Development of practical skills in chemistry

Welcome to Practical Chemistry for OCR B (Salters)

Practical skills are at the very heart of chemistry. In the OCR Chemistry B (Salters) AS Level course (Specification H033), your experimental abilities are directly assessed in both written examination papers: H033/01 (Foundations of chemistry) and H033/02 (Chemistry in depth). At least 15% of the total marks across these papers test practical knowledge, planning, data analysis, and evaluation.

Don't worry if experimental questions sometimes feel daunting! Practical chemistry follows logical rules. Once you master the standard apparatus, safety protocols, error calculations, and key laboratory techniques, you will be able to tackle these exam questions with complete confidence.

1. Planning and Experimental Design

Every successful chemical investigation begins with a solid plan. In your exams, you will often be asked to design an experiment or identify the key variables involved.

Understanding Experimental Variables

To design a fair and valid test, you must clearly distinguish between three types of variables:
• Independent Variable: The factor that you deliberately change (for example, the concentration of an acid or the temperature of a reaction).
• Dependent Variable: The factor that you measure to obtain your results (for example, the time taken for a cross to disappear or the volume of gas produced).
• Control Variables: All other factors that must be kept strictly constant throughout the experiment (such as the total volume of solution, temperature, or surface area of a solid catalyst). If a control variable changes, you cannot be certain which factor caused your dependent variable to change!

Risk Assessments and Laboratory Safety

Safety questions test your ability to recognize chemical hazards and suggest realistic control measures.
• Common Chemical Hazard Categories: Toxic, Corrosive, Flammable, Oxidizing, Harmful / Irritant, and Environmental Hazard.
• Standard Control Measures:
- Toxic or harmful gases/vapours: Carry out the reaction inside a well-ventilated fume cupboard.
- Corrosive or irritant liquids (e.g. concentrated acids and alkalis): Wear safety goggles and nitrile gloves.
- Flammable organic liquids (e.g. alcohols, volatile solvents): Heat using an electric heating mantle or a water bath; never heat with an open Bunsen burner flame.

Key Takeaway for Planning: Always state what you change (independent), what you measure (dependent), what you keep constant (control), and state specific safety precautions linked directly to the hazards named in the question.

2. Core Quantitative Apparatus and Measurement Standards

Precision and accuracy in the laboratory depend on choosing the right piece of equipment and recording your data using official standard conventions.

Volumetric Glassware and Standards

Different pieces of glassware have specific capacities and measurement uncertainties:
• Standard Volumetric Flask: Typically has a capacity of \(250.0\text{ cm}^3\) (typical tolerance \(\pm 0.20\text{ cm}^3\)). Used for preparing standard solutions of accurately known concentration.
• Transfer Pipette: Typically delivers a fixed volume of \(25.0\text{ cm}^3\) (typical tolerance \(\pm 0.06\text{ cm}^3\)). Used to transfer exact aliquots into a conical flask.
• Burette: Graduated in \(0.10\text{ cm}^3\) increments. A single reading has an uncertainty of \(\pm 0.05\text{ cm}^3\).

Critical Exam Rule for Burette Readings: All initial and final burette readings must be recorded to 2 decimal places ending in either .00 or .05 (e.g., \(0.00\text{ cm}^3\), \(21.45\text{ cm}^3\), or \(23.40\text{ cm}^3\)). Never write a single decimal place like \(23.4\text{ cm}^3\)!

Titration Concordancy

When carrying out a titration, results must be reproducible:
• Concordant Titres: Titres that are within \(\pm 0.10\text{ cm}^3\) of each other.
• Calculating the Mean Titre: You must only average the concordant titres. Always reject the rough (trial) titre and any non-concordant runs before calculating the mean.

Mass Measurements: Weighing by Difference

To accurately weigh a solid using a 2 decimal place (\(\pm 0.01\text{ g}\)) or 3 decimal place (\(\pm 0.001\text{ g}\)) digital balance, use the weighing by difference technique:
1. Weigh the weighing boat containing the solid sample and record the mass (\(m_1\)).
2. Tip the solid into the beaker.
3. Re-weigh the empty weighing boat containing any residual traces of solid and record the mass (\(m_2\)).
4. Calculate the exact mass transferred:
\(\text{Mass transferred} = m_1 - m_2\)

Key Takeaway for Apparatus: Record all burette values to 2 d.p. (.00 or .05), average only titres within \(\pm 0.10\text{ cm}^3\), and use weighing by difference to eliminate transfer losses.

3. Mathematical Calculations and Experimental Uncertainties

Calculating Percentage Uncertainty

Every measuring device has an inherent uncertainty. To calculate the percentage uncertainty in a measurement, use the formula:
\(\text{Percentage Uncertainty} = \left( \frac{\text{Absolute Uncertainty} \times \text{number of readings}}{\text{Quantity measured}} \right) \times 100\)

Single Reading vs. Two-Reading Measurement:
• A fixed pipette involves a single measurement: \(\text{number of readings} = 1\).
• A burette titre requires taking an initial reading and a final reading, so it involves 2 readings:
\(\text{Percentage Uncertainty}_{\text{titre}} = \left( \frac{2 \times 0.05}{\text{Titre}} \right) \times 100\)
• A temperature change (\(\Delta T = T_{\text{final}} - T_{\text{initial}}\)) or a mass by difference also involves 2 readings, meaning the absolute uncertainty of the thermometer or balance must be multiplied by 2.

Calorimetric Calculations

When calculating enthalpy changes from temperature measurements in a calorimeter:
1. Calculate heat energy transferred (\(q\)) in Joules:
\(q = mc\Delta T\)
Where \(m\) = mass of solution (in \(\text{g}\), assuming \(1\text{ cm}^3 = 1\text{ g}\)), \(c = 4.18\text{ J g}^{-1}\text{ K}^{-1}\), and \(\Delta T\) = temperature change in \(\text{K}\) or \(^\circ\text{C}\).

2. Convert to the molar enthalpy change of reaction (\(\Delta H\)) in \(\text{kJ mol}^{-1}\):
\(\Delta H = -\frac{q}{n \times 1000}\)
Where \(n\) is the amount in moles of the limiting reactant. Remember: exothermic reactions must have a negative \(\Delta H\) sign!

Percentage Yield and Atom Economy

To evaluate how efficient an organic synthesis or chemical process is, two key metrics are used:
• Percentage Yield: Compares what you actually obtained to the theoretical maximum:
\(\text{Percentage Yield} = \left( \frac{\text{Actual yield}}{\text{Theoretical yield}} \right) \times 100\)

• Atom Economy: Evaluates the proportion of reactant mass converted into the desired product:
\(\text{Atom Economy} = \left( \frac{M_r \text{ of desired product}}{\sum M_r \text{ of all reactants}} \right) \times 100\)

Key Takeaway for Calculations: Double the uncertainty whenever finding a difference (titres, temperature changes, mass by difference), remember the negative sign for exothermic \(\Delta H\), and keep intermediate values unrounded in your calculator to avoid rounding errors.

4. Data Presentation, Graphing, and Significant Figures

Table Conventions

• Column Headers: Must always be presented as Quantity / unit using a forward slash (solidus), for example: \(\text{Time } / \text{ s}\), \(\text{Volume } / \text{ cm}^3\), or \(\text{Temperature } / \text{ }^\circ\text{C}\).
• Data Consistency: All raw readings in a single column must be recorded to the same number of decimal places matching the resolution of the instrument.

Graphing Rules for Success

• Sensible Scales: Use linear, regular scale increments (such as 1, 2, 5, or 10 units per large grid square). Never use awkward increments like 3, 6, or 7.
• Use the Grid: Your plotted points must occupy more than 50% of the graph grid along both the x-axis and y-axis.
• Line of Best Fit: Draw a smooth, continuous line or curve with an even balance of points on either side. Do not force the line through anomalous points.
• Determining Rates of Reaction: To find the reaction rate at a specific time, draw a tangent to the curve at that point and calculate the gradient:
\(\text{Gradient} = \frac{\Delta y}{\Delta x}\)

Significant Figures (SF)

Your final numerical answer must be rounded to the same number of significant figures as the least precise piece of raw data provided in the question. Avoid rounding intermediate steps during multi-step calculations.

Key Takeaway for Data Presentation: Format table headers with a solidus (\(\text{Quantity } / \text{ unit}\)), use scales of 1, 2, or 5 covering over half the grid, and match final significant figures to the least precise raw data.

5. Core AS Practical Techniques in Salters Contexts

Heating Under Reflux vs. Distillation

Reflux and distillation are fundamental organic techniques used across the Salters units (such as when synthesizing haloalkanes or oxidizing alcohols).

Reflux:
• Purpose: Allows prolonged heating of a reaction mixture without losing volatile reactants or products.
• Setup: A pear-shaped or round-bottom flask fitted vertically with a Liebig condenser.
• Key Rules:
- Add anti-bumping granules to promote smooth, even boiling and prevent violent bubbling.
- Condenser water must enter at the bottom and leave at the top (counter-current flow ensures the jacket remains completely full of cold water).
- The top of the condenser must remain open. Never put a stopper in the top of a reflux condenser! Doing so creates a sealed system where pressure will build up, risking an explosion.

Distillation:
• Purpose: Separates a volatile product from a reaction mixture based on differences in boiling points.
• Setup: Flask fitted with a still head, thermometer (with the bulb positioned right opposite the side-arm condenser entrance), condenser sloping downwards, and a collecting vessel.

Purification of an Organic Liquid Product

Follow these standard sequential steps to purify a synthesized liquid product (e.g., a haloalkane):
1. Separating Funnel: Pour the crude mixture into a separating funnel. Allow layers to separate based on density, then run off the lower aqueous layer.
2. Washing with \(\text{NaHCO}_3\text{(aq)}\): Add aqueous sodium hydrogencarbonate to remove any residual acid impurities. Shake the funnel and frequently invert and open the tap to release the build-up of \(\text{CO}_2\) gas pressure.
3. Drying the Organic Layer: Add an anhydrous inorganic salt, such as anhydrous \(\text{MgSO}_4\) or anhydrous \(\text{CaCl}_2\). Swirl until the drying agent stops clumping and the liquid becomes completely clear.
4. Final Redistillation: Filter off the drying agent and redistill the liquid, collecting the fraction that boils within a narrow range around the target compound's known boiling point.

Qualitative Inorganic Analysis

You must know the exact reagents, observations, and equations for identifying common ions:

1. Test for Halide Ions (\(\text{Cl}^-\), \(\text{Br}^-\), \(\text{I}^-\)):
• Acidify the sample with dilute \(\text{HNO}_3\) (to remove any interfering carbonate ions), then add \(\text{AgNO}_3\text{(aq)}\):
- Chloride (\(\text{Cl}^-\)): White precipitate (\(\text{AgCl}\)); dissolves in dilute \(\text{NH}_3\text{(aq)}\).
- Bromide (\(\text{Br}^-\)): Cream precipitate (\(\text{AgBr}\)); insoluble in dilute \(\text{NH}_3\), but dissolves in concentrated \(\text{NH}_3\text{(aq)}\).
- Iodide (\(\text{I}^-\)): Yellow precipitate (\(\text{AgI}\)); insoluble in both dilute and concentrated \(\text{NH}_3\text{(aq)}\).
Memory Aid: Halides get darker as you go down the group (White \(\rightarrow\) Cream \(\rightarrow\) Yellow), and their silver precipitates become progressively harder to dissolve in ammonia!

2. Test for Carbonate Ions (\(\text{CO}_3^{2-}\)):
• Add dilute acid (e.g. \(\text{HNO}_3\) or \(\text{HCl}\)).
• Observation: Rapid effervescence (fizzing). Bubble the generated gas through limewater; the limewater turns cloudy/milky due to \(\text{CO}_2\).

3. Test for Sulfate Ions (\(\text{SO}_4^{2-}\)):
• Acidify with dilute \(\text{HCl}\) or dilute \(\text{HNO}_3\) (to eliminate carbonates), then add \(\text{BaCl}_2\text{(aq)}\) or \(\text{Ba(NO}_3)_2\text{(aq)}\).
• Observation: Formation of a dense white precipitate of barium sulfate (\(\text{BaSO}_4\)).

Key Takeaway for Techniques: Remember condenser water always enters at the bottom, vent separating funnels frequently when washing with \(\text{NaHCO}_3\), and use dilute nitric acid before adding silver nitrate for halide tests.

6. Summary of Top Exam Pitfalls to Avoid

• Burette Decimal Precision: Always write burette readings to two decimal places ending in `.00` or `.05`. Writing \(24.3\text{ cm}^3\) loses marks; write \(24.30\text{ cm}^3\).
• Concordancy Errors: Never include non-concordant titres (those differing by more than \(0.10\text{ cm}^3\)) or the initial rough titre in your mean.
• Uncertainty Multiplication: Don't forget that measurements requiring two readings (temperature change, mass by difference, titration titre) have double the absolute error.
• Reflux Diagrams: Never draw a stopper in a reflux condenser, and make sure water enters at the bottom and leaves at the top.
• Accuracy vs. Precision vs. Reliability: Repeating an experiment and calculating a mean improves reliability by reducing the effect of random errors; it does not eliminate systematic errors or change the intrinsic accuracy of the equipment!