Topic 6: Energetics

Know that the enthalpy change, ∆H, is the heat energy change measured at constant pressure and that standard conditions are 100 kPa and a specified temperature, usually 298 K.

Use —the enthalpy change, ∆h, is the heat energy change measured at constant pressure and that standard conditions are 100 kpa to connect the rule to the data and decision in the question.

This matters because —the enthalpy change, ∆h, is the heat energy change measured at constant pressure and that standard conditions are 100 kpa determines what can be inferred or chosen; begin with the stated conditions and keep the conclusion tied to the evidence.

Example: apply —the enthalpy change, ∆h, is the heat energy change measured at constant pressure and that standard conditions are 100 kpa to one small, clearly defined case, show the key step or comparison, and explain the result in words.

Boundary: —The enthalpy change, ∆H, is the heat energy change measured at constant pressure and that standard conditions are 100 kPa is not a universal recommendation. Check the syllabus scope, assumptions, units and the limits of the evidence before generalising.

Know that, by convention, exothermic reactions have a negative enthalpy change and endothermic reactions have a positive enthalpy change.

Use —that, by convention, exothermic reactions have a negative enthalpy change and endothermic reactions have a positive enthalpy to connect the rule to the data and decision in the question.

This matters because —that, by convention, exothermic reactions have a negative enthalpy change and endothermic reactions have a positive enthalpy determines what can be inferred or chosen; begin with the stated conditions and keep the conclusion tied to the evidence.

Example: apply —that, by convention, exothermic reactions have a negative enthalpy change and endothermic reactions have a positive enthalpy to one small, clearly defined case, show the key step or comparison, and explain the result in words.

Boundary: —That, by convention, exothermic reactions have a negative enthalpy change and endothermic reactions have a positive enthalpy is not a universal recommendation. Check the syllabus scope, assumptions, units and the limits of the evidence before generalising.

Be able to construct and interpret enthalpy level diagrams, showing exothermic and endothermic enthalpy changes.

Use —construct and interpret enthalpy level diagrams, showing exothermic and endothermic enthalpy changes to connect the rule to the data and decision in the question.

This matters because —construct and interpret enthalpy level diagrams, showing exothermic and endothermic enthalpy changes determines what can be inferred or chosen; begin with the stated conditions and keep the conclusion tied to the evidence.

Example: apply —construct and interpret enthalpy level diagrams, showing exothermic and endothermic enthalpy changes to one small, clearly defined case, show the key step or comparison, and explain the result in words.

Boundary: —Construct and interpret enthalpy level diagrams, showing exothermic and endothermic enthalpy changes is not a universal recommendation. Check the syllabus scope, assumptions, units and the limits of the evidence before generalising.

Know the definition of standard enthalpy change of: i reaction, ∆rH ii formation, ∆fH iii combustion, ∆cH iv neutralisation, ∆neutH v atomisation, ∆atH.

Use —the definition of standard enthalpy change of: i reaction, ∆rh ii formation, ∆fh iii combustion, ∆ch iv neutralisation to connect the rule to the data and decision in the question.

This matters because —the definition of standard enthalpy change of: i reaction, ∆rh ii formation, ∆fh iii combustion, ∆ch iv neutralisation determines what can be inferred or chosen; begin with the stated conditions and keep the conclusion tied to the evidence.

Example: apply —the definition of standard enthalpy change of: i reaction, ∆rh ii formation, ∆fh iii combustion, ∆ch iv neutralisation to one small, clearly defined case, show the key step or comparison, and explain the result in words.

Boundary: —The definition of standard enthalpy change of: i reaction, ∆rH ii formation, ∆fH iii combustion, ∆cH iv neutralisation is not a universal recommendation. Check the syllabus scope, assumptions, units and the limits of the evidence before generalising.

Be able to use experimental data to calculate: i energy transferred in a reaction recalling and using the expression: energy transferred (J) = mass (g) × specific heat capacity (J g-1 °C-1) × temperature change (°C) ii enthalpy change of the reaction in kJ mol⁻¹ This will be limited to experiments where substances are mixed in an insulated container and combustion experiments using a suitable calorimeter.

Use —experimental data to calculate: i energy transferred in a reaction recalling and using the expression: energy transferred to connect the rule to the data and decision in the question.

This matters because —experimental data to calculate: i energy transferred in a reaction recalling and using the expression: energy transferred determines what can be inferred or chosen; begin with the stated conditions and keep the conclusion tied to the evidence.

Example: apply —experimental data to calculate: i energy transferred in a reaction recalling and using the expression: energy transferred to one small, clearly defined case, show the key step or comparison, and explain the result in words.

Boundary: —Experimental data to calculate: i energy transferred in a reaction recalling and using the expression: energy transferred is not a universal recommendation. Check the syllabus scope, assumptions, units and the limits of the evidence before generalising.

Know Hess’s Law and be able to apply it to: i constructing enthalpy cycles ii calculating enthalpy changes of reaction using data provided, or data selected from a table or obtained from experiments.

Use —hess’s law to connect the rule to the data and decision in the question.

This matters because —hess’s law determines what can be inferred or chosen; begin with the stated conditions and keep the conclusion tied to the evidence.

Example: apply —hess’s law to one small, clearly defined case, show the key step or comparison, and explain the result in words.

Boundary: —Hess’s Law is not a universal recommendation. Check the syllabus scope, assumptions, units and the limits of the evidence before generalising.

CORE PRACTICAL 2 Determination of the enthalpy change of a reaction using Hess’s Law.

Use —core practical 2 determination of the enthalpy change of a reaction using hess’s law to connect the rule to the data and decision in the question.

This matters because —core practical 2 determination of the enthalpy change of a reaction using hess’s law determines what can be inferred or chosen; begin with the stated conditions and keep the conclusion tied to the evidence.

Example: apply —core practical 2 determination of the enthalpy change of a reaction using hess’s law to one small, clearly defined case, show the key step or comparison, and explain the result in words.

Boundary: —CORE PRACTICAL 2 Determination of the enthalpy change of a reaction using Hess’s Law is not a universal recommendation. Check the syllabus scope, assumptions, units and the limits of the evidence before generalising.

Be able to evaluate the results obtained from experiments and comment on sources of error and uncertainty and any assumptions made in the experiments Students will need to consider experiments where substances are mixed in an insulated container and combustion experiments using, for example, a spirit burner and be able to draw suitable graphs and use cooling curve corrections.

Use —evaluate the results obtained from experiments and comment on sources of error and uncertainty and any assumptions made to connect the rule to the data and decision in the question.

This matters because —evaluate the results obtained from experiments and comment on sources of error and uncertainty and any assumptions made determines what can be inferred or chosen; begin with the stated conditions and keep the conclusion tied to the evidence.

Example: apply —evaluate the results obtained from experiments and comment on sources of error and uncertainty and any assumptions made to one small, clearly defined case, show the key step or comparison, and explain the result in words.

Boundary: —Evaluate the results obtained from experiments and comment on sources of error and uncertainty and any assumptions made is not a universal recommendation. Check the syllabus scope, assumptions, units and the limits of the evidence before generalising.

Understand the terms ‘bond enthalpy’ and ‘mean bond enthalpy’, and be able to use bond enthalpies to calculate enthalpy changes, understanding the limitations of this method.

Use —the terms ‘bond enthalpy’ and ‘mean bond enthalpy’ to connect the rule to the data and decision in the question.

This matters because —the terms ‘bond enthalpy’ and ‘mean bond enthalpy’ determines what can be inferred or chosen; begin with the stated conditions and keep the conclusion tied to the evidence.

Example: apply —the terms ‘bond enthalpy’ and ‘mean bond enthalpy’ to one small, clearly defined case, show the key step or comparison, and explain the result in words.

Boundary: —The terms ‘bond enthalpy’ and ‘mean bond enthalpy’ is not a universal recommendation. Check the syllabus scope, assumptions, units and the limits of the evidence before generalising.

Be able to calculate mean bond enthalpies from enthalpy changes of reaction.

Use —mean bond enthalpies from enthalpy changes of reaction to connect the rule to the data and decision in the question.

This matters because —mean bond enthalpies from enthalpy changes of reaction determines what can be inferred or chosen; begin with the stated conditions and keep the conclusion tied to the evidence.

Example: apply —mean bond enthalpies from enthalpy changes of reaction to one small, clearly defined case, show the key step or comparison, and explain the result in words.

Boundary: —Mean bond enthalpies from enthalpy changes of reaction is not a universal recommendation. Check the syllabus scope, assumptions, units and the limits of the evidence before generalising.

Understand that bond enthalpy data gives some indication about which bond will break first in a reaction, how easy or difficult it is and therefore how rapidly a reaction will take place at room temperature.

Use —bond enthalpy data gives some indication about which bond will break first in a reaction, how easy or difficult it is to connect the rule to the data and decision in the question.

This matters because —bond enthalpy data gives some indication about which bond will break first in a reaction, how easy or difficult it is determines what can be inferred or chosen; begin with the stated conditions and keep the conclusion tied to the evidence.

Example: apply —bond enthalpy data gives some indication about which bond will break first in a reaction, how easy or difficult it is to one small, clearly defined case, show the key step or comparison, and explain the result in words.

Boundary: —Bond enthalpy data gives some indication about which bond will break first in a reaction, how easy or difficult it is is not a universal recommendation. Check the syllabus scope, assumptions, units and the limits of the evidence before generalising.