Energy

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Energy

• Energy: capacity to do work
• Work: exertion of force overcoming resistance or producing molecular change
• Exothermic reaction gives out heat
• Endothermic reaction absorbs heat

Direct + Indirect uses of Energy

Direct

• Burning petrol to power a car
• Burn car to power a steam train
• Burn natural gas to use a stove to cook eggs Indirect
• Takes energy to process crude oil to separate petrol. Then consumers use the petrol to power the car
• Burning coal to spin turbines to generate electricity to charge a laptop. Consumers use the energy stored in the laptop
• Truck bringing of diesel to transport eggs to consumers

Greenhouse Effect

• Complete combustion: reacts with air to produce carbon dioxide and water
• Incomplete combustion: reacts with air to produce carbon monoxide and water, or carbon and water

With the burning of fossil fuels, we put a lot of carbon dioxide into the air. 

Cause + Effect enhanced

1. Extra burning of fossil fuels
2. Enhanced greenhouse effect
3. Average global temperatures rise
4. Ice melting and sea level rises

• Some sunlight is absorbed by greenhouse gases. However, the kinetic energy formed by the greenhouse gasses moving and the absorbed sunlight, heats up the earth

Combustion

• Combustion: substance burns in the presence of oxygen
  • Reaction between a substance and oxygen that releases heat and light energy
• Fuel: substance that reacts with oxygen (combusts) to produce useful energy
• Many fractions obtained from crude oil are used as fuels because they contain hydrocarbons that burn easily and release a large amount of useful energy
• Complete combustion: lots of oxygen present
  • Hydrocarbon + oxygen —> Carbon dioxide + water + energy
• If there is a shortage of air(oxygen), incomplete combustion takes place
• Incomplete combustion: limited oxygen present
  • Hydrocarbon + oxygen —> Carbon dioxide + water + energy
  • Still yields carbon dioxide and water
  • However, it produces toxic by-products, in CO and C
  • Also releases less energy than complete oxygen
  • Carbon monoxide is poisonous because it reduces the ability of blood to carry oxygen, and reduces the ability of blood to expel carbon dioxide

Balancing equations: Combustion

1. Balance C
2. Balance H
3. If oxygen is an odd number, times everything by 2

Heat of combustion is used to refer to the amount of heat released during a combustion reaction

• Involves combustion of a measured mass of alcohol to heat a measured mass of water over a measured temperature rise
• It is assumed that the total heat released by the combustion reaction will be absorbed by the water

The heat of combustion can be calculated with:$\Delta H=mc\Delta t$

• ∆H: change in enthalpy (j)
• m: mass of heated substance (g)
• c: specific heat capacity of heated substance (˚C)
• ∆T: change in temperature of heated substance

Energy in Chemical Change

Understanding Energy

• Energy: capacity to do work
• Measured in Joules
• Can be transferred or transformed
• Can therefore come in different forms
  • Potential energy
  • Chemical energy
  • Nuclear energy
  • Mechanical energy
  • Elastic energy
  • Gravitational energy
  • Kinetic energy
  • Heat energy
  • Light energy
  • Electrical energy
  • Sound energy
• Enthalpy: changes taking place in a chemical reaction
• Total energy present in a reaction
• △H = H(products) - H(reactants)
• H: enthalpy
• △H: change in enthalpy

2 prefixes: system and surroundings

System: the area in which the chemical reaction happens

Surroundings: Contents outside the system that energy is transferred to/from

2 types of reactions

Exothermic reactions: transfers energy from system to surroundings

• Energy is released
• Bonds are broken
• Heat is transferred/released from system to surroundings

Endothermic reactions: transfers energy from surroundings to system

• Heat is transferred/absorbed from surroundings to system

Energy is stored in chemical bonds, known as chemical potential energy, or bond energy

Exothermic reactions have a negative value for △H, meaning enthalpy has decreased for the duration of the reaction

• Happens if chemical potential energy is converted to particle kinetic energy, e.g. increase in temperature
• Overall chemical potential energy is conserved in the form of heat
• However, this raises the system’s temperature, so heat flows away from the system to the surroundings
• The loss of heat energy from the system to the surroundings means that enthalpy has decreased

Endothermic reactions have a positive value for △H, meaning enthalpy has increased for the duration of the reaction 

• Particle kinetic energy is converted to chemical potential energy
• Reduction of particle kinetic energy means reacting system’s temperature will fall
• Heat flows from the surroundings to the system, increasing enthalpy

Representing Energy Changes in Chemical Reactions

• Enthalpy change can be shown graphically using an energy profile graph, or it can be written along with the equation 
• Sometimes the heat absorbed or released during the reaction is written into the equation, rather than giving the enthalpy change
• When a catalyst is used, less energy is needed for a reaction to occur
• Activation energy: amount of energy required to start/sustain reaction
• catalyst: substance that lowers activation energy of the reaction, and is not consumed during the reaction