chemical energetics

Question 1

Whether a reaction is endothermic or exothermic depends on the difference between

Answer 1

the energy needed to break bonds and the energy released when the new bonds are formed.

Question 2

Endothermic

Answer 2

If more energy is absorbed than is released, this reaction is endothermic.

More energy is required to break the bonds than that gained from making the new bonds.

The change in energy is positive since the reactants have more energy than the products.

The symbol ΔH (delta H) is used to show the change in heat energy. H is the symbol for enthalpy, which is a measure of the total heat of reaction of a chemical reaction.

Therefore an endothermic reaction has a positive ΔH

Question 3

Exothermic

Answer 3

If more energy is released than is absorbed, then the reaction is exothermic.

More energy is released when new bonds are formed than energy required to break the bonds in the reactants.

The change in energy is negative since the reactants have less energy than the products.

Therefore an exothermic reaction has a negative ΔH value.

Question 4

exothermic enrgy level diagram

Answer 4

Question 5

endothermic energy level diagram

Answer 5

Question 6

in EXothermic reactions

Answer 6

heat Exits the system

Question 7

ENdothermic reactions

Answer 7

heat ENters the system.

Question 8

Exothermic reactions always… , whereas endothermic reactions take …

Answer 8

• give off heat and they feel hot
• heat in and they feel cold.

Question 9

Energy level diagrams

Answer 9

These are graphical representations of the heat changes in chemical reactions (see above).

The enthalpy of the reactants and products is displayed on the y-axis.

The reaction pathway is shown on the x-axis.

Arrows on the diagrams indicate whether the reaction is exothermic (downwards pointing) or endothermic (upwards pointing).

Question 10

Enthalpy change during an exothermic reaction explanation:

Answer 10

During an exothermic reaction, energy is given out.

This means that the energy of the products will be lower than the energy of the reactants, so the change in enthalpy (ΔH) is negative.

This is represented on the energy-level diagram above with a downwards arrow as the energy of the products is lower than the reactants.

Question 11

Enthalpy change during an endothermic reaction explanation:

Answer 11

During an endothermic reaction, energy is absorbed.

This means that the energy of the products will be higher than the energy of the reactants, so the change in enthalpy (ΔH) is positive.

This is represented on the energy-level diagram above with an upwards arrow as the energy of the products is higher than the reactants.

Question 12

Calculate the energy of a reaction using bond energies

Energy of reaction calculations

Method

Equation

Answer 12

Each chemical bond has a specific bond energy associated with it.

This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed.

This energy can be used to calculate how much heat would be released or absorbed in a reaction.

To do this it is necessary to know the bonds present in both the reactants and products.

Question 13

Answer 13

Question 14

Answer 14

Question 15

Energy from Fuels

Answer 15

A fuel is a substance which releases energy when burned.

When the fuel is a hydrocarbon then water and carbon dioxide are produced in combustion reactions.

Propane for example undergoes combustion according to the following equation:

C3H8 + 5O2 → 3CO2 + 4H2O ΔH = -2219 kJ/mol

The efficiency of a fuel refers to how much energy is released per unit amount.

We can measure the efficiency of fuels by calorimetry.

A known mass of the fuel is combusted and used to heat up a known mass of water to calculate its heat of combustion.

Different fuels heat the water by different amounts and they can be analysed and compared in this way.

Question 16

Calorimetry experiment

Answer 16

Method:

Using a measuring cylinder, put 100 cm3 of water into a copper can.

Measure and record the initial temperature of the water.

Fill the spirit burner with test substance and measure and record its mass.

Place the burner under the copper can and light the wick.

Stir the water constantly with the thermometer and continue heating until the spirit burner burns out.

Measure and record the highest temperature of the water.

Question 17

Hydrogen as a fuel

Answer 17

Hydrogen is used in rocket engines and in fuel cells to power some cars.

Hydrogen has a series of advantages and disadvantages regarding its use as a fuel.

Advantages:

It releases more energy per kilogram than any other fuel (except for nuclear fuels).

It does not pollute as it only produces water on combustion, no other product is formed.

Disadvantages:

Expensive to produce and requires energy for the production process.

Difficult and dangerous to store and move around (usually stored as liquid hydrogen in highly pressurised containers).

Question 18

Radioactive Isotopes as Fuels

Answer 18

Uranium-235 undergoes decay and gives off heat energy which nuclear power stations harness.

The heat it produces is used to heat water to steam, which in turn is used to power turbines to generate electricity.

Nuclear fuel energy is clean as it does not produce pollutants such as CO2 or oxides of nitrogen or sulfur.

But nuclear power plants are expensive to build and maintain as well as being potentially dangerous in the event of an accident as radioactive materials may be released.