RP 2 - Measuring Enthalpy Change Flashcards
What’s Hess’ Law?
The enthalpy change for a chemical reaction is always the same, regardless of the route from reactants to products. For a Hess’ Cycle the sum of the clockwise enthalpy changes equals the sum of the anti-clockwise enthalpy changes
How can you reduce the uncertainty in the mass measurement? (2)
- Use a balance with a greater resolution
- Use a larger mass
What’s percentage uncertainty and how do you calculate it?
Percentage Uncertainty in a Measurement = absolute uncertainty divided calculated value multiplied by 100
Explain how you calculate enthalpy change experimentally
- q=mcdeltaT
- Where ‘m’ is the mass of the solution that changes temperature (1 g = 1 cm^3), ‘c’ is the specific heat capacity (usually of water), ‘deltaT’ is the temperature change, measured using a thermometer and ‘q’ is the heat energy taken in or released (in joules)
- Convert this number from J to kJ and divide this converted number by the number of moles of the limiting reactant
- Add a sign to show whether enthalpy change is exothermic or endothermic
Why may an experimental value for enthalpy change be different to the theoretical value? (3)
- Heat loss to apparatus/surroundings
- Incomplete combustion
- Non-standard conditions
- Evaporation if alcohol/water
How do you prevent heat loss to surroundings/apparatus? (2)
- Insulate the beaker by placing it in a polystyrene cup with a lid
- Avoid large temperature differences between surrounding and calorimeter
- Use a bomb calorimeter
Other than preventing heat loss, how can the accuracy of this experiment be improved?
- Read the thermometer at eye level to avoid parallax errors
- Stir the solution so the temperature is evenly distributed
- Use a digital thermometer for more accurate and faster readings
- Use greater concentrations and masses, leading to a greater temperature change and thus smaller uncertainty
What’s accuracy?
How close the data gathered is to the actual value
