M3 ,, Enthalpy Changes

Question 1

Activation energy definition?

Answer 1

The activation Energy is defined as the minimum energy which particles need to collide to start a reaction

Question 2

Exothermic?

Answer 2

exothermic change energy is transferred from the system (chemicals) to the surroundings.
The products have less energy than the reactants
In an exothermic reaction the ∆H is negative
combustion of fuels and the oxidation of carbohydrates such as glucose in respiration

Question 3

Endothermic?

Answer 3

energy is transferred from the surroundings to the system (chemicals). They require an input of heat energy e.g. thermal decomposition of calcium carbonate
The products have more energy than the reactants
the ∆H is positive

Question 4

Enthalpy change standard conditions?

Answer 4

100 kPa pressure
298 K (room temperature or 25oC)
Solutions at 1mol dm-3
all substances should have their normal state at 298K

Question 5

Enthalpy change of reaction?

Answer 5

the enthalpy change when the number of moles of reactants as specified in the balanced equation react together

Question 6

Standard Enthalpy change of formation?

Answer 6

enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa), all reactants and products being in their standard states

Question 7

Enthalpy change of combustion?

Answer 7

the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions. (298K and 100kPa), all reactants and products being in their standard states

Question 8

Enthalpy change of neutralisation?

Answer 8

standard enthalpy change of neutralisation is the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water.

Question 9

Enthalpy change formula?

Answer 9

energy change = mass of solution x heat capacity x temperature change

Q (J) = m (g) x cp (J g-1K-1) x T ( K)

Question 10

Calorimetric method?

Answer 10

 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 2-3 minutes
 At minute 3 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

Question 11

Calorimetry - why is reaction slow?

Answer 11

If the reaction is slow then the exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the reaction

To counteract this we take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together.

Question 12

How to get a better average temp - calorimeter

Answer 12

We also take the temperature of the reactants for a few minutes before they are added together to get a better average temperature. If the two reactants are solutions then the temperature of both solutions need to be measured before addition and an average temperature is used.

Question 13

Errors in calorimetry method?

Answer 13

• 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.

Question 14

Errors in measuring Enthalpy of combustion?

Answer 14

• • • • • •
Energy losses from calorimeter Incomplete combustion of fuel Incomplete transfer of energy Evaporation of fuel after weighing
Heat capacity of calorimeter not included
Measurements not carried out under standard conditions as H2O is gas, not liquid, in this experiment

Question 15

Bond Enthalpy?

Answer 15

The mean bond enthalpy is the enthalpy change when one mole of bonds of (gaseous covalent) bonds is broken (averaged over different molecules)

Question 16

Bond breaking + bond making ?

Answer 16

Bond breaking absorbs energy and bond making releases energy

Question 17

Bond Enthalpy equation?

Answer 17

H = total bond energies broken - total bond energies made

Question 18

Hess law?

Answer 18

Hess’s law states that total enthalpy change for a reaction is independent of the route by which the chemical change takes place