ENERGETICS

Question 1

define enthalpy change:ΔH

Answer 1

Enthalpy change is the amount of heat energy taken in or given out during any change in a system provided the pressure is constant.

Question 2

Define standard enthalpy change of formation?ΔfH

Answer 2

Enthalpy change that occurs when when 1 mole of a compound is formed from its constituent elements under standard conditions and all reactants and products in their standard states.

Question 3

Define standard enthalpy change of combustion?ΔcH

Answer 3

the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions and all reactants and products being in their standard states.

Question 4

what are the standard conditions for temperature,pressure and concentration?

Answer 4

Tempreture-298k(25° c)
pressure-100kpa(100,000pa)
concentration-1mol dm-3

Question 5

Explain what happens in a exothermic reaction:

Answer 5

• energy is transferred into surrounding
• The products have less energy than the reactants
• negative ΔH

Question 6

Explain what happens in a endothermic reaction:

Answer 6

• Heat is taken in from the surroundings to the system, increasing the temperature of the surroundings
• positive ΔH
• products have more energy than reactants

Question 7

what is the formula for standard enthalpy change of formation?

Answer 7

△fH=(sum of △fH of product)-(sum of △fH of reactant)

Question 8

what is the equation for standard enthalpy change of combustion?

Answer 8

ΔcH=(sum of ΔcH of reactant)-(sum of ΔcH of product)

Question 9

formula for energy change:

Answer 9

q = mcΔT
q=heat exchanged with surroundings (J)
m=mass of the substance heated/cooled (g)
c=specific heat capacity (J g-1 K–1)  
ΔT=change in temperature(K or °C )

Question 10

General method to calculate enthalpy change?

Answer 10

1. Using q = m x cp x ΔT calculate energy change for quantities used
2. Work out the moles of the reactants used
3. Divide q by the number of moles of the reactant not in excess to give ΔH
4. A)dd a sign and unit (divide by a thousand to convert Jmol-1 to kJmol-1

Question 11

Define Hess’ Law

Answer 11

The enthalpy change of a reaction is independent of the route taken

Question 12

What is calorimetry?

Answer 12

Calorimetry is the process of measuring the amount of heat given off or taken in during a chemical reaction

Question 13

method for calorimetry: required practical 2

Answer 13

1-washes the equipment(cup and pipettes etc)with the solutions to be used
2-dry the cup after washing
3-put polystyrene cup in a beaker for insulation and support
4-Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
5-clamp thermometer into place making sure the thermometer bulb is immersed in solution
6-measure the initial temperatures of the solution or both solutions if 2 are used. Do this at regular intervals like every minute for 2-3 minutes until a trend is observed
7-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.
8-If using a solid reagent then use ‘before and after’ weighing method
9-stirs mixture (ensures that all of the solution is at the same temperature)
10-Record temperature every minute after addition for several minutes

Question 14

How do you improve temperature measurement in calorimetry practical?

Answer 14

• measure & record initial temperature of the solution for a few minutes before addition?
• measure &record the temperature after the addition at regular intervals like each minute for 2-3 minute

Question 15

How do improve the temperature determination for a for a calorimetry practical?

Answer 15

• plot a graph of temperature against time
• extrapolate the temperature curve/line back to the time the reactants were added together to determine ΔT at the point of addition

Question 16

Errors in the calorimetry method:

Answer 16

-incomplete combustion
-evaporation
-density of solution is taken to be the same as water.
-The method assumes all
solutions have the heat capacity of water.
-energy transfer from surroundings (usually loss)