Chapter 9- how chemical reacts PDF

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Chapter 9- how chemical reacts
Chapter 9- how chemical reacts...

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Chapter No 9

Chemical Reactions Are Represented By Chemical Equations: Chemical Equation It is a short way for illustrating a chemical reaction,

Reactant Substances that are about to chemically react

Product Substances that are formed from a chemical reaction

Coefficients Integers used to indicate the ratio by which reactants react and products form.

In 3NO there is 3 show coefficient

Example: 1,

Answer=six oxygen molecule Example2,

We can show the above reaction by a equation like,

Law of mass conservation

Which states that matter is neither created nor destroyed during a chemical reaction? Instead, atoms simply change partners to form new material

9.2 Counting Atoms and Molecules by Mass

Relative mass Different atoms have different masses. Their relative masses are given in the periodic table C =the mass of one carbon atom is approximately 12amu.

Formula mass The sum of the atomic masses of the elements in a chemical formula .Unit=amu Formula mass of O2

Atomic mass of 0(oxygen) = 15.999amu Formula mass of 02 = 32 amu

Formula mass of CO2 Atomic mass of C(carbon) = 12amu Formula mass of 02 = 32 amu Formula mass of CO2= 44amu

Conversion of amu into gram 1 atomic mass unit (amu) = 1.661 X 10-24 gram. Two amu equals how many grams= 2x1.661 X 10-24 = 3.322 × 10-24 grams

9.3--Converting Between Grams and Moles

1mole of He =

4.003 g He =6.02 x 1023 He atoms

1mole of Ne =

20.180 g Ne =6.02 x 1023 Ne atoms Avogadro’s Number 6.02 x 1023 =1mole

Mole The atomic mass of an element expressed in grams is equal to one mole of that element. 1C= (12amu) =12gm=1Mole The formula mass of a compound expressed in grams is equal to one mole of that compound. 18 g of H2O = 1 mole of H2O

1 mole of H2O = 6.02 x 1023 molecules

Molar mass The mass of one mole of a substance (unit=gm) Pb = 207.2 g (Pb = 207.2 grams/mole)

H2O = 18 g (H2O = 18 grams/mole )

9.4 Chemical Reactions Can Be Exothermic or Endothermic

Exothermic Said of reaction in which there is a net release of energy

Endothermic Said of a reaction in which there is a net absorption of energy

Bond energy

The amount of energy required to pull two bonded atoms apart, which is the same as the amount released when they are brought together Unit: kJ/mole

Net enrgy value is negative so reaction will be exothermic

Net result is positive so reaction is endothermic

9.5 Chemical Reactions Are Driven by the Spreading of Energy

Entropy is a measure of this natural spreading of energy. Wherever there is a spreading of energy, there is a corresponding increase in entropy.

The Sun is a prime example of the fact that energy has a natural tendency to disperse. The Second Law of Thermodynamics Heat naturally flows from a higher-temperature object to a lower temperature object because, in doing so, energy is dispersed from where it is concentrated (a hot pan) to where it is spread out (the cooler kitchen).

Sunlight + 6 CO2 (g) + 6 H20 (g) ----0> C6H1206(s) + 6 02(g)

Spreading energy=increase in entropy Concentrated energy=decrease in entropy

Unit:9.6 Chemical Reactions Can Be Slow or Fast Reaction rate How quickly the concentration of reactants decreases

Factors effecting on the rate of reaction 1. Collisions among molecules; successful collision lead to increase rate of reaction.

2. Concentration: Increasing concentration increases the number of collisions per second so rate will be increase

3.Pressure: increase the pressure ,increase the rate of reaction because molecule has less space and collision will increase

4.Temprature :At high temperature rate of reaction will be increase

Activation energy, Ea Not all collisions lead to products…so they need a high amount of energy that is known as Activation energy Ea The minimum energy required to overcome the initial breaking of bonds in reactants

Instructor: Shaher bano Bajwa...