Chapter 6: Chemical Reactions Flashcards
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What is a Chemical Reaction?
- A process in which one or more reactants, are converted to one or more different products.
- Occur when chemical bonds between atoms are formed or broken
- A chemical reaction rearranges the atoms of the reactants to create different products.
reactants 🡢 products
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What is Chemical Equation?
- Detailed information to describe any chemical reaction
- Reactions can be represented using words or symbols; which display all the reactants, products and other relevant information
Word Equation vs Formula Equation
Worded Equation: calcium carbonate + sulphuric acid 🡢 calcium sulphate + carbon dioxide gas + water
Formula Equation: CaCO3 + H2SO4 🡢 CaSO4 + CO2 + H2O
Products vs Reactants
- Reactants: LHS; starting substances (e.g. calcium carbonate and sulphuric acid)
- Products: RHS; end substances (e.g. calcium sulphate, carbon dioxide gas, and water)
States of Matter
- Solids (s)
- Liquids (l)
- Gas (g)
- Aqueous (aq) (disolved in water)
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How to Identify a Chemical Change?
- Change in colour
- Smelling a gas or bubbles
- New solid (precipitate) formed in a clear solution
- Energy is absorbed or produced in the form of heat or light
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How to Identify a Physical Change?
- Chemical nature is not altered; nothing new is produced
- However, there may be a change in state (solid, liquid, gas), size, and shape
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Law of Conservation of Mass
- “The Mass of a completely isolated system will remain constant”
- Total Mass of Reactants = Total Mass of Products
- Mass cannot be created or destroyed, only rearranged or transformed
Balancing Chemical Reactions
Example (propane + oxygen)
* C3H8 + O2 🡢 H2O + CO2
LHS: 3 carbons, 8 hydrogens, 2 oxygens
RHS: 1 carbon, 2 hydrogens, 3 oxygens
Becomes…
C3H8 + 5O2 🡢 4H2O + 3CO2
LHS: 3 carbons, 8 hydrogens, 10 oxygens
RHS: 3 carbon, 8 hydrogens, 10 oxygens
Hydrogen (H2) gas + oxygen gas (O2) 🡢 pure water (H2O)
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What is Chemical Energy?
- Energy stored in the chemical bonds of a substance
- Chemical reactions always involve energy changes as making bonds and breaking bonds involve energy changes
- All molecules store energy in the form of chemical potential energy
- However, during a chemical reaction the atoms in the molecules are rearranged and so energy may be released
Chemical potential energy: energy from fossil fuels, batteries, and food
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Activation Energy
For all chemical reactions, activation energy is the energy supplied to break the bonds of the reactants for a chemical reaction to occur
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Steps for Endothermic or Exothermic Reaction
- Step 1: Energy must be SUPPLIED to break chemical bonds of reactants
- Step 2: Energy is RELEASED when new chemical bonds are formed in the products
*A reaction is exothermic if more energy is released than supplied. If more energy is supplied than is released, then the reaction is endothermic *
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Difference between Exothermic and Endothermic
- All reactions (exothermic and endothermic) first require energy from surroundings to break bonds in the reactant molecules and secondly make bonds to form new product molecules therefore releasing energy back into the surroundings.
- The difference is whether the amount of energy released when the new bonds form is greater or less than the energy required to break the old bonds.
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Exothermic
- Reactions that release energy
- More energy is released when the bonds are formed in the products, than energy used to break bonds in the reactants. Therefore, a net release of energy.
- Heat (energy) given off; temperature of the substance rises; HOT
Reactants 🡢 products + energy or heat
* E.g. burning of gasoline
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Endothermic
- Reactions that absorb energy
- More energy is absorbed to break the bonds in the reactants than released by the bonds being formed in the products. Therefore, energy is absorbed.
- Heat (energy) taken in; temperature of the substance drops; COLD
*Reactants + energy or heat 🡢 products
* E.g. cooking of pancakes
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Examples of Exothermic Reactions
- Combustion of fuels
- Yeast & Hydrogen Peroxide
- Epson salts & water
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Examples of Endothermic Reactions
- Photosynthesis
- Acedic Acid & Sodium Bicarbonate
Reactions with oxygen tend to be…
Exothermic
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Reactions with Hydrocarbons
- Hydrocarbons are substances made of only hydrogen and carbon atoms
- Can be a gas such as natural gas or methane (CH4) or a liquid like octane in petrol (C8H18)
- Hydrocarbons are excellent fuels because they burn in oxygen to produce large amounts of energy
- The combustion of hydrocarbons produces carbon dioxide and water
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Corrosion
- Exothermic
- Most metals combine with the oxygen in the air to form metal oxides: a process called corrosion
*Metal + oxygen gas 🡢 metal oxide
* E.g. Rusting of iron: 4Fe + 3O2 🡢 2Fe2O3
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Respiration
- Exothermic
- Chemical reaction that occurs in the cells of all living things
- Involves the breakdown of glucose with oxygen to release energy
- The energy produced by respiration is used by the organism to live, move and grow. Respiration is slow compared with combustion, so it does not produce large amounts of heat or light, however, it is enough to keep your body warm.
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Equation for Respiration
Word: glucose + oxygen 🡢 carbon dioxide + water + energy
Formula: C6H12O6 + 6O2 🡢 6CO2 + 6H2O + Energy
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Types of Chemical Reactions
- Combination
- Decomposition
- Precipitation
- Combustion
- Neutralisation
- Metal displacement
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Combination
- A chemical reaction where two reactants combine to form one product
- Where A + B 🡢 AB
H2O + SO3 🡢 H2SO4
The water is now joined to the sulphur trioxide to form di-hydrogen sulphur tetra-oxide (sulfuric acid)
E.g. Magnesium + oxygen 🡢 magnesium oxide
Sodium + chlorine gas 🡢 sodium chloride
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Decomposition
- A chemical reaction where one reactant breaks apart into two or more products
- Where AB 🡢 A + B
H2CO3 🡢 H2O + CO2
The hydrogen carbonate is separated into water and carbon dioxide
E.g. Elephant’s toothpaste: hydrogen peroxide 🡢 water + oxygen
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Metal Displacement
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- A metal displacement reaction occurs when a more reactive metal is placed in an aqueous solution of ions of less reactive metal. The more reactive metal become the ions in solution and the less reactive metal ions become the solid.
- A(s) + BC (aq) 🡢 B(s) + AC(aq)
Fe(s) + CuSO4 (aq) 🡢 Cu(s) + FeSO4(aq)
The iron trades places with the copper and is now with the sulphate
E.g. Copper solution and zinc metal
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Precipitation
- When two clear solutions produce an insoluble salt (i.e produce a solid ionic compound) (double displacement)
- “Clear” can still have colour
AB(aq) + CD(aq) 🡢 AD(s) + CB(aq)
AgNo3(aq) + NaCl(aq) 🡢 AgCl(s) + NaNo3(aq)
The silver trades places with the sodium and is now joined to the chloride while the sodium is now joined to the nitrate.
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Combustion
- Describes a rapid chemical reaction with oxygen in which a substance burns to release energy in the form of heat and light. Therefore, all combustion reactions can be classified as exothermic reactions.
CH4 + 2O2 → CO2 + 2H2O
The methane (fuel) is burned in the presence of oxygen to form carbon dioxide and water
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Combustion of Fossil Fuels
- Fossil fuels are made up of carbon-based molecules
- Gas stoves and Bunsen burners use methane and ethane to burn with oxygen, and petrol is a mixture of different hydrocarbons, mainly octane
- The complete combustion of these fuels produces water and carbon dioxide. This occurs when there is excess oxygen
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Incomplete Combustion
- Occurs when there is limited oxygen, and the products are either carbon (soot) or carbon monoxide (CO) instead of carbon dioxide (CO2).
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Neutralisation
- Occurs when an acid reacts with a base to form a salt and water:
- Acid + base 🡢 salt + water
2NaOH + H2SO4 🡢 2H2O + Na2SO4
The sodium hydroxide reacts with the sulfuric acid to form sodium sulfate and water (neutral)
Acids and the Salt formed
- Hydrochloric (HCl) = Chloride CL-
- Nitric (HNO3) = Nitrate NO3-
- Sulfuric (H2SO4) = Sulphate SO42-
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Rates of Chemical Reactions
The speed of different chemical reactions varies hugely. Some reactions are very fast, and others are very slow. The speed of a reaction is called the rate of the reaction.
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Reactions, Particles, and Collisions
- Reactions occur when particles collide with a certain amount of energy
- Minimum amount of energy for particles to react is called the activation energy
- Rate of reaction depends on:
1. The frequency of the collisions between particles
2. The energy with which particles collide - If the particles collide with less energy than the activation energy 🡢 They will not react, just bounce of each other
Changing the Rate of Reactions
- Factors that affect the rate of reactions:
1. Increased temperature
2. Increased concentration of dissolved reactants, and increased pressure of gaseous reactants
3. Increased surface area of solid reactants
4. Increased agitation (stirring)
5. Use of a catalyst
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Temperature
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- An increase in temperature will increase the speed of reaction. Increasing the temperature increases speed of particles moving which results in particles colliding more frequently. Gives the particles more energy. This results in chemical bonds breaking and atoms reforming more regularly.
- A decrease in temperature will decrease the speed of reaction.
E.g. Putting cookies in the oven to cook them. Putting milk in the fridge to slow the reaction that makes milk sour.
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Concentration
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- The amount of reactants present in a volume.
- Dilute solution has fewer molecules in the same amount of volume
- An increase in concentration will increase the speed or rate of reaction as the particles are more likely to collide and react when there is a high concentration.
- A decrease in concentration will decrease the speed of reaction.
E.g. Using more gas on a stove. Coating iron to limit the amount of oxygen that can react with the iron.
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Surface Area
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- An increase in surface area will increase the speed of reaction. The reaction takes place to the outside of the solid. If the solid is broken down into pieces the inside of the lump is now also exposed and able to be reacted.
- Increase in pieces 🡢 increase in surface area 🡢 Faster reaction
E.g. Medicine capsules contain powder so it can be absorbed faster into the blood stream.
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Agitation (Stirring)
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- An increase in agitation will increase the speed of reaction. Agitation ensures that reactants are kept in contact.
E.g. When you stir a cup of coffee to make the sugar dissolve.
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Catalysts
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- Catalysts are chemicals that speed up reactions but are not consumed
- The addition of a catalyst increases the speed of a reaction. They reduce the amount of energy that is required to convert the reactants into products. Therefore, make it easier for reactant molecules to collide and form products
E.g. Catalytic converter in car exhausts speed up the reaction between CO (toxic) and O2 to form CO2 (not toxic).
- Enzymes are biological catalysts. They are natural molecules that hold reactant molecules together until they rearrange to form products
E.g. Amylase is found in saliva and helps to break down starch in carbohydrates.
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Solubility
- A substance is said to be soluble if it dissolves.
E.g. sugar is soluble in water. - A substance is insoluble if it does not dissolve.
E.g. chalk is insoluble in water. - A solute is the added substance.
- A solvent is a substance which dissolves a solute.
- A solution = solute + solvent
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Ionic Compounds
A compound formed by cations and anions held together by their mutual attraction, thus forming an ionic bond
Cations
Positively charged ion
Anions
Negatively charged ion
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Ions
A charged atom that has lost or gained electrons
