9.2 Measuring enthalpy changes

Question 1

Where does kelvin start

Answer 1

At absolute zero
0°
-273°C

Question 2

Is the change in temperature in °C and K different?

Answer 2

No. It is the same.

Question 3

What quantities are needed to calculate the energy change of the surroundings?

Answer 3

Mass, specific heat capacity and temperature change.

Question 4

How is mass measured?

Answer 4

By weighing

Question 5

What is mass usually measured in?

Answer 5

grams

Question 6

What is specific heat capacity?

Answer 6

The energy needed to raise the temperature of 1g of a substance by 1K

Question 7

Do good conductors have high or low heat capacities?

Answer 7

low

Question 8

Do insulators have high or low values of

Answer 8

high

Question 9

What is the heat capacity of water

Answer 9

c =4.18J^-1K^-1

Question 10

How is temperature change determined

Answer 10

From the thermometer readings

ΔT =T(final) - T(initial)

Question 11

Equation to calculate the energy (heat) in joules

Answer 11

q = mcΔT

Question 12

Equation for the combustion of methanol

Answer 12

CH3OH(l) + 1 1/2 O2(g) –> CO2(g) + 2H2O

Question 13

Experiment for burning liquid fuels

Answer 13

1. Use measuring cylinder, measure out 150cm^3 of water. Pour water into beaker, record initial temperature of water to nearest 0.5 degrees.
2. Add methanol to spirit burner. Weigh spirit burner containing methanol
3. Place the spirit burner under the beaker. Light burner and burn methanol while stirring water with thermometer
4. After about 3 mins, extinguish flame and immediately record the max. temperature reached by the water
5. Re-weigh spirit burner containing methanol. Assume that wick has not been burnt

Question 14

Why does a data book value differ from an experimental value

Answer 14

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

Question 15

How to compare negative numbers for experimental and data book values

Answer 15

Say less exothermic or less negative rather than just x value is less

Question 16

How can you minimise errors from heat loss and incomplete combustion

Answer 16

The use of draught screens and an input of oxygen

Question 17

How to determine an enthalpy change of reactions between two solutions or one solution and a solid

Answer 17

Using plastic cups made of polystyrene foam which is cheap, light and offer some insulation against heat loss to surroundings

Question 18

What counts as the surroundings in reactions between aqueous solutions

Answer 18

The solution itself

Question 19

Reaction between two aqueous solutions

Answer 19

Particles within solutions may react when they collide, and any energy transfer is between chemical particles and water molecules in the solution

Question 20

Method of experiment measuring the enthalpy change for the reaction

Answer 20

1. Pipette 25.0cm^3 of 1.00mol dm^-3 CuSO4 into a polystyrene cup. Weigh out excess of zinc powder.
2. Start stop-clock and take temperature of the solution every 30s until the temperature stays constant.
3. Add zinc to solution and stir mixture. Record the temperature every 30 seconds until temperature has fallen for several mins.
4. 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