Chp 6 - Chemical Energetics

Question 1

What is an Exothermic reaction

Give an example

Answer 1

A reaction in which energy is given out or exits out to the surroundings (temperature of environment increases)

Examples:
Combustion of fuels
Reaction of acids and metals
Neutralisation reactions

Ask teacher why neutralisation reactions

Question 2

What is an endothermic reaction

Give an example

Answer 2

A reaction in which energy is taken in or enters in from surroundings (temperature of environment decreases)

The energy change is positive

Examples:
Thermal decomposition of carbonates
Electrolysis
First stages of photosynthesis

Question 3

How do you decide whether a reaction is Exothermic or endothermic

Answer 3

Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when the new bonds are formed

Question 4

How do you know if something is endothermic or not

Answer 4

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 products have more energy than the reactants

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 value

Question 5

What is enthalpy

Answer 5

This is the total amount of heat energy that a chemical system contains and considers both reactants and products.

Question 6

How do you know if a reaction is Exothermic or not

Answer 6

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 products have less energy than the reactants

Therefore an exothermic reaction has a negative ΔH value

Question 7

What are energy level diagrams

Answer 7

These are graphical representations of the heat changes in chemical reactions

Question 8

On energy level diagrams where is the enthalpy of reactants and products in

Answer 8

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

Question 9

On energy level diagrams where is the reaction pathway shown

Answer 9

The reaction pathway is shown on the x-axis

Question 10

On a energy level diagram what arrow means an Exothermic reaction

Answer 10

A downwards pointing arrow indicates that a reaction is Exothermic

Question 11

On a energy level diagram what arrow means an endothermic reaction

Answer 11

An upwards pointing down arrow represents an endothermic reaction

Question 12

What is an Exothermic reaction Pt 2

Answer 12

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 13

What is an endothermic reaction Pt 2

Answer 13

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 14

What is bond energy

Answer 14

This is the energy required to break one mole of covalent bonds between two particular atoms and is measured in kJ per mole.

Or it’s also the amount of energy given out when the bond is formed

Question 15

What is the energy of reaction calculations

Answer 15

This is the energy that 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 16

What is the method of finding out how much heat has been released or absorbed in a reaction.

Answer 16

Equation

Energy change = Energy taken in - Energy given out

Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’

Add together the bond energies for all the bonds in the products – this is the ‘energy out’

Calculate the energy change: Energy change = energy in – energy out

Question 17

How do you find out how much how much heat can be release or absorbed in the reaction which is an exothermic reaction

Hydrogen and chlorine react to form hydrogen chloride gas:

H2 + Cl2 → 2HCl

Answer 17

H-H = bond energy = 436
Cl-Cl= bond energy = 243
H-Cl= bond energy = 432

Energy in = 436 + 243 = 679 KJ / Mole

Energy out = 2 x 432 (As there are to HCLs)= 864 KJ / Mole

Energy change = 679 – 864 = -185 KJ / Mole

*The energy change is negative, showing that energy is released to the surroundings so it is an exothermic reaction.

Question 18

How do you find out how much how much heat can be release or absorbed in the reaction which is an endothermic reaction.

Hydrogen Bromide decomposes to form Hydrogen and Bromine:

2 x ( H - Br ) → H - H + Br - Br

Answer 18

H-Br = bond energy = 366
H-H = bond energy = 436
Br-Br = bond energy = 193

Energy in = 2 x 366 = 732 KJ / Mole

Energy out = 436 + 193 = 629 KJ / Mole

Energy change = 732 – 629 = +103 KJ / Mole

*The energy change is positive, showing that energy is taken in from the surroundings, so is an endothermic reaction.

Question 19

What H-H + Cl + Cl become

Answer 19

H-H + Cl-Cl —> H-CL + H-CL

Question 20

What is fuel

Answer 20

A fuel is a substance which releases energy when burned

Question 21

What happens when a fuel is a hydrocarbon

Answer 21

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 (Exothermic reaction)

Question 22

What does the efficiency of fuel refer to

Answer 22

The efficiency of a fuel refers to how much energy is released per unit amount
We can measure the efficiency of fuels by calorimetry

Question 23

What is a calorimetry

Answer 23

It’s the measurement of heat transfers in chemical reactions.

Question 24

How do you calculate a substances heat combustion through calimentry

Answer 24

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