10.03.01 Conservation of Mass
Syllabus
The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.
This means that chemical reactions can be represented by symbol equations which are balanced in terms of the numbers of atoms of each element on both sides of the equation.
Students should understand the use of the multipliers in equations in normal script before a formula and in subscript within a formula.
Some reactions may appear to involve a change in mass but this can usually be explained because a reactant or product is a gas & its mass has not been taken into account. For example: when a metal reacts with Oxygen the mass of the Oxide produced is greater than the mass of the metal or in thermal decompositions of metal Carbonates Carbon Dioxide is produced & escapes into the atmosphere leaving the metal Oxide as the only solid product.
Students should be able to explain any observed changes in mass in non-enclosed systems during a chemical reaction given the balanced symbol equation for the reaction and explain these changes in terms of the particle model.
Whenever measurements are made there's always some uncertainty about the result.
Students should be able to:
represent the distribution of results and make estimations of uncertainty
use the range of a set of measurements about the mean as a measure of uncertainty.
What does that mean?
Balanced equations
You'll have seen lots of chemical (symbol) equations) like this...
The above is an equation for burning Methane (CH4) in Oxygen (O2), which produces Carbon Dioxide (CO2) and Water Vapour (H2O)
But when you count the atoms on either side...
... we can see it's unbalanced, its not possible for Hydrogen atoms to disappear or for Oxygen atoms to appear from nowhere.
Hopefully you'd know to balance the Hydrogen atoms first...
But that we'd still need to balance the Oxygen atoms...
This equation is now balanced because it has the same number of each type of element on both sides of the equation.
Which means that the total mass on the left equals the total mass on the right.
This is an example of the Law of Conservation of Mass - mass can't be created or destroyed, just redistributed.
You should be able to do the same with the following equations...
QUESTIONS
Why some reactions seem to disobey the law of conservation of mass?
#1 Losing Gases
If we put marble chips in acid and leave the flask on a balance the mass decreases until the reaction stops.
But that's because one of the products is a gas (CO2) - which escapes the flask and takes away its mass.
But it doesn't break the law because the combined mass of all the reactants equals the combined mass of all the products.
#2 Gaining Gas
Rusting is an example of a reaction where a solid (Iron) reacts with a gas (Oxygen).
So, it gains the mass of the Oxygen.
And becomes heavier.
But it doesn't break the law because the combined mass of all the reactants equals the combined mass of all the products.
.
Uncertainty
If you did the rusting experiment you might find that the results didn't quite match what you were expecting.
But unless you're doing the experiment wrong it's likely to be a result of uncertainty.
A digital balance that measures to 1 decimal place might read 23.1 g
But we don't know if it should be reading up to 23.149999999 g because it would round this to 23.1 g
And we don't know if it should be reading down to 22.05 g because it would round this to 23.1 g too
So, the reading is only 23.1 +/- 0.05 g
The uncertainty of the measurement is +/- 0.05 g whatever we do.
Unless we get a balance accurate to more decimal places.
But even this has an uncertainty of +/- 0.005 g
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