All of the processes involved in science have corresponding energy changes, however there are some chemical and physical changes that are of particular interest, either historically, or because they can be used to obtain other information about energy changes that are difficult to find directly.
The actual value of a specific change can be affected by the temperature at which it is measured, so a standard set of conditions is defined to ensure that the data produced in an experiment is relevant. Standard state conditions are 1 atmosphere pressure and molar quantities.
Note: This is not to be confused with S.T.P. which, in terms of the gas laws, is defined as 273K and 1 atmosphere pressure. Energy measurements are usually quoted under standard conditions and represented by the subway sign (shown right, more correctly called a superscript plimsoll line). The temperature, while not formally included in standard state conditions is quoted and is usually 298K. In common with many theoretically calculated values, reaction enthalpies often differ from values found experimentally. In this section we examine the reasons behind experimental deviation.
Syllabus reference R1.1.4Reactivity 1.1.4 - The standard enthalpy change for a chemical reaction, ΔH⦵, refers to the heat transferred at constant pressure under standard conditions and states. It can be determined from the change in temperature of a pure substance.
- Apply the equations Q = mcΔT and ΔH = − Q/n in the calculation of the enthalpy change of a reaction.
Guidance
- The units of ΔH⦵ are kJ mol–1.
- The equation Q = mcΔT and the value of c, the specific heat capacity of water, are given in the databooklet.
Tools and links
- Tool 1, Inquiry 1, 2, 3 - How can the enthalpy change for combustion reactions, such as for alcohols or food, be investigated experimentally?
- Tool 1, Inquiry 3 - Why do calorimetry experiments typically measure a smaller change in temperature than is expected from theoretical values?
Calorimetry
Literally, the measurement of heat, calorimetry decribes the procedure of measuring the heat change in a chemical or physical process. There are several types of experiment that can be performed, but they all involve measurement of temperature before and after the change.
Extensive and intensive property
Energy is an extensive property in that the amount of energy involved in a process is directly dependent on the amount of matter. In other words, increase the amount of matter and you increase the amount of energy.
Intensive properties of matter do not change with the amount of matter. For example, the density of a substance remains the same regardless of the mass.
Standardisation of data
To make the data gathered in calorimetry consistent, we choose to define both the conditions and the amount of matter involved.
Standard conditions refer to molar amounts of matter, solution concentrations of 1 mol dm-3 and ambient pressure of 1 atmosphere (101.3 kPa). These standard conditions are denoted using a superscript plimsole sign, o.
Hence, the standard combustion enthalpy is defined as the energy released when 1 mol of a substance is burned in excess air or oxygen. It is writtern as ΔHco.
Heat capacity
All systems are made up of components and substances that have different structures and different fundamental particles. Consequently they have differing abilities to absorb heat energy, and produce different temperature changes on absorption of energy. This is called the heat capacity of the system. It literally means the capacity, or absorbing ability that a substance, or system, has for heat energy.
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Example: A calorimeter can absorb 100kJ of energy with a resultant increase in temperature of 1ºC. This means that whenever the calorimeter absorbs 100kJ its temperature increases by 1ºC. In other words, its heat capacity is 100 kJ ºC-1. If it were to absorb 200kJ then its temperature would increase by 2ºC, if it were to absorb 300kJ it would increase in temperature by 3ºC, etc etc. |
If the heat capacity of a system is known, or found by, calibration (using heating coils, or known chemical reactions), this can then be used to measure the energy released by other chemical reactions.
Specific heat capacity
The amount of energy that a given mass of substance (either 1 g or 1 kg) can absorb that produces a 1ºC increase in temperature is called it's specific heat capacity. This is sometimes given the symbol 'shc' or simply 'c' .
The specific heat capacity of water is 4.18 kJ kg-1 ºC-1. This means that when 1 kg of water absorbs 4.18 kJ of energy its temperature will increase by 1ºC.
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Example: Calculate the amount of energy needed to raise the temperature of a bathtub containing 100kg of water by 20ºC. (ignore the energy required to heat the material of the bathtub itself.) E = mcΔT Energy needed = 100kg x 4.18 kJ kg-1 ºC-1 x 20ºC Energy needed = 100 x 4.18 x 20 = 8360 kJ |
Water has a relatively high specific heat capacity. Metals, such as copper, have much lower specific heat capacities, so the temperature rise is greater for the same input of energy. The following table shows the specific heat capacity of some substances and the effect in terms of temperature change when 100g of each substance is provided with 1 kJ of energy.
kJ/kg ºC
The above table illustrates the large capacity that water has to absorb energy when compared to metals.
Water having a low relative molecular mass has more particles per unit mass able to absorb energy, while maintaining the average energy at a lower value. The temperature of any substance is proportional to the average energy of the particles in the material.
Experimental determination of energy change
There are two main methods used to determine energy change:
- By direct measurement of the reaction mixture
- Collection of the energy released by absorption in water.
Example calorimetry experiments
1. The enthalpy of combustion of an alcohol.
2. The enthalpy of solution of an ionic compound