Energetics

Question 1

define enthalpy change

Answer 1

Enthalpy change is the change in heat energy at constant pressure

Question 2

what are the standard conditions

Answer 2

• 100 kPa (or 1 atmosphere, 1 atm)pressure
• 298 K (25 °C) temperature
• 1.0 mol dm-3 concentration for all solutions

Question 3

what type of reaction is it if a bond is broken

Answer 3

endothermic

Question 4

what type of reaction is it if a bond is made

Answer 4

exothermic

Question 5

define activation energy

Answer 5

minimum energy required to start a reaction

Question 6

define mean bond enthalpy

Answer 6

energy required to break one mole of covalent bonds into gaseous atoms averaged over a range of different compounds

Question 7

The standard enthalpy of formation Δf Hθ is:

Answer 7

*The enthalpy change when one mole of substance is formed
*from its constituent elements under standard conditions
*with all reactants and products being in their standard states.

Question 8

State why the enthalpy of formation of Na(s) is zero.

Answer 8

Na is an element

Question 9

: State why the enthalpy of formation of liquid Na is not zero.

Answer 9

: Na(l) is not the standard state of Na

Question 10

: Why may the enthalpy of combustion of a compound be difficult to measure?

Answer 10

: Incomplete combustion may occur

Question 11

Hess’ Law

Answer 11

Hess’ Law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.

Question 12

: Give one reason why the bond enthalpy that you calculated above is different from the mean bond enthalpy quoted in a data book.

Answer 12

The data book value is averaged over a range of different compounds (not just CH3CHF2 molecules).

Question 13

The enthalpy change of combustion of methanol was measured using a calorimeter. 0.32 g of methanol was burnt and the temperature of 200 cm3 of water changed from 28 ºC to 32 ºC. The specific heat capacity of water is 4.18 J K−1 g−1.

CH3OH(l) + O2(g) -> CO2(g) + 2H2O(l)

Calculate the enthalpy change

Answer 13

• 200 cm3 of water = 200 g of water
• ΔT = 32 – 28 = 4
• Q = 200 × 4.18 × 4 = 3360 J
• Q must be in kJ. 3360 J = 3.36 kJ
• n = moles of fuel combusted = Mass/Mr = 0.32 g / 32 =
  0.01 moles
• ΔH = Q/n = 3.36 / 0.01 = 336 kJ mol−1
• As the temperature of the water increased, the reaction
  must be exothermic, so the enthalpy change must have a
  negative sign
• ΔH = −336 kJ mol−1

Question 14

Steps to Measure an Enthalpy Change Using a Cooling Curve

Answer 14

• Record the temperature for a suitable time (3 minutes)
  before adding reactants together
• To establish an accurate initial temperature
• Mix reactants then record temperature every minute until
  a trend is seen
• Plot a graph of temperature against time
• Extrapolate the cooling curve back to the point of addition
• To establish a theoretical temperature change accounting
  for heat loss

Question 15

State the meaning of the term standard enthalpy of combustion.

Answer 15

• Enthalpy change when one mole of a substance burns completely in oxygen
• With all substances in their standard states

Question 16

A student determines the enthalpy change for the reaction between
calcium carbonate and hydrochloric acid.
CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
The student follows this method:
* measure out 50 cm3 of 1.00 mol dm–3 aqueous hydrochloric acid
using a measuring cylinder and pour the acid into a 100 cm3 glass
beaker
* weigh out 2.50 g of solid calcium carbonate on a watch glass and tip
the solid into the acid
* stir the mixture with a thermometer
* record the maximum temperature reached.

The student uses the data to determine a value for the enthalpy change.
Explain how the experimental method and use of apparatus can be improved to provide more accurate data.
Describe how this data from the improved method can be used to determine an accurate value for the temperature change.

Answer 16

Stage 1: Apparatus
1a. Use a burette/pipette (instead of a measuring cylinder)
1b. Use a polystyrene cup (instead of a beaker) / insulate beaker
1c. Reweigh the watchglass after adding the solid 1d: Use powdered solid

Stage 2: Temperature Measurements
2a. Measure and record the initial temperature of the solution for a few
minutes before addition
2b. Measure and record the temperature after the addition at regular
intervals (eg each minute) for 8+ minutes/until a trend is observed

Stage 3: Temperature Determination
3a. Plot a graph of temperature against time
3b. Extrapolate to the point of addition
3c. Determine ΔT at the point of addition

Question 17

which way do arrows go in enthalpy of combustion

Answer 17

down

Question 18

which way do arrows go in enthalpy of formation

Answer 18

up