Energetics (Physical) (complete)

Question 1

What is enthalpy?
Define enthalpy change.
Name the standard conditions for enthalpy change.

Answer 1

• Enthalpy is a measure of the heat content of a substance at constant pressure.
• Enthalpy change - Change in the heat content at constant pressure.
• 100kPa, and a stated temperature (usually 298K)

Question 2

Define Standard Enthalpy of formation.

Define Standard Enthalpy of combustion.

Answer 2

• The Enthalpy change when one mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions.
• The Enthalpy change when one mole of a substance is burned completely in oxygen with all reactants and products in standard states under standard conditions.

Question 3

Give the equation for how you would calculate the amount of energy given out (or absorbed) by a reaction.

Hence, give the equation for calculating the enthalpy change of a reaction.

Answer 3

q = mc(delta)T

m = mass of substance heated (g)
c = specific heat capacity (Jg-1K-1)
(delta)T = temperature rise

Enthalpy change (per mole) = q / number of moles reacting

Question 4

Why are enthalpy change results collected using calorimetry always imperfect?

Answer 4

• Due to heat loss being a major problem which can lead to error in the results.

Question 5

Describe the method to calculate enthalpy change of reactions taking place in solution.

Answer 5

• The reaction is carried out in an insulated polystyrene cup.
• Add the (first) solution to the cup, and start a timer, placing an insulated lid over the cup with a thermometer through the top. Take the temperature of the solution every minute for 3 minutes starting at 0.
• On the fourth minute, add either the second solution or the solid, depending on the experiment. Don’t take the temperature this minute, but stir very quickly to ensure the reaction is carried out as quickly as possible.
• Take the temperature of the reaction every minute until the temperature starts decreasing very obviously.
• Record each temperature measured in a table.
• Draw a graph of the temperature recorded, and extrapolate the line to show a value for the fourth minute.
• This is your total temperature change.

Question 6

Name Hess’s Law

Give the cycle for calculating Enthalpy of Formation of a reaction.

Give the cycle for calculating Enthalpy of combustion of a reaction.

Answer 6

• The enthalpy change for a reaction is independent of the route taken.

reactants —> products
—> elements —>

reactants —> products
—> oxides <—

Question 7

Give the cycle for calculating Enthalpy change of a Bond Enthalpies reaction.

Answer 7

reactants —> products
—> gas atoms <—

(reactants —> gas atoms) = (include energy to form substances into gases if not already)