Edexcel IGCSE Chemistry 复习笔记 1.5.1 Word & Chemical Equations

Nothing created - nothing destroyed

• • However, the same atoms are always present before and after reaction
  • They have just joined up in different ways
  • Atoms cannot be created or destroyed, so if they exist in the reactants then they absolutely must be in the products!New substances are made during chemical reactions
• Because of this the total mass of reactants is always equal to the total mass of products
• This idea is known as the Law of Conservation of Mass

Conservation of Mass

• The Law of Conservation of Mass enables us to balance chemical equations, since no atoms can be lost or created
• • Write word equations for reactions outlined in these notes
  • Write formulae and balanced chemical equations for the reactions in these notesYou should be able to:

Word Equations

• These show the reactants and products of a chemical reaction using their full chemical names
• The reactants are those substances on the left-hand side of the arrow and can be thought of as the chemical ingredients of the reaction
• They react with each other and form new substances
• The products are the new substances which are on the right-hand side of the arrow
• The arrow (which is spoken as “goes to” or “produces”) implies the conversion of reactants into products
• Reaction conditions or the name of a catalyst (a substance added to make a reaction go faster) can be written above the arrow
• An example is the reaction of sodium hydroxide (a base) and hydrochloric acid produces sodium chloride (common table salt) and water:

sodium hydroxide + hydrochloric acid ⟶ sodium chloride + water

Representing reactions as equations

• Chemical equations use the chemical symbols of each reactant and product
• When balancing equations, there has to be the same number of atoms of each element on either side of the equation in accordance with the Law of Conservation of Mass
• A symbol equation uses the formulae of the reactants and products to show what happens in a chemical reaction
• A symbol equation must be balanced to give the correct ratio of reactants and products:   

S + O₂ → SO₂

• This equation shows that one atom of sulfur (S) reacts with one  molecule of oxygen (O₂) to make one molecule of sulfur dioxide (SO₂)
• The following non-metals must be written as molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂
• To balance an equation you work across the equation from left to right, checking one element after another
• If there is a group of atoms, for example a nitrate group (NO₃^–) that has not changed from one side to the other, then count the whole group as one entity rather than counting the individual atoms
• • Acid-base neutralisation reaction:Examples of chemical equations:

NaOH (aq) + HCl (aq)  ⟶ NaCl (aq) + H₂O (l) 

• • Redox reaction:

2Fe₂O₃ (aq) + 3C (s) ⟶ 4Fe (s) + 3CO₂ (g)

• In each equation there are equal numbers of each atom on either side of the reaction arrow so the equations are balanced

• Don't forget to add state symbols when writing balanced equations:

Balancing Equations

The best approach is to practice lot of examples of balancing equations

• By trial and error change the coefficients (multipliers) in front of the formulae, one by one checking the result on the other side
• Balance elements that appear on their own, last in the process

Answer

Answer