Energetics (Edexcel IGCSE Chemistry (Modular): Unit 1): Flashcards

The assumptions made are:

1. The specific heat capacity of the solution equals that of pure water (4.18 J/g/°C)

2. The density of the solution equals that of pure water (1 g/cm³)

3. The heat capacity of the container is ignored

4. Heat losses are negligible

5. The reaction is complete

True.

A polystyrene cup acts as an insulator, reducing heat loss to the surroundings. A vacuum flask or metal can can also be used.

In a combustion calorimetry experiment, the spirit burner is weighed before and after burning to find the mass of fuel used.

An insulated container is used to reduce heat loss to the surroundings. This makes the measured temperature rise more accurate and closer to the true enthalpy change of the reaction.

Specific heat capacity is the energy needed to raise the temperature of 1 g of a substance by 1 °C. The specific heat capacity of water is 4.18 J/g/°C.

False.

Some heat is lost to the surroundings and some is absorbed by the calorimeter itself, meaning the measured temperature rise is lower than the true value.

The two main sources of error are:

1. Heat losses to the surroundings and the calorimeter

2. Incomplete combustion of the fuel

Complete the table showing the steps in a combustion calorimetry experiment.

In combustion calorimetry, a fuel is burned to heat water in a copper can. In reactions in solution, two reagents are mixed in an insulated container and the temperature change of the mixture is measured.

Heat energy change (Q) is the amount of heat energy released or absorbed in a reaction. It is calculated using Q = m × c × ΔT and is measured in joules (J).

In Q = m × c × ΔT:

• Q = heat energy change (J)

• m = mass of the substance being heated (g)

• c = specific heat capacity (J/g/°C)

• ΔT = temperature change (°C)

To calculate heat energy change, you need to know the mass of the substance, its specific heat capacity and the temperature change.

True.

The specific heat capacity of water (4.18 J/g/°C) is used in calorimetry calculations. It is assumed that the solution being heated has the same specific heat capacity as pure water.

In an exothermic reaction, energy is released to the surroundings and the temperature increases. By convention, the sign of Q is negative because the system has lost energy.

Molar enthalpy change (ΔH) is the heat energy released or absorbed per mole of substance in a reaction. It is calculated using ΔH = Q ÷ n and is measured in kJ/mol.

To calculate the molar enthalpy change from Q: first calculate Q (in J) using Q = m × c × ΔT. Then divide by the number of moles (n): ΔH = Q ÷ n. Finally, convert Q to kJ by dividing by 1000 before calculating ΔH.

False.

Combustion reactions are exothermic, so energy is released. This means ΔH is negative for all combustion reactions.

To convert joules to kilojoules, divide by 1000. The units of molar enthalpy change are kJ/mol.

Energy per gram is Q divided by the mass of fuel burned. Energy per mole is Q divided by the number of moles of fuel. The molar value allows fair comparison between different fuels.

An energy level diagram is a graphical representation showing the relative energies of reactants and products in a chemical reaction. The x-axis shows reaction progress and the y-axis shows energy.

False.

In an exothermic reaction, the products are at a lower energy level than the reactants. The arrow on the diagram points downwards.

A downward arrow shows that the reaction is exothermic (products have less energy than reactants). An upward arrow shows that the reaction is endothermic (products have more energy than reactants).

On an energy level diagram, the difference in height between the reactants and products represents the overall enthalpy change of the reaction. This value is negative for an exothermic reaction.

In an exothermic reaction, the products have less energy than the reactants. Energy is released to the surroundings, so the overall change in energy is negative.

True.

In an endothermic reaction, energy is taken in from the surroundings. The products have more energy than the reactants, so the enthalpy change is positive.

The x-axis shows the progress of the reaction (from reactants to products). The y-axis shows the energy of the species involved.

Bond energy is the amount of energy required to break a chemical bond, or the amount of energy released when that bond is formed. It is measured in kJ.

True.

Breaking bonds requires energy to be taken in from the surroundings. Remember: bond breaking is ENDothermic — the bond comes to an END.

Bond breaking requires energy to be taken in from the surroundings. Because energy is absorbed rather than released, bond breaking is an endothermic process.

Bond breaking is endothermic because energy must be taken in. Bond making is exothermic because energy is released.

Enthalpy change (ΔH) = energy taken in − energy given out. If the result is negative, the reaction is exothermic; if positive, the reaction is endothermic.

Calculate the total energy needed to break all bonds in the reactants (energy in) and the total energy released when forming all bonds in the products (energy out). If energy out > energy in, the reaction is exothermic; if energy in > energy out, it is endothermic.

False.

In an exothermic reaction, the energy released when forming bonds is greater than the energy needed to break them. This gives a negative overall enthalpy change.

A negative ΔH value means the reaction is exothermic. The products have less energy than the reactants, and energy has been released to the surroundings.

Complete the table about bond breaking and bond making.

Displayed formulas show every individual bond in the molecule. This makes it easier to count the correct type and number of bonds being broken and formed, and to apply the balancing numbers correctly.

A calorimeter is an insulated container used to measure temperature changes during a chemical reaction. In this practical, a polystyrene (Styrofoam) cup with a lid is used.

The aim is to study the calorimetry of the reaction between HCl and NaOH by measuring how temperature changes as different volumes of acid are added to a fixed volume of alkali.

True.

The NaOH volume (25 cm³) stays fixed while increasing volumes of HCl (5, 10, 15, 20, 25 cm³) are added in turn, and the highest temperature is recorded each time.

In the practical, the initial temperature of the NaOH solution is recorded before adding the HCl, and the highest temperature reached by the mixture is recorded after stirring.

Results should be recorded in a table (volume of acid vs temperature). A graph is then plotted with a line of best fit to identify the volume of acid that gives the greatest temperature change.

The volume of acid that produces the largest temperature change is the point at which the most energy is released. A larger temperature difference indicates more energy transferred in the reaction.