What is enthalpy in chemistry?

Enthalpy is a thermodynamic property that represents the total heat content of a system. It is often denoted by the symbol H and is used to quantify the energy changes that occur during chemical reactions at constant pressure. The change in enthalpy (ΔH) indicates whether a reaction is endothermic or exothermic. In an endothermic reaction, energy is absorbed from the surroundings, resulting in a positive ΔH, while in an exothermic reaction, energy is released, leading to a negative ΔH. Understanding enthalpy is crucial for predicting the energy dynamics of chemical processes and is a fundamental concept in thermochemistry.

How do you calculate energy change?

To calculate the energy change during a chemical reaction, one typically uses the formula ΔH = total energy of bonds broken - total energy of bonds formed. This involves determining the energy required to break the bonds in the reactants and the energy released when new bonds are formed in the products. For example, if the total energy to break bonds is 2736 kJ and the energy released from forming bonds is 3466 kJ, the energy change would be calculated as ΔH = 2736 kJ - 3466 kJ, resulting in a negative value, indicating an exothermic reaction. This calculation is essential for understanding the energy efficiency and feasibility of chemical reactions.

What is an endothermic reaction?

An endothermic reaction is a type of chemical reaction that absorbs energy from its surroundings, typically in the form of heat. This absorption of energy results in a decrease in temperature of the surrounding environment, making the reaction feel cold to the touch. In terms of enthalpy change (ΔH), endothermic reactions have a positive ΔH value, indicating that energy is required for the reaction to proceed. A common example of an endothermic reaction is the decomposition of calcium carbonate, which has a ΔH of +178 kJ per mole. Understanding endothermic reactions is important in various applications, including chemical synthesis and thermodynamic studies.

Hess's Law is a principle in thermochemistry that states that the total enthalpy change of a chemical reaction is independent of the pathway taken to achieve that reaction. This means that whether a reaction occurs in one step or multiple steps, the overall change in enthalpy will be the same. Hess's Law is particularly useful for calculating enthalpy changes for reactions that are difficult to measure directly. By using known enthalpy changes from related reactions, one can apply Hess's Law to find the enthalpy change for the desired reaction. This law is foundational in understanding the conservation of energy in chemical processes and is widely used in thermodynamic calculations.

What is calorimetry used for?

Calorimetry is a scientific technique used to measure the amount of heat absorbed or released during a chemical reaction or physical change. This method involves using a calorimeter, an instrument designed to isolate the reaction from its surroundings, allowing for accurate measurements of temperature changes. The energy transferred during the reaction can be calculated using the formula Q = mcΔT, where Q is the heat energy, m is the mass of the substance, c is the specific heat capacity, and ΔT is the change in temperature. Calorimetry is essential in determining the enthalpy changes of reactions, such as combustion, and is widely used in chemistry, biology, and materials science to study energy transformations.

AQA 1.4 Energetics REVISION

Allery Chemistry・3 minutes read

Chris Harris presents a revision aid for AQA Chemistry, focusing on enthalpy changes during chemical reactions, with endothermic reactions absorbing energy and exothermic reactions releasing it, and highlights methods for calculating these changes through calorimetry and Hess's Law. Key examples include the energy changes for methane combustion and neutralization reactions, emphasizing the importance of understanding bond energies and enthalpy values in chemical processes.

Insights

Chris Harris explains that enthalpy change (ΔH) is a key concept in understanding energy shifts during chemical reactions, with endothermic reactions absorbing energy and exothermic reactions releasing it; for example, the decomposition of calcium carbonate has a ΔH of +178 kJ per mole, while the combustion of ethane has a ΔH of -1560 kJ per mole, illustrating the contrasting energy dynamics.
The text emphasizes the practical application of calorimetry to measure enthalpy changes, demonstrating how energy transfer can be quantified through experiments, such as heating water with burning ethanol, which resulted in a calculated ΔH of -364.5 kJ per mole; this method allows for real-world understanding of theoretical concepts like Hess's Law, which states that the total enthalpy change is independent of the reaction pathway.

Get key ideas from YouTube videos. It’s free

Recent questions

What is enthalpy in chemistry?
Enthalpy is a thermodynamic property that represents the total heat content of a system. It is often denoted by the symbol H and is used to quantify the energy changes that occur during chemical reactions at constant pressure. The change in enthalpy (ΔH) indicates whether a reaction is endothermic or exothermic. In an endothermic reaction, energy is absorbed from the surroundings, resulting in a positive ΔH, while in an exothermic reaction, energy is released, leading to a negative ΔH. Understanding enthalpy is crucial for predicting the energy dynamics of chemical processes and is a fundamental concept in thermochemistry.
How do you calculate energy change?
To calculate the energy change during a chemical reaction, one typically uses the formula ΔH = total energy of bonds broken - total energy of bonds formed. This involves determining the energy required to break the bonds in the reactants and the energy released when new bonds are formed in the products. For example, if the total energy to break bonds is 2736 kJ and the energy released from forming bonds is 3466 kJ, the energy change would be calculated as ΔH = 2736 kJ - 3466 kJ, resulting in a negative value, indicating an exothermic reaction. This calculation is essential for understanding the energy efficiency and feasibility of chemical reactions.
What is an endothermic reaction?
An endothermic reaction is a type of chemical reaction that absorbs energy from its surroundings, typically in the form of heat. This absorption of energy results in a decrease in temperature of the surrounding environment, making the reaction feel cold to the touch. In terms of enthalpy change (ΔH), endothermic reactions have a positive ΔH value, indicating that energy is required for the reaction to proceed. A common example of an endothermic reaction is the decomposition of calcium carbonate, which has a ΔH of +178 kJ per mole. Understanding endothermic reactions is important in various applications, including chemical synthesis and thermodynamic studies.
What is Hess's Law?
Hess's Law is a principle in thermochemistry that states that the total enthalpy change of a chemical reaction is independent of the pathway taken to achieve that reaction. This means that whether a reaction occurs in one step or multiple steps, the overall change in enthalpy will be the same. Hess's Law is particularly useful for calculating enthalpy changes for reactions that are difficult to measure directly. By using known enthalpy changes from related reactions, one can apply Hess's Law to find the enthalpy change for the desired reaction. This law is foundational in understanding the conservation of energy in chemical processes and is widely used in thermodynamic calculations.
What is calorimetry used for?
Calorimetry is a scientific technique used to measure the amount of heat absorbed or released during a chemical reaction or physical change. This method involves using a calorimeter, an instrument designed to isolate the reaction from its surroundings, allowing for accurate measurements of temperature changes. The energy transferred during the reaction can be calculated using the formula Q = mcΔT, where Q is the heat energy, m is the mass of the substance, c is the specific heat capacity, and ΔT is the change in temperature. Calorimetry is essential in determining the enthalpy changes of reactions, such as combustion, and is widely used in chemistry, biology, and materials science to study energy transformations.

Summary

00:00

Understanding Enthalpy Changes in Chemistry

The video is a revision aid for AQA Chemistry, focusing on energetics, presented by Chris Harris from alerts.com, with PowerPoint materials available for purchase via a link in the description.
Enthalpy change (ΔH) measures energy change during a chemical reaction at constant pressure, expressed in kJ per mole, with standard conditions defined as 100 kPa and 298 K (25°C).
Endothermic reactions absorb energy, feeling colder; ΔH is positive, e.g., the decomposition of calcium carbonate has ΔH of +178 kJ per mole.
Exothermic reactions release energy, feeling warmer; ΔH is negative, e.g., burning ethane produces carbon dioxide and water with ΔH of -1560 kJ per mole.
Bond breaking requires energy (endothermic), while bond making releases energy (exothermic); ΔH is positive for breaking bonds and negative for forming them.
Mean bond enthalpy averages energy required to break bonds, calculated by dividing total energy for breaking multiple bonds by the number of bonds.
To calculate energy change for methane combustion, total energy to break bonds (2736 kJ) minus energy released from forming bonds (3466 kJ) results in ΔH of -730 kJ per mole.
Calorimetry measures enthalpy change of combustion by burning fuel to heat water; energy transferred is calculated using the formula Q = mcΔT, where Q is in joules.
For a calorimetry experiment with 100 g of water heated from 23°C to 57°C using 1.8 g of ethanol, the energy transferred is 14212 J, resulting in ΔH of -364.5 kJ per mole.
In a neutralization reaction, mixing 25 cm³ of 1 M HCl with 25 cm³ of 1 M NaOH raised the temperature from 20°C to 26°C, yielding an energy change of 1.254 kJ for the reaction.

13:02

Calculating Enthalpy Changes in Reactions

To calculate moles, multiply concentration (1 mole/dm³) by volume (25 cm³ converted to 0.025 dm³), resulting in 0.0250 moles.
The enthalpy change is calculated by dividing energy change (-1.254 kJ) by moles (0.0250), yielding an enthalpy change of -5016 kJ/mol.
Hess's Law states that the total enthalpy change is independent of the reaction pathway, applicable through formation and combustion cycles.
In a formation cycle, elements in standard states are at the bottom, and the enthalpy of formation (ΔHf) is calculated by balancing the equation and substituting values.
For combustion of methanol, the enthalpy of formation is -234 kJ/mol, and for products, it’s -966 kJ/mol, leading to a combustion enthalpy of -732 kJ/mol.
In a combustion cycle, products (CO₂ and H₂O) are at the bottom, and the enthalpy of combustion for pentane is calculated using the combustion data provided.
The enthalpy of formation for pentane is determined by calculating the total enthalpy changes, resulting in -177 kJ/mol, confirming the reaction is exothermic.