Chemistry AT2 DEPTH STUDY 2026 PREP

Experimental quality

The quality of an experiment is assessed by three main parameters: accuracy, validity and reliability. 

• Accuracy is a feature of the results. 

• Reliability is a feature of the results. 

• Validity is a feature of the method

Reliability

refers to the consistency achieved between the results of repeated trials under the same conditions – is evaluated by repeating the experiment multiple times and comparing corresponding results. If the results obtained are repeatable → the experiment can be evaluated as reliable

How to improve reliability

Use consistent equipment and follow the same procedure under the same conditions each time.

Validity

refers to the appropriateness of the methodology of a scientific experiment for testing the hypothesis or fulfilling the aim by controlling variables and correctly identifying the independent and dependent variables

How to ensure validity

• Address the aim directly – make sure the method is designed specifically to answer the inquiry question or test the hypothesis

• Control all variables except independent variable 

• Choose appropriate equipment and materials suited to the measurements or observations you need

• Define variables clearly

Accuracy (experimental accuracy)

refers to the proximity of experimentally obtained results to the theoretically expected results

How to improve accuracy

• Calibrate the equipment where possible, so they can give correct readings

• Taking an average of multiple trials (after discarding outliers) to minimise random error

• Use precise equipment (choose measuring tools with fine scales or small increments) – known as instrument accuracy

• Measure carefully e.g. take readings at eye level (for liquid meniscus) to minimise parallax errors

• Use appropriate techniques e.g. clean your equipment between trials

Systematic error

• Consistent

• Reduces accuracy (how close you are to the true value)

• Result from a flaw in the experimental design or faulty measuring equipment 

• Because the error occurs consistently, repeating the experiment and averaging the results will not fix the problem

Random error

• Variable 

• Reduces precision (how close your measurements are to each other)

• Result from unknown and unpredictable changes in the experiment, often related to the limitations of the measuring equipment or slight human variations.

• To minimise its effect: take multiple trials, discard outlier and calculate the average

Endothermic reactions

chemical reactions that absorb energy or ‘enters’ the reaction system, typically in the form of heat, from their surroundings, causing the surroundings to cool down (e.g. photosynthesis)

• If the total chemical energy of the products is > the total energy of reactants: → energy will be absorbed from the surrounds

Exothermic reactions

chemical reactions which release energy or ‘exits’ the reaction system, typically in the form of heat, to the surroundings, causing the surroundings to warm up (e.g. respiration)

• If the total chemical energy of the products is < the total energy of reactants: → energy will be released from the system into the surroundings.

Heat of Combustion

the enthalpy change that occurs when a specific amount of a substance burns completely in oxygen

Specific heat capacity (Jg^-1 K^-1 or Jg^-1 °C^-1)

is the measure of the amount of energy needed to increase the temperature of a specific quantity of that substance.

• Is the reflection of the types of bonds holding molecules, ions or atoms together in a substance 

• The higher the specific heat capacity, the more effectively a material stores heat energy 

  • E.g. water has a specific heat capacity of 4.18 Jg^-1 K^-1 – means that 4.18 J of heat energy is required to increase the temperature of 1 g of water by 1 K. The relatively high value is due to the hydrogen bonds between water molecules

Calorimeter

a device that measures the energy changes occurring during chemical and physical reactions

Calorimetry

measurement of heat changes in a system

Efficiency of a calorimeter

a measure of how efficiently heat energy is transferred from the combustion of a material to the heating of the water.

Dissolution

the process of separating positive and negative ions from a solid ionic compound to form hydrated ions when an ionic compound dissolves in water

Ion-dipole attraction

attraction between an ion and a polar molecule (e.g. water)

Enthalpy

chemical energy of a substance under constant pressure

Enthalpy Change (heat of reaction kJmol^-1)

measure of the amount of energy absorbed or released during a chemical reaction, hence refers to the exchange of heat energy between the system and its surroundings under constant pressure (ΔH) 

• Can occur during physical and chemical change

Enthalpy Change in Exothermic Reactions

H_p < H_r: energy is released from its system into the surroundings 

ΔH < 0: reaction is exothermic

Enthalpy Change in Endothermic Reactions

H_p > H_r: energy must be absorbed from its surroundings

• ΔH < 0: reaction is endothermic

ΔH > 0: reaction is endothermic

Thermochemical Equation

The amount of energy (kJ) signified by the ΔH value corresponds to the mole amounts specified by the coefficients in the equation – if the coefficients of the equation are changed the ΔH also changes. 

C₆H₁₂O_{6 (aq)} + 6O_{2 (g)} → 6CO_{2 (g)} + 6H₂O_(l)     ΔH = -2803 kJmol^-1

When 1 mol of glucose reacts with 6 mol of oxygen to produce 6 mol of carbon dioxide and 6 mol of water, 2803kJ of energy is released.

Reversing the chemical equation changes the sign but not the magnitude of ΔH

If the coefficient of the substance in the combustion is 2 or more, the heat of combustion must be multiplied by this no. to determine ΔH.

E.g. 2C₄H_{10 (g)} + 13O_{2 (g)} → 8CO_{2 (g)} + 10H₂O _(l)    ΔH = 2 x -2886 = -5772 kJmol^-1

Activation energy

the minimum amount of energy colliding reactant particles need in order for successful collisions to occur that lead to a reaction

Energy Profile Diagram

represents the energy changes that occur during the course of a reaction

Activation complex

the peak of the activation energy and is the transitional state between reactants and products

Bond energy

the amount of energy required to break 1 mol of a stated bond to its constituent gaseous atoms under standard state conditions (25°C, 1 bar)

*Bond breaking requires energy – endothermic

  Bond formation releases energy – exothermic

Enthalpy of vapourisation (ΔH_vap)

known as latent heat of heat vapourisation and is the amount of energy required to transform 1 mol of a substance from a liquid to a gas at its boiling point– measured at one atmosphere pressure (atm)

• E.g. 40.7 kJ of heat energy is required to vaporise 1 mol of liquid water at 100℃ to 1 mol of steam – ΔH_vap of water is 40.7 kJmol^-1 and process is endothermic (heat absorbed by water)

H₂O_(l) → H₂O_(g)     ΔH = +40.7 kJmol^-1

• The enthalpy change in the condensation of 1 mol of steam to 1 mol of water – exothermic (heat released during condensation)

H₂O_(g) → H₂O_(l)     ΔH = -40.7 kJmol^-1

Enthalpy of Fusion (ΔH_fus)

known as latent heat of fusion and is the energy associated with melting 1 mol of a solid to its liquid form at its melting point – measured at 1 atm

• E.g. enthalpy of fusion for water at its melting point is 6.01 kJmol^-1

H₂O_(s) → H₂O_(l)     ΔH = +6.01 kJmol^-1

This change represents the change in intermolecular bonding 

• Melting and vapourisation requires energy – endothermic process

• Solidification and condensation releases energy – exothermic process

Hess’s Law

The energy cycles of Hess’s Law shows that no matter what pathway of a chemical reaction is, the difference in enthalpy between the two substances is independent of the route taken for the conversion