9.2 Measuring enthalpy changes
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Where does kelvin start
At absolute zero 0° -273°C
Is the change in temperature in °C and K different?
No. It is the same.
What quantities are needed to calculate the energy change of the surroundings?
Mass, specific heat capacity and temperature change.
How is mass measured?
By weighing
What is mass usually measured in?
grams
What is specific heat capacity?
The energy needed to raise the temperature of 1g of a substance by 1K
Do good conductors have high or low heat capacities?
low
Do insulators have high or low values of
high
What is the heat capacity of water
c =4.18J^-1K^-1
How is temperature change determined
From the thermometer readings ΔT =T(final) - T(initial)
Equation to calculate the energy (heat) in joules
q = mcΔT
Equation for the combustion of methanol
CH3OH(l) + 1 1/2 O2(g) –> CO2(g) + 2H2O
Experiment for burning liquid fuels
• Use measuring cylinder, measure out 150cm^3 of water. Pour water into beaker, record initial temperature of water to nearest 0.5 degrees. • Add methanol to spirit burner. Weigh spirit burner containing methanol • Place the spirit burner under the beaker. Light burner and burn methanol while stirring water with thermometer • After about 3 mins, extinguish flame and immediately record the max. temperature reached by the water • Re-weigh spirit burner containing methanol. Assume that wick has not been burnt
Why does a data book value differ from an experimental value
Data book values are obtained using much more sophisticated apparatus than a spirit burner and a beaker of water Heat loss to surroundings Incomplete combustion Evaporation of substance from wick Non-standard conditions
How to compare negative numbers for experimental and data book values
Say less exothermic or less negative rather than just x value is less
How can you minimise errors from heat loss and incomplete combustion
The use of draught screens and an input of oxygen
How to determine an enthalpy change of reactions between two solutions or one solution and a solid
Using plastic cups made of polystyrene foam which is cheap, light and offer some insulation against heat loss to surroundings
What counts as the surroundings in reactions between aqueous solutions
The solution itself
Reaction between two aqueous solutions
Particles within solutions may react when they collide, and any energy transfer is between chemical particles and water molecules in the solution
Method of experiment measuring the enthalpy change for the reaction
• Pipette 25.0cm^3 of 1.00mol dm^-3 CuSO4 into a polystyrene cup. Weigh out excess of zinc powder. • Start stop-clock and take temperature of the solution every 30s until the temperature stays constant. • Add zinc to solution and stir mixture. Record the temperature every 30 seconds until temperature has fallen for several mins. • Plot graph of temperature against time To correct for cooling, extrapolate cooling curve section of graph back to time when zinc was added. Draw vertical line from time solutions mixed to extrapolated cooling curve