Energy changes and fuels

Question 1

What is enthalpy (H)?

Answer 1

The heat content that is stored in a chemical system.

Question 2

What is an exothermic reaction?

Answer 2

A reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings (-ΔH).

Question 3

What is an endothermic reaction?

Answer 3

A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings (+ΔH).

Question 4

Why are exothermic reactions important?

Answer 4

• Oxidation of fuels: Oxidation of hydrocarbons by combustion is to H₂O and CO₂ is an exothermic reaction. We use this reaction in cars, gas stoves to cook food, as well as gas boilers to make hot water and keep warm in the winter.
• Respiration: Oxidation of glucose by respiration to H₂O and CO₂ is the most important exothermic reaction to life. This reaction provides energy for living organisms in order to stay alive.

Question 5

Why are endothermic reactions important?

Answer 5

• The thermal decomposition of CaCO₃ to CaO and CO₂ is a very important endothermic reaction in agriculture. CaO, or lime, is spread over fields in order to neutralise acidic soil to obtain maximum crop yield.
• Photosynthesis is also a very important endothermic reaction. Plants use photosynthesis to convert CO₂ and H₂O into C₆H₁₂O₆. This process removes Co₂ from the air as well as provides our main source of energy.

Question 6

What is an enthalpy profile diagram?

Answer 6

A diagram for a reaction to compare the enthalpy of reactants with the enthalpy of products.

Question 7

What is activation energy?

Answer 7

The minimum energy required to start a reaction by the breaking of bonds.

Question 8

What is the equation used to quantify enthalpy changes?

Answer 8

Q = mcΔT (Energy = mass x specific heat capacity x change in temperature).

Question 9

What is specific heat capacity?

Answer 9

The energy required to raise the temperature of 1g of a substance by 1°C.

Question 10

What are ‘standard conditions’?

Answer 10

1. 298 K (25ºC) temperature.
2. 100 KPa (1 atm) pressure.
3. 1 mol.dm-3 concentration (when applicable).

Question 11

What is Standard Enthalpy change of Combustion (ΔHc°)?

Answer 11

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states.

Question 12

What is Standard Enthalpy change of Formation (ΔH_f°)?

Answer 12

The enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions.

Question 13

What is Standard Enthalpy change of Reaction (ΔH_r°)?

Answer 13

The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.

Question 14

What is Hess’ Law?

Answer 14

If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change (ΔH) is the same for each route.

Question 15

What is a Specific Bond Enthalpy?

Answer 15

The enthalpy change that takes place when breaking by homolytic fission 1 mol of a given bond in the molecules of a gaseous species.

Question 16

What is an Average Bond Enthalpy?

Answer 16

The average enthalpy change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of a gaseous species.

Question 17

Why are ΔHr° different from the ΔH calculated using average bond enthalpies?

Answer 17

1. Average bond enthalpies are only averages and they do not take into account different bonding environments that may lead to slightly different bond enthalpies.
2. Average bond enthalpies are for gaseous bonds. Not all species are gaseous under standard conditions, therefore extra enthalpy changes will take place when converting the gaseous species into its standard state.

Question 18

What is a hydrocarbon?

Answer 18

A compound that only contains the elements carbon and hydrogen.

Question 19

What effect does the length of the hydrocarbon chain have on its boiling point?

Answer 19

The longer the hydrocarbon chain, the more atoms it’s made out of, the more electrons are in the hydrocarbon molecule. More electrons means stronger fluctuations in the electron cloud which create stronger instantaneous dipoles and induced dipoles, resulting in stronger Van der Waals’ interactions between molecules; more heat energy is required to break the molecules apart, so longer hyrocarbon molecules have higher boiling points.

Question 20

What effect does branches of hydrocarbons have on its boiling point?

Answer 20

The more branched the hydrocarbon chain, the fewer the points of contact between molecules. This results in weaker Van der Waals’ interactions between molecules and less heat energy required to break the molecules apart, so more branced hydrocarbons ave lower boiling points.

Question 21

Why does each carbon atom in a hydrocarbon have a tetrahedral shape around it?

Answer 21

Each carbon makes 4 bonds and is therefore surrounded by 4 regions of electron density. These regions repel each other equally, forming a tetrahedral shape around each carbon atom with bonding angles of 109.5°.

Question 22

What is fractional distillation?

Answer 22

The separation of the components in a liquid mixture into fractions which differ in boiling point (and hence chemical composition) by means of distillation, typically using a fractional column.

Question 23

What are the processes of fractional distillation?

Answer 23

1. The liquid mixture is vapourised and passed into a fractional column which decreases in temperature up the column.
2. Longer hydrocarbons have higher boiling points and condense at higher temperatures near the bottom and are then tapped off.
3. Shorter hydrocarbons have lower boiling points and condense at lower temperatures near the top and are tapped off.
4. Shorter hydrocarbons are usually burned diractly as fuels whereas longer hydrocarbons are further processed to make petrochemicals.

Question 24

What are the dangers of incomplete combustion of hydrocarbons?

Answer 24

Incomplete combustion of hydrocarbons produces carbon monoxide (CO). CO is highly toxic as it binds onto haemoglobin in the blood and prevents oxygen from being carried.

Question 25

What is the use of cracking?

Answer 25

• To break down longer, saturated hydrocarbons a mixture of shorter hydrocarbons including alkanes and alkenes.
• These can be burned as fuels or used to make polymers.

Question 26

What are the conditions required for catalytic cracking?

Answer 26

Catalytic cracking takes place high temperatures of around 450°C, low pressures and the presence of zeolite catalyst.

Question 27

What are the uses of reforming?

Answer 27

• Reforming converts straight chained hydrocarbons into cyclic hydrocarbons and hydrogen.
• Cyclic hydrocarbons have higher octane ratings, so are more efficient fuels. Hydrogen can be used as a feedstock material in the chemical industry or can be directly burned as a fuel.

Question 28

What are the conditions required for reforming?

Answer 28

Reforming takes place under high temperature of around 500°C, where gaseous reactants are passed over platinum catalyst suspended on a bed of aluminium oxide.

Question 29

What is the use of isomerisation?

Answer 29

• Isomerisation converts straight chained hydrocarbons into branced isomers.
• This improves the octane rating of the hydrocarbon.

Question 30

What conditions are required for isomerisation?

Answer 30

Isomerisation takes place under high pressures of around 20 atm (2,000 KPa) and temperature of 250°C in the presence of platinum catalyst on a zeolite base.

Question 31

What are the disadvantages of using non-renewable fossil fuels?

Answer 31

• Fossil fuels will eventually run out as they are a non-renewable resource.
• Burning fossil fuels releases CO₂ into the air which is a greenhouse gas that is contributing towards global warming.

Question 32

What solutions are there to these problems?

Answer 32

• Biofuels are being produced from bioethanol which is a renewable rewable resource that well never run out.
• Other renewable sources of energy that don’t have carbon emissions should be used more. E.g. wind power, solar power, hydroelectric power, geothermal power…