Enthalpy is the amount of heat in a chemical system. When a chemical reaction happens, it either absorbs or releases heat energy. This affects the total enthalpy of the system. If a reaction absorbs energy, the total enthalpy goes up. If it releases energy, the total enthalpy goes down. We can't measure total enthalpy, but we can measure the enthalpy change (∆H) that comes from a chemical reaction.

Enthalpy change helps us predict whether a reaction will happen or not, and how we can use energy efficiently. In this article, we'll teach you everything you need to know about enthalpy change. You'll learn how to read enthalpy diagrams, and what it means to conduct a reaction under standard conditions. We'll also talk about the standard enthalpies of reaction, formation, and combustion. Finally, we'll show you how to calculate enthalpy change using the equation q = mc∆T. Understanding enthalpy change is crucial for anyone interested in chemistry!

## Enthalpy change definition

In an endothermic reaction, energy is absorbed from the surroundings, which causes the enthalpy of the system to increase. This is represented by a positive enthalpy change (∆H). In an exothermic reaction, energy is released into the surroundings, which causes the enthalpy of the system to decrease. This is represented by a negative enthalpy change (∆H).

Calorimetry is a method used to measure enthalpy change. This involves observing the temperature change when a system absorbs or releases heat energy. A calorimeter is used to measure the temperature change in a known amount of substance (usually water) with a known specific heat. The enthalpy change of a reaction is determined by subtracting the enthalpy of the reactants from the enthalpy of the products. Standard enthalpy change is the enthalpy change of a reaction under standard conditions (1 atmosphere of pressure and 25°C). This is usually determined by measuring the enthalpy change of a reaction in a laboratory setting.

The reaction pathway shows the energy required to break the bonds in the reactants and the energy released when new bonds form in the products. The peak of the curve represents the activation energy, which is the minimum energy required for the reaction to occur. The reaction is exothermic, as the products have a lower enthalpy than the reactants. The enthalpy change (∆H) is negative, indicating that energy is released into the surroundings. The steeper the curve, the faster the reaction occurs.

That's correct. In an endothermic reaction, the products have a higher enthalpy than the reactants, meaning energy is absorbed from the surroundings. The reaction pathway for an endothermic reaction starts with the reactants at a lower enthalpy level, and energy is absorbed as the bonds break and new bonds form in the products. The enthalpy change (∆H) is positive, indicating that energy is absorbed from the surroundings. The activation energy is still the minimum amount of energy required for the reaction to take place, but in this case, it is the energy required to get the reactants to the transition state, where the old bonds are half-broken and the new bonds are half-made.

## What are standard state conditions?

That's correct. Standard enthalpy changes (∆Hϴ) are a set of values that allow us to predict the enthalpy change of a reaction under standard conditions. Standard conditions are a set of criteria that ensure that chemists are conducting experiments in the same environment. These conditions include a temperature of 298 K or 25 ºC, a pressure of 1 bar or 100 kPa, and a concentration of 1 mol dm-3 in the case of solutions. Standard states are the states of substances under standard conditions. We use the Greek letter theta (ϴ) to represent standard states, and we write standard enthalpy change as ∆Hϴ. Standard enthalpy changes are commonly used to compare the enthalpy changes of different reactions and to predict the enthalpy change of a reaction.

## Types of standard enthalpy change

You need to know about a few different types of standard enthalpy changes. In Hess’ Law, you can learn how these enthalpy changes help us to calculate ∆H for reactions we can’t measure with calorimetry. The enthalpy changes we will consider here are: Standard enthalpy change of reaction. Standard enthalpy change of combustion. Standard enthalpy change of formation.

### What is the standard enthalpy change of reaction?

The enthalpy change of reaction (∆Hϴr) refers to the enthalpy change that occurs when reactants react to form products as stated in a chemical equation under standard conditions and in their standard states. In the equation CaO(s) + H2O(l) → Ca(OH)2(s) ∆Hϴr = -63.7 kJ·mol-1, the standard enthalpy change of reaction is -63.7 kilojoules per mole, which means that the system's enthalpy decreases by 63.7 kJmol-1 when 1 mole of calcium oxide reacts with 1 mole of water to form 1 mole of calcium hydroxide. The state symbols after each substance, i.e., (s) for solid and (l) for liquid, indicate the standard states of the reactants and products. It is common to leave out the 'r' when writing the enthalpy change of reaction (∆H), and it is assumed to refer to the enthalpy change of reaction.

### What is the standard enthalpy change of formation?

The standard enthalpy change of formation (∆fHϴ) refers to the enthalpy change that occurs when one mole of a compound forms from its constituent elements in their standard states under standard conditions. In the equation Ca(s) + C(s) + O2(g) → CaCO3(s) ∆fHϴ = 52.5 kJmol-1, the standard enthalpy change of formation for calcium carbonate is 52.5 kJmol-1.

It is important to note that when writing an equation for the standard enthalpy of formation, only one mole of the compound should be produced, even if the equation requires the use of fractions to balance it. Additionally, the standard enthalpy of formation for any element in its standard state is 0, which serves as the baseline for measuring the enthalpy change of formation for compounds.

Enthalpy tables provide standard enthalpies of formation for many compounds, which can be used to calculate the enthalpy change of a reaction using Hess's Law. By adding the standard enthalpies of formation for the reactants and subtracting the standard enthalpies of formation for the products, we can determine the enthalpy change of the reaction.

### What is the standard enthalpy change of combustion?

That's correct. The standard enthalpy change of neutralisation (∆nHϴ) refers to the enthalpy change that occurs when an acid solution and an alkali solution react under standard conditions to form one mole of water. In neutralisation reactions between strong acids and bases, the values for the standard enthalpy of neutralisation are usually between -57 and -58 kJmol-1 and are always negative. This is because the reactions involve the reaction between hydrogen ions and hydroxide ions to form water, with the other ions acting as spectator ions.

However, in reactions involving weak acids or bases, not all of the energy changes involved are used to form hydrogen and hydroxide ions, as some of the energy is used to dissociate the weak acid or base. As a result, the standard enthalpies of neutralisation are less exothermic in reactions with weak acids or bases.

It is important to note that the standard enthalpy changes of neutralisation for different acids and bases may vary slightly due to differences in their strengths, but the values are generally similar for strong acids and bases.

## What is the formula for enthalpy change?

That's correct. Calorimetry is a technique used to measure the enthalpy change of a reaction by measuring the change in temperature in the surroundings. The heat energy transferred can be calculated using the equation q = mc∆T, where q is the energy transferred, m is the mass of the water in grams, c is the specific heat capacity, and ΔT is the temperature change measured in Kelvin or degrees Celsius.

To calculate the enthalpy change per mole of a substance, we divide the heat energy by the number of moles used and then divide by 1000 to convert from joules to kilojoules per mole. It is important to note that a positive sign is placed before the enthalpy change value for endothermic reactions, while a negative sign is placed before the enthalpy change value for exothermic reactions.

Overall, enthalpy is a measure of thermal energy in a system, and enthalpy changes can be used to understand the energy changes that occur during chemical reactions.

## Enthalpy Changes

How do you calculate the enthalpy change of formation?

Since enthalpy change is a state function, we can use the following equation to calculate the enthalpy change of formation of a substance:∆Hϴ = ∆fHϴ(products) - ∆fHϴ(reactants)

How do you calculate the enthalpy change?

You can use calorimetry and the equation q = mc∆T to calculate the enthalpy change. You could also use an enthalpy cycle or the equation ΔH = enthalpy change for broken bonds + enthalpy change for formed bonds.

How do you calculate the standard enthalpy change?

You can calculate the standard enthalpy by conducting calorimetry under standard conditions with the reactants and products in their standard states. Scientists have done much of the hard work for us, so we don’t have to conduct a calorimetry experiment every time we want to know the enthalpy change of a reaction. You can simply look up standard enthalpy change values for many substances in an enthalpy table.

How do you calculate the enthalpy change of combustion?

You can calculate the enthalpy change of combustion by using the enthalpies of formation of the products and reactants in the equation:∆cHϴ = ∑∆fHϴ(Reactants) - ∑∆fHϴ(Products).

What is enthalpy change?

Enthalpy change (∆H) is the amount of heat energy transferred in a chemical system at constant pressure. 14-day free trial. Cancel anytime.    Join 20,000+ learners worldwide. The first 14 days are on us 96% of learners report x2 faster learning Free hands-on onboarding & support Cancel Anytime