Module 1 – Development of Practical Skills In Chemistry Flashcards
What is calorimetry?
Calorimetry is a process used to find the amount of energy released or used in a chemical reaction.
What type of reaction releases energy to the surroundings?
Exothermic
What type of reaction takes in energy from the surroundings?
Endothermic
What is a neutralisation reaction?
A reaction between an acid and a base, forming a salt and water.
What is meant by the term enthalpy change of neutralisation?
The enthalpy change that accompanies the formation of 1 mole of water from a neutralisation reaction under standard conditions.
What is the neutralisation reaction between hydrochloric acid and sodium hydroxide?
Hydrochloric acid + sodium hydroxide → sodium chloride + water
HCl + NaOH → NaCl + H2O
Why might a neutralisation reaction be carried out in a styrofoam cup?
A styrofoam cup is a better thermal insulator than a glass beaker, so less heat will be lost to the surroundings, making the calculated temperature change more accurate.
What equation is used to calculate the energy absorbed by the solution?
Q = mcΔT
Where: m - mass of the solution (g), c - specific heat capacity of the solution (the value for water is used), ΔT - temperature change (°C or K), Q - heat energy (J)
What does ‘m’ represent in the equation Q = mcΔT?
‘m’ represents the mass of the solution in grams (g).
What does ‘c’ represent in the equation Q = mcΔT?
‘c’ represents the specific heat capacity of the solution.
What does ‘ΔT’ represent in the equation Q = mcΔT?
‘ΔT’ represents the temperature change in degrees Celsius (°C) or Kelvin (K).
What does ‘Q’ represent in the equation Q = mcΔT?
‘Q’ represents the heat energy absorbed or released in joules (J).
How is the enthalpy change of neutralisation calculated?
Q ÷ mol
How can the number of moles be calculated?
mol = volume x concentration for the hydrochloric acid used
How can you find the maximum temperature reached during a neutralisation reaction?
Plot a graph of temperature (y axis) against time (x axis) and extrapolate to the time that the reactants were combined.
What are the possible sources of error when investigating the enthalpy change of neutralisation?
- Heat transfer to and from surroundings.
- Incomplete combustion if using fuel.
- Inaccuracy in temperature measurement.
How can the errors of heat transfer and incomplete combustion be reduced?
A bomb calorimeter could be used to combat these two errors.
How can inaccuracies in temperature measurement be reduced?
Use an electronic thermometer.
Describe an experiment to investigate the enthalpy change of neutralisation of a reaction between hydrochloric acid and sodium hydroxide
- Add 25cm3 HCl (aq) to a polystyrene cup. Place the cup in a glass beaker. Measure 25cm3 NaOH(aq).
- Start the timer. Record the initial temperature of the HCl at minute intervals for 3 minutes.
- At the fourth minute, add the sodium hydroxide and do not record the temperature.
- Continue recording the temperature at minute intervals from 5 to 10 minutes. Stir the solution when measuring the temperature.
What is a control variable?
A variable that is kept constant throughout the experiment
What is an independent variable?
A factor that is changed during the experiment to see the effect it has on another factor
What is a dependent variable?
A factor that is measured or observed in the experiment due to the changing of the independent variable
When plotting a graph what variables go on what axis?
The independent variable is on the x-axis
The dependent variable is on the y-axis
How do you measure the gradient of either a tangent or line of best fit?
Gradient = change in y-coordinate/change in x-coordinate
What is an anomaly?
The deviation of a value from its expected value (i.e. a value that doesnt fit the trend)
Why should anomalies be ignored when calculating a mean?
Not including anomalies in the mean value will make it much more accurate
What is precision?
A set of precise measurements will have very little spread about the mean
However precision gives no idea of how close values are to the actual, true value - only how close values are to each other
What is accuracy?
The more accurate the data, the closer it is to the actual value
What is uncertainty?
The uncertainty in a measurement is the interval within which the actual value is expected to lie
What is percentage uncertainty and how do you calculate it?
Percentage uncertainty in a measurement: 100 x absolute uncertainty/calculated value
What are different ways you can improve apparatus to give more accurate results?
Increase the number of readings
Control external variables
Use measuring devices/equipment with greater precision
What apparatus is used in a titration to accurately measure volumes?
Burette to measure the volume of solution added
Pipette and pipette filler to measure a specific volume of solution into the conical flask
Describe the ‘weighing-by-difference’ method?
Add roughly the desired mass of a substance to a weighing bottle and weigh
Transfer the substance to a beaker/conical flask
Reweigh the weighing bottle
The difference between the masses is the mass of the substance
Describe how to carry out an acid-base titration
Use a pipette to add 25cm3 of acid to a conical flask. Add a few drops of indicator
Pour alkali into the burette. Record the initial burette volume
Complete a trial titre. The conical flask should be swirled constantly above a white tile. Stop adding the alkali as soon as the end point is reached. Record the final burette volume
Repeat the titration until two constant concordant results are obtained. Add alkali drop by drop near the end point
(25cm3 of alkali may be initially put into the conical flask if the burette is filled with acid)
When transferring a solution from one beaker to another, how can you ensure as much of the dissolved substance has been transferred as possible?
Use distilled water to wash any leftover solution from the old apparatus into the new beaker
How do you accurately fill a volumetric flask to the graduated mark?
Fill so the bottom of the meniscus rests on the graduated mark The meniscus is the curve of the liquid
What is a standard solution?
A solution of known concentration
Describe how to make a standard solution of NaHCO3
Weigh out a specified amount of NaHCO3
Add a small volume of distilled water to the beaker containing the NaHCO3 to dissolve it
Use a funnel to add the solution to a 250cm3 volumetric flask
Rinse the beaker with distilled water and add the washings to the flask
Make the flask up to the graduation mark with distilled water
Stopper and shake the flask
Suggest 2 indicators that could be used in an acid-base titration
Phenolphthalein
Methyl orange
State the colour of phenophthalein in: a) Acidic conditions b) Basic conditions
a) Colourless
b) Pink
State the colour of methyl orange in: a) Acidic conditions b) Basic conditions
a) Red
b) Yellow (Orange at the end point of a titration)
What are concordant results in a titration?
Titres that are within 0.1 cm3 of each other
What degree of precision should burette readings be recorded to?
To the nearest 0.05cm3
Which results are used when calculating a mean titre?
2 concordant results (within 0.10cm3 of each other)
When completing a titration, what piece of apparatus is used to make the colour change easier to observe?
A white tile placed under the conical flask
What 3 things are required to take an accurate burette reading?
Read from the bottom of the meniscus
Allow any liquid on the walls of the burette to settle before taking a reading
Ensure there are no air bubbles
What is the end point of a titration?
The first point at which the indicator changes colour permanently
Why is the conical flask swirled during a titration?
To ensure all the reactants are combined so that the reaction is complete
Why is it better to have a titre volume of 25 cm3 than 10 cm3?
The larger the titre volume, the smaller the percentage error
What equation links number of moles and concentration?
number of moles = concentration x volume
If you know the volume of acid required to neutralise an alkali, how could you calculate the concentration of the acid, given the alkali concentration and volume?
Calculate the number of moles of the alkali using the known volume and concentration
Use the chemical equation to work out the ratio of acid and alkali that react and hence work out how many moles of acid have reacted
Divide the moles of acid by the volume used in neutralisation
What is a ‘clock reaction’?
A reaction in which the concentration of a reagent quickly changes after a certain time period, causing a colour change
Briefly describe a method that could be followed to find the order of reaction with respect to iodine ions in the iodine clock reaction
- mix potassium iodide, sodium thiosulfate and starch solutions in a conical flask
- add potassium peroxodisulphate and start the timer
- record the time taken for the solution to turn blue-black
- repeat steps 1-3 with varying concentrations of KI
How can the order of a reactant be deduced from the rate of reaction?
The rate of reaction is proportional to 1/time. Use changes in the rate of reaction and concentrations to deduce the order with respect to each reagent
Write the word and chemical equations for the reaction between potassium iodide and potassium peroxodisulphate (K2S2O8)
Potassium peroxodisulphate + potassium iodide -> potassium sulfate + iodine
K2S2O8 + 2KI -> 2K2SO4 + I2
What equation links moles, volume and concentration?
Moles = volume x concentration
How can the initial rate be calculated for an iodine clock reaction?
Initial rate = Moles of iodine / Time taken for colour change
Give 2 possible sources of error in an iodine clock reaction
- Innacurate timing of the appearance of blue colour -> could have two students timing the experiment and calculate an average value
- Adding starch slightly increases the volume which affects the concentrations of the reactants and thus the amount they chance over time
Suggest some control variables for the iodine clock experiment
- Total volume of reactants
- Volume of starch and thiosulfate used
- The concentration of all the reagents (except potassium iodide solution)
- Temperature (as this can also affect the rate of reaction)
Write the rate equation for the reaction between iodide ions and peroxodisulphate ions?
Rate = k [I-] [S2O82-]
What is the overall order of the reaction between iodide ions and peroxodisulphate ions?
2
