Required Practical 2 - Measuring Enthalpy change

Measuring an Enthalpy change (Flame Calorimetry)

Measuring an Enthalpy change (Flame Calorimetry)

Give the 9 steps for Flame Calorimetry

1. Measure 100 cm³ of distilled water into a copper calorimeter and record the initial temp

2. Weigh the spirit burner containing alcohol

3. Place the calorimeter above the spirit burner and use draught shields to minimise heat loss

4. Light spirit burner and stir water constantly

5. Extinguish the burner after the temp has risen by 30-40^oC

6. Record the temperature of the water

7. Reweigh the spirit burner (lid on) to find the mass of alcohol burner

8. Calculate the energy transferred using q=mct

9. Find moles burnt

Give 4 ways to reduce Uncertainty

1. Insulate

2. Stir

3. Digital Thermometer

4. Account for heat loss by extrapolating the graph

Give 3 reasons to keep the lid of the spirit burner when not in use

• (+ each of their effect)

1. Prevent evaporation of alcohol = Changes mass so effects enthalpy change

2. Reduce exposure to heat = Reduces fire risk

3. Prevents Oxygen entering = Prevents degradation of alcohol

Give the 3 main errors that occur in this experiment (+ reasons)

1. Heat loss to surroundings = Decrease temp rise - making it seem less exothermic

2. Incomplete combustion = Decrease energy + forms soot

3. Fuel/Water evaporation = Mass inaccurate to how much was truly burnt - changing the enthalpy value

What is the formula for energy transfer? (units)

q=mcΔT

• q= Joules

• m= kg

• c= j/kg

• T= Kelvins

How do you calculate enthalpy change?

ΔH= -q/n

n= number of moles burnt of alcohol

ΔH of this experiment will be greater or less than what is quoted from reliable values (+ reason)

Underestimate:

• Heat exposes to surroundings (metal, air) most to water (not all) meaning decrease temperature and decreasing in energy loss and enthalpy

Measuring Enthalpy change (Solution Calorimetry)

Measuring Enthalpy change (Solution Calorimetry)

Give the 6 steps for Solution calorimetry

1. Measure 25 cm³ of 0.4 moldm^-3 copper sulfate using a measure cylinder and pour it into a polystyrene cup placed in a beaker for insulation

2. Record temp of the solution every 30 seconds for 3 minutes to obtain a baseline

3. At the 3rd minute add 4g of iron powder, stir the mixture, continue recording the temperature every 30 seconds for 5 more minutes

4. Plot a temp-time graph, with 2 lines of best fit

5. Extrapolate both lines to the time of mixing, difference between them gives a more accurate temp change

6. Calculate energy change, then calculate

Give 4 ways to reduce Uncertainty

1. Insulate

2. Stir

3. Digital Thermometer

4. Account for heat loss by extrapolating the graph

Give 4 sources of error in this experiment (+effects)

1. Heat loss to surroundings - heat goes to air, container walls and lid

2. Incomplete mixing - ΔT not uniform

3. Cup absorbs heat

4. When repeating the experiments, the initial temp wont be constant

What is the number of moles for solution? (+use)

The limiting reagents moles:

• Solution energy change divided by moles

If you doubled the mass of iron power used, how will it effect the results of this experiment? (reason)

Enthalpy change will remain constant:

• CuSo₄ is the limiting reagent. as fewer moles than iron, so the number of moles reacted is constant