Energy from reactions
Energy changes take place during chemical reactions. Exothermic reactions give out thermal energy and endothermic reactions take in thermal energy. These changes can be measured experimentally or calculated before being analysed. Knowing the amount of energy involved in a reaction can be used to ensure that resources are used efficiently.
Measurimg energy transfers; Heat energy can be given out or taken in from the surroundings during chemical reactions. The amount of energy transferred can be measured. This is called calorimetry.
Energy changes from combustion
To do the experiment:
measure cold water into a calorimeter (a metal or glass container)
record the starting temperature of the water
heat the water using the flame from the burning fuel
record the final temperature of the water
Energy from reactions 2
The spirit burner containing the fuel is usually weighed before and after the experiment - in this way, the mass of the fuel burned can be found. Knowing the mass of fuel burnt and the temperature change in the water, it is then possible to calculate the energy released by the fuel. This method also works for finding the amount of energy released by foods.The biggest source of error is usually heat loss to the surroundings. This can be reduced by insulating the sides of the calorimeter and adding a lid.
energy changes from reactions in solution
To do this experiment:
add a known volume of the first reactant (in solution) to the insulated container
record the starting temperature of the liquid
add the second reactant (either in solution or as a solid powder)
replace the lid and stir the reaction mixture
record the maximum temperature that the reaction mixture reaches
calculating energy changes
The amount of energy transferred during a chemical reaction (either from the burning of a fuel or a chemical reaction in solution) can be calculated using the equation:
Q = mc ΔT
Q = the heat energy transferred (joule, J)
m = the mass of the liquid being heated (grams, g)
c = the specific heat capacity of the liquid (joule per gram degree Celsius, J/g°C)
ΔT = the change in temperature of the liquid (degree Celsius, °C)
The specific heat capacity of water is 4.2 J/g°C. This value is also used when the liquid being heated is not water. For example if an acid, alkali or other solution is being used.Energy is normally measured in joules, J. However sometimes the amount of energy can be given in other units, including kilojoules, kJ (1 kJ = 1000 J), kJ per mole and kJ per gram.The energy content of food is often measured in calories and calories per gram. In the exam, you will be given a conversion factor if you are asked to convert from calories to joules.
Bonds and chemical reactions
During a chemical reaction:
Energy is needed to break bonds, and energy is released when bonds are made.
Exothermic reactions give out heat energy to the surroundings. Exothermic reactions have a negative energy change.Some examples of exothermic reactions are:
Bonds and chemical reactions 2
Activation energy and catalysts
Simple energy level diagrams only show the energy levels at the beginning and end of a reaction. Energy levels change gradually during a reaction, and this can be shown using a curve between the reactant and product energy levels.
This is an exothermic reaction because the energy level of the reactants is higher than the energy level of the products. However, the energy curve goes up from the reactants’ energy level to begin with, then drops to the products’ energy level. This is because many reactions need an input of energy to start the reaction off. This is energy is called the activation energy. It is represented on an energy level diagram as the difference between the reactants’ energy level and the top of the curve. For example, burning methane in a Bunsen burner:
methane + oxygen → carbon dioxide + water
CH4 + 2O2 → CO2 + 2H22O
The activation energy must be supplied in the form of a flame or a spark to get the methane to ignite. Once the reaction begins, it gives out energy to the surroundings so it is exothermic.
Activation energy and catalysts 2
A catalyst is a substance that speeds up the rate of a chemical reaction without being used up in the reaction.
Catalysts can do this because they provide a different pathway for the reaction to follow. This pathway has lower activation energy than the one followed by the uncatalysed reaction. As a result, a greater proportion of reacting particles have enough energy to react.
Lowering the activation energy has many advantages. It means that reactions happen more quickly and are more economical in terms of the energy required for industrial-scale reactions.
Hydrogen as a fuel
Hydrogen is often seen as an environmentally-friendly alternative to fossil fuels. Some car manufacturers have developed cars than run on hydrogen rather than petrol or diesel.
There are two ways in which hydrogen is used to power cars:
- Burning hydrogen directly in the engine
Water is the only product formed when hydrogen burns:
hydrogen + oxygen → water
2H2 + O2 → 2H2O
There are no carbon dioxide emissions that could contribute to global warming.
- Hydrogen fuel cells
In a hydrogen fuel cell, hydrogen reacts with oxygen without burning. The energy released is used to generate electricity, which is used to drive an electric motor.
Hydrogen as a fuel 2
Problems with hydrogen
At the moment, most hydrogen is made by reacting steam with coal or natural gas - both non-renewable resources.
Hydrogen can also be made by passing electricity through water. Unfortunately, most electricity is generated using coal and other fossil fuels, so pollution from burning these fuels happens at the power station. Pollution therefore still occurs.
However, some countries are producing hydrogen using electricity from renewable sources, such as geothermal energy in Iceland.
Advantages and Disadvantages of Hydrogen as a fuel
- unlike petrol and diesel, hydrogen does not generate carbon dioxide when burnt
- hydrogen fuel cells are very efficient
- few filling stations sell hydrogen
- hydrogen must be compressed and liquefied, and then stored in tough, insulated fuel tanks
- atmospheric pollution may be generated during the production of hydrogen
- hydrogen fuel cells do not work at very low temperatures, and they may also require a platinum catalyst (platinum is expensive and prone to contamination by impurities)