درس Chemical Changes العلوم للصف الثامن نخبة منهج انجليزي

درس Chemical Changes العلوم للصف الثامن نخبة منهج انجليزي

New Vocabulary

chemical reaction

reactants

products

chemical equation

coefficient

balanced chemical equation
mole

molar mass

Review Vocabulary

chemical formula : shorthand that gives the elements in a compound and their ratios

 

Lavoisier and the Conservation of Mass

A chemical reaction is a change in which one or more substances are converted into new substances

 The substances present at the beginning of the reaction, the substances that react, are called reactants

 The new substances produced are called products 

Lavoisier and the Conservation of Mass •

In the 1770s, French chemist Antoine Lavoisier determined that the total mass of the products of a reaction always equal the total mass of the reactants

• This discovery became known as the law of conservation of mass

 

Writing Equations

A chemical equation is a way to describe a chemical reaction using chemical formulas and other symbols

 For example, here is a description of a chemical reaction : Nickel (ll) chloride, dissolved in water , plus sodium hydroxide, dissolved in water , produces solid nickel (ll) hydroxide plus sodium
chloride, dissolved in water

The chemical equation for this reaction is

Coefficients

The numbers in front of the formulas in a chemical equation are called coefficients Coefficients represent the number of units of each substance taking part in a chemical reaction

When no coefficient appears , a coefficient of 1 is assumed

Balancing Equations

 A balanced chemical equation has the same number of atoms of each element on both sides of the arrow

 This way of representing chemical reactions reflects the law of conservation of mass

Balancing Equations

How can you balance the chemical equation for the reaction between magnesium and oxygen Step 1 Write the chemical equation without coefficients

Mg(s) + 02(g) MgO(s)

Step 2 Count the atoms in reactants and products Magnesium is balanced. Oxygen is not

 

Balancing Equations

Step 3 Choose coefficients that balance the equation. Try adding a coefficient of 2 to MgO(s)

Mg(s) + 02(g) 2MgO(s)

Step 4 Recheck the numbers of each type of atom on each side of the equation. Oxygen is now balanced but magnesium is not. Add a coefficient of 2 to Mg(s) to balance the equation

2Mg(S) + 02 (g) 2MgO(S)

Understanding Chemical Equations

 Chemists commonly do not work with individual atoms, molecules and ions. They work with larger quantities of reactants, with masses that can be expressed in grams

Chemists use a counting unit to relate mass and number of particles. One mole is the amount of substance that contains 6.02 x 1023 particles of that substance

 

Understanding Chemical Equations

 The following table shows how moles, mass, and number of particles are related for this chemical equation, showing the reaction between carbon and oxygen to form carbon dioxide:

C(s) + 02(g) C02(g)

Understanding Chemical Equations

The mass in grams of one mole of a substance is called its molar mass

 The molar mass of an element that exists as individual atoms, in g / mol, is the same as its atomic mass in amu. The molar mass of a compound is the sum of the masses of its atoms

 

Understanding Chemical Equations

 You can use molar mass as a conversion factor to calculate the number of moles , given the mass How many moles make up 50.0 g of N02(g)

Understanding Chemical Equations

 You can use molar mass as a conversion factor to calculate the mass, given the number of moles What is the mass of 0.2020 mol of N02(g)