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Rate of chemical change Rate of chemical change is expressed in terms of Concentration of reactants or products.

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Presentation on theme: "Rate of chemical change Rate of chemical change is expressed in terms of Concentration of reactants or products."— Presentation transcript:

1 Rate of chemical change Rate of chemical change is expressed in terms of Concentration of reactants or products

2 Collision Theory Reaction Rate depends on: 1) The number of collision per unit time between the reacting species. 2) The fraction of these collisions that are successful in producing a new molecule.

3 Activation energy The reactants must gain enough energy to climb an energy hill They form an activated complex or a transition state before they cascade down becoming products

4 Will a Reaction take place? Only if the collisions have enough energy, and have the right orientation

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6 Concentration & Reaction Rate The more concentrated the solution, the more chance of a collision The more collisions, the more likely an activated complex will form Click on picture to see movie

7 Particle size: small particle size increases collisions Large amounts of surface area Small amounts of surface area Movie

8 The effect of temperature on rate A high temperature means particles will collide more often A high temperature means particles collide with more force

9 Effect of a catalyst A catalyst provides an alternative route for the reaction with a lower activation energy

10 Effect of a Catalyst

11 Reversable Reactions and K eq K eq = equilibrium constant K eq = [productC] c [productD] d [reactantA] a [reactantB] b K eq changes only when the temperature changes. aA + bB cC + dD

12 Reversible reactions reach a state of dynamic equilibrium N 2 + 3H 2 2 NH 3 K eq = [NH 3 ] 2 [H 2 ] 3 [ N 2 ]

13 LeChateliers principle A system at equilibrium resists change. If it is exposed to stress (change) it will shift the equilibrium point to try to reduce the stress (change) [Co(H2O) 6]+2 + 4Cl- [CoCl 4 ]-2 + 6H2O Pink blue

14 LeChateliers principle If you add a reactant – the equilibrium shifts toward the product If you add a product – the equilibrium shifts toward the reactant If you take away a reactant – the equilibrium shifts toward the reactants If you take away a product – the equilibrium shifts toward the product side

15 LeChateliers principle & heat If the reaction is exothermic (heat is a product), So, adding heat shifts the reaction to the reactant side If the reaction is endothermic (heat is a reactant) – adding heat shifts the reaction to the product side

16 LeChateliers principle & Pressure If the reaction contains gas particles, pressure is considered a stress condition 2SO 2 + O 2 2SO 3 Adding pressure shift the equilibrium to the side with fewer gas particles will have less pressure

17 Entropy: chaos S = S p -S r If S is +, entropy change is favored S = S p -S r Parts of a whole Unmix Unexplode unmelt The universe favors chaos

18 Is the reaction spontaneous This is a tug of war between entropy and enthalpy G = H - T S ( T is in kelvin) If Gibbs free energy is a negative number, the reaction is spontaneous If Enthalpy is negative, (exothermic), and entropy is + (chaos favored) the reaction is always spontaneous If one of the conditions is not favored – it becomes a tug of war


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