enthalpy changes

Question 1

What does the system mean in a chemical reaction

Answer 1

The atoms and bonds involved in the chemical reaction

Question 2

Explain the law of conservation

Answer 2

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

Question 3

What energy change is breaking bonds associated with?

Answer 3

Endothermic reaction

Question 4

What energy change is making bonds associated with?

Answer 4

Exothermic reaction

Question 5

What is an endothermic reaction

Answer 5

A reaction with an overall positive enthalpy change

Question 6

What is an exothermic reaction

Answer 6

A reaction with an overall negative enthalpy change

Question 7

What does an exothermic reaction graph look like

Answer 7

Reactants are higher than products

Question 8

What does an endothermic reaction graph look like

Answer 8

Reacts are lower than products

Question 9

What does activation energy mean

Answer 9

The minimum energy required for a reaction to take place

Question 10

What are the standard conditions

Answer 10

100 kPa

298 K

Question 11

Define enthalpy change of formation

Answer 11

The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions

Question 12

Define enthalpy change of combustion

Answer 12

The energy change that takes place when 1 mole of a substance is completely combusted

Question 13

Define enthalpy change of neutralisation

Answer 13

The energy change that takes place when 1 mole of water is formed from a neutralisation reaction

Question 14

What does enthalpy change of reaction mean?

Answer 14

The energy change associated with a given reaction

Question 15

Why might the experimental method for enthalpy determination not be accurate

Answer 15

• Heat s lost to the surroundings
• Not in standard conditions
• Reaction may not go to completion

Question 16

What does average bond enthalpy mean

Answer 16

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

Question 17

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

Answer 17

• Bond enthalpies area a mean for the same bond across different molecules
• Standard enthalpy of combustion and formation just apply to that molecule so are more accurate

Question 18

How to calculate the enthalpy change of a reaction using average bond enthalpies?

Answer 18

(bond enthalpies of reaction) - (bond enthalpies of products)

Question 19

How do you calculate the heat energy released by a reaction?

Answer 19

q = m c ∆T

Question 20

What does “ q = m c ∆T “ mean

Answer 20

• q = energy change of water (J)
• m = mass of water (g)
• c = specific heat capacity of water (4.18 Jg^-1K^-1)
• ∆T = temperature change of the water (C)

Question 21

How do you calculate the enthalpy change of combustion?

Answer 21

Thermal energy (kJ) / number of moles