Back to Thermochemical Equations
EnergyA system's capacity to do work. changes which accompany chemical reactions are almost always expressed by thermochemical equations, such as
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) (25°C, 1 atmAbbreviation for atmosphere, a unit of pressure equal to 101.325 kPa or 760 mmHg. pressureForce per unit area; in gases arising from the force exerted by collisions of gas molecules with the wall of the container.)
ΔHm = –890 kJ (1)
Here the ΔHm (delta H subscript m) tells us whether heatEnergy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles. energy is released or absorbed when the reaction occurs as written, and also enables us to find the actual quantity of energy involved. By convention, if ΔHm is positive, heat is absorbed by the reaction; i.e., it is endothermicIn chemical thermodynamics, describes a process in which energy is transferred from the surroundings to the system as a result of a temperature difference.. More commonly, ΔHm is negative as in Eq. (1), indicating that heat energy is released rather than absorbed by the reaction, and that the reaction is exothermicDescribes a process in which energy is transferred to the surroundings as a result of a temperature difference.. This convention as to whether ΔHm is positive or negative looks at the heat change in terms of the matterAnything that occupies space and has mass; contrasted with energy. actually involved in the reaction rather than its surroundings. In the reaction in Eq. (1), the C, H, and O atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. have collectively lost energy and it is this loss which is indicated by a negative value of ΔHm.
It is important to notice that ΔHm is the energy for the reaction as written. In the case of Equation (1), that represents the formation of 1 mol of carbon dioxide and 2 mol of water. The quantity of heat released or absorbed by a reaction is proportional to the amount of each substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. consumed or produced by the reaction. Thus Eq. (1) tells us that 890.4 kJ of heat energy is given off for every moleThat chemical amount of a substance containing the same number of units as 12 g of carbon-12. of CH4 which is consumed. Alternatively, it tells us that 890.4 kJ is released for every 2 moles of H2O produced. Seen in this way, ΔHm is a conversion factorA relationship between two units of measure that is derived from the proportionality of one quantity to another; for example, the mass of a substances is proportional to its volume and the conversion factor from volume to mass is density. enabling us to calculate the heat absorbed or released when a given amount of substance is consumed or produced. If q is the quantity of heat absorbed or released and n is the amount of substance involved, then
EXAMPLE 1 How much heat energy is obtained when 1 kg of ethane gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container., C2H6, is burned in oxygen according to the equation:
2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(l)
ΔHm = –3120 kJ (3)
SolutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture. The massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object. of C2H6 is easily converted to the amount of C2H6 from which the heat energy q is easily calculated by means of Eq. (2). The value of ΔHm is –3120 kJ per per 2 mol C2H6. The road map is
= − 51 879 kJ = − 51.88 MJ
Note: By convention a negative value of q corresponds to a release of heat energy by the matter involved in the reaction.
The quantity ΔHm is referred to as an enthalpyA thermodynamic state function, symbol H, that equals internal energy plus pressure x volume; the change in enthalpy corresponds to the energy transferred as a result of a temperature difference (heat transfer) when a reaction occurs at constant pressure. change for the reaction. In this context the symbol Δ (delta) signifies change in” while H is the symbol for the quantity being changed, namely the enthalpy. We will deal with the enthalpy in some detail in Chap. 15. For the moment we can think of it as a property of matter which increases when matter absorbs energy and decreases when matter releases energy.
It is important to realize that the value of ΔHm given in thermochemical equations like (1) or (3) depends on the physical state of both the reactants and the products. Thus, if water were obtained as a gas instead of a liquidA state of matter in which the atomic-scale particles remain close together but are able to change their positions so that the matter takes the shape of its container in the reaction in Eq. (1), the value of ΔHm would be different from -890.4 kJ. It is also necessary to specify both the temperatureA physical property that indicates whether one object can transfer thermal energy to another object. and pressure since the value of ΔHm depends very slightly on these variables. If these are not specified [as in Eq. (3)] they usually refer to 25°C and to normal atmospheric pressure.
Two more characteristics of thermochemical equations arise from the law of conservation of energy. The first is that writing an equation in the reverse direction changes the sign of the enthalpy change. For example,
H2O(l) → H2O(g) ΔHm = 44 kJ (4a)
tells us that when a mole of liquid water vaporizes, 44 kJ of heat is absorbed. This corresponds to the fact that heat is absorbed from your skin when perspiration evaporates, and you cool off. CondensationThe process in which a liquid forms from gas or vapor of the same substance. A chemical reaction in which two molecules combine to form a very small molecule and a larger molecule than either of the two reactants. of 1 mol of water vapor, on the other hand, gives off exactly the same quantity of heat.
H2O(g) → H2O(l) ΔHm = –44 kJ (4b)
To see why this must be true, suppose that ΔHm [Eq. (4a)] = 44 kJ while ΔHm [Eq. (4b)] = –50.0 kJ. If we took 1 mol of liquid water and allowed it to evaporate, 44 kJ would be absorbed. We could then condense the water vapor, and 50.0 kJ would be given off. We could again have 1 mol of liquid water at 25°C but we would also have 6 kJ of heat which had been created from nowhere! This would violate the law of conservation of energy. The only way the problem can he avoided is for ΔHm of the reverse reaction to be equal in magnitude but opposite in sign from ΔHm of the forward reaction. That is,
ΔHm forward = –ΔHm reverse