Chapter 4 (Energetics)

Question 1

What is Hess’s law?

Answer 1

The enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.

Question 2

What is an endothermic reaction?

Answer 2

When more energy is taken in to make bonds than released when breaking bonds.

Temperature decreases

Question 3

What is an exothermic reaction?

Answer 3

When more energy is released by breaking bonds than making bonds.

Temperature increases

Question 4

What unit is used for energy change?

Answer 4

Kilojoules per mole

KJ mol-1

Question 5

What are the standard conditions for measuring enthalpy change?

Answer 5

Pressure of 100kPa
Temperature of 298K
Solutions at 1 mol dm-3

Question 6

What is enthalpy change?

Answer 6

The measurement of heat change at a constant pressure.

Question 7

What is the standard molar enthalpy of formation?

Answer 7

The enthalpy change when one mole of substance is formed from its constituent elements under standard condition, all reactants and products being in standard state.

Question 8

What is the standard molar enthalpy of combustion?

Answer 8

The enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions, all reactants and products being in their standard state.

Question 9

What is specific heat capacity?

Answer 9

The amount of heat needed to raise the temperature of 1g of substance by 1K.

Question 10

What are the units for specific heat capacity?

Answer 10

Joules per gram per kelvin

J g-1 K-1

Question 11

What is the equation for enthalpy change?

Answer 11

Enthalpy change = mass of substance x specific heat capacity x temperature change

q = MC∆T

Question 12

Which reaction has a negative ∆H?

Answer 12

Exothermic

Question 13

Which reaction has a positive ∆H?

Answer 13

Endothermic

Question 14

What is created during incomplete combustion?

Answer 14

Soot (carbon), carbon monoxide and water

Question 15

What is the enthalpy of formation of an element?

Answer 15

0 kJ mol-1

Question 16

Describe the method for calculating calorimetry

Answer 16

1) Measure out desired volumes of solution with volumetric pipettes and transfer to insulated cup.
2) Use a thermometer to measure initial temperature and then repeat at even intervals such as every minute for 3 minutes.
3) After 3 minutes transfer the 2nd reagent to a cup. If a solid reagent is used then add the solution to the cup first then add solid after being weighed on balance (weigh before and after of container containing solid).
4) Stir the mixture and record the temperature every minute minutes for several minutes.
5) Plot on a graph and extrapolate data to when reactants were added.

Question 17

What is the general equation for a neutralisation reaction?

Answer 17

acid + alkali > salt + water

Question 18

What are possible errors for calorimetry experiment?

Answer 18

• Energy transferred from surroundings (usually lost)
• Approximation in specific heat capacity of solution. Method assumes that all solutions have same specific heat capacity as water.
• Neglecting the specific heat capacity of the calorimeter (ignore energy absorbed by apparatus)
• Reaction or dissolving may be slow or incomplete.
• Density is taken to be the same as water.