3.1.4 Energetics

Enthalpy change?

The heat energy change at constant pressure

Enthalpy change of combustion?

• The heat energy change when 1 mole of a substance is completely burnt in oxygen

• All under standard conditions

• All reactants and products being in their standard states

Enthalpy change of Formation?

• The heat energy change when 1 mole of a substance is formed from its constituent elements

• All under standard conditions

• All reactants and products being in their standard states

Enthalpy change of a reaction?

• The enthalpy change when reactants react in their molar quantities given in the reaction equation

• All under standard conditions

• All reactants and products being in their standard states

Standard conditions numbers for

• pressure

• Temperature

• 100kpa

• 298 K

Exothermic reaction?

• Energetic is transferred from the system (chemicals) to the surroundings

• Products has less energy then the reactants

Endothermic reaction?

• Energy is transferred from the surroundings to the system (chemicals)

• The products have more energy than the reactants

Energy change equation?

• 2

• Energy change = mass of solution X heat capacity X temperature change

• Q (j) = m(g) X C(g-1k-1) X change T (K)

• /_\ H (KJmol-1) = q (KJ)/ n

Heat capacity of pure water?

• 4.18 J g^-1 K^-1

• In any reaction where the reactants are dissolved in water we assume that the heat capacity is the same as pure water

Hess’s Law?

The total enthalpy change (at constant pressure) in a reaction is independent of the route taken

Enthalpy change of formation equation?

Enthalpy change = Products - Reactants

Enthalpy change of combustion equation?

Enthalpy change = Reactants - Products

Mean Bon enthalpy equation?

Enthalopy change = sum of bond energies broken (reactants) - sum of bond energies made (products)

Mean bond enthalpy?

The average energy change (at constant pressure) for that typ of bond over different molecules

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.

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

A student planned and carried out an experiment to determine the enthalpy of reaction

when magnesium metal displaces zinc from aqueous zinc sulfate.

Mg(s) + Zn2+(aq) ⟶ Mg2+(aq) + Zn(s)

The student used this method:

• A measuring cylinder was used to transfer 50 cm3 of a 1.00 mol dm−3 aqueous solution of zinc sulfate into a glass beaker.

• A thermometer was placed in the beaker.

• 2.08 g of magnesium metal powder were added to the beaker.

• The mixture was stirred and the maximum temperature recorded.

The student recorded a starting temperature of 23.9 °C and a maximum temperature of 61.2 °C.

Suggest how the students’ method, and the analysis of the results, could be improved in order to determine a more accurate value for the enthalpy of reaction. Justify your suggestions.

Do not refer to the precision of the measuring equipment. Do not change the

amounts or the concentration of the chemicals.

Stage 1 Improved insulation

1a Insulate the beaker or use a polystyrene cup or a lid

1b To reduce heat loss

Stage 2 Improved temperature recording

2a Record the temperature for a suitable time before adding the metal

2b To establish an accurate initial temperature

OR

2c Record temperature values at regular time intervals

2d To plot the temperature results against time on a graph

Stage 3 Improved analysis of results

3a Extrapolate the cooling back to the point of addition

3b To establish a (theoretical) maximum temperature OR temperature change (e.g.

at the 4th minute) OR adjust for the cooling /apply a cooling correction

3a and 3b could be seen on an extrapolated sketch graph

(Note– IGNORE use of measuring equipment with greater precision)

Calorimetry - general method?

• washes the equipment (cup and pipettes etc) with the solutions to be used

• dry the cup after washing

• put polystyrene cup in a beaker for insulation and support

• measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup

• clamp thermometer into place making sure the thermometer bulb is immersed in solution

• measure the initial temperatures of the solution or both solutions if 2 are used. Do this every minute for 2-4 minutes

• at minute 4 transfer second reagent to cup. If a solid reagent is used then add the solution to the cup first and then add the solid weighed out on a balance.

• if using a solid reagent then use ‘before and after’ weighing method

• stirs mixture (ensures that all of the solution is at the same temperature)

• record temperature every minute after addition for several minutes

Errors in the method?

• energy transfer from surroundings (usually loss)

• approximation in specific heat capacity of solution. The method assumes all

solutions have the heat capacity of water.

• neglecting the specific heat capacity of the calorimeter- we ignore any energy

absorbed by the apparatus.

• reaction or dissolving may be incomplete or slow.

• Density of solution is taken to be the same as water.