بوربوينت 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)