not the water
• the conditions aren’t standard e.g. water vapour instead of liquid
• experiment takes a lot of time so not all the heat energy transferred from the water to air is accounted for

Question 27

How do you calculate the enthalpy change?

Answer 27

enthalpy of products - enthalpy of reactants

Question 28

What sign does an endothermic reaction have?

Answer 28

positive because enthalpy of products are higher than reactants

Question 29

What sign does an exothermic reaction have?

Answer 29

negative because enthalpy of products is less than reactants

Question 30

What are the points to remember when drawing an enthalpy level diagram?

Answer 30

• only need to label the vertical axis as enthalpy, H
• horizontal axis could be labelled as progress of reaction or extent of reaction but not necessary
• formulae and state symbols should be given for reactants and products
• values for enthalpy change should be given, including correct sign
• not essential to show activation energy