Air and Combustion - Chemistry questions and answers

 AIR AND COMBUSTION  QUESTIONS

QUESTION 1.

Study the experiment setup represented by the diagram below and answer the question that follows.

a) Explain what would be observed if red and blue litmus papers were dipped into the water at the end of the experiment.   

b) Write an expansion in terms of X and Y to show the (%) percentage of gas used by the burning candle

ANSWER

(a) The blue litmus paper would turn pink/ red. Red litmus paper remains red. The carbon (IV) oxide produced when the candle burns dissolve in water to form a solution of weak carbonic acid. 

(b) x- y/x (100%) 

QUESTION 2.

The diagram below represents two iron nails with some parts wrapped tightly with zinc and copper strips respectively.

 What observations would be made at the exposed points A and B if the wrapped nails are left in the open for several months? Explain.

ANSWER

Observation: At A No rusting takes place.

Explanation: Zinc is more reactive than iron. It reacts with oxygen in the presence to iron hence preventing it from rusting. It acts as a sacrificial metal 

Observation at B The nail is covered by a reddish-brown substance/coating/rust Explanation: Copper is less reactive than iron. Iron combines first with oxygen in presence of moisture and rust.  

QUESTION 3.

When magnesium is burnt in the air it reacts with oxygen and nitrogen gas giving white ash. Write two equations for the two reactions that take place.

ANSWER

2Mg(s) + O2(g) → 2MgO(s) 

3 Mg(s) + N2(g) →Mg3N2(s) 

QUESTION 4.

In an experiment, rods of metals P, Q and R were cleaned with a sandpaper and placed in a beaker containing water. Another set of rods was also cleaned and placed in a beaker containing dilute acid.

 

After placing the rods in the two liquids bubbles of gas were seen around some of the rods as shown in the diagram below.  

a) Why was it necessary to clean the rods with sandpaper before dipping them into the liquids?  

b) Arrange the three metals in order of their reactivity starting with the most reactive. 

ANSWER

(a) To remove the layer of oxide on their surfaces which could inhibit the reaction 

(b) Q, R,P 

QUESTION 5.

Oxygen reacts with the elements phosphorous, sulphur, and chlorine to form oxides in which the element is in its highest oxidation number. 

The table below gives the oxide of sulphur and its highest oxidation number. Complete the table for phosphorous and chlorine. 

(Atomic number p=15, s=16, Cl= 17) 

ANSWER

Oxide: Highest oxidation number 

P2O5 (+5) 

Cl2O7 (+7) 

QUESTION  6.

Write an equation for the reaction that takes place when carbon (II) Oxide gas is passed over heated Lead (II) Oxide.

ANSWER

CO(g) + PbO(s) →Pb(s) + CO2(g)

QUESTION 7.

1997: pp 1A q. 1 

The setup below was used to study some properties of air.

State and explain two observations that would be made t the end of the experiment.

ANSWER

-Iron will be covered by a reddish-brown substance/coating/rust 

-Water in the test tube rise and water in a beaker drops 

Explanation: 

Iron Combines with oxygen in a presence of moisture to form hydrated Iron (III) oxide/rust, water rises up to occupy the space which was occupied by oxygen in the tube.  

QUESTION 8.

Give the formula of an oxide that reacts with both dilute Hydrochloric acid and hot concentrated sodium hydroxide.

ANSWER

Al2O3 (Aluminium Oxide) 

QUESTION 9.

In an experiment, a certain volume of air was passed repeatedly from a syringe over heated excess zinc powder as shown in the diagram below.

  

 

The experiment was repeated using excess magnesium powder. In which of the experiments was the change in the volume of air greatest? Give reasons. 

ANSWER

The change was greatest with Magnesium. 

Both react with oxygen gas to form oxides, but magnesium also reacts with nitrogen to form magnesium nitrate (Mg3N2).

QUESTION 10.

State and explain the change in mass that occurs when the following substances are separately heated in open crucibles. 

i) Copper metal 

ii) Copper (II) Nitrate 

ANSWER

(i) Mass increase: Oxygen combines with the copper metal to form copper (II) Oxide. 

(ii) Mass decrease: copper Nitrate decomposes to give gases that escape leaving behind copper (II) oxide. 

QUESTION 11.

The diagram below shows an iron bar, which supports a bridge. The iron is connected to a piece of magnesium metal.

  

Explain why it is necessary to connect the piece of magnesium metal to the iron bar.

ANSWER

Magnesium is above iron in the reactivity series. It supplies electrons to the iron bar hence preventing it from rusting/ cathode protection. 

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