Chemical Kinetics Notes

Reaction Speed and How It Works

  • Kinetics: This is all about how fast chemical changes happen. Think of it like the speed a car drives.

  • Thermodynamics: This is about if a chemical change is likely to happen, or if it really wants to. Think of it like knowing if a car has enough gas to reach its destination.

  • Key point: Just because a reaction can happen (thermodynamics) doesn't mean it will happen quickly (kinetics). A car might have enough gas for a long trip, but it could still drive very slowly.

  • Initial rate: This is how fast a reaction is going right at the very beginning, when you first mix everything.

  • Reaction example (A → B): Imagine you have a pile of 'A' blocks that are turning into 'B' blocks.

    • Over time, you'll have fewer 'A' blocks and more 'B' blocks.

    • Usually, the speed of making 'B' blocks slows down as you run out of 'A' blocks.

    • The fastest speed is right at the start, when you have the most 'A' blocks.

  • Things we'll learn about:

    • How fast reactions truly are.

    • Comparing the 'average speed' of a reaction to its 'speed right now'.

    • A special "recipe" called the 'general rate law' that tells us how concentration affects speed.

    • 'Reaction orders' which tell us how much each ingredient's amount affects the speed.

Measuring Speed: Start Speed vs. Average Speed

  • Average speed: This is the speed measured over a whole period of time. Like if you say you drove 60 miles in one hour, that's your average speed.

  • Speed right now (instantaneous rate): This is the speed at one exact moment in time, like what your car's speedometer shows you right now.

  • Car example: If a car travels from mile marker 240 to mile marker 300 in one hour:

    • average speed=change in distancechange in time=300−2401 hour=60 miles per hour.\text{average speed} = \frac{\text{change in distance}}{\text{change in time}} = \frac{300 - 240}{1 \text{ hour}} = 60 \text{ miles per hour}.

  • Chemistry example: We do the same thing for reactions! We measure how much a chemical changes over a certain amount of time to find its speed.

What Makes Reactions Go Faster or Slower?

  • How much stuff (Concentration of reactants):

    • If you cram more reactant molecules into a small space, they'll bump into each other more often.

    • More bumps mean more chances to react, so the reaction goes faster.

    • Think of a crowded hallway: you'll bump into people more often than in an empty one.

  • Temperature (how hot it is):

    • When things are hotter, molecules move around much faster and crash into each other harder.

    • Faster, harder crashes mean more successful reactions, so the reaction speeds up.

    • Imagine bouncing balls: if you throw them harder, they hit other balls more often and with more force.

  • Surface area (if it's a solid):

    • If you have a solid chemical, only the parts on its outside can react.

    • If you break it into smaller pieces, or grind it into a powder, you expose much more of its surface.

    • More exposed surface means more places for reactions to happen, so it goes faster (e.g., sugar cube vs. granulated sugar dissolving).

  • Special helpers or hinderers (Catalysts and inhibitors):

    • Catalysts: These are like reaction cheerleaders! They help reactions go faster by finding an easier path or making the bumps more effective, but they don't get used up themselves.

    • Inhibitors: These are like reaction roadblocks! They slow down reactions by getting in the way or making the bumps less effective.

Quick Review: Speed vs. Likelihood, and Reaction Rules

  • Kinetics vs. Thermodynamics (super simple):

    • Kinetics: "How FAST is it?"

    • Thermodynamics: "Will it happen at ALL? (and does it want to?)."

    • Something might really want to happen (like a ball rolling downhill), but if there's a big bump (high energy needed to start), it might take a long time to get going.

  • General rate law and Reaction orders:

    • General rate law: This is a formula that tells us exactly how the speed of a reaction is affected by the amounts of each chemical you put in.

    • Reaction orders: These are special numbers in that formula. They tell you how important the amount of each specific chemical is in changing the overall reaction speed. We find these numbers by doing experiments.

Defining Reaction Speed

  • How we define "rate" for A → B:

    • The reaction speed is usually measured by how quickly the new chemical (B) appears, or how quickly the old chemical (A) disappears.

    • We always want the speed (rate) to be a positive number.

    • rate=change in amount of Bchange in time\text{rate} = \frac{\text{change in amount of B}}{\text{change in time}}

    • If we look at chemical A disappearing, we'd say: rate=−change in amount of Achange in time\text{rate} = - \frac{\text{change in amount of A}}{\text{change in time}}. We put a minus sign because A is disappearing, and we want the rate to be a positive number.

  • How we calculate changes: We always take the final amount minus the starting amount of a chemical, and the final time minus the starting time.

    • For example, if chemical B changes: change in B=final amount of B−initial amount of B\text{change in B} = \text{final amount of B} - \text{initial amount of B}.