Chemical reactions, Year 8: more practice

4

HintAtoms are rearranged, never made or destroyed.

The whyThe same atoms are present before and after the reaction. They are just joined to different partners.

4.4

HintNothing is lost: the missing mass left as a gas.

The whyMass is conserved, so the gas must account for the difference: 10.0 − 5.6 = 4.4 g.

20

HintAdd the masses of the reactants.

The whyIn a closed system the total mass stays the same, so 12 + 8 = 20 g.

Carbon dioxide

HintCarbonates lose a gas when they are strongly heated.

The whyCalcium carbonate → calcium oxide + carbon dioxide. This is how quicklime is made.

It is made of only one kind of atom, so there is nothing simpler to break into

HintThink about what decomposition has to produce.

The whyThermal decomposition splits a compound into simpler substances. An element contains only one kind of atom.

Copper oxide

HintThe metal gains the -ide ending to show oxygen has joined it.

The whyOxidation is the gain of oxygen. Copper turns black as copper oxide forms.

The energy stored in the reacting chemicals

HintThink about what a battery holds before it is used.

The whyReactants hold stored chemical energy. In an exothermic reaction some of it is released as heat.

Sodium, zinc, iron

HintThe soft metal that is kept under oil is the most reactive here.

The whyThe order in the series is sodium, then zinc, then iron. Most of the reactions are quicker the higher up the series a metal is.

It is very unreactive

HintMost metals are found joined to other elements in ores.

The whyUnreactive metals do not combine easily with oxygen or other elements, so they can survive uncombined for millions of years.

Yes, because zinc is more reactive than copper

HintCheck which metal is higher in the reactivity series.

The whyA more reactive metal takes the place of a less reactive one in its compound: zinc + copper sulfate → zinc sulfate + copper.