Unit 5: Concept 1 Writing equations

Chemical Reactions

  • Physical change: change in a substance that doesn’t change its identity

    • Ex. Boiling, melting, vaporizing, grinding, etc.

  • Chemical change: change of a substance into another substance; when a chemical reaction occurs 

    • Ex. Burning, oxidizing, rotting, fermenting, etc.

    • Chemical reaction: process by which substances collide with enough energy that new bonds are formed between atoms, thus creating new substances.

      • Do NOT have a change in matter.

      • Do have a change in energy.

  • Collision theory: Reacting particles must collide in order for a chemical reaction to occur

    • AND do it with enough force to change something.

      • Meaning that they CAN collide without actually reacting if there isn’t enough energy

    • The rate of a chemical reaction depends on the frequency of the collisions.

    • When a chemical change occurs, a chemical reaction occurs. 

    • Evidence of a chemical reaction:

  • Energy change in the system 

    • Ex. Release of light

    • Ex. Heat released or absorbed,  a sudden temperature change

  • Sudden color change

  • Odor change

  • Gas release

  • Sudden appearance of a solid

    • The solid that forms is known as a precipitate

Law of Conservation of Mass

  • Law of Conservation of Mass: matter cannot be created or destroyed – it can only change forms. 

    • This is true even in a chemical change/reaction!

  • Ex. If there are 25 g of reactants, there must be 25 g of products made.

  • Ex. If there are 2 oxygen atoms in the reactants, there must be 2 oxygen atoms in the products.

    • This is where learning to balance equations becomes critical!

Writing Equations

  • Equations represent chemical reactions.

  • They can be written in word form or formula form. 

    • Example of words: 

Hydrogen gas and oxygen gas react to form liquid water.

  • Example of formulas: 

2H2 (g) + O2 (g)  2H2O (l)

  • Reactants = starting substances; “ingredients” 

  • Products = ending substances; what is made

  • + = separates one or more reactants or products

  •  = yields; produces

  • Coefficient = tells us the number of each substance needed

    • These can change! The subscripts cannot.

    • If only 1 is needed, the 1 is not written (Ex. Cl2)

    • How to read this: the ratio of sodium to chlorine is 2:1 in this reaction.

  • The other notation represents the state of matter of each substance.

    • (s) = solid

    • (l) = liquid

    • (g) = gas

    • (aq) = aqueous 

      • This means dissolved in water

  • Putting it all together…

    2Na(s) + Cl2(g) 2NaCl(s)

    2 solid sodium atoms 

    reacts with 

    1 chlorine gas molecule

    to yield 

    2 solid sodium chlorides

Balancing Equations

  • Equations must be balanced to keep from violating the Law of Conservation of Mass.

  • Coefficients are used to balance equations. 

    • Why?