Chem Chapter 6: Chemical Reactions

chemical reactions

a process where changes in the chemical formulas of the substances involved

physical change

processes that do not alter the chemical formulas of the starting materials, such as the boiling of water

chemical change

the chemical formulas of the starting materials and final products differ, like a burning candle (chemical reaction)

physical properties 

any attribute of a substance that can be measured or described without changing the chemical formula of the substance

ie. density, mass, color, boiling point, solubility

chemical properties

the ability of a substance to undergo a chemical reaction

law of mass conservation

states in any change, the mass of the final products equals the mass of the starting molecules

2nd principle

number of atoms of each element remains constant

chemical equation

uses chemical formulas and coefficients (has reactants on the left and products on the right)

ex. Ca + S = CaS

reactants

chemical formulas to left of arrow; starting substances in a chemical reaction

products

chemical formulas to right of arrow; substance formed in chemical reaction

coefficient

shows number of formula units of each substance

2Na + 2 = Na₂S

• tells us that we have 2 independent sodium atoms

subscript

means how many of those element atoms have been bonded

ex. 2Na + S = Na₂S

• have 2 bonded sodium atoms

mass relationships in a chemical reaction

we can combine a balanced equation with the atomic weights of the elements to calculate the mass of each product formed in a chemical reaction

ex. 2Na + 2 = Na₂S

2 NA = 45.98g 

S = 32.06g 

atom ratio of 2:1 (can also be 2:1 mole ratio)

2:1 mole ratio

1 mol S / 2 mol Na

heat reaction

amount of heat absorbed or given off by a chemical reaction, based on the relative masses of the reactants as obtained from the balanced equation

can use relationship between heat and mass to find the amount of heat that will be produced when we use any mass of reactant

weight of reactant x given heat / mass of 1 mole of reactant

exothermic reaction

a reaction that produces heat (reactions that raise the temp of surroundings)

ex. rusting of metals, burning of all types of fuels

endothermic

absorb heat from their surroundings (include heat as a reactant in the equation)

ΔH

amount of heat / to represent the heat of reaction without writing in the equation

heat is a product

exothermic reaction; heat of reaction is written as negative 

ex. ΔH = -1342kCal

heat is a reactant

endothermic reaction; heat of reaction is written as a positive

ex. ΔH = 2.8kCal

nutritive value

the # of calories that the human body obtains from a gram of a specified nutrient'

ex. 1g of any carb supplies roughly 4kcal of energy to our bodies

carbs

1g of carb supplies 4kcal

proteins

1g of protein supplies 4kcal

fats

1g of fats supplies 9kcal

calorie

(cal, C) another term for nutritive value

mass of nutrient x nutrient value = calories

calorie content formula (have to round to nearest 10 cal)

combustion reaction

chemical process we call burning / the reaction of a chemical compound with oxygen (O₂ ) and converts the compound into small oxygen containing molecules (Co₂ , H2O, SO₂ )

• producing Co₂ , H2O from compounds made from carbon, hydrogen, and oxygen

multiply by 2

• have to multiply all coefficients by 2 before balancing oxygen bc its an odd #

• doubling coefficients always produces an even # of oxygen atoms 

photosynthesis

process of making glucose and oxygen from Co₂ and water using sunlight to supply energy needed

photosynthesis reaction

6Co₂ + 6H2O + 686kcal = C₆ H₁₂ O₆ + 6O₂

Respiration

the combustion of carbon containing molecules by living organisms

respiration reaction

(combustion of glucose)

C₆ H₁₂ O₆ + 6O_{2 =} 6Co₂ + 6H2O + 686kcal

carbon cycle

cycle formed from respiration and photosynthesis

rule of reaction affected by 3 factors

1. how often reactant molecules collide with each other (increase rate by increasing the concentration of reactant molecules)

2. how much energy the molecules have when they collide

3. how much energy the molecules need to react

how often reactant molecules collide with each other

increase rate by increasing the concentration of reactant molecules

how much energy the molecules have when they collide

increase temp of reactants = increase in rate of reaction

• activation energy

activation energy

the minimum energy that reactant molecules must have to react (increase temp = increase in kinetic energy of molecules = increase in activation energy)

how much energy the molecules need to react

high reaction energies = slow rate, low activation energy = faster rate

high activation energy

slow rate

low activation energy

fast rate

energy diagrams

show the potential energy of the molecules throughout the reaction

exothermic reaction (ED)

energy of product is lower than energy of reactants

endothermic reaction (ED)

energy of product is above energy of reactants

intermediate

first step product then becomes the reactant for second step

catalyst

substance that speeds up a chemical reaction but is not consumed by reaction

• main job is to lower the activation energy of a reaction to create a faster reaction rate

reversible reaction

a chemical reaction that can occur in either the forward or background direction

• forms an equilibrium mixture

equilibrium mixture

a stable mixture of products and reactants that is formed by a reversible reaction (forward and reverse reactions occur at the same rate)

chemical equilibrium

a stable mixture of products and reactants in which the rates of the forward reaction and reverse reaction are equal