During any chemical reaction, heat can be absorbed from the environment or released into the environment through the reaction. The heat exchange between a chemical reaction and its environment is known as the enthalpy of reaction or H. However, H cannot be measured directly; Instead, scientists use the change in temperature of a reaction over time to find the change in enthalpy over time (denoted as ΔH). Using the ΔH data, a scientist can determine whether a reaction gives off heat (or "is exothermic") or absorbs heat (or "is endothermic"). Usually ΔH = m x s x ∆T, where m is the mass of the reactants, s is the specific heat of the product, and ΔT is the change in temperature of the reaction.
Steps
Method 1 of 3: Solve Enthalpy Problems
Step 1. Determine the products and reactants of the reaction
Any chemical reaction involves two categories of chemicals: products and reagents. Products are the chemical matter created by the reaction, while reactants are the chemicals that interact, combine, or break down to make the product. In other words, the reactants in a reaction are like the ingredients in a recipe, while the products are like the finished dish. To find the ΔH of a reaction, you must first identify its products and reactants.
- As an example, let's say we want to find the enthalpy of reaction for the formation of water from hydrogen and oxygen: 2H2 (Hydrogen) + O2 (Oxygen) → 2H2O (Water). In this equation, H2 and OR2 are the reagents and H2OR is the product.
Step 2. Determine the total mass of the reactants
Next, you must find what the masses of the reactants are. If you don't know the masses and you don't have a scientific scale to weigh the reactants, you can use their molar masses to find their actual masses. Molar masses are constants that you can find in standard periodic tables (for individual elements) and in other chemistry resources (for molecules and compounds). You simply multiply the molar mass of each reactant by the number of moles you must use to find the masses of the reactants.
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In our water example, our reactants are hydrogen and oxygen gases, which have molar masses of 1 and 16 g / mol, respectively. Since we use 2 moles of hydrogen (represented by the coefficient "2" in the following equation for H2) and 1 mole of oxygen (represented by no coefficient close to O2), we can calculate the total mass of the reactants as follows:
2 × (1 g / mol) + 1 × (16 g / mol) = 2 g / mol + 16 g / mol = 18 g / mol.
Step 3. Find the specific heat of your product
Below you will find the specific heat of the product you are analyzing. Each element or molecule has a specific heat value associated with it: these values are constant and are usually found in chemistry materials (such as in the boxes at the back of a chemistry textbook). There are several different ways to measure specific heat, but for our formula, we are going to use the value measured in units of joules / gram ° C.
- Keep in mind that if the equation has multiple products, you will need to perform an enthalpy calculation for the reaction of each component used to produce each of the products, and then add them together to find the enthalpy for the entire reaction.
- In our example, the end product is water, which has a specific heat of approximately 4.2 joules / gram ° C.
Step 4. Find the difference in temperature after the reaction
Next, we are going to find the ΔT, which is the given temperature change from before the reaction until the reaction ends. To calculate this value, subtract the initial temperature (or T1) of the reaction from the final temperature (or T2). As with most work in chemistry, you should use the temperature in degrees Kelvin (K) but using degrees Celsius C will give you the same results.
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For our example, let's say our reaction was at 185K at the start, but it had cooled to 95K by the time it ended. In this case the ΔT must be calculated as follows:
∆T = T2 - T1 = 95 K - 185 K = - 90 K
Step 5. To solve it, use the formula ∆H = m x s x ∆T
Once you have m, the mass of the reactants, s, the specific heat of the product, and ΔT, the temperature change in your reaction, you have everything to find the enthalpy of the reaction. You just have to insert the values in the formula ∆H = m x s x ∆T y to solve the multiples. The answer will be given in units of joules of energy (J).
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For our example problem, we would find that the enthalpy of reaction is defined as follows:
ΔH = (36 g) x (4, 2 JK-1 g-1) x (-90 K) = - 13, 608 J
Step 6. Determine if your reaction gains or loses energy
One of the most common reasons why ΔH is calculated in various reactions is to determine whether the reaction is exothermic (loses energy and gives off heat) or endothermic (gains energy and absorbs heat). If the sign of your final ΔH answer is positive, the reaction is endothermic. On the other hand, if the sign is negative, the reaction is exothermic. The higher the number itself, the more exo or endo thermal is the reaction. Beware of strongly exothermic reactions; These can sometimes present a large release of energy, which if fast enough can cause an explosion.
- In our example, our final answer is -13,608 J. Since the sign is negative, we know that our reaction is exothermic. This makes sense; H2 me2 are gases, while H2Or, the product is a liquid. Hot gases (in the form of steam) release energy into the environment in the form of heat to cool it to the point where they can form liquid water, which means that the formation of H2Or it is exothermic.
Method 2 of 3: Estimate Enthalpy
Step 1. Use bond energies to estimate enthalpy
Almost all chemical reactions involve the formation or breaking of bonds between atoms. Since energy cannot be destroyed or created in a chemical reaction, if we know the energy required to form or break bonds in the reaction, we can estimate the enthalpy change for the entire reaction with great precision by adding these bonding energies..
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For example, consider the reaction H2 + F2 → 2HF. In this case, the energy required to break up the H atoms in the H2 moving them away from the molecule is 436 kJ / mol, while the energy required for F2 it is 158 kJ / mol. Finally, the energy required to form HF between H and F is = -568 kJ / mol. We multiply this by 2 because the product in the equation is 2 HF, giving us 2 and the times; -568 = -1136 KJ / mol. Adding all this we get:
436 + 158 + -1136 = - 542 kJ / mol.
Step 2. Use enthalpies of formation to calculate enthalpy
The enthalpies of formation establish ΔH values that represent the enthalpy changes of the reactions that are used to create specific chemicals. If you know the enthalpies of formation needed to create products and reactants in an equation, you can add them together to estimate enthalpy in the same way that you would for bonding energies as described above.
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For example, consider reaction C2H5OH + 3O2 → 2CO2 + 3H2O. In this case, we know that the enthalpies of formation for the following reactions
C2H5OH → 2C + 3H2 + 0.5O2 = 228 kJ / mol
2C + 2O2 → 2CO2 = -394 x 2 = -788 kJ / mol
3H2 + 1.5 O2 → 3H2O = -286 x 3 = -858 kJ / mol
Since we can add these equations to get C2H5OH + 3O2 → 2CO2 + 3H2Or, the reaction for which we are trying to find the enthalpy, we can simply add the enthalpies of the formation reactions above to find the enthalpy of this reaction as follows:
228 + -788 + -858 = - 1418 kJ / mol.
Step 3. Don't forget to change the signs when you solve the equations
It is important to note that when you use enthalpies of formation to calculate the enthalpy of a reaction, it is necessary to invert the sign of the enthalpy of formation each time you solve for any component in the reaction equation. In other words, you have to convert your formation reaction equations one or more times in order to get all the products and reactants in position to cancel them correctly, and reverse the sign of the enthalpies of the formation reactions that you have solved for.
- In the example above, notice that the formation reaction we used for C2H5OH is the other way around. C2H5OH → 2C + 3H2 + 0.5O2 C is shown2H5OH, it doesn't form. Because we invert the equation in order to get all the products and reactants to cancel correctly, we invert the sign in the equation for the enthalpy of formation to have 228 kJ / mol. Actually, the enthalpy of formation for C2H5OH is -228 kJ / mol.
Method 3 of 3: Experimental Observation of Enthalpy Changes
Step 1. Take a clean container and fill it with water
It's easy to see the principles of enthalpy in action with a simple experiment. To ensure that the reaction in the experiment works without any external contamination clean and sterilize the container you will be using. Scientists use special closed vessels called calorimeters to measure enthalpy, but reasonable results can be achieved with any small glass jar. Regardless of the container you use, fill it with clean running water at room temperature. It is also important that you carry out the reaction in a place that has room temperature.
For this experiment you must use a small container. We are going to test the alteration of enthalpy effects with Alka-Seltzer in water, so that the less water used, the more evident the change in temperature will be
Step 2. Insert a thermometer into the container
Attach a thermometer and adjust it in the container so that the end of the temperature reader is below the water level. Take a reading of the water temperature, for our purposes the water temperature will represent the T1 which is the initial temperature of the reaction.
Let's say we measure the temperature of the water and find it to be exactly 10 degrees C. In a few steps, we are going to use this sample temperature to demonstrate the principles of enthalpy
Step 3. Add an Alka-Seltzer tablet to the container
When you are ready to begin the experiment, drop a single Alka-Seltzer tablet into the water. You will immediately notice that it begins to bubble and fizz. As the tablet dissolves in the water, it is broken down by the chemicals formed which are bicarbonate (HCO3-) and citric acid (which reacts as hydrogen ions, H+). These chemicals react to form water gas and carbon dioxide in the 3HCO reaction.3− + 3H+ → 3H2O + 3CO2.
Step 4. Measure the temperature when the reaction ends
Monitor the reaction as it progresses; the Alka-Seltzer tablet should dissolve gradually. As soon as the tablet finishes its reaction (or appears to have slowed to a crawl), measure the temperature again. The water should be slightly cooler than before. If it is warmer, the experiment may be affected by some external factor (such as, for example, if the room you are in is especially hot).
For our example experiment, let's say the water temperature is 8 degrees C after the tablet finishes fizzing
Step 5. Estimate the enthalpy of the reaction
In an ideal experiment, when you add the Alka-Seltzer tablet to water, it forms water and carbon dioxide gas (the latter of which you can see as bubbles) and causes the temperature of the water to drop. From this information, an endothermic reaction would be expected; that is, a reaction that absorbs energy from the surrounding environment. Dissolved liquid reactants need extra energy to make the jump to the gaseous product, so it takes the energy in the form of heat from its surroundings (in this case, water). This causes the water temperature to drop.