# Tutor profile: Elena S.

## Questions

### Subject: Chemistry

A reaction has a positive change in enthalpy and entropy. At 30 degrees Celsius, it is non-spontaneous, while at 60 degrees Celsius, it becomes spontaneous. Why does increasing the temperature of this reaction cause it to become spontaneous?

A reaction is spontaneous when the change in Gibbs Free Energy (delta G) is negative. The formula to calculate change in Gibbs Free Energy is (change in enthalpy)-(Temperature)(change in entropy). Thus, it is unfavorable for enthalpy to be positive but favorable for entropy to be positive. When the favorable entropy value is multiplied by a larger quantity, the equation will be more likely to result in a negative delta G. Thus, having a larger temperature value will cause the favorable change in entropy to become more important, resulting in a spontaneous reaction. At a low temperature, the unfavorable enthalpy outweighs the entropy, resulting in a positive, non-spontaneous delta G.

### Subject: Biology

If an animal cell does not have access to oxygen, how does it continue generating ATP through glycolysis?

Glycolysis is an anaerobic process, meaning it does not require oxygen to run. It does, however, require NAD+, the oxidized form of NADH. In the electron transport chain, NADH is oxidized to NAD+, which the cell can later use in glycolysis. When a cell does not have oxygen, the electron transport chain cannot run and no NAD+, which is necessary for glycolysis, is produced. Thus, in anaerobic conditions, the cell must oxidize NADH by reducing pyruvate (a carbohydrate) to form lactic acid in a process called fermentation. This NAD+ is now available as an input to glycolysis, and glycolysis provides a net output of 2 ATP.

### Subject: Statistics

A zoologist wants to see if there is a difference in the mean lifespan of chimpanzees that are held in captivity and those that roam free in the wild. She collects a random sample of 50 chimpanzees held in zoos from around the world, and a random sample of 50 chimpanzees that live in the wilderness. What statistical test should she use to compare the mean lifespans of these populations? Are all of the requirements to run this test met?

The zoologist should use a two-sample t-test in her study (two-tailed since does not hypothesize that one group's mean will be greater than the other). Two-sample t-tests are used to compare means from two samples when the standard deviation is not known. The conditions to run this test are met. Both samples are stated to be randomly selected. The Central Limit Theorem (CLT) applies because both samples are greater than 30, which fulfills the normality condition. Samples are independent because the population of all chimpanzees in captivity and all chimpanzees in the wilderness is greater than 500 (50x10).

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