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energy mostly where?
- contained within a molecule is found mostly in its bonds.
- Each type of bond has its own specific energy
- Bond energy values: average energy contained in the bonds between two atoms.
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what does decrease energy mean?
decrease in energy, the bonds are more stable.
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hess law
- total energy change resulting from a chemical reaction is dependent only on the initial and final states, and is independent of the reaction route.
- e.g. reactions occur in “steps” and form intermediate
- substances before forming the final products of the reaction.
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hess law
- the total energy change resulting from a chemical reaction is dependent only on the initial and final states,
- independent of the reaction route.
- reactions can be added together to obtain an overall reaction.
- e.g. Some reactions occur in “steps” and form intermediate
- substances before forming the final products of the reaction.
- apple + press - water -> apple concentrate
- apple concentrate + water -> apple juice
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- apples + pressure -> apple juice
- net reaction is same
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Standard Enthalpy of Formation
- reaction that gives the quantity of energy associated with the formation of one mole of a substance from its elements
- Standard Enthalpies of Reaction: reaction that gives the energy change associated with the reaction of one mole of a substance.
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explain how mass, heat capacity, and change in temperature of a substance determine the amount of heat gained or lost by the substance;
- specific heat capacity:heat energy needed to raise the temperature of an object by 1 K
- high specific heat capacities take a lot of heat energy and therefore a long time to heat up and also a long time to cool down.
- more mass = longer to heat up/cool down
- change in temp: more heat enrgy gained/lost
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Rate of chemical reactions
- speed at which a chemical reaction takes place
- usually expressed as the change in concentration of a reactant or product per unit of time.
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collision theory
two species to collide - they have to collide the right way around, and they have to collide with enough energy for bonds to break
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Activation Energy
minimum energy required before a reaction can occur.
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Potential Energy
- energy due to position, composition, or arrangement
- potential to do work when released.
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using collision theory, how the 6 factors affect rate reaction
- The nature of the reactant(s): proceed differently depending on which substances are reacting
- Generally liquids/gases react faster than solids
- Ions in solution tend to react quickly, Reaction rates tend to decrease in molecules with strong covalent bonds esp. if large
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concentration of the reactant(s)
- As the concentration of reactants increases, the number of particles of reactants, per unit of volume, increases; the number of collisions per second also increases. = the rate of reaction will increase
- reaction rate is largest at the beginning for reactants and then decreases as the concentration of reactants gets used up
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surface area of the reactants
- All chemical reactions occur on the surface of the reactants.
- If reactants is broken down into smaller pieces, increases s.a. which can react
- number of collisions between the particles of reactant increases and the rate of reaction also increases.
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Temperature
- High temperatures create faster speeds in the molecules comprising the reactants; causes particles to collide with more force (or higher kinetic energies) = increased probabilities of a chemical reaction à + collision
- reduced in fridge
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Pressure
- increase in the overall pressure cause an increase in the number of collisions between the particles of reactant
- accelerating the rate of chemical reaction, and vice-versa for a decrease of the overall pressure on the system.
- only applies reactants are gases.
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Effect of a catalyst
- alters the rate of a chemical reaction without itself undergoing any permanent chemical change. Not used up in reaction
- A catalyst does not cause a chemical reaction.
- A catalyst does not affect the quantity of substances produced during the reaction.
- work by:
- Adsorption - ·
- Certain particles of reactant accumulate on the surface of the catalyst, increasing the concentration of the reactants locally in that region. easier for the other particles of reactant to react with them.
- Intermediate products - unstable compounds or ions formed by catalysts with certain reactants -> decompose to products
- lower activation energy
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Reaction Mechanisms
- Reactions happen invisibly and any intermediate products may not be obvious
- chemical reactions can occur in a series of steps, with intermediate products, care must be take in determining reaction rates from the overall reaction equation.
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rate determining step
- slowest reaction in a reaction mechanism.
- NO2(g) + CO(g) → NO(g) + CO2(g)
- NO2 + NO2 → NO + NO3 (slow step)
- NO3 + CO → NO2 + CO2 (fast step)
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Rate Law
- Most chemical reaction rates vary with reactant concentrations
- experimental, relation b/twn concentration + reaction rate
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rate equation
rate = k [NO]2[H2]
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Reaction Order
Higher reaction order = reactant has greater impact on the rate of reaction
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