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.