Thermochemistry is all about the energy changes that happen alongside chemical reactions.
Chemical reactions either release heat (exothermic) or absorb heat (endothermic) – in many instances, the heat can be used to do useful work such as in a battery or engine.
Definitions:
Heat is the energy transferred as a result of temperature differences.
Work is done as a result of motion against an opposing force.
The universe is made up of the system (the part of the universe being studied) and the surroundings (the rest of the universe). This is useful in helping us follow the flow of energy and matter during a chemical reaction.
Neither energy nor matter (atoms) can be created of destroyed during a chemical reaction so either they are moving from the system to the surroundings, or vice versa.
These are both examples of open systems (both energy and matter can be exchanged with the surroundings). In a closed system, only energy can be exchanged with the surroundings as the system is sealed. In an isolated system, neither energy or matter is exchanged, maybe because the reaction is happening inside a vacuum flask.
Heat transfers result in a temperature change – different substances need different amounts of heat energy to raise the temperature of 1g of that substance by 1K. This is called the specific heat capacity (C) of the substance.
Water has a specific heat capacity of 4.18 JK-1g-1 which means it takes 4.18J of heat to raise the temperature of 1g of water by 1K.
Practice questions
- Calculate the energy needed to raise the temperature of 25cm3 of water from 0°C to 25°C, given that the specific heat capacity is 4.18 JK-1g-1 and water has a density of 1 gcm-3 (you need to know this too!).
- Calculate the energy needed to raise the temperature of 0.5kg of copper by 50°C given that its specific heat capacity is 0.38 JK-1g-1.
Answers
- 2612.5J 2. 9500J
Since most chemical reactions happen in an open system, they are happening at constant pressure (atmospheric pressure doesn’t change). The heat transferred during a reaction at constant pressure is called the enthalpy change, ΔH.
Heat changes during reactions are the the result of bonds being broken (an endothermic process) and made (an exothermic process).
- in an exothermic reaction, ΔH is negative because from the point of view of the chemical system, energy is lost to the surroundings (whose temperature increases)
- in an exothermic reaction, more energy is released when new bonds form in the products than was used to break the bonds in the reactants.
- activation energy, Ea, is the energy needed to break bonds in the reactants and kick start the reaction – this is why we need a match to light the methane (often there is enough energy in the surroundings to kick start the reaction e.g. magnesium reacting with acid)
- in an endothermic reaction, ΔH is positive because the reaction has taken in energy from the surroundings (whose temperature decreases)
We can’t measure the enthalpy of a substance directly but we can measure the enthalpy change for a reaction
ΔH = H(products) – H(reactants)
The standard enthalpy change for a reaction is given by:
