Energetics and Hess's Law

Question 1

What is enthalpy change?

Answer 1

The enthalpy change of a reaction is the heat exchange with the surroundings at constant pressure

Question 2

What type of energy change occurs when the volume is constant?

Answer 2

Internal energy change (ΔU)

Question 3

What type of energy change occurs when pressure is constant?

Answer 3

Enthalpy change ( ΔH)

Question 4

What does an energy level diagram look like for an exothermic reaction?

Answer 4

Reactants above products
Downwards arrow (from reactants to products)
Negative ΔH

Question 5

What does an energy level diagram look like for an endothermic reaction?

Answer 5

Products above reactants
Upwards arrow (from reactants to products)
Positive ΔH

Question 6

Amount of substance as a factor of enthalpy change

Answer 6

The amount of energy transferred depends on the amount of substance used. Doubling the amount of substance doubles the energy transferred

Question 7

Temperature as a factor of enthalpy change

Answer 7

ΔH is usually quoted at 298K, and changes as temperature changes

Question 8

Pressure as a factor of enthalpy change

Answer 8

Usually quoted at 100kPa (1 atm)

Question 9

Standard state as a factor of enthalpy change

Answer 9

Some elements exist in two or more forms in the same physical state, and the standard state is the most stable state and is the one used when calculating enthalpy change

Question 10

What is the definition for standard enthalpy of reaction?

Answer 10

The standard enthalpy change of reaction refers to the enthalpy change using the amounts shown in the equation under standard conditions (298K, 100kPa, solutions with concentration 1 moldm^-3)

Question 11

What is the definition of standard enthalpy of formation of a compound?

Answer 11

The standard enthalpy change of a compound is the enthalpy change that takes place when one mole of compound is formed from its elements in their standard states under standard conditions

Question 12

What is the standard enthalpy of formation a measure of?

Answer 12

How stable a compound is relative to its constituent elements. The more negative the value, the more stable it is in relation to its elements

Question 13

What is the definition of standard enthalpy of combustion of a substance?

Answer 13

The standard enthalpy of combustion is the enthalpy change that takes place when one mole of substance is burnt completely in oxygen under standard conditions

Question 14

What is the definition of standard enthalpy of neutralisation?

Answer 14

The standard enthalpy of neutralisation of an acid with an alkali is the enthalpy change that takes place when an amount of acid or alkali is neutralised to form one mole of water. The reaction is carries out in dilute solution at 298K and 100kPa

Question 15

What is the definition of standard enthalpy of atomisation?

Answer 15

The standard enthalpy of atomisation of an element is the enthalpy change that takes place when 1 mole of gaseous ions is made from the element in its standard state under standard conditions

Question 16

How can a simple calorimeter be used to measure enthalpy of combustion of ethanol?

Answer 16

• Record the initial and final temperature of the water in the calorimeter
• Use E= mcΔT to calculate energy change
• Record mass of spirit burner before and after to work out what mass of ethanol has combusted and then calculate moles

Question 17

What are the sources of error when calculating enthalpy change using a simple calorimeter?

Answer 17

• Heat is lost to the surroundings
• Only the specific heat capacity of water is used in the calculation when some heat is also used to heat the copper calorimeter which is not factored in
• Assumed no heat is transferred into calorimeter
• Assumed 100% complete combustion (incomplete combustion means not all heat has been released)
• Some ethanol (for example) may have evaporated rather than combusted

Question 18

Why are polystyrene cups often used when measuring enthalpy changes of reactions in solution?

Answer 18

They are good insulators and have low heat capacity and so absorb little heat themselves

Question 19

What is Hess’s Law?

Answer 19

The total enthalpy change accompanying a chemical change is independent of the route by which the chemical change takes place

Question 20

Bond breaking is…

Answer 20

Endothermic

Question 21

Bond making is…

Answer 21

Exothermic

Question 22

What is the definition of bond enthalpy/bond dissociation enthalpy?

Answer 22

The enthalpy change when one mole of bonds in a molecule is broken, the molecules and their resulting fragments being in the gaseous state

Question 23

What is the definition of mean bond enthalpy?

Answer 23

The mean bond enthalpy is the average enthalpy change when one mole of bonds is broken, the molecules and their resulting fragments being in the gaseous state

Question 24

Why are mean bond enthalpies useful?

Answer 24

• They allow enthalpy changes in reactions to be estimated
• Strength of bonds can be compared
• Allow structure and bonding to be understood
• Allow mechanisms of chemical reactions to be understood