Energy Changes in Chemical Reactions - SS1 Chemistry Past Questions and Answers - page 3
The enthalpy change of a reaction is affected by:
The stoichiometry of the reaction
The physical states of the reactants and products
The temperature at which the reaction occurs
All of the above
The enthalpy change of a reaction can be represented by which symbol?
ΔT
ΔP
ΔH
ΔV
Define enthalpy change and discuss its significance in chemical reactions. Explain how enthalpy change is related to the heat absorbed or released during a reaction. Provide examples.
Enthalpy change is the heat energy exchanged during a chemical reaction at constant pressure. It represents the difference in enthalpy (H) between the products and the reactants and is denoted as ΔH. Enthalpy change is significant as it provides information about the heat absorbed or released during a reaction and helps determine the overall energy change of the system.
In an exothermic reaction, the products have lower enthalpy than the reactants, resulting in a negative ΔH value. This indicates that the reaction releases heat energy to the surroundings. For example, the combustion of methane (CH4) is an exothermic reaction with a negative enthalpy change. It releases heat and is commonly used for heating or cooking purposes.
In an endothermic reaction, the products have higher enthalpy than the reactants, resulting in a positive ΔH value. This indicates that the reaction absorbs heat energy from the surroundings. An example of an endothermic reaction is the process of photosynthesis in plants, where carbon dioxide (CO2) and water (H2O) react in the presence of sunlight to produce glucose and oxygen. This reaction requires energy input in the form of sunlight to proceed.
Enthalpy change is related to the heat absorbed or released during a reaction through the equation ΔH = q, where q represents the heat exchanged between the system and the surroundings. For an exothermic reaction, q is negative, indicating that heat is released by the system. For an endothermic reaction, q is positive, indicating that heat is absorbed by the system.
Calorimetry is a technique used to measure enthalpy changes experimentally. It involves the use of a calorimeter, a device that can accurately measure the heat exchanged during a reaction. By measuring the temperature change of a known mass of a substance (often water) surrounding the reaction, the heat released or absorbed can be determined. Calorimetry allows for the determination of enthalpy changes in a wide range of reactions and is an essential tool in studying thermodynamics.
Discuss the applications of calorimetry in various fields, including chemistry, biology, and energy research. Provide examples of specific applications.
Calorimetry has diverse applications in various fields, including chemistry, biology, and energy research. Some specific applications include:
- Chemistry:
- Reaction kinetics: Calorimetry is used to study reaction rates and determine the heat of reaction. By measuring the heat evolved or absorbed as a function of time, the rate at which a reaction proceeds can be determined.
- Thermodynamic studies: Calorimetry is instrumental in determining enthalpy changes (ΔH) and other thermodynamic parameters of chemical reactions. This information provides insights into the feasibility, spontaneity, and energy efficiency of chemical processes.
- Calorimetric titration: Calorimetry can be used in titration experiments to measure the heat released or absorbed during a chemical reaction, allowing for precise determination of reaction stoichiometry and equilibrium constants.
- Biology:
- Metabolic studies: Calorimetry is employed in metabolic studies to measure the heat produced by living organisms during various physiological processes. This information helps understand energy balance, nutrient utilisation, and metabolic rates in organisms.
- Food science: Calorimetry is used to determine the energy content of food by measuring the heat released during its combustion. This data is important for nutritional labelling and assessing the caloric value of different food products.
- Energy research:
Fuel analysis: Calorimetry is used to determine the calorific value of fuels, including fossil fuels and biofuels.
Hess's law states that the overall enthalpy change of a reaction is equal to:
The sum of the enthalpy changes of the individual steps of the reaction.
The average of the enthalpy changes of the individual steps of the reaction.
The difference between the enthalpy changes of the reactants and products.
The enthalpy change of the slowest step of the reaction.
Which of the following statements about Hess's law is true?
It is only applicable to reactions that occur at constant pressure.
It can be used to calculate the enthalpy change of a reaction using calorimetry data.
It can only be applied to reactions that occur in the gas phase.
It cannot be used to determine the enthalpy change of a reaction if the stoichiometry is unknown.
The heat of formation of a compound is defined as:
The heat is absorbed or released when the compound decomposes.
The heat is absorbed or released when the compound reacts with another compound.
The heat absorbed or released when one mole of the compound is formed from its elements in their standard states.
The heat is absorbed or released when one mole of the compound is dissolved in water.
The heat of formation of an element in its standard state is:
Zero.
Negative.
Positive.
Indeterminate.
The heat of formation of a compound can be calculated using:
Calorimetry data.
Hess's law.
Bond energies.
Standard enthalpies of reaction.
Explain Hess's Law
Hess's Law states that the total enthalpy change of a chemical reaction is independent of the pathway taken from the initial to the final state. In other words, the enthalpy change of a reaction depends only on the initial and final states and is not affected by the intermediate steps.
Hess's Law is used to calculate enthalpy changes of reactions by manipulating and combining known enthalpy changes of other reactions. This can be achieved by using a series of intermediate reactions whose enthalpy changes are known or can be determined.