2017 PMOD CHEM EXAM

Questions and Answers

what occurs as the atomic number increases for the Group 17 elements?

(a) atomic radii decrease

(b) melting points decrease

(d) the elements become more reactive

Describe all the types of bonding present between particles in the solution of copper (II) chloride (aq)

Ion – dipole bonding between ions and water

Hydrogen bonding between water molecules

Dispersion forces between all particles Can also include dipole – dipole forces, but not required for mark

(a) Using a labelled diagram, describe the bonding present in solid potassium.

Particles arranged in a regular lattice (drawn or labelled)

Positively charged potassium ions drawn and labelled (K+)

Delocalised electrons drawn and labelled

metallic bonding is present in solid potassium, where the potassium cations are surrounded by a sea of delocalised electrons

Explain why solid potassium can conduct electricity, whereas solid potassium chloride cannot.

Metal: Delocalised electrons can flow through structure to carry current Ionic solid: IONS cannot move / fixed in lattice so cannot carry current (NO MOBILE CHARGED PARTICLES (IONS!!! DO NOT SAY ELECTRONS) Signup and view all the answers

Some students carried out an investigation into the effect of concentration of nitric acid on the rate of this reaction, by combining 20.0 mL of dilute nitric acid of various concentrations, with 0.10 g of magnesium ribbon. (b) Explain why each of the trials produced about the same volume of gas.

Nitric acid must be in excess / magnesium must be the limiting reagent mass of Mg is constant, vol of acid constant but concentration changes so moles of acid changes Volume of hydrogen gas depends on amount of magnesium used so is the same in each trial Signup and view all the answers

Use collision theory to explain the effect of increasing the concentration of nitric acid on the rate of this reaction.

MORE particles of nitric acid in a GIVEN VOLUME Frequency of collisions between reactants increases Frequency / number of successful collisions increases (more reactant particles will collide with correct orientation and enough energy to overcome activation energy) THEREFORE, ROR INCREASS Signup and view all the answers

Another group of students in the same class carried out their investigation using the same concentrations of ethanoic acid, CH3COOH, instead of nitric acid. Describe how their results would be different, and explain why this is the case. (4)

Time taken for reaction is longer (not less gas in a given time – this is not 1 what the method measures) Ethanoic acid is a weak acid / only a small proportion of molecules are 1 ionised Concentration of H+ is lower THEREFOREM Rate of reaction is SLOWER Signup and view all the answers

How do you know if the reaction is endothermic? What does this tell you about the relative bond energies of the reactants and products in the reaction?

As the enthalpy change is positive Energy is absorbed from the surroundings Energy needed to break bonds in reactants is greater than energy released when forming bonds in the products Signup and view all the answers

How do you know if the reaction is exothermic? What does this tell you about the relative bond energies of the reactants and products in the reaction?

as enthalpy change is negative energy is released into the surroundings energy needed to break the bonds in reactants is LESS THAN energy released when forming bonds in products Signup and view all the answers

The rate of photosynthesis varies significantly with the light and weather conditions. With reference to activation energy, • explain why plants cannot photosynthesise at night. • predict how the rate of photosynthesis during the day would be affected by temperature conditions. • explain why changing the temperature affects the rate of photosynthesis.

Photosynthesis cannot take place at night as there is not enough light present Without light, cannot overcome activation energy for the reaction Rate of photosynthesis increases as temperature increases Higher temperature increases (average) energy of molecules which increases proportion of successful reactant particle collisions Signup and view all the answers

Explain why hydrogen fluoride’s boiling point is significantly higher than the boiling points of the other hydrogen halides.

Hydrogen fluoride forms hydrogen bonds whereas the others can’t Hydrogen bonds make the intermolecular bonding in HF stronger than the intermolecular bonding in the others Greater amount of energy needed to overcome stronger intermolecular bonds so boiling point is higher Signup and view all the answers

Explain the trend observed in the boiling points of hydrogen chloride, hydrogen bromide and hydrogen iodide (exclude hydrogen fluoride). Refer to all types of intermolecular bonds present in your answer.

HCl, HBr and HI all form dipole – dipole forces (These get weaker down the group due to decreasing difference in polarity – but this is not required) All three also form dispersion forces As you go down the group, dispersion forces get stronger due to increasing number of electrons in the molecule The SUM of the dipole – dipole and dispersion forces is what must be OVERCOME for the substance to boil Boiling point increases down the group (from HCl to HBr to HI) therefore the increasing strength of dispersion forces must be responsible for this trend Signup and view all the answers

Describe the purification of mine water by distillation. Include the physical properties of the components that allow this method to work.

Mine water is heated to (at least) 100oC so that the water boils Steam is collected and condensed Ionic salts are left behind as their boiling points are much higher than that of water Signup and view all the answers

Explain why distillation would not be suitable as a method to purify mine water on an industrial scale.

Large energy / fuel / electricity / heat requirement Signup and view all the answers

Explain why laboratory filtration is not able to separate the dissolved ions from mine water.

Dissolved ions are small enough to pass through the pores in filter paper Filtration can only remove larger insoluble particles Signup and view all the answers

define structural isomer

Structural isomers have the same molecular formula but atoms are bonded in a different order Signup and view all the answers

define geometric isomer

Geometric isomers have the same molecular formula, atoms bonded in the same order, but different arrangement in space (around the C=C double bond) Signup and view all the answers

During the manufacture of the isomers of 1,2-dichloroethene, samples of the reactants and products are frequently tested, to ensure that the reaction is giving the desired products. One of the tests carried out on the hydrocarbon reactant and the range of products is a test for unsaturation. (d) Explain the difference between a saturated and an unsaturated hydrocarbon. Suggest a test that could be used to establish whether the substances are saturated or unsaturated, and give expected observations that would confirm saturation and unsaturation. Write a balanced equation for the reaction that would take place in the test, for one of the compounds discussed in this question.

Saturated hydrocarbons contain as many hydrogen atoms as possible on the carbon chain (double the number of carbons +2) Unsaturated hydrocarbons contain fewer hydrogen atoms than maximum for the carbon chain (only double the number of carbon atoms) Test for unsaturation: shake with bromine water / solution (not pure Br2) (Also accept shake with iodine solution) Saturated substances: mixture remains orange Unsaturated substances: mixture decolourises / turns colourless / fades Any correct example of an addition reaction between one of the examples and Br2. Can use molecular or structural formulae. Eg: C2H4 + Br2 → C2H4Br2 Signup and view all the answers

Study Notes

Group 17 Elements and Atomic Number

As atomic number increases in Group 17, elements become less reactive due to greater atomic size and increased shielding effect.
Physical states transition from gases (fluorine, chlorine) to solids (iodine, astatine) as atomic number rises.

Bonding in Copper(II) Chloride Solution

Copper(II) chloride (aq) consists of ionic bonds between copper and chloride ions.
In solution, ions are dissociated and surrounded by water molecules, exhibiting ion-dipole interactions.
Presence of electrostatic attractions facilitates conductivity in the solution.

Solid Potassium Bonding and Conductivity

Solid potassium comprises metallic bonds which allow free movement of delocalized electrons.
Solid potassium can conduct electricity due to mobility of electrons.
Solid potassium chloride, with its ionic structure, cannot conduct electricity as ions are fixed in a crystalline lattice.

Investigation with Nitric Acid and Magnesium Ribbon

Trials produced similar gas volumes because the amount of magnesium and acid remained constant, yielding a maximum reaction.
Collision theory suggests that higher concentrations increase the frequency of effective collisions, thus enhancing reaction rate.

Ethanoic Acid vs. Nitric Acid Reaction Results

Ethanoic acid is a weak acid with fewer ions compared to nitric acid, leading to slower reaction rates.
Reduced ion concentration impacts the overall rate of reaction due to lower collision frequency.

Endothermic and Exothermic Reactions

A reaction is endothermic if it absorbs heat, indicating that products have higher energy and broken bonds require more energy than formed.
A reaction is exothermic if it releases heat, suggesting that formed bonds are stronger and lower in energy than reactant bonds.

Photosynthesis, Light, and Temperature

Plants cannot photosynthesize at night due to the absence of light, which is essential for energy absorption in photosynthesis.
Photosynthesis rates during the day can drop with decreasing temperatures due to reduced kinetic energy potentially lowering effective collisions.
Temperature changes affect enzyme activity, impacting metabolic processes and gas exchange in photosynthesis.

Boiling Points of Hydrogen Halides

Hydrogen fluoride has a higher boiling point than other hydrogen halides due to strong hydrogen bonding.
The boiling points of hydrogen chloride, bromide, and iodide decrease due to weaker van der Waals forces.

Distillation of Mine Water

Distillation is effective for purification as it separates components based on boiling point differences, allowing water vapor to condense and leave impurities behind.
Industrial scale distillation is often unsuitable due to high energy costs and inefficient processing of large volumes.
Laboratory filtration cannot separate dissolved ions due to their small size and the nature of the solvent.

Isomers

Structural isomers differ in the arrangement of atoms and connectivity, while geometric isomers vary in spatial orientation.
Saturated hydrocarbons contain only single bonds, whereas unsaturated hydrocarbons contain at least one double or triple bond.

Testing for Unsaturation

A test for saturation vs. unsaturation is the addition of bromine water; a color change indicates unsaturation.
Saturated hydrocarbons do not react with bromine water, maintaining their color, while unsaturated ones decolorize it.
Example reaction: C2H4 + Br2 → C2H4Br2 (bromination of ethylene).

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Description

Explore the trends that occur as the atomic number increases for Group 17 elements, also known as the halogens. This quiz covers changes in properties such as electronegativity, atomic radius, and reactivity. Test your understanding of these fundamental concepts in chemistry!