Describing Chemical Reactions

	Describing Chemical Reactions
	Date:	Name:

In a chemical reaction, one or more substances change into different substances.

For example, when iron and sulfur are heated together a chemical reaction occurs that transforms them into iron (II) sulfide. When these two elements react they release more energy than was put in by the heating.

This is one way to write information about a chemical reaction, but it is not efficient.

A ________________________ can also be used:
Iron	+ Sulfur		iron (II) sulfide	+ energy
A ________________________ is even more efficient:
Fe	+ S		FeS	+ energy

Define these terms:

Chemical reaction
Reactant
Product
Word Equation
Chemical Equation

Label the yield arrow, reactants and products in the word equation and the chemical equation above.

Chemical equations can provide a lot of information in a very little space. For example, not only does it tell us whether ________________ is used or given off, it also tells us the ________________ of the reactants and products.

State symbol	Meaning
(s)
(l)
(g)
(aq)

Example 1: Write the word equation and the chemical equation using the description of the reaction:

When solid copper (II) carbonate absorbs energy it produces carbon dioxide gas and solid copper (II) oxide.
Word equation:

Chemical Equation:

Example 2: Write the word equation and the chemical equation using the description of the reaction:

A piece of zinc is submerged into a test tube containing a copper (II) sulphate solution. Copper metal settles to the bottom of the now warmer test tube now containing a zinc sulphate solution.
Word equation:

Chemical Equation:

Questions

Q1. Compare the following terms. Remember to use a comparison word (e.g. however, while, but) when making a comparison.

a) word equation and chemical equation

b) reactant and product

c) solid and aqueous

Q2. a) How can you use chemical equations to determine whether a reaction needed heat to proceed?

b) How can you use chemical equations to determine whether a reaction produced heat?

Q3. What is the purpose of the “yield arrow” in a chemical equation?

Q4. Write word equations for the following reactions

a) Copper reacts with oxygen to form copper (II) oxide.

b) Baking soda (sodium hydrogen carbonate) combines with acetic acid (vinegar) to create water, carbon dioxide and sodium acetate.

c) Hydrogen peroxide is exposed to light and breaks down into water and oxygen gas.

Q5. Consider the following reaction: HNO3 (aq) + NaHCO3 (s)  NaNO3 (aq) + H2O (l) + CO2 (g)

a) Name the reactants.

b) Name the products.

c) What evidence suggests a chemical change is taking place?

Q6. Write a chemical equation from the following information: Nickel is produced when carbon is burned with nickel(II) oxide. Carbon monoxide gas is also produced from this reaction.

	Conserving Mass in Chemical Reactions
	Date:	Name:

The Law of Conservation of Mass

This very important law makes sense when you think about it: ____________________

_______________________________________________________________________
Consider this reaction:

C + O₂  CO₂

There is one carbon atom and two oxygen atoms before the reaction and after the reaction. None of the atoms have been lost. No extra atoms have been created. This chemical equation is said to be balanced.

Consider the following skeleton equation:

H₂ + Cl₂  HCl
This equation is not balanced. It appears that a hydrogen atom and a chlorine atom are “lost” after the reaction takes place. How can we make sure this equation obey the Law of Conservation of Mass?
Let’s look at the atoms visually:

When hydrogen gas reacts with chlorine gas to make hydrochloric acid, not just one molecule is produced but two.

Another way to show this is by using a _______________________:

H₂ + Cl₂  ___ HCl


Now this equation is balanced: it has the same number of atoms for each element on the reactant side and the product side.

In the next section you will learn how to balance more complicated skeleton equations.

Questions

Q1. Explain why the following situations don’t disobey the Law of Conservation of Mass.

a) The mass of a hamburger decreases as it is cooked.

b) You are often lighter in the morning than you were when you went to bed.

Q2. A 25.0 g sample of compound A is mixed with a 40.0 g sample of compound B. A chemical reaction occurs. Once the reaction is complete, the final mixture has a mass of 55.0 g. What happens to the “missing” mass? How much mass is “missing”?

Q3. Explain the difference between a “skeleton equation” and a “balanced equation”. Give an example of each.