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Reaction Time: The time from the onset of a stimulus until a response is observed.
 Science Education: Essentials of Cognitive Psychology

Measuring Reaction Time and Donders' Method of Subtraction

JoVE Science Education

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University

The ambition of experimental psychology is to characterize the mental events that support the human ability to solve problems, perceive the world, and turn thoughts into words and sentences. But people cannot see or feel those mental events; they cannot be weighed, combined in test tubes, or grown in a dish. Wanting to study mental life, nonetheless, Franciscus Donders, a Dutch ophthalmologist in the early 1800s, came up with a property that he could measure—even back then: he measured the time it took for human subjects to perform simple tasks, reasoning that he could treat those measurements as proxies for the time it takes to complete the unobservable mental operations involved. In fact, Donders went one step further, developing a basic experimental paradigm known as the Method of Subtraction. It simply asks a researcher to design two tasks that are identical in nearly every way, excepting a mental operation hypothesized to be involved in one of the tasks and omitted in the other. The researcher then measures the time it takes to complete each task, and by subtracting the outcomes, he extracts an estimate of the time it takes to execute the one mental operation of interest. In this way, the method allows a researcher

 JoVE Chemistry

Conducting Miller-Urey Experiments

1School of Chemistry and Biochemistry, Georgia Institute of Technology, 2Earth-Life Science Institute, Tokyo Institute of Technology, 3Institute for Advanced Study, 4Astromaterials Research and Exploration Science Directorate, NASA Johnson Space Center, 5Goddard Center for Astrobiology, NASA Goddard Space Flight Center, 6Geosciences Research Division, Scripps Institution of Oceanography, University of California at San Diego

JoVE 51039

 Science Education: Essentials of General Chemistry

Determining Rate Laws and the Order of Reaction

JoVE Science Education

Source: Laboratory of Dr. Neal Abrams — SUNY College of Environmental Science and Forestry

All chemical reactions have a specific rate defining the progress of reactants going to products. This rate can be influenced by temperature, concentration, and the physical properties of the reactants. The rate also includes the intermediates and transition states that are formed but are neither the reactant nor the product. The rate law defines the role of each reactant in a reaction and can be used to mathematically model the time required for a reaction to proceed. The general form of a rate equation is shown below:     where A and B are concentrations of different molecular species, m and n are reaction orders, and k is the rate constant. The rate of nearly every reaction changes over time as reactants are depleted, making effective collisions less likely to occur. The rate constant, however, is fixed for any single reaction at a given temperature. The reaction order illustrates the number of molecular species involved in a reaction. It is very important to know the rate law, including rate constant and reaction order, which can only be deter

 JoVE Biology

Enhanced Reduced Representation Bisulfite Sequencing for Assessment of DNA Methylation at Base Pair Resolution

1Department of Medicine, Weill Cornell Medical College, 2Institute for Computational Biomedicine, Weill Cornell Medical College, 3Department of Physiology and Biophysics, Weill Cornell Medical College, 4Department of Pathology, University of Michigan

JoVE 52246

 JoVE Behavior

Vision Training Methods for Sports Concussion Mitigation and Management

1Neurology and Rehabilitative Medicine, University of Cincinnati, 2Division of Sports Medicine, Department of Orthopaedic Surgery, University of Cincinnati, 3Department of Athletics, University of Cincinnati, 4Department of Neurosurgery, University of Cincinnati, 5College of Education, Criminal Justice, and Human Services, University of Cincinnati, 6Division of Sports Medicine, Cincinnati Children's Hospital Medical Center

JoVE 52648

 Science Education: Essentials of General Chemistry

Spectrophotometric Determination of an Equilibrium Constant

JoVE Science Education

Source: Laboratory of Dr. Michael Evans — Georgia Institute of Technology

The equilibrium constant, K, for a chemical system is the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their respective stoichiometric coefficients. Measurement of K involves determination of these concentrations for systems in chemical equilibrium. Reaction systems containing a single colored component can be studied spectrophotometrically. The relation between absorbance and concentration for the colored component is measured and used to determine its concentration in the reaction system of interest. Concentrations of the colorless components can be calculated indirectly using the balanced chemical equation and the measured concentration of the colored component. In this video, the Beer's law curve for Fe(SCN)2+ is determined empirically and applied to the measurement of K for the following reaction: Four reaction systems with different initial concentrations of reactants are investigated to illustrate that K remains constant irrespective of initial concentration

 JoVE Developmental Biology

Understanding Early Organogenesis Using a Simplified In Situ Hybridization Protocol in Xenopus

1Developmental and Stem Cell Biology, Hospital for Sick Children, 2Children's Health Research Institute, University of Western Ontario, 3Department of Physiology and Pharmacology, University of Western Ontario, 4Neurosciences and Mental Health, Hospital for Sick Children, 5Department of Paediatrics, University of Western Ontario

JoVE 51526

 Science Education: Essentials of Organic Chemistry

Conducting Reactions Below Room Temperature

JoVE Science Education

Source: Laboratory of Dr. Dana Lashley - College of William and Mary

Demonstration by: Matt Smith

When new bonds are formed in the course of a chemical reaction, it requires that the involved species (atoms or molecules) come in very close proximity and collide into one another. The collisions between these species are more frequent and effective the higher the speed at which these molecules are moving. A widely used rule of thumb, which has its roots in the Arrhenius equation1, states that raising the temperature by 10 K will approximately double the rate of a reaction, and raising the temperature by 20 K will quadruple the rate: (1) Equation (1) is often found in its logarithmic form: (2) where k is the rate of the chemical reaction, A is the frequency factor (relating to frequency of molecular collisions), Ea is the activation energy required for the reaction, R is the ideal gas constant, and T is the temperature at which the r

 Science Education: Essentials of General Chemistry

Le Châtelier's Principle

JoVE Science Education

Source: Laboratory of Dr. Lynne O'Connell — Boston College

When the conditions of a system at equilibrium are altered, the system responds in such a way as to maintain the equilibrium. In 1888, Henri-Lewis Le Châtelier described this phenomenon in a principle that states, "When a change in temperature, pressure, or concentration disturbs a system in chemical equilibrium, the change will be counteracted by an alteration in the equilibrium composition." This experiment demonstrates Le Châtelier's principle at work in a reversible reaction between iron(III) ion and thiocyanate ion, which produces iron(III) thiocyante ion: Fe3+(aq) + SCN- (aq) FeSCN2+ (aq) The concentration of one of the ions is altered either by directly adding a quantity of one ion to the solution or by selectively removing an ion from the solution through formation of an insoluble salt. Observations of color changes indicate whether the equilibrium has shifted to favor formation of the products or the reactants. In addition, the effect of a temperature change on the solution at equilibrium can be obs

 JoVE Biology

Protocols for Implementing an Escherichia coli Based TX-TL Cell-Free Expression System for Synthetic Biology

1Department of Biology, California Institute of Technology, 2Department of Bioengineering, California Institute of Technology, 3Synthetic Biology Center, Department of Bioengineering, Massachusetts Institute of Technology, 4School of Physics and Astronomy, University of Minnesota

JoVE 50762

 Science Education: Essentials of Organic Chemistry

Preparing Anhydrous Reagents and Equipment

JoVE Science Education

Source: Laboratory of Dr. Dana Lashley - College of William and Mary
Demonstrated by: Timothy Beck and Lucas Arney

Many reactions in organic chemistry are moisture-sensitive and must be carried out under careful exclusion of water. In these cases the reagents have a high affinity to react with water from the atmosphere and if left exposed the desired reaction will not take place or give poor yields, because the reactants are chemically altered. In order to prevent undesired reactions with H2O these reactions have to be carried out under an inert atmosphere. An inert atmosphere is generated by running the reaction under nitrogen gas, or in more sensitive cases, under a noble gas such as argon. Every component in such a reaction must be completely anhydrous, or free of water. This includes all reagents and solvents used as well as all glassware and equipment that will come into contact with the reagents. Extremely water-sensitive reactions must be carried out inside of a glovebox which provides a completely sealed off anhydrous environment to work under via a pair of gloves which protrudes out to one of the sides of the chamber.

 JoVE Biology

Reconstitution Of β-catenin Degradation In Xenopus Egg Extract

1Department of Cell and Developmental Biology and Program in Developmental Biology, Vanderbilt University Medical Center, 2Division of Gastroenterology, Hepatology & Nutrition and Division of Developmental Biology, Cincinnati Children's Hospital Medical Center, 3Vanderbilt Ingram Cancer Center, Vanderbilt University School of Medicine

JoVE 51425

 JoVE Biology

Quantitative, Real-time Analysis of Base Excision Repair Activity in Cell Lysates Utilizing Lesion-specific Molecular Beacons

1Department of Pharmacology & Chemical Biology, University of Pittsburgh School of Medicine, 2Hillman Cancer Center, University of Pittsburgh Cancer Institute, 3Department of Experimental Therapy, The Netherlands Cancer Institute, 4Department of Human Genetics, University of Pittsburgh School of Public Health

JoVE 4168

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