01 / 20

Apply Chemical
Reactions

Understand how atoms rearrange, why mass is conserved, and how to read and balance equations like \( 2H_2 + O_2 \rightarrow 2H_2O \).

GED Science

What Is a Chemical Reaction?

A chemical reaction is a process where one or more substances (the reactants) are transformed into one or more new substances (the products). The atoms don't vanish or appear: they simply rearrange into new combinations.

REACTANTS (\(2H_2 + O_2\))
PRODUCTS (\(2H_2O\))
H H H H + O O O H H O H H

Signs a Reaction Happened

Color Change
A new substance often has a different color than the starting materials.
Gas Produced
Bubbles or fizzing show a gas is forming, like \( CO_2 \).
Temperature Shift
Heat is released or absorbed as bonds break and form.
Precipitate Forms
A solid appears when two clear liquids are mixed.

Reading an Equation

\( 2H_2 + O_2 \rightarrow 2H_2O \)
Reactants Products 2 coefficient 2 subscript
Coefficient
The big number in front. It tells how many molecules. You may change it to balance.
Subscript
The small number below. It tells atoms per molecule. Never change it to balance.

Law of Conservation of Mass

Reactants 36 g Products 36 g Balanced

Because mass is conserved, the number of each kind of atom must be the same on both sides of the arrow. This is why we balance equations.

Common Reaction Types

General FormExample
Synthesis\( A + B \rightarrow AB \)\( 2H_2 + O_2 \rightarrow 2H_2O \)
Decomposition\( AB \rightarrow A + B \)\( 2H_2O \rightarrow 2H_2 + O_2 \)
Single Replacement\( A + BC \rightarrow AC + B \)\( Zn + 2HCl \rightarrow ZnCl_2 + H_2 \)
Combustionfuel \( + O_2 \rightarrow CO_2 + H_2O \)\( CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O \)

Energy: Exothermic vs Endothermic

Exothermic
Releases energy (heat or light) to the surroundings. The container feels warm. Example: burning wood.
Endothermic
Absorbs energy from the surroundings. The container feels cold. Example: a cold pack.

How to Balance an Equation

1
Count atoms of each element on both sides of the arrow.
2
Adjust coefficients (the big front numbers) one element at a time. Never change subscripts.
3
Save hydrogen and oxygen for last; balance metals and other elements first.
4
Recount to confirm every element matches on both sides.
5
Reduce coefficients to the smallest whole-number ratio if possible.

Example 1: Balance Water

Balance: $$ H_2 + O_2 \rightarrow H_2O $$

\( H_2 + O_2 \rightarrow H_2O \) (not balanced)
O: 2 left, 1 right ✕
\( 2H_2 + O_2 \rightarrow 2H_2O \) ✓

Example 2: Methane Combustion

Balance: $$ CH_4 + O_2 \rightarrow CO_2 + H_2O $$

1
Carbon: 1 left, 1 right. Already balanced.
2
Hydrogen: 4 left. Put a 2 on water: \( 2H_2O \) gives 4 H.
3
Oxygen: right now has 2 + 2 = 4 O. Put a 2 on \( O_2 \).

Example 3: Conservation Check

If 10 g of hydrogen reacts completely with 80 g of oxygen to form water, how much water is produced?

\( H_2 + O_2 \)
10 + 80 g →
Water (\( H_2O \))
= 90 g

Common Mistakes

✕ Changing Subscripts
Changing \( H_2O \) to \( H_2O_2 \) makes a different substance. Only change coefficients.
✕ Forgetting to Recount
Fixing one element can unbalance another. Always recount all atoms at the end.
✕ Confusing Phys & Chem
Melting ice is physical. A new substance forming is chemical.
✕ Ignoring Conservation
Products can't weigh more or less than reactants in a closed system.

Quick Summary

IdeaKey Rule
Reactants & ProductsLeft of arrow react to form the right side
Conservation of MassAtoms in = atoms out; mass is unchanged
BalancingChange coefficients, never subscripts
ExothermicReleases energy; feels warm
EndothermicAbsorbs energy; feels cold

Question 1 of 5

In the equation \( 2H_2 + O_2 \rightarrow 2H_2O \), what does the 2 in front of \( H_2 \) represent?

Question 2 of 5

According to the Law of Conservation of Mass, if 12 g of carbon reacts fully with 32 g of oxygen, what is the mass of the \( CO_2 \) produced?

Question 3 of 5

Which observation is the best evidence that a chemical reaction occurred?

Question 4 of 5

When balancing \( H_2 + O_2 \rightarrow H_2O \), what should you do?

Question 5 of 5

A reaction makes its container feel cold to the touch. This reaction is best described as:

Reactions in Everyday Life

Cooking
Heat triggers reactions that brown food and create new flavors.
Photosynthesis
Plants combine \( CO_2 \) and water using light to make sugar and \( O_2 \).
Rusting
Iron + oxygen slowly form iron oxide, a brittle new solid.

You Can Now Apply Reactions

1
Identify reactants, products, and signs of a reaction.
2
Apply conservation of mass to find unknown masses.
3
Balance equations by adjusting coefficients only.
4
Classify reactions by type and by energy (exo/endo).