WHAT IS THERMOCHEMICAL EQUATION???
Thermochemical equations are just like other balanced equations except they also specify the heat flow for the reaction. The heat flow is listed to the right of the equation using the symbol ΔH. The most common units are kilojoules, kJ. Here are two thermochemical equations:
H2 (g) + ½ O2 (g) → H2O (l); ΔH = -285.8 kJ
HgO (s) → Hg (l) + ½ O2 (g); ΔH = +90.7 kJ
HOW TO WRITE THERMOCHEMICAL EQUATION???
- Coefficients refer to the number of moles. Thus, for the first equation, -282.8 kJ is the ΔH when 1 mol of H2O (l) is formed from 1 mol H2 (g) and ½ mol O2.
- Enthalpy changes for a phase change, so the enthalpy of a substance depends on whether is it is a solid, liquid, or gas. Be sure to specify the phase of the reactants and products using (s), (l), or (g) and be sure to look up the correct ΔH from heat of formation tables. The symbol (aq) is used for species in water (aqueous) solution.
- The enthalpy of a substance depends upon temperature. Ideally, you should specify the temperature at which a reaction is carried out. When you look at a table of heats of formation, notice that the temperature of the ΔH is given. For homework problems, and unless otherwise specified, temperature is assumed to be 25°C. In the real world, temperature may different and thermochemical calculations can be more difficult.
LAWS AND RULES NEEDED TO BE APPLIED WHEN USING THERMOCHEMICAL EQUATION???
- ΔH is directly proportional to the quantity of a substance that reacts or is produced by a reaction.
Enthalpy is directly proportional to mass. Therefore, if you double the coefficients in an equation, then the value of ΔH is multiplied by two. For example:
H2 (g) + ½ O2 (g) → H2O (l); ΔH = -285.8 kJ
2 H2 (g) + O2 (g) → 2 H2O (l); ΔH = -571.6 kJ
- ΔH for a reaction is equal in magnitude but opposite in sign to ΔH for the reverse reaction.
example:
HgO (s) → Hg (l) + ½ O2 (g); ΔH = +90.7 kJ
Hg (l) + ½ O2 (l) → HgO (s); ΔH = -90.7 kJ
This law is commonly applied to phase changes, although it is true when you reverse any thermochemical reaction.
- ΔH is independent of the number of steps involved.
This rule is called Hess's Law. It states that ΔH for a reaction is the same whether it occurs in one step or in a series of steps. Another way to look at it is to remember that ΔH is a state property, so it must be independent of the path of a reaction.
If Reaction (1) + Reaction (2) = Reaction (3), then ΔH3 = ΔH1 + ΔH2
WHAT IS HESS’S LAW???
For a chemical equation that can be written as the sum of two or more steps, the enthalpy change for the overall equation equals the sum of the enthalpy changes for the individual steps. In other words, no matter how you go from given reactants to products (whether in one step or several), the enthalpy change for the overall chemical change is the same.
The heat transferred in a given change is the same whether the change takes place in a single step or in several steps.
ΔHrxn for a given reaction is the same whether that reaction takes place directly through one step or via several different reactions.
S(s) + O2 (g) ---> SO2 (g) ΔH = -296 kJ
SO2 (g) + 1/2 O2 (g) ---> SO3 (g) ΔH = -98.9 kJ
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S(s) + 1 1/2 O2 (g) ---> SO3 (g) ΔH = -394.9 kJ
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S(s) + 1 1/2 O2 (g) ---> SO3 (g) ΔH = -394.9 kJ
In order to use Hess's Law:
1) If a rxn is reversed, the sign of ΔH is reversed.
S(s) + O2 (g) ---> SO2 (g) ΔH = -296 kJ
SO2 (g) ---> S(s)+ O2 (g) ΔH = +296 kJ
2) If the coefficients in a balanced equation are multiplied by some number, the value of ΔH must be multiplied by that same number.
S(s) + O2 (g) ---> SO2 (g) ΔH = -296 kJ
2 S(s) + 2 O2 (g) ---> 2 SO2 (g) ΔH = (2)(-296 kJ) = -592 kJ
WHAT IS THE CHARACTERISTICS OF THERMOCHEMICAL EQUATION???
- Factors affecting the amount of heat evolved or absorbed:
- The amount of reactants and products.
- Physical state of the reactants and products.
- Pressure, temperature and volume.
- Characteristic of a thermochemical equation:
- Thermochemical equation is properly balanced.
- Thermochemical equation gives the value of corresponding to the quantities of substances given by the equation.
- The physical states of the reactants and products are represented by the symbols (s), (l), (g) and (aq), for solid, liquid, gas and aqueous states respectively.
WHAT IS CALORIMETRY???
Calorimetry is the process of measuring the amount of heat released or absorbed during a chemical reaction. Calorimetry is a way of determining whether or not a reaction is exothermic, releases heat, or endothermic, absorbs heat. Calorimetry is a large aspect of thermodynamics, temperature measurements, because it gives the ability to collect data under specific conditions. Calorimetry is also a large part of everyday life, it even controls the metabolic rates in people, and controls our own bodies heat. Calorimetry can take place in any closed container which does not exchange heat with the surrounding environment. Any such container which is closed off from the environment is called a calorimeter.
Heat Capacity
Concepts
When heat is transferred to an object, the temperature of the object increases. When heat is removed from an object, the temperature of the object decreases. The relationship between the heat ( q ) that is transferred and the change in temperature ( ΔT ) is
q = C ΔT = C ( Tf - Ti )
The proportionality constant in this equation is called the heat capacity ( C ). The heat capacity is the amount of heat required to raise the temperature of an object or substance one degree. The temperature change is the difference between the final temperature ( Tf ) and the initial temperature( Ti ).
Quantity | Symbol | Unit | Meaning |
| heat | q | joule (J) | Energy transfer that produces or results from a difference in temperature |
| temperature | T | oC or K | Measure of the kinetic energy of molecular motion |
| temperature change | ΔT | oC or K | Difference between the final and initial temperatures for a process |
| heat capacity | C | J oC-1 or J K-1 | Heat required to change the temperature of a substance one degree |
A calorimeter is an experimental device in which a chemical reaction or physical process takes place. The calorimeter is well-insulated so that, ideally, no heat enters or leaves the calorimeter from the surroundings. For this reason, any heat liberated by the reaction or process being studied must be picked up by the calorimeter and other substances in the calorimeter.
A thermometer is typically inserted in the calorimeter to measure the change in temperature that results from the reaction or physical process. A stirrer is employed to keep the contents of the calorimeter well-mixed and to ensure uniform heating.
The calorimeter shown below contains some water and is equipped with a thermometer, a stirrer, and a heating element. When activated, an electric current is passed through the heating element to generate heat, which is transferred to the calorimeter. In this simulation, you can set the heating rate (units of J sec-1) and the heating time (units of sec).
A calorimeter is a device used to measure the quantity of heat flow in a chemical reaction. Two of the most common types of calorimeters are the coffee cup calorimeter and the bomb calorimeter.
WHAT IS COFFEE CUP CALORIMETER???
A coffee cup calorimeter is essentially a polystyrene (Styrofoam) cup with a lid. The cup is partially filled with a known volume of water and a thermometer is inserted through the lid of the cup so that its bulb is below the water surface. When a chemical reaction occurs in the coffee cup calorimeter, the heat of the reaction if absorbed by the water. The change in the water temperature is used to calculate the amount of heat that has been absorbed (used to make products, so water temperature decreases) or evolved (lost to the water, so its temperature increases) in the reaction.
Heat flow is calculated using the relation:
q = (specific heat) x m x Δt
where q is heat flow, m is mass in grams, and Δt is the change in temperature. The specific heat is the amount of heat required to raise the temperature of 1 gram of a substance 1 degree Celsius. The specific heat of water is 4.18 J/(g·°C).
For example, consider a chemical reaction which occurs in 200 grams of water with an initial temperature of 25.0°C. The reaction is allowed to proceed in the coffee cup calorimeter. As a result of the reaction, the temperature of the water changes to 31.0°C. The heat flow is calculated:
qwater = 4.18 J/(g·°C) x 200 g x (31.0°C - 25.0°C)
qwater = +5.0 x 103 J
In other words, the products of the reaction evolved 5000 J of heat, which was lost to the water. The enthalpy change, ΔH, for the reaction is equal in magnitude but opposite in sign to the heat flow for the water:
ΔHreaction = -(qwater)
Recall that for an exothermic reaction, ΔH < 0; qwater is positive. The water absorbs heat from the reaction and an increase in temperature is seen. For an endothermic reaction, ΔH > 0; qwater is negative. The water supplies heat for the reaction and a decrease in temperature is seen.
WHAT IS BOMB CALORIMETER???
A coffee cup calorimeter is great for measuring heat flow in a solution, but it can't be used for reactions which involve gases, since they would escape from the cup. The coffee cup calorimeter can't be used for high temperature reactions, either, since these would melt the cup. A bomb calorimeter is used to measure heat flows for gases and high temperature reactions.
A bomb calorimeter works in the same manner as a coffee cup calorimeter, with one big difference. In a coffee cup calorimeter, the reaction takes place in the water. In a bomb calorimeter, the reaction takes place in a sealed metal container, which is placed in the water in an insulated container. Heat flow from the reaction crosses the walls of the sealed container to the water. The temperature difference of the water is measured, just as it was for a coffee cup calorimeter. Analysis of the heat flow is a bit more complex than it was for the coffee cup calorimeter because the heat flow into the metal parts of the calorimeter must be taken into account:
qreaction = - (qwater + qbomb)
where qwater = 4.18 J/(g·°C) x mwater x Δt
The bomb has a fixed mass and specific heat. The mass of the bomb multiplied by its specific heat is sometimes termed the calorimeter constant, denoted by the symbol C with units of joules per degree Celsius. The calorimeter constant is determined experimentally and will vary from one calorimeter to the next. The heat flow of the bomb is:
qbomb = C x Δt
Once the calorimeter constant is known, calculating heat flow is a simple matter. The pressure within a bomb calorimeter often changes during a reaction, so the heat flow may not be equal in magnitude to the enthalpy change.
In a Coffee-Cup Calorimeter, reactants are mixed in a solution, usually water, and measure the temperature change. Since styrofoam acts as a very effective insulator, little heat transfer takes place between the cup and the surrounding air. Everything within the cup, including the cup itself, is considered an isolated system. The heat of reaction is clearly defined as "the quantity of heat that would be exchanged with the surroundings in restoring the calorimeter to its initial temperature." But the calorimeter cannot be restored back to its original conditions, which is why we take the negative quantity of heat producing the temperature change in the calorimeter.
Which means,
Note that,
Temperature can be in Kelvin or Celcius, and the change in temperature, , is calculated by taking .
