Three-quarters of Earth's surface is covered with a substance that is a very effective heat-storage material—water. Liquid H2O can absorb and store a tremendous amount of heat energy without becoming too hot itself. The effectiveness of a substance at storing heat energy depends on a parameter called specific heat.
Specific heat is a measure of how much energy something absorbs compared to how hot it gets. More precisely, the specific heat of a substance is the amount of energy it takes to raise the temperature of 1 gram of that substance by 1 degree Celsius.
Watch a short video from that demonstrates a "trick" that depends on the high specific heat of water.
from Saint Mary's University
Quantifying Specific Heat
Quantitative experiments show that 4.18 Joules of heat energy are required to raise the temperature of 1g of water by 1°C. Thus, a liter (1000g) of water that increased from 24 to 25°C has absorbed 4.18 J/g°C x 1000g x 1°C or 4180 Joules of energy. For comparison, alcohol (ethanol) has a lower specific heat: it takes only 2.2 Joules of energy to increase the temperature of one gram of ethanol by one degree Celsius.
To calculate the amount of heat energy gained or lost by a substance, multiply the mass of the substance by its specific heat constant multiplied by the change in temperature.
q = m x C x ΔT
where: q = heat energy in Joules (J) m = mass of the substance in grams (g) C = specific heat for that substance in Joules per gram per degree Celsius (J/g°C)
ΔT = change in temperature in degrees Celsius (°C) Change is calculated by subtracting initial temperature from final temperature (Tf - Ti)
Page 2
- View this brief animation, made up of 18 separate images of colored satellite heat data. Use the color key at the top of the animation to help determine the ocean temperature. Focus on interpreting the current as a supply of heat energy to the Gulf of Mexico.
To view this video please enable JavaScript, and consider upgrading to a web browser that supports HTML5 video
Gulf of Mexico's Loop Current - Next, visit the NOAA View Data Exploration Tool opens in new window The link should take you to the Sea Surface Temperature data map.
- Zoom in to the Gulf of Mexico so that you can see the same area of ocean you saw in the earlier animation.
- On the left panel, click Weekly Data to see the data as weekly data averages, and use the date slider to back the map data up to at least a year from current data. Then click the box labeled Data Values – this will allow you to use your mouse to view the temperature where your mouse hovers on the data map. Hide
- Then hit the play button and watch at least a full year of the current. It may take a few minutes to play smoothly, but watch how the warmer water moves within the Gulf of Mexico. Next, slide the starting date to sometime around January of 2005 and watch how the Loop Current changes through the year. Take note of how it compares to the year you looked at and how it compares to the earlier animation.
- Consider the three forms of visual data - the image, the color animation, and the NOAA animation. What are some benefits of using one instead of the others? Do you get a better sense of what's happening by using all three?
« Previous Page Next Page »
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.
| 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.
top
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.
| 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.) 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.
| Material | specific heat capacity kJ kg-1 ºC-1 | Temperature change when 100g receives 1 kJ |
| Aluminium | 0.897 | 11ºC |
| Copper | 0.385 | 26ºC |
| Gold | 0.129 | 77ºC |
| Iron | 0.450 | 22ºC |
| Water | 4.18 | 2.4ºC |
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.
top
Page 2
Page 3
Syllabus ref: 5.1
Chemistry deals with observation of the world around us and explanation of the things that happen. When chemical and physical processes happen they often do so with a change in the temperature of the medium, or surroundings. The study of energetics, or thermodynamics, seeks to explain the underlying causes of this phenomenon and to use it to make predictions as to why processes do, or don't occur.
| Nature of science: Fundamental principle-conservation of energy is a fundamental principle of science. Making careful observations-measurable energy transfers between systems and surroundings. |
Understandings
Essential idea: The enthalpy changes from chemical reactions can be calculated from their effect on the temperature of their surroundings.
Heat is a form of energy. Temperature is a measure of the average kinetic energy of the particles.
Total energy is conserved in chemical reactions.
Chemical reactions that involve transfer of heat between the system and the surroundings are described as endothermic or exothermic.
The enthalpy change (ΔH) for chemical reactions is indicated in kJ mol-1.
ΔH values are usually expressed under standard conditions, given by ΔH°, including standard states.
Applications and skills
Calculation of the heat change when the temperature of a pure substance is changed using q=mcΔT
A calorimetry experiment for an enthalpy of reaction should be covered and the results evaluated.
Page 4
The energy concept
It is an observation that when substaces are heated their temperature increases. Although this may seem like an obvious statement, it is important to understand the processes that are involved. To do this we have to invoke the concepts of energy and temperature.
Energy is a concept that is very difficult to define without using examples. Certain forms of energy are easy to understand due to common experience, such as kinetic energy.
We appreciate that a moving body possesses 'kinetic energy' and that if something gets in its way then there will be collision and an effect (breakage, noise, deformation etc).
As the kinetic theory tells us that all particles are in motion then we can refer kinetic energy to this motion and say that the particles have energy due to their movement. The total energy is the product of the average kinetic energy of a particle multiplied by the number of particles.
However, particles are not only in translational motion, they are also capable of vibrations and rotations. The total amount of motional energy they possess is a combination of these.
When a substance is given energy in the form of heat, the particles of the material respond by increasing their kinetic energy. The vibrations, rotations and translations of the particles in the material increase. We say that the substance has become 'hotter', although in fact all that has happened is that the particles are now moving faster.
In order to quantify this increase in particular kinetic energy, we must have a means of measuring and from here arises the idea of temperature scales and thermometers.
top
Temperature
Temperature is a measure the average heat energy content of the system under study.
The concept of 'heat' is easy to understand, as we have senses capable of detecting changes in heat. We say that something is warm or cold. However, the sensation of heat is just that, a sensation, something interpreted by the brain and understood by experience. What is actually occurring is a transfer of vibrations from the heat source to the sensors in the skin. These sensors produce an electrical signal that travels through nerve fibres to the brain, where we experience the sensation.
Our idea of 'heat' is simply the degree of vibrations experienced by the sensory cells. Clearly, this is OK for biological necessities, but hardly empirical. Scientists soon realised the need of enumerating this hot/cold reflex, and invented the thermometer.
Recognising that certain materials respond to heat/cold by expansion/contraction, it was possible to enclose a sample of a suitable material into a glass tube and to watch it expand or contract as the local environment temperature changed. A scale was needed that was common experience for people all over the world. The two key reference points on the scale chosen were the freezing point (melting point) and boiling point of water. A scale was constructed between these two points and subdivided into 100 units (the Celsius scale). Nowadays the Celsius scale is the scale of choice for most purposes.
The thermometer responds to the average motion of the particles in the environment being measured. It should be recognised that when the thermometer registers 80ºC, it is informing us that the motion of the particles in the substance being measured has caused motion of the particles in the mercury, resulting in the mercury in the thermometer expanding to the 80º mark on the Celsius scale.
The Celsius scale is a relative scale, relative to the melting and boiling points of water. A more scientific scale is the absolute scale of temperature measurement in which the magnitude of a degree is the same as 1 degree Celsius, but the zero of the scale starts at absolute zero. This is also called the Kelvin scale; the unit of measurement is the Kelvin, K.
|
273 Kelvin = 0º Celsius 373 Kelvin = 100º Celsius |
Absolute zero is the temperature at which there is no particle motion whatsoever. It is equal to -273.16 ºC under current definitions.
Quick check time
top
Heat energy
The total energy of a sample is a function of both the temperature and the mass of particles present. The temperature gives a measure of the average kinetic energy of the particles in the system.
| Example: Which contains the most energy, a bathful of lukewarm water or a spark at 2000ºC? A spark may have a temperature of 2000ºC, but it contains less energy than a bathful of lukewarm water. |
In reality, it is difficult to measure the absolute quantity of kinetic energy contained within a sample and it is of limited value. We are more concerned with changes in the heat energy, as this also reflects changes in the chemical energy of the system, as we shall see in the next section.
| It is important to remember that all substances at the same temperature have the same average kinetic energy. |
top