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Metals and acids end of unit test answers

CHEM 1.4 End of Unit Test – Model Answers
Question 1

Iron is one of the most important industrial metals due to its use in the manufacture of steel. Iron will readily react in the atmosphere to produce reddish-brown rust. For this rust to form the presence of both oxygen and water is required. The rusting process can be sped up by the presence of either salt or acid.

Below is an experiment designed to test which method is best for preventing rust on a steel (iron) nail.
Discuss why the nail wrapped in copper foil would have more rust than the nail wrapped in magnesium ribbon.

Include in your answer:
• Any observations for each of the above reactions
• An explanation for the observation in each test tube
• All relevant balanced equations for each of the above experiments
• Suggestions for how the above idea could be used commercially (hint: you must discuss the uses of iron and link these to the above experiment)

Observation Test tube with just nail in (A) – rusty.
Middle test tube (B) – no rust on nail.
Test tube with nail wrapped in copper (C) – rusty.

Explanation (A) Iron in steel reacts in the presence of oxygen, water and carbon dioxide to form iron(III) oxide.
(B) Magnesium is more reactive than iron, so will react with oxygen first. This is magnesium acting as a sacrificial metal
(C) Copper is a less reactive metal than iron. It will not react with the oxygen or water, so iron reacts first, faster than copper.

Balanced equations (A) 4 Fe + 3 O2 --> 2 Fe2O3
(B) 2 Mg + O2 -->2 MgO
(C) No equation for copper (4 Fe + 3 O2--> 2 Fe2O3)

Commercial use Sacrificial protection used in boats. A piece of more reactive metal is attached to the steel hull of a ship and so the more reactive metal will react first, protecting the iron in the steel.

Question 2
A student carried out an experiment with sulfuric acid (H2SO4) and calcium carbonate (CaCO3), as detailed below:
In test tube A she mixed 1 mL of sulfuric acid with a 2 gram lump of calcium carbonate.
In test tube B she mixed 1 mL of sulfuric acid with 2 grams of powdered calcium carbonate.
The sulfuric acid in both experiments was the same concentration.

Discuss, in terms of particles, how changing the surface area of the calcium carbonate affected the rate of reaction.
Include in your answer:
• A comparison of the observations you would make during these reactions
• An explanation of your observations
• A balanced equation for the chemical reaction taking place.

Comparison of observations - In both test tubes heat was produced, bubbles of gas and CaCO3 would disappear. The reaction in test tube A would be slower than the same reaction in test tube B.

Explanation of observations – Test tube B contained powdered CaCO3 which has a higher surface area than the lump of CaCO3. This means that more calcium carbonate particles are immediately available to react with the acid particles so there will be more collisions per unit time and so the rate of reaction will be faster.

Balanced equation – CaCO3 + H2SO4 --> CaSO4 + CO2 + H2O

 
Question 3
Discuss the similarities and differences between the reactions of magnesium and aluminium in oxygen and in hydrochloric acid.
Include in your answer:
• Any observations made during these reactions
• An explanation of why these similarities and differences occur
• All relevant balanced equations

Observations of reaction with oxygen – Mg burns with a bright white light. A white/grey powder is formed. Al reacts immediately with oxygen to form a white oxide layer.

Observations of reaction with HCl – Mg reacts with HCl to produce a gas and the magnesium will disappear. Al does not react with HCl.

Explanation of differences – Mg is more reactive than Al so reacts with oxygen more quickly.
Al does not react with HCl due to the impenetrable oxide layer on the surface, so the metal itself does not come into contact with the acid.

Explanation of similarities – Mg is more reactive than Al but both react with oxygen.

Balanced equations – 2 Mg + O2 --> 2 MgO
4Al + 3 O2 --> 2 Al2O3
Mg + 2 HCl --> MgCl2 + H2
No reaction for Al and HCl

Question 4
Calcium carbonate is commonly known as limestone, and is often used in fertilizers to reduce the acidity of soil.

Discuss why limestone can be used to reduce the acidity of soil.

Include in you answer:
• The property of limestone that allows it to reduce acidity.
• Any relevant equations.
• A definition of a neutralisation reaction

Property of limestone that allows it to reduce acidity – limestone is basic. It will react with any acid particles in the soil, neutralising them and so reducing the acidity of the soil. The salt and water produced are neutral so the pH of the soil will increase (acidity will decrease).

Any relevant equations acid + metal carbonate --> metal salt + carbon dioxide + water
e.g. CaCO3 + 2 HCl --> CaCl2 + CO2 + H2O
Or... CaCO3 + 2 H+_--> Ca2+ + CO2 + H2O
A definition of a neutralisation reaction – the number of H+ ions and OH- ions will be equal so pH = 7

Question 5
Cork rockets can be made using a plastic bottle and a cork. The cork is fitted into the neck of the bottle. Propulsion is generated by reacting baking soda (sodium bicarbonate) with acid. The carbon dioxide produced in the reaction builds up pressure inside the bottle causing the cork to fire out of the neck of the bottle when a particular pressure is reached.

For a particular class experiment with the cork rocket, the time that elapsed between mixing the chemicals in the bottle and the cork firing out was measured.
A student in the class suggested that they had to wait too long.
Another student suggested that it would be possible to reduce the time elapsed between the loading and the firing of the rocket. The teacher agreed.

Discuss ALL possible factors that you could change in the experiment, in chemical terms, to decrease this time. Include in your answer:
• clear reference to the particles involved
• link the steps you take to their effect on the reaction.

Increase surface area of baking soda. Particles – more sodium bicarbonate particles immediately available to react, so more collisions per unit time.
Effect on reaction – rate of reaction will increase and so the time taken to fire the cork will decrease.

Increase concentration of acid. Particles – more acid particles per unit volume, so more collisions per unit time between acid particles and sodium bicarbonate particles.
Effect on reaction – rate of reaction will increase and so the time taken to fire the cork will decrease.

Increase temperature. Particles – all particles will have more kinetic energy and so be moving faster, which means that there will be more collision per unit time.
Effect on reaction – rate of reaction will increase and so the time taken to fire the cork will decrease.

Metals and acids questions

Question 1
The students created a chemical volcano using the baking soda and vinegar.
Discuss what is happening in this reaction. Use the information in the table in (a) to help you.
Include in your answer
• the type of reaction that has occurred
• observations that would be made
• the products of the reaction
• an appropriate balanced equation.

ANSWER:
Type of reaction:
The reaction is a neutralisation reaction because the acid is being turned into a new substance that is neither acid of base..that has a pH =7

Observations:
Fizzing would be observed as CO2 gas is produced and the sodium hydrogen carbonate would disappear
Word equation: (must be all in words – no formulae)
Sodium hydrogen carbonate (or sodium bicarbonate) + ethanoic / acetic acid-->sodium ethanoate / acetate + water + carbon dioxide.
Balanced equation: NaHCO3 + CH3COOH -->CH3COONa + H2O + CO2
(NaCH3COO also accepted.)


Question 2
(a) A student put some water in a beaker, added a few drops of litmus solution, and then added a small piece of calcium metal. Calcium reacts vigorously in water.
(i) Describe TWO observations that would be made when calcium is added to the water in the beaker and explain why they occur.

ANSWER:
Fizzing / bubbling / effervescence
• Explanation: hydrogen / H2 / gas is produced
OR
Litmus / water / solution turns blue or blue-purple / purple-blue
• Explanation: an alkali Ca(OH)2 forms
OR
White solid forms or solution goes cloudy
• Explanation: calcium hydroxide is not very soluble it forms suspension.
OR
Calcium / metal disappears.
• Explanation: has formed a compound in solution / forms a solution / has reacted with the water / forms product that dissolves / disperses in the water
OR
The beaker contents get warmer
• Explanation: exothermic reaction / reaction releases heat energy
OR
Ca pieces sink, float up, sink (and cycle repeats)
Explanation: light H2 gas forms on Ca & helps it floats up etc (but not floats / fizzes around surface)


(ii) Write a balanced equation for the reaction of calcium in water.

ANSWER
Ca + 2H2O --> Ca(OH)2 + H2

b)
THREE metals labelled X, Y and Z need to be identified based on some characteristic physical and chemical properties.The possible metals are:

iron, magnesium, sodium, copper, zinc

Identify metals X, Y and Z and justify your answer using the reactions described in the table opposite. Include the products of any reactions in your answer.

ANSWER

X is sodium because it is shiny grey and the reactions with oxygen, water and acid are all very strong / vigorous. Sodium is the only metal in the list that has such vigorous reactions. It is high in the activity series. Sodium reacting with oxygen in air would produce sodium oxide. Sodium reacting with water would produce sodium hydroxide and hydrogen gas. Sodium reacting with acid would produce sodium sulfate and hydrogen gas.

Y is copper because it is pink-brown in colour and has no reaction with oxygen, water or acid, reflecting the position of copper on the activity series. Copper is a very unreactive metal.

Z is magnesium because it is shiny grey and it burns with a characteristic / distinctive bright (white) light in oxygen to form magnesium oxide. It has a relatively high position in the activity series but is not as reactive as metal X/Na. Magnesium reacting with water would produce magnesium hydroxide and hydrogen gas. Magnesium reacting with acid would produce magnesium sulfate and hydrogen gas.
(Equations can be used to identify products. They do not have to be balanced. However, if formulae are used they should be correctly written.)

Question 3
Magnesium ribbon reacts with dilute hydrochloric acid in a conical flask, which is connected to an inverted measuring cylinder in a trough of water. The volume of gas produced is measured over a few minutes, and the results are used to sketch a graph.

(a) Explain why, in terms of particles involved, the line on the graph levels off over time.

ANSWER
• The reaction has stopped and no more gas is produced.
• The reactants are being converted into products. This means there are fewer particles available to collide as the reaction progresses.
• The rate of the reaction will decrease and eventually the reaction will stop.
• Incorrect chemistry referring to changes in energy or speed of particles or “gas levelling off” or changes to activation energy negated some answers.


(b) Discuss the effect on the rate of reaction when repeating this experiment using the same mass of magnesium powder with the dilute hydrochloric acid.

ANSWER
• The reaction rate will increase.
• There are more reactant particles immediately available to react because the surface area of the powder is greater than that of the ribbon. There is greater exposure of the magnesium particles so more can collide at any one time.
• This means there will be more (effective) collisions per second / more frequent collisions between the Mg / powder and the acid, leading to an increase in the rate of the reaction.

Metal Reactivity