In all chemical reactions, heat can be received from the surroundings or released into the surroundings. The exchange of heat between a chemical reaction and its environment is known as the enthalpy of the 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 (written as H). With H, a scientist can determine if a reaction gives off heat (or is "exothermic") or receives heat (or is "endothermic"). In general, H = m x s x T, where m is the mass of the reactants, s is the specific heat of the products, and T is the change in temperature in the reaction.
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Method 1 of 3: Solving Enthalpy Problems
Step 1. Determine the reaction of your products and reactants
Any chemical reaction involves two chemical categories - products and reactants. Products are chemical substances that result from reactions, while reactants are chemical substances that combine or split to produce products. In other words, the reactants of a reaction are like the ingredients of a food recipe, while the products are the finished food. To find the H of a reaction, first, identify the products and reactants.
For example, say we are going to find the enthalpy of the reaction for the formation of water from hydrogen and oxygen: 2H2 (Hydrogen) + O2 (Oxygen) → 2H2O (Water). In this equation, H2 and O2 is the reactant and H2O is a product.
Step 2. Determine the total mass of the reactants
Next, find the mass of your reactants. If you don't know its mass and can't weigh it on a scientific scale, you can use its molar mass to find its actual mass. Molar mass is a constant that can be found in the regular periodic table (for single elements) and other chemical sources (for molecules and compounds). Just multiply the molar mass of each reactant by the number of moles to find the mass of the reactants.
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In the water example, our reactants are hydrogen and oxygen gases, which have molar masses of 2 g and 32 g. Since we are using 2 moles of hydrogen (judging by the coefficient of 2 in H2) and 1 mole of oxygen (judging by the absence of coefficients in O2), we can calculate the total mass of the reactants as follows:
2 × (2g) + 1 × (32g) = 4g + 32g = 36g
Step 3. Find the specific heat of your product
Next, find the specific heat of the product you are analyzing. Each element or molecule has a specific specific heat: this value is a constant and is usually found in chemistry learning resources (for example, in the table at the back of a chemistry textbook). There are different ways to calculate specific heat, but for the formula we're using, we're using the unit Joule/gram °C.
- Note that if your equation has many products, you'll need to calculate the enthalpy for the reactions of the elements used to produce each product, then add them up to find the overall enthalpy for the reaction.
- In our example, the final product is water, which has a specific heat of approx. 4.2 joules/gram °C.
Step 4. Find the difference in temperature after the reaction occurs
Next, we will find T, the change in temperature before and after the reaction. Subtract the initial temperature of the reaction (or T1) from the final temperature after the reaction (or T2) to calculate it. As in most chemical work, the Kelvin (K) temperature is used (although Celsius (C) will give the same result).
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For our example, let's say the initial temperature of the reaction is 185K but cools to 95K when the reaction is complete. In this problem, T is calculated as follows:
T = T2 – T1 = 95K – 185K = -90K
Step 5. Use the formula H = m x s x T to solve
If you have m, the mass of the reactants, s, the specific heat of the products, and T, the change in temperature of the reaction, then you are ready to find the enthalpy of the reaction. Plug your values into the formula H = m x s x T and multiply to solve. Your answer is written in energy units, namely Joules (J).
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For our example problem, the enthalpy of the reaction is:
H = (36g) × (4.2 JK-1 g-1) × (-90K) = -13,608 J
Step 6. Determine whether your reaction is receiving or losing energy
One of the most common reasons for calculating H for various reactions is to determine if the reaction is exothermic (loses energy and releases heat) or endothermic (gains energy and absorbs heat). If the sign of your final answer for H is positive, then the reaction is endothermic. Meanwhile, if the sign is negative, the reaction is exothermic. The larger the number, the greater the exo- or endothermic reaction. Be careful with strong exothermic reactions - they sometimes release large amounts of energy, which, if released very quickly, can cause an explosion.
In our example, the final answer is -13608J. Since the sign is negative, we know that our reaction is exothermic. This makes sense - H2 and O2 is a gas, while H2O, the product, is a liquid. The hot gas (in the form of steam) must release energy to the environment in the form of heat, to cool it down to form a liquid, that is, the reaction to form H2O is exothermic.
Method 2 of 3: Estimating the Enthalpy Size
Step 1. Use bond energies to estimate the enthalpy
Almost all chemical reactions involve the formation or breaking of bonds between atoms. Since in chemical reactions, energy cannot be destroyed or created, if we know the amount of energy required to form or break bonds in a reaction, we can estimate the enthalpy change for the overall reaction with a high degree of accuracy by adding up these bond energies.
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For example, the reaction used H2 + F2 → 2HF. In this equation, the energy required to break down the H atoms in the H. molecule is2 is 436 kJ/mol, while the energy required for F2 is 158 kJ/mol. Finally, the energy required to form HF from H and F is = -568 kJ/mol. We multiply by 2 because the product in the equation is 2 HF, so that's 2 × -568 = -1136 kJ/mol. Adding them all together, we get:
436 + 158 + -1136 = -542 kJ/mol.
Step 2. Use the enthalpy of formation to estimate the enthalpy
The enthalpy of formation is a set of values H that represents the enthalpy change of a reaction to produce a chemical substance. If you know the enthalpy of formation required to produce the products and reactants in the equation, you can add them up to estimate the enthalpy like bond energies described above.
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For example, the equation used C2H5OH + 3O2 → 2CO2 + 3H2O. In this equation, we know that the enthalpy of formation for the following reaction is:
C2H5OH → 2C + 3H2 +0.5O2 = 228 kJ/mol
2C + 2O2 → 2CO2 = -394 × 2 = -788 kJ/mol
3H2 +1.5 O2 → 3H2O = -286 × 3 = -858 kJ/mol
Since we can sum these equations to get C2H5OH + 3O2 → 2CO2 + 3H2O, from the reaction we are trying to find the enthalpy, we only need to add up the enthalpy of the reaction of formation above to find the enthalpy of this reaction, as follows:
228 + -788 + -858 = -1418 kJ/mol.
Step 3. Don't forget to change the sign when reversing the equation
It's important to note that when you use the enthalpy of formation to calculate the enthalpy of a reaction, you must change the sign of the enthalpy of formation whenever you reverse the reaction equation for the elements. In other words, if you invert one or more of your equations for the formation of a reaction so that the products and reactants cancel each other out, change the sign of the enthalpy of the formation reaction you are swapping.
In the example above, note that the formation reaction we used for C2H5OH upside down. C2H5OH → 2C + 3H2 +0.5O2 show C2H5OH is split, not formed. Since we reversed this equation so that the products and reactants cancel each other out, we changed the sign of the enthalpy of formation to give 228 kJ/mol. In fact, the enthalpy of formation for C2H5OH is -228 kJ/mol.
Method 3 of 3: Observing the Enthalpy Change in Experiments
Step 1. Take a clean container and fill it with water
It is easy to see the principle of enthalpy with a simple experiment. To ensure that your experimental reaction is not contaminated with external substances, clean and sterilize the containers you intend to use. Scientists use special sealed containers called calorimeters to measure enthalpy, but you can get good results with any glass or small test tube. Whatever container you use, fill it with clean, room temperature water. You should also experiment in a room with a cold temperature.
For this experiment, you will need a fairly small container. We will examine the effect of the enthalpy change of Alka-Seltzer on water, so the less water you use, the more pronounced the temperature change will be
Step 2. Insert the thermometer into the container
Take a thermometer and set it in the container so that the tip of the thermometer is under the water. Read the temperature of the water - for our purposes, the temperature of the water is denoted by T1, the initial temperature of the reaction.
Let's say we measure the temperature of water and the result is 10 degrees C. In a few steps, we will use these temperature readings to prove the principle of enthalpy
Step 3. Add one Alka-Seltzer to the container
When you're ready to start the experiment, drop an Alka-Seltzer into the water. You will notice immediately the grain is bubbling and hissing. When the beads dissolve in water, they break down into the chemical bicarbonate (HCO.).3-) and citric acid (which reacts in the form of hydrogen ions, H+). These chemicals react to form water and carbon dioxide gas in the equation 3HCO3− + 3H+ → 3H2O + 3CO2.
Step 4. Measure the temperature when the reaction is complete
Watch as the reaction proceeds - the Alka-Seltzer granules will slowly dissolve. As soon as the grain reaction ends (or has slowed down), measure the temperature again. The water should be colder than before. If it's warmer, the experiment may be affected by outside forces (for example, if the room you're in is warm).
For our experimental example, let's say the temperature of the water is 8 degrees C after the grains stop fizzing
Step 5. Estimate the enthalpy of the reaction
In an ideal experiment, when you drop an Alka-Seltzer grain into water, it forms water and carbon dioxide gas (the gas can be observed as a hissing bubble) and causes the temperature of the water to drop. From this information, we guess the reaction is endothermic - that is, it absorbs energy from the surrounding environment. The dissolved liquid reactants require additional energy to produce a gaseous product, so they absorb energy in the form of heat from the surroundings (in this experiment, water). This causes the water temperature to decrease.
