Name: Steps for Balancing Chemical Equations (II)
Uploaded: 2019-07-19
Duration: 5 min 1 s
Description: Balancing a chemical equation is important because it gives the exact number of moles of the reactants and the products. Let's dive into this video.

Question

Four groups of students in Mrs Kiran's class were investigating how the mass of magnesium changes when it burns. Burning is a phenomenon where a substance reacts with oxygen. They noted that it burns with a white sparkling flame similar to the sparks we see when we burn crackers.

At the end of the experiment, they noted the initial mass of magnesium before burning and the mass of magnesium at the end of burning. These data are shown below in the table.

Group	Mass of magnesium in the starting	Mass of magnesium at the end
$1$	$0.51$	$0.65$
$2$	$0.52$	$0.72$
$3$	$0.50$	$0.35$
$4$	$0.51$	$0.75$

Three groups analysed the increase in the mass of the metal at the end of the reaction. Could you state the reason for this increase? Also, mention whether the reaction is exothermic or endothermic.
Which group has not performed the experiment correctly?
Which law of chemical reaction can be explained with the help of this experiment? What other data is needed to prove the law correctly.

Accepted Answer

Magnesium burns in the presence of oxygen with a white dazzling light to form magnesium oxide. It is an exothermic reaction as it produces heat and light. The word equation for the reaction is as follows:

Magnesium + Oxygen $\to$ Magnesium oxide

By the combination of metal with oxygen, a layer of new compound - magnesium oxide is formed over the metal surface. That is why the mass of the metal is observed to be increased after the reaction.

Since, after the reaction, the mass of the metal should have increased, but group $3$ has observed a decrease in the mass of the metal. Hence, they did not perform the experiment correctly.

The law of conservation of mass has been tried to be explained here with the experiment. The increased mass of metal is due to the combined mass of metal and the mass of the oxygen reacted. The law of conservation of mass states that in a chemical reaction the mass of the reactant is equal to the mass of the products formed.

In the table the mass of the second reactant, oxygen is missing. If the mass of oxygen used would have been tabulated, we could have easily proved that the law of mass conservation applies to the chemical reactions.

Important Questions on Chemical Reactions and Equations

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