Physical Week 3

Question 1

What is the enthalpy change?

Answer 1

Enthalpy change, represented by the symbol ΔH, is the heat released or absorbed during a chemical reaction conducted at a constant pressure. It is a measure of the energy change in the system.

Question 2

What is the standard enthalpy of formation?

Answer 2

Standard enthalpy change of formation, denoted as ΔH°f, is the energy change when one mole of a compound forms from its elements under standard conditions and standard states. It represents the enthalpy change associated with the formation of a compound.

Question 3

What is the standard enthalpy change?

Answer 3

Standard enthalpy change, denoted as ΔH°, refers to the enthalpy change for a reaction that occurs under standard conditions. Standard conditions include a temperature of 298 K (25°C), a pressure of 1 bar (100 kPa), and a concentration of 1 mol dm-3 for solutions.

Question 4

What are energy diagrams?

Answer 4

Energy diagrams, also known as reaction coordinate diagrams, are graphical representations of the energy changes that occur during a chemical reaction. They show the energy of the reactants, products, and the transition state.

Question 5

What is the standard enthalpy change of combustion?

Answer 5

Standard enthalpy change of combustion is the energy change when one mole of a compound completely burns in oxygen under standard conditions and standard states. It represents the enthalpy change associated with the combustion of a compound.

Question 6

Define exothermic.

Answer 6

Exothermic refers to a type of chemical reaction or process in which heat energy is released. During an exothermic reaction, the products have lower energy than the reactants, and the excess energy is released as heat into the surroundings.

Question 7

Define endothermic.

Answer 7

Endothermic refers to a type of chemical reaction or process in which heat energy is absorbed. During an endothermic reaction, the products have higher energy than the reactants, and the system takes in energy from the surroundings in the form of heat.

Question 8

What is enthalpy change and how is it represented?

Answer 8

Enthalpy change (ΔH) refers to the heat released or absorbed during a reaction at constant pressure. It is represented by the symbol ΔH.

Question 9

What is the standard enthalpy change and when is it used?

Answer 9

The standard enthalpy change (ΔH°r) is used for reactions under specific standard conditions. It refers to the heat released or absorbed during a reaction at constant pressure.

Question 10

How can energy diagrams represent exothermic and endothermic changes?

Answer 10

Energy diagrams show that exothermic reactions have lower energy products, while endothermic reactions have higher energy products.

Question 11

What are exothermic and endothermic reactions?

Answer 11

Exothermic reactions release heat energy, while endothermic reactions absorb heat energy.

Question 12

What is the standard enthalpy change of combustion?

Answer 12

The standard enthalpy change of combustion is the energy change when one mole of a compound completely burns in oxygen under standard conditions.

Question 13

Can standard enthalpy changes of formation be determined for any compound?

Answer 13

Yes, standard enthalpy changes of formation can be determined for any compound, even if direct synthesis from elements isn’t feasible.

Question 14

What is practical calorimetry?

Answer 14

Practical Calorimetry is a specific experiment in which substances are mixed inside an insulated container, and the changes in temperature are observed. It involves steps such as washing the equipment, preparing the substances, setting up the thermometer, taking initial measurements, introducing the second reagent, stirring and monitoring, accounting for slow reactions, and enhancing accuracy through pre-temperature readings.

Question 15

What is calorimetry?

Answer 15

Calorimetry is a technique used to measure the heat transfer that occurs during a chemical reaction or a physical change. It involves using a calorimeter, which is an insulated container, to isolate the reaction or change from the surroundings.

Question 16

What is the reaction rate?

Answer 16

Reaction Rate is the speed at which a chemical reaction takes place. It is determined by the rate at which reactant molecules collide and the energy required for successful collisions.

Question 17

What is extrapolation?

Answer 17

Extrapolation is a mathematical technique used to estimate values beyond the range of known data points. It involves extending a curve or a line to predict values outside the observed range.

Question 18

What is a practical error?

Answer 18

Error refers to the difference between a measured value and the true value or the target value. In the context of calorimetry, errors can occur due to various factors such as energy transfer from the surroundings, variations in heat capacity, neglecting the calorimeter’s heat capacity, incomplete reactions or dissolving, and assuming the density of the solution is equal to that of water.

Question 19

What is meant by heat transfer?

Answer 19

Heat Transfer is the process of energy exchange between two objects or systems due to a temperature difference. It can occur through three mechanisms: conduction, convection, and radiation.

Question 20

What is heat capacity?

Answer 20

Heat Capacity is the amount of heat energy required to raise the temperature of a substance by one degree Celsius or one Kelvin. It is a property that depends on the mass and composition of the substance.

Question 21

What is accuracy?

Answer 21

Accuracy refers to the closeness of a measured value to the true value or the target value. In the context of calorimetry, accuracy is important in obtaining reliable results.

Question 22

How can accuracy be enhanced in measuring temperature changes during chemical reactions?

Answer 22

Take temperature readings of the reactants for a few minutes before mixing and use an average.

Question 23

What factors are considered when calculating enthalpy change from experimental data?

Answer 23

When calculating enthalpy change from experimental data, the heat capacity of water is considered and assumed to match pure water.

Question 24

What are some potential errors that can occur in the practical calorimetry experiment?

Answer 24

Some potential errors that can occur in the practical calorimetry experiment include energy transfer from the surroundings leading to loss.

Question 25

What is involved in measuring enthalpies of combustion with calorimetry?

Answer 25

Measuring enthalpies of combustion with calorimetry involves burning a fuel and heating water in a metal container.

Question 26

What are the steps involved in the practical calorimetry experiment?

Answer 26

The steps involved in the practical calorimetry experiment include preparing the substances, setting up a thermometer, taking initial measurements, introducing a second reagent, stirring and monitoring the mixture, accounting for slow reactions, and enhancing accuracy through pre-temperature readings.

Question 27

What is often neglected in the specific heat capacity of the calorimeter?

Answer 27

The specific heat capacity of the calorimeter is often neglected, ignoring energy absorbed by the apparatus.

Question 28

What should be done if the reaction proceeds slowly during the experiment?

Answer 28

Take readings at regular intervals and extrapolate the temperature curve back to the time when the reactants were mixed together.

Question 29

What is one limitation (relating to the heat capacity of solutions) of the method used in practical calorimetry?

Answer 29

One limitation is that the method assumes all solutions have the same heat capacity as water, which may not always be accurate.

Question 30

If you are conducting an experiment involving a slow reaction, how would you accurately measure the temperature rise?

Answer 30

Take readings at regular intervals and extrapolate the temperature curve back to the time when the reactants were mixed together.

Question 31

What are some potential issues with reactions or dissolving in practical calorimetry?

Answer 31

Some potential issues are that reactions or dissolving may not complete fully or could be slow.

Question 32

What is combustion?

Answer 32

Combustion is a chemical reaction in which a substance reacts with oxygen to produce heat and light. It is typically a rapid oxidation reaction that releases energy in the form of heat and light.

Question 33

What is the heat of combustion?

Answer 33

Heat of combustion is the amount of heat energy released when one mole of a substance undergoes complete combustion. It represents the enthalpy change for the combustion reaction and is typically expressed in units of kJ/mol.

Question 34

What is Hess’ Law?

Answer 34

Hess’s Law states that regardless of how a chemical change occurs, the overall energy change remains constant. It states that the total energy change in a reaction is independent of the pathway taken to convert reactants into products.

Question 35

What is the main principle of Hess’s Law in the context of calculating the standard enthalpy change of forming benzene?

Answer 35

Creating two routes on the diagram, keeping with the arrow flow and ensuring the path arrows never go against the equation arrows beneath them.

Question 36

How can the enthalpy change of forming benzene be calculated using Hess’s Law?

Answer 36

The enthalpy change of forming benzene can be calculated using Hess’s Law by adding the enthalpy values of carbon and hydrogen.

Question 37

How can Hess’s Law be applied to calculate energy changes?

Answer 37

Hess’s Law can be applied to calculate energy changes by using interconnected reactions or by simplifying calculations using known energy changes from other reactions.

Question 38

How can the change in enthalpy (ΔH) be determined using Hess’s Law?

Answer 38

Using Hess’s Law, the change in enthalpy (ΔH) can be determined by adding the enthalpy values of the reactants and subtracting the enthalpy values of the products.

Question 39

What is the principle behind Hess’s Law?

Answer 39

The principle behind Hess’s Law is that the energy changes along different pathways are equal.

Question 40

How can the heat of reaction be calculated using combustion data?

Answer 40

Combustion data can be used to calculate the heat of reaction by subtracting the enthalpy values of the products from the enthalpy values of the reactants.

Question 41

If you are given the combustion energy values for benzene, carbon, and hydrogen, how would you calculate the standard enthalpy change of forming benzene using Hess’s Law?

Answer 41

Create two routes on the diagram, keeping with the arrow flow: Route 1: ΔH = -3267 + 6(-394) + 3(-286) Route 2: ΔH = 6(-394) + (-3267 - 3(-286))

Question 42

What type of reactions can enthalpy changes be calculated for using Hess’s Law?

Answer 42

Enthalpy changes can be calculated for reactions involving elements in their standard states.

Question 43

What does Hess’s Law state about the overall energy change in a chemical reaction?

Answer 43

Hess’s Law states that the overall energy change in a chemical reaction remains constant regardless of the pathway taken.