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In any chemical reaction, heat can be either absorbed or released to the surroundings. The temperature of exchange between a chemical reaction and its surroundings is called the enthalpy of the reaction, denoted by H. However, H cannot be measured directly, measuring the change in temperature instead. The magnitude of the reaction over time is used to calculate the change in enthalpy over time (denoted ∆H ). Knowing the ∆H of a reaction, we can determine whether the reaction is endothermic (the heat of the reaction is taken from the environment) or exothermic (the heat of the reaction is released to the environment). Where m is the mass of the reactant, s is the specific heat of the product, ∆T is the change in temperature during the reaction, we have ∆H = m x s x ∆T .
Steps
Solving the Enthalpy Problem
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Identify reactants and products. Every chemical reaction consists of reactants and products. Products are chemical substances produced by chemical reactions, and reactants are substances that “interact, combine, or break down” to form products. In other words, the reactants of a chemical reaction can be thought of as the ingredients of a recipe, while the product is the finished dish. To find the ∆H of a reaction, we first need to determine the reactants and products of that reaction.
- For example, find the enthalpy of the reaction that forms water from hydrogen gas and oxygen gas. 2H 2 (Hydrogen gas) + O 2 (Oxygen gas) → 2H 2 O (Water). In this reaction, H 2 and O 2 are the reactants, H 2 O are the products.
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Determine the total mass of the reactants. The next step is to determine the mass of the reactant. If you do not know this value, or cannot measure it, you can use specific gravity to determine their actual mass. Density is a constant that can be found in the periodic table (for an element) or based on other sources (for molecules or compounds). By multiplying the density by the number of mp of the reactant, you will find the mass of the reactant.
- In the above example, the reactants are hydrogen gas and oxygen gas, with densities of 2 grams and 32 grams, respectively. Since we use 2 mp of hydrogen (determined by the factor “2” before H 2 in the reaction, and 1 mp of oxygen (determined by the factor “1” before H 2 in the reaction), we get the total The mass of reactants is as follows:
2 × (2g) + 1 × (32g) = 4g + 32g = 36g
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Determine the specific heat capacity of the product. Next, we need to determine the specific heat of the product of the reaction we are considering. Each element or molecule has its own value of heat capacity: this value is deterministic and is often given in chemical sources, such as textbook appendices. There are many ways to determine the specific heat, however, according to the formula we are using, this value is given in units of Jun/gram °C.
- Note, if the reaction equation has multiple products, you need to calculate the enthalpy for the component reactions to form each product, then add these values together to get the enthalpy of the entire reaction. .
- In the example of a reaction that forms water from hydrogen and oxygen gas, the final product of the reaction is water, the specific heat of water is about 4.2 Jun/gram °C .
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Determine the temperature variation of the reaction. In this step, we will calculate ∆T, i.e. the change of temperature before and after the reaction. ∆T is the difference between the post-reaction temperature (T2) and the initial reaction temperature (T1). As with most chemistry problems, we need to use degrees Kelvin (degrees K), although degrees Celsius can also be used and will give the same result.
- For the above water forming reaction, if the initial temperature of the reaction is 185K and when the reaction ends, the temperature is 95K. Thus, ∆T is calculated as follows:
∆T = T2 – T1 = 95K – 185K = -90K
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Use the formula ∆H = m x s x ∆T. Once we have the m-value, i.e. the mass of reactants, the s-value, the specific heat of products, and ∆T, the temperature difference before and after the reaction, we can calculate the enthalpy of the reaction. react by substituting the above values into the formula ∆H = m x s x ∆T, unit is Jun (J).
- With the above example, we will calculate the enthalpy of the reaction as follows:
∆H = (36g) × (4.2 JK-1 g-1) × (-90K ) = -13,608 J
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Determine the thermal properties of the reaction. One of the common reasons for calculating the ∆H of reactions is to see if it is exothermic (loss of energy as heat) or endothermic (gaining energy and absorbing heat). If the sign of the enthalpy ∆H result is positive, this is an endothermic reaction. Conversely, if the sign of ∆H is negative, it is an exothermic reaction. The larger this value is, the greater the gain or exothermicity of the reaction. Care must be taken with strongly exothermic reactions because these reactions can give off a large amount of energy and, if the reaction is rapid, can cause an explosion.
- In the example under consideration, the final result we get is -13608 J. Since the sign is negative, this reaction is ‘ exothermic . This makes perfect sense because — H 2 and O 2 are gaseous, while H 2 O, the product of the reaction, is liquid. Hot gases (which exist in the form of vapors) need to radiate energy to the environment as heat to a certain extent to convert to liquid form, that is, the formation of H 2 O will generate heat.
Estimating Enthalpy
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Use binding energy to estimate enthalpy. Almost all chemical reactions involve the making or breaking of bonds between atoms. Since in a chemical reaction energy can only be created or lost, if we know the energy required to make (or break) the bonds in a reaction, then add them all up. , we can estimate the enthalpy change of the entire reaction accurately.
- For example, consider the reaction H 2 + F 2 → 2HF. In this case, the energy to break the bond between two H atoms of H 2 molecule is 436 kJ/mp, the energy required to form F 2 is 158 kJ/mp. [1] X Research Source So, the energy required to form HF from H and F is: -568 kJ/mp. [2] X Research Source Multiply this by 2, since the product of the reaction is 2 HF, we get 2 × -568 = -1136 kJ/mp. Adding all these energy values together we get:
436 + 158 + -1136 = -542 kJ/mp .
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Use standard enthalpy to estimate enthalpy. Standard enthalpy is a definite value of ∆H that characterizes the change in enthalpy in a reaction forming a substance. If you know the standard enthalpy values needed to form products and reactants in a chemical reaction, you can add them together to estimate the enthalpy as for the bond energy usage outlined above.
- For example, consider the reaction C 2 H 5 OH + 3O 2 → 2CO 2 + 3H 2 O. In this case, we know the standard enthalpy values of the component reactions as follows: : [3] X Research source rescue
C 2 H 5 OH → 2C + 3H 2 + 0.5O 2 = 228 kJ/mp
2C + 2O 2 → 2CO 2 = -394 × 2 = -788 kJ/mp
3H 2 + 1.5 O 2 → 3H 2 O = -286 × 3 = -858 kJ/mp
We can add these component reactions to get the reaction equation as C 2 H 5 OH + 3O 2 → 2CO 2 + 3H 2 O, this is the reaction we are looking for enthalpy, so we can Add the enthalpy of the component reactions above to get the enthalpy of the total reaction as follows:
228 + -788 + -858 = -1418 kJ/mp .
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You need to remember to change the sign when switching sides of the equations. An important point to remember is that when using the formation enthalpy of the half-reactions to calculate the enthalpy of the total reaction, you need to change the sign of the enthalpy when switching sides of the reaction’s components. In other words, if you reverse the formation reactions so that the reactants or products can be eliminated, you need to change the enthalpy sign of the reactions that have been reversed.
- In the above example, we can see that the reaction to form C 2 H 5 OH is used in the opposite direction. C 2 H 5 OH → 2C + 3H 2 + 0.5O 2 shows that C 2 H 5 OH is decomposed but not formed. Since we have reversed the direction of the reaction so that we can reasonably cancel out the components, we need to change the sign of the enthalpy of the reaction, so we get a value of 228 kJ/mp. Actually, the enthalpy of the reaction forming C 2 H 5 OH is -228 kJ/mp.
Observing Enthalpy Variations Experimentally
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Take a clean jar and fill it with water. The variation of enthalpy can be easily observed with simple experiments. Clean and disinfect the test vessel you want to use to ensure that no impurities are mixed into the reaction. Scientists use a sealed container called a calorimeter to measure enthalpy, but using a small glass jar can also help you observe the same phenomenon. Fill the jar you cleaned with clean room temperature water. The experiment should also be carried out indoors where the temperature is cool.
- To perform this experiment, prepare a small neutral flask. We’ll examine the enthalpy change effect of Alka-Seltzer (or any effervescent tablet) in water. So, the less water you use, the easier it is to notice a change in temperature.
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Place a thermometer in the jar. Take a thermometer and fix it to the jar so that the bulb of the thermometer is below the water level in the jar. Read the temperature of the water; We take this temperature as the initial temperature T1.
- Suppose we measure the temperature of water as 10 degrees Celsius. In the next steps we will use this value to describe the principle of enthalpy.
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Take an Alka-Seltzer pill and drop it into the bottle. When you’re ready, drop an Alka-Seltzer pill into the bottle. The pill will bubble up immediately. When the tablet is dissolved in water, it decomposes to form bicarbonate (HCO 3– ) and citric acid (citric acid reacts as hydrogen ion H + ). These substances will react with each other to form water and CO 2 according to the equation 3HCO 3− + 3H + → 3H 2 O + 3CO 2 .
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Measure the temperature at the end of the reaction. Observe the reaction that occurs; The tablet will slowly dissolve into the water. As soon as the tablet dissolves, measure the temperature again. You will see that the temperature of the water will now be lower than the original T1 temperature. If you measure a higher temperature then the reaction has been affected by external factors (for example, the room where you performed the experiment is too hot).
- Assume the temperature when the tablet is completely dissolved is 8 degrees Celsius.
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Estimate the enthalpy of the reaction. In the case of the experiment achieving ideal conditions, when you drop the Alka-Seltzer pellet into the water, the water and CO2 gas (which is the rising bubble) will be formed and cause the temperature of the water to drop. From this information, we can infer that this is an endothermic reaction; i.e., this reaction will receive heat from the surroundings. The liquid reactants that dissolve during the reaction need energy to convert to gaseous products, so these will take energy in the form of heat from the environment (in this case, water). Therefore, the temperature of the water will decrease.
- In this example experiment, the temperature of the water was reduced by 2 degrees Celsius after the Alka-Selzter tablet had completely dissolved. This is completely consistent with the statement we make, that this is an exothermic reaction.
Advice
- The calculations here use the temperature unit Kelvin (K) – a temperature scale similar to Celsius. To convert from Celsius to Celsius, simply add or subtract 273: K = °C + 273.
- You do not need to use Alka-Seltzer, any effervescent tablet can be used in the above experiment.
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wikiHow is a “wiki” site, which means that many of the articles here are written by multiple authors. To create this article, 10 people, some of whom are anonymous, have edited and improved the article over time.
There are 7 references cited in this article that you can view at the bottom of the page.
This article has been viewed 106,558 times.
In any chemical reaction, heat can be either absorbed or released to the surroundings. The temperature of exchange between a chemical reaction and its surroundings is called the enthalpy of the reaction, denoted by H. However, H cannot be measured directly, measuring the change in temperature instead. The magnitude of the reaction over time is used to calculate the change in enthalpy over time (denoted ∆H ). Knowing the ∆H of a reaction, we can determine whether the reaction is endothermic (the heat of the reaction is taken from the environment) or exothermic (the heat of the reaction is released to the environment). Where m is the mass of the reactant, s is the specific heat of the product, ∆T is the change in temperature during the reaction, we have ∆H = m x s x ∆T .
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