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ISU CHE 141 - Exam 1 Study Guide

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CHE 141 1st EditionExam# 1 Study Guide Lectures: 1 - 9Lecture 1 (January 12)Reaction Rates I. Four factors influence the rate of reaction: 1. Physical stateof the reactants -Reactions in the solid phase tend to be very slow while those in liquid and gas phases are generally faster. 2. Concentrationof reactants-Increasing concentration of particles in a given volume increases the frequency of collisions3. Temperature-Average KE of particles increases with T, thus likelihood of collisions increases. 4. Presence of Catalysts-Catalysts accelerate reactions without being consumed in the processVocab- II. A rate of change is the change that occurs in a given interval of time- An average rate of change is a rate of change measured from the starting time to the finishing time - Speed is a rate of change we use daily:o Speed= change in distance/change in time- The speed of a reaction is called the reaction rateo This is the rate at which reactants are consumed and products are formed during a chemical reactiono The reaction rate is expressed at the change in concentration of a reactant or product in a certain amount of timeo Reaction rate=change in concentration/change in time- We can write a rate of reaction for each reactant and product in a chemical equationo Rate of formation=change in product/change in timeo Rate of consumption=-change in reactant/change in time- Formulas- III. Rule for any reactiono aA+bBcC+dD- The reaction rate is given byo Rate=-1/a x change in [A]/change in time=-1/b x change in [B]/change in time=1/c x change in [C]/change in time=1/d x change in [D]/change in timeo This type of expression is known as relative reaction rate as it is the rate derived from the reaction stoichiometry- A straight line is defined by y=mx+c where y=y variable (dependent), x=x variable (independent), c=constant (y-intercept) and m=gradient (slope)- Gradient the slope of a straight lineo M=change in y/change in x, rise/runo A positive gradient (or slope) has m>0o A negative gradient (or slope) has m<0 o A horizontal line has m=0 (constant function)o A vertical line is parallel to the y axis and has infinite slop (undef)- Tangent a straight line that touches a curve at a single point without intersecting it- Draw a straight line at the point of interest, keeping the angle between the tangent and thecurve the same on both sides- Instantaneous rate the rate of a reaction at a specific instant during the course of a reaction- The instantaneous rate is obtained from the straight-line tangent that touches the curve of concentration as a function of the time at the point of interest- The initial rate is the instantaneous rate determined for t=0Lecture 2 (January 14)Concentration and Reaction rates- I. Reactions occur due to successful collisions between reactant molecules- The more reactant molecules present in a given space the higher the frequency of collisions- At t=0, the concentration of reactants is a maximum and the changes in reactant/product concentrations are rapid so the rate is at a maximum- As reaction proceeds, reactant concentrations decrease so frequently of collisions of reactants decreases and thus the rate at which products are formed decreaseReaction Order- II. The order of a reaction with respect to a reactant tells us the relationship between the initial concentration of that reactant and the rate of the reaction - The reaction order can only be determined experimentally- Zero Order with respect to [X], rate is independent of [X], doubling [X] has no effect on rate- First order with respect to [X], rate is directly proportional to [X], doubling [X] doubles rate- Second order with respect to [X], rate is to the square of [X], doubling [X] quadruples rate- Formlulas- III. From this we can write a rate law - A rate law is an equation that defines the experimentally determined relation between the concentrations of reactants in a chemical reaction and the rate of that reaction; it cannot be predicted by merely looking at the chemical equation- For a generic chemical reaction with 2 reactants A+BC, the rate law expression is Rate=k[A]^m[B]^n- Where m is the partial reaction order with respect to A and n is the partial reaction order with respect to B- The sum of the reaction orders m+n is the overall reaction order- The proportionality constant k, in this expression is called a rate constant- The rate constant is a proportionality constant which related the rate of a reaction to the concentration of reactants- We can determine the rate constant by substituting the initial concentrations and initial rate from an experiment into the rate law- When comparisons between experiments are difficult to make, a mathematical approach can be useful- For the reaction A+BC, where A is kept constant and B is variedo Rate 1/Rate 2=([B]1/[B]2)^n- Where B is constant and A is variedo Rate 1/Rate 2=([A]1/[A]2)^m- An exponent in a rate law (order) may be an integer, fraction, zero, or even negative - Rate laws and reaction orders are different from relative rates and must be determined experimentally- The value of the rate constant k does not change with concentration of reactants- The units of the rate constant k depend on the order of reaction and must be derived directly from the rate lawLecture 3 (January 16)Time and Reaction Rates- Reaction rate changes with concentration (kinetic molecular theory)- Reactant concentrations change over the course of the reaction- Rate of reaction changes over the course of the reactionIntegrated Rate Laws: First Order Reactions- Consider the decomposition reactiono Xproducts- This reaction is first order (depends only on the concentration of one reactant, X) so fromthe relative rate Rate=-change in [X]/change of t=k[X]- This is a differential rate equation that relates the rate of change of concentration to the concentration itself- Integration of the differential rate equation produces the corresponding integrated rate law, which relates the concentration to time: Rate=-change in [X]/change in t=k[X] then rearrange in terms of k (change in t) so change in [X]/[X]=-k (change in t), at t=0 [X]=[X]0 rearranging in terms of ln[X] which is ln[X]=-kt+ln[X]0- This is the integrated rate law for the first order reaction: ln[X]=-kt+ln[X]0- k=rate constant- [X]0=initial concentration of reactant X- This is sometimes written in terms of [X] using the exponential function [X]=[X]0e^-ktMathback- Logarithms are basically another way of writing indices- When


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