Science Chapter 9-Chemical Formulas & Equations

4 ways a chemical reaction can be sped up:

1) surface area (more reactant particles can interact at the same time) 2) temperature (reactant particles collide faster and more often) 3) concentration and pressure (increases the likelihood of reactant particle collisions) 4) the presence of catalysts (less energy is needed to set the reaction off)

This BALANCED equation shows three things

1) the law of conservation of mass has been preserved 2) chemical reactions occur only when specific amounts of each of the reactants are available. It takes two silver atoms (Ag) for every hydrogen sulfide molecule (H2S) to react. 3) The products of a chemical reaction are produced in specific amounts. Every time the reaction between 2Ag + H2S takes place, exactly one molecule of silver sulfide (Ag2S) and one molecule of hydrogen (H2) are produced.

For example, the equation 2H2O + O2 > H2O is unbalanced. Changing the subscript in the products to H2O2 appears to balance it, but the substance is not water anymore, it's hydrogen peroxide, which is violently explosive!

Add a coefficient to balance the equation: 2H2 + O2 > 2H2O

Ag + H2S >>>>> Ag2S + H2

The law of conservation of mass states that the new matter cannot be created but this equation shows more silver (Ag) atoms in the products than the reactants. Therefore, this equation is UNBALANCED.

You CANNOT balance a chemical equation by changing the subscript.

The subscript indicates how many atoms of an element are within a compound or molecule. Changing the subscript changes the substance chemically.

What does the 2 in front of the element silver (Ag) on the reactants side tell us?

There are two silver atoms present. This now equals the number of silver atoms present in the products, and the equation is balanced. 2Ag + H2S >>>>> Ag2S + H2

Enzyme

a catalyst that speeds up chemical reactions in living cells.

Combustion

a chemical reaction in which a substance combines with oxygen and releases energy.

Endothermic reaction

a chemical reaction that absorbs thermal energy. An example would be mixing citric acid and sodium bicarbonate with water. The reaction absorbs thermal energy from the water, causing it to become cold.

Exothermic reaction

a chemical reaction that releases thermal energy. An example would be burning something such as paper or wood. As the carbon in these substances combines with oxygen, thermal energy is released, producing heat and light.

Chemical formula

a combination of chemical symbols and subscripts (numbers) that tells what elements and how many atoms of each are found in a molecule or compound.

Coefficient

a number places in front of an element symbol or chemical formula in a chemical equation. It indicates how many atoms or molecules of the substance are present in the reactants or products.

Chemical reaction

a process that produces chemical change, or a process in which atoms of one or more substances rearrange to form one or more new substances.

Catalyst

a substance that increases reaction rates by lowering the activation energy of a reaction.

Inhibitor

a substance that slows or even stops a chemical reaction. Preservatives in food products are chemical inhibitors.

Decomposition

a type of chemical reaction in which a single compound breaks down to form two or more different substances.

Replacement

a type of chemical reaction in which one element replaces another element in a compound.

Synthesis

a type of chemical reaction in which two or more substances combine and form one compound.

Double-replacement

a type of chemical reaction where the negative ions in two compounds switch places, forming two new compounds.

Chemical equation

an equation that tells the reactants, products and the proportion of each substance in a chemical reaction, or a description of a reaction using element symbols and chemical formulas.

Exothermic reactions can occur slowly or rapidly

an example of a SLOW exothermic reaction would be the one that takes place between plaster of Paris and water. Thermal energy is released but it happens over an extended period of time. A RAPID exothermic reaction can be seen in the explosive combustion of gun powder.

How can chemical equations be balanced?

by adding coefficients to the equation to balance them out.

Activation energy

the minimum amount of energy needed to start a chemical reaction.

Products

the things that come out of a chemical reaction. Products are written on the right in a chemical equation.

Reactants

the things that go into a chemical reaction. Reactants are written on the left in a chemical equation.

The law of conservation of mass

the total mass of the reactants before a chemical reaction is the same as the total mass of the products after the chemical reaction.

2Ag + H2S ---> Ag2S + H2

this equation shows that there are now equal numbers of all elements and atoms on each side. Therefore, this equation is BALANCED.