Conservation of Mass (pg 125)

Question 1

What is meant by ‘Conservation of Mass?

Answer 1

Mass is neighter created nor destroyed in chemical reactions. In other words, the mass of any one element at the beginning of a reaction will equal the mass of that element at the end of the reaction

Question 2

In a chemical Reaction, mass is always conserved, what do this mean?

Answer 2

During a chemical reaction no atoma are destroyed and no atoms are created.

This mean thee are the same number and types of atoms on each side of a reaction equation.

Because of this, no mass is lost or gained - We say that mass in conserved during a reaction

Question 3

Explain this reaction, why we say that mass is conserved?

2Li + F2 > 2LiF

Answer 3

In the above reaction, there are 2 lithium atoms and 2 fluorine atoms on each side of the equation.

By adding up the relative formlar masses of the substances on each side of a alanced symbol equation, you can see hat mass is conserved. The total Mr of all the reactants equals the total Mr of the products.

Question 4

How do you show that Mass is conserved in the reaction:

2Li + F2 > 2LiF.

Answer 4

1) add up the relative formula masses on the left-hand side of the equation
2 x Mr(Li) + 2 x Mr(F) = (2 x 7) + (2 x 19) = 14 + 38 = 52

2) add up the relative formla masses on the right-hand side of the equation
2 x Mr(LiF) = 2 x (7 + 19) = 2 x 26 = 52

The total Mr on the left-hand side of the equation is equal to the total Mr on the right-hand side, so mass is conserved.

Question 5

If the mass seems to change, there is usually what involved?

Answer 5

Gas

Question 6

In some experiments you might observe a change of mass of an unsealed reaction vessel during a reaction. There are usually two explanations for this, what are they?

Explaination 1?

Answer 6

Explanation 1:

If the mass increases, it’s probably because one of the reactants is a gas that’s found in air (e.g. oxygen) and all the products are solids, liquids or aqueous.

Before the reaction, the gas is floating around in the air. it’s there, but it’s not contained in the reaction vessel, so you can’t account for its mass.

When the gas reacts to form part of the product, it becomes contained inside the reaction vessell - so the total mass of the stuff inside the reaction vessel increases.

for example; when a metal reacts with oxygen in an unsealed container, the mass of the container increases. The mass of the metal oxide produced equals the total mass of the metal and the oxygen that reacted from the air.

Metal(s) + oxgen(g) &raquo_space;> Metal Oxide(s)

Question 7

In some experiments you might observe a change of mass of an unsealed reaction vessel during a reaction. There are usually two explanations for this, what are they?

Explanation 2?

Answer 7

Explanation 2:

If the mass decreases, it’s probably because one of the products in a as and all the reactants are solids, liquids or aqueous.

Before the reacion, all the reactants are contained in the reaction vessel.

If the vessell isn’t enclosed, then the gas can escape from the reaction vessell as it’s formed. Its no longer contained in the reaction vessell, so you can’t account for its mass - the total mass of the stuff inside the reaction vessell decreases.

For example, when a metal carbonate thermally decomposes to form a metal oxide and carbon dioxide gas, the mass of the reaction vessell will decrease if it isn’t sealed. But in reality, the mass of the metal oxide and the carbon dioxide produced will equal the mass of the metal carbonate that decomposed.

Metal carbonate (s)&raquo_space;> metal oxide(s) + carbon dioxide(g)

Remember from the particle model on page 120, that a gas will expand to fill any container it’s in. So if the reaction vessell isn’t sealed, the gas expands out from the vessell and escapes into the air around.

Question 8

So that mass is conserved in the reaction:

H2SO4(aq) + 2NaOH(aq) &raquo_space;»> Na2So4(aq) + 2H2O(l) Ar(H) = 1, Ar(O) = 16, Ar(Na) = 23, Ar(s) = 32

(5 marks

Answer 8

Total mass on the left hand side = Mr(H2SO4) + 2 x Mr(NaOH) Mr of H2SO4 = (2 x 1) +32+ (4x16) = 98 2 x Mr of NaOH = 2 x (23 + 16 + 1) = 80

so total mass on the left hand side = 98+80 = 178 (2 marks for 178, mark for either 98 or 80).

Total mass on right hand side = Mr(Na2SO4) + 2 x Mr(H2O) Mr of Na2SO4 = (2x23) + 32 + (4x16) = 142 2xMr of H2O = 2 [(2x1) + 16] = 36 142+36 = 178
(2 marks for 178, 1 mark for either 142 or 36).

The total Mr on the left hand side is equal to the total Mr on te right hand side, so mass is conserved (1 mark)